Advantages of all in-room terminal unit systems include the following:

In-room terminal unit systems with central ventilation air have these additional advantages:

Basic calculations for airflow, temperatures, relative humidity, loads, and psychrometrics are covered in Chapters 1, 17, and 18 of the 2017 ASHRAE Handbook—Fundamentals. Use caution in determining peak load conditions for each space served by a terminal unit. Rather than depending on guidelines for typical lighting, ventilation, infiltration, equipment, and occupancy levels, the designer should try to get realistic values based on the particular owner’s use plans for the facility. If the client has an existing or similar facility, visiting it to understand the actual occupancy hours, concentration of equipment, and occupancy should help avoid unrealistic assumptions. Incorporating effects of planned energy-saving features (e.g., daylighting; high-efficiency/low-heat-producing lighting; shading apparatus for privacy, glare, or solar radiant control; fan and outdoor air modulation; full-building energy management control strategies) can prevent oversizing of terminal units and resulting potential loss of humidity control. Determining areas where the normal base load will be a small percentage of the concentrated usage load and understanding the usage schedule for the area can allow equipment selection and control strategies to maximize energy savings and still provide excellent comfort control in both extremes.

For example, in a zone with a terminal unit sized for 7000 Btu/h (common for offices), a simple change of four 100 W incandescent light bulbs to compact fluorescents could reduce the space’s heat load by about 18% of the unit’s capacity. If this is the consistent peak load of the space for future years, the terminal unit is oversized. Unless a central ventilation pretreatment system handles the entire latent load of the space and the outdoor air, humidity control will be lost in the space.

Integrated building design (IBD) techniques should be used to ensure the building envelope provides adequate energy efficiency, airtightness, and moisture penetration control to allow terminal units to control the indoor environmental conditions without need for excessive moisture control in each space. Close cooperation of all parties during design is necessary to create an overall building design that minimizes the required mechanical systems’ energy consumption while achieving good indoor conditions. For details on IBD, see Chapter 58 of the 2019 ASHRAE Handbook—HVAC Applications.

Computer programs generally can model primary ventilation systems as well as secondary in-room terminal systems. Most, however, do not allow the user to assign the room latent load as part of the primary ventilation system capacity requirement. Therefore, the designer needs to either manually determine final capacity requirements for both the primary and in-room units, or use overrides and manual inputs to redistribute the loads within the computer program after initial sensible and latent loads as well as block and peak conditions have been determined.

The design refrigeration load is determined by considering the entire portion or block of the building served by the air-and-water system at the same time. Because the load on the secondary-water system depends on the simultaneous demand of all spaces, the sum of the individual room or zone peaks is not considered.

Some in-room terminal units provide only heating to the end space. Equipment such as cabinet or unit heaters, radiant panels, radiant floors, and finned-tube radiators are designed for heating only. Extreme care must be used with these systems if they are incorporated into a two-pipe changeover piping distribution system, or any other system in which secondary water being piped is not consistently over 100°F. The heating coils in these units are not designed to handle condensation, and there is no drain pipe in the unit. If cold water is provided to these units, dripping condensation from units, valves, or piping may damage building finishes or saturate the insulation, leading to mold growth. Ball valves tied into the automatic temperature control (ATC) system and/or aquastats should be provided to prevent water at temperatures below space air dew point from reaching heating-only terminal units.

Generally, the supply air volume from the central apparatus is constant and is called primary or ventilation air to distinguish it from recirculated room air or secondary air. The quantity of primary air supplied to each space is determined by the amount of outdoor air required by codes and other applicable guidelines for ventilation. If in-room terminal units are heating-only units, then the primary-air system must also provide the required sensible cooling capacity at maximum room cooling load. The air may be from outdoors, or may be mixed outdoor and return air. During the cooling season, air is dehumidified sufficiently in the central conditioning unit to maintain required humidity conditions and to prevent condensation on the in-room terminal unit cooling coil. (Both outdoor air and space latent loads should be handled by the central unit.) Centrally supplied air can be supplied at a low enough dew point to absorb moisture generated in the space, but as a minimum should be supplied at a condition so that the room terminal unit has to remove only the space-generated latent load (this is only appropriate with unit ventilators with the capability to handle the latent space loads). In winter, moisture can be added centrally to limit dryness.

As the primary air is dehumidified, it is also cooled. The air should be cool enough when delivered to offset part of the room sensible loads without overcooling the spaces. If the necessary amount of cooling (i.e., that needed to dehumidify primary air to handle all of the space and outdoor air latent loads) is likely to overcool the end spaces, the primary air should be reheated to minimize any overcooling possibilities. A heating coil located in the reheat position may be required in the central (primary) air handler, as well as a preheater in areas with freezing weather. Alternatively, reheating can occur at the terminal units, where only a minimal number of spaces might be overcooled.

When outdoor air is introduced from a central ventilation system, it may be connected to the inlet plenum of some in-room terminal units (fan-coils, unit ventilators, active chilled beams, or induction units), or introduced directly into the space. If introduced directly, ensure that this air is pretreated, dehumidified, and held at a temperature approximately equal to room temperature so as not to cause occupant discomfort when the space unit is off. Caution should always be used to prevent overcooling or loss of humidity control from ventilation air, which can lead to condensation problems on surfaces as well as discomfort for occupants.

In the ideal in-room terminal unit design, the cooling coil is always dry, greatly extending terminal unit life and eliminating odors and the possibility of bacterial growth in the unit in the occupied space, as well as limiting condensation issues in the spaces. In this case, in-room terminals may be replaced by radiant panels (see Chapter 6) or chilled beams and panels, as the primary air controls the space humidity. Therefore, the moisture content of primary air must be low enough to offset the room’s latent heat gain and to maintain a room dew point low enough to preclude condensation on the secondary cooling surface.

Even though some systems operate successfully with little or no condensate, a condensate drain is recommended. In systems that shut down during off hours, start-up load may include a considerable dehumidification load, producing moisture to be drained away. In climates with elevated outdoor air dew points during space-cooling periods, piped condensate removal systems that can be maintained regularly should always be included.

Central equipment size is based on the block load of the entire building at the time of the building peak load, not on the sum of individual in-room terminal unit peak loads. Cooling load should include appropriate diversity factors for lighting and occupant loads. Heating load is based on maintaining the unoccupied building at design temperature, plus an additional allowance for pickup capacity if the building temperature is set back at night. For additional information, see Chapter 3.

If water supply temperatures or quantities are to be reset at times other than at peak load, the adjusted settings must be adequate for the most heavily loaded space in the building. Analysis of individual room load variations is required.

If the side of the building exposed to the sun or interior zone loads requires chilled water in cold weather, consider using condenser water with a water-to-water heat exchanger or a four-pipe system. Varying refrigeration loads require the water chiller to operate satisfactorily under all conditions.

As with any HVAC system, the amount of ventilation air required depends on the number of occupants in the space as well as other factors (see ASHRAE Standard 62.1). The rate of airflow per person or per unit area is also usually dictated by state codes, based on activity in the space and contaminant loads. If the amount of ventilation air required is considerable (i.e., 10% or more of a space’s total supply air volume), the designer needs to consider how the excess air will move out of the space and the building. Means of preventing overpressurization may have to be provided, depending on building tightness, amount of exhaust, and other considerations. Additionally, the designer should consider heat recovery for the rejected air.

As with all systems, the initial cost of an in-room terminal system varies widely, depending on location, local economy, and contractor preference (even for identical systems). For example, a through-wall unit ventilator system is less expensive than fan-coil units with a central ventilation system, because it does not require extensive ductwork distribution. The operating cost depends on the system selected, the designer’s skill in selecting and correctly sizing components, and efficiency of the duct and piping systems. A terminal unit design without a central ventilation system is often one of the less expensive systems to install, but in most situations will not operate without some condensation and humidity issues.

Because in-room terminal equipment is in each occupied space, maintenance may be more time consuming, depending on the size of the facility. The equipment and components are less complex than other equipment. A common method of repairing in-room terminal units is to simply disconnect and replace a nonfunctioning unit, minimizing the time spent in the occupied space. The nonfunctioning unit can then be repaired in a workshop and used as a spare. The number of individual units is much greater than in many systems, therefore increasing the number of control valves. If a building automation system (BAS) is used, the number of control points is increased as well, which raises the first cost and maintenance costs of the controls.

The engineer’s early involvement in design of any facility can considerably reduce the building’s energy consumption; collaboration between the architect and engineer allows optimization of energy conservation measures, including the building envelope, as well as selection of an HVAC system that is energy efficient with minimal operational costs. In practice, however, a system might be selected based on low first cost or to perform a particular task. In general, terminal units can save energy if the BAS controls operation of the units and can deenergize them if the space is unoccupied. This adds significant cost to the control system, but can save on operational costs. If a central ventilation system is used, energy recovery in the air handler should be considered to minimize operation costs.

The choice of two- or four-pipe piping system and the design of the piping system is another energy-impacting decision. Pumping horsepower, insulation to control heat losses, and the number of yearly changeovers all affect the appropriateness and available energy savings of each type of piping system.

Life-cycle costs include the first, operational, maintenance, and replacement costs over a predetermined length of time. With an in-room terminal system, a major portion of the cost of the system is the insulated piping infrastructure needed to provide the primary and secondary equipment with water (or refrigerant). When properly selected, installed, and maintained, these components have a very long expected life. The in-room terminal units are generally of simple design and complexity and can be readily repaired. As a general rule, the less condensate that occurs at the in-room units, the longer their life span. Selection of good-quality central ventilation equipment can result in quite lengthy life spans for these units. Operational concerns that affect costs are discussed throughout this chapter. Consult Chapter 37 of the 2019 ASHRAE Handbook—HVAC Applications for more information on equipment life expectancies and predicting maintenance costs.

Terminal units have common components; mainly, a fan, coil(s), filter, dampers, and controls (although some units only have coils and controls).

Terminal units are available in many different configurations; however, not all configurations are available for all types of terminal units. The designer should evaluate the air pathways of the specific units being considered, because some allow raw outdoor air to bypass the filter and/or the coils. Additionally, on some unit ventilators with face and bypass, the raw outdoor air is allowed to stratify in the bypass section, again bypassing the coils. Depending on climate conditions and filtration requirements, these configurations may not be appropriate.

Low-profile vertical units are available for use under windows with low sills; however, in some cases, the low silhouette is achieved by compromising features such as filter area, motor serviceability, and cabinet style.

Floor-to-ceiling, chase-enclosed units are available in which the water and condensate drain risers are part of the factory-furnished unit. Stacking units with integral prefabricated risers directly one above the other can substantially reduce field labor for installation, an important cost factor. These units are used extensively in hotels and other residential buildings. For units serving multiple rooms (single-occupancy suite-type spaces), the supply and return air paths must be isolated from each other to prevent air and sound interchange between rooms.

Perimeter-located units give better results in climates or buildings with high heating requirements. Heating is enhanced by under-window or exterior wall locations. Vertical units with finned risers can operate as convectors with the fans turned off during night setback, and overheating can become an issue.

Horizontal overhead units may be fitted with ductwork on the discharge to supply several outlets. A single unit may serve several rooms (e.g., in an apartment house where individual room control is not essential and a common air return is feasible). Units must have larger fan motors designed to handle the higher static pressure resistance of the connected ductwork.

Horizontal models conserve floor space and usually cost less, but when located in furred ceilings, they can create problems such as condensate collection and disposal, mixing return air from other rooms, leaky pans damaging ceilings, and difficult access for filter and component removal. In addition, possible condensate leakage may present air quality concerns.

Other sections in this chapter discuss specific configurations for the type of unit, and indicate additional chapters in the volume that provide diagrams and additional information.

Four-pipe distribution of secondary water has dedicated supply and return pipes for chilled and hot water. The four-pipe system generally has a high initial cost compared to a two-pipe system but has the best system performance. It provides (1) all-season availability of heating and cooling at each unit, (2) no summer/winter changeover requirement, (3) simpler operation, and (4) hot-water heating that uses any heating fuel, heat recovery, or solar heat. In addition, it can be controlled at the terminal unit to maintain a dead band between heating and cooling so simultaneous heating and cooling cannot occur.

Three-pipe distribution uses separate hot- and cold-water supply pipes. A common return pipe carries both hot and cold water back to the central plant. The terminal unit control introduces hot or cold water to the common unit coil based on the need for heating or cooling. This type of distribution is not recommended because of its energy inefficiency from constantly reheating and recooling water, and it does not comply with most recognized energy codes.

Condenser water systems are very similar to the two-pipe changeover with partial electric strip heat. The supply and return pipes carry water at more moderate temperatures than those of typical chilled or hot water. The heat pumps use the water as a heat source for heating and as a heat sink in cooling mode, allowing various in-room heat pumps to meet room needs during intermediate seasons. If additional heat is needed in a room, a small electric strip heater in the terminal unit can provide it. When the outdoor temperature drops sufficiently to require additional heating capacity, the water system must be changed over to hot water. Chapters 9, 14, and 49 contain additional information about condenser water piping systems and heat pumps.

Floor-mounted units have various ventilation air ductwork connections, including from the back or a ducted collar on the top of the cabinet. Ceiling-mounted and stacking units can be mounted completely exposed, partially exposed in a soffit, fully recessed, or concealed. Ventilation air connections can be made in the back or top of the unit.

For existing building retrofit, it is often easier to install piping and wiring for a terminal unit system than the large ductwork required for an all-air system. Common fan-coil system applications are hotels, motels, apartment buildings, and office buildings. Fan-coil systems are used in many hospitals, but they are less desirable because of the low-efficiency filtration and difficulty in maintaining adequate cleanliness in the unit and enclosure. In addition, limits set by the American Institute of Architects’ Guidelines for Design and Construction of Hospital and Health Care Facilities (AIA 2001) do not allow air recirculation in certain types of hospital spaces.

Unit ventilator systems are most frequently used in classrooms, which need a high percentage of outdoor air for proper ventilation. Unit ventilators are often located under a window along the perimeter wall. They are available in a two-pipe configuration with changeover, two-pipe with electric heating, four-pipe, or with heating and DX coils for spaces (e.g., computer rooms) that may require year-round cooling. Limited ductwork may be allowed, allowing for higher and often exposed ceiling systems. These units can often be misused as shelving in classroom; books and paperwork may be stacked on top of them, impeding airflow to the space. Also, ventilation air intake louvers can become choked by vegetation if they are not properly maintained. In addition, because the fans are sized to accommodate 100% ventilation air, they are typically noisier than fan-coils, although recent developments have led to new units that are much quieter.

For existing building retrofit, it is easiest to replace unit ventilators in kind. If a building did not originally use unit ventilators, installing multiple ventilation air intake louvers to accommodate the unit ventilators may be cost prohibitive. Likewise, installing a different type of system in a building originally fitted with unit ventilators requires bricking up intake louvers and installing exposed ductwork (if there is no ceiling plenum) or creating a ceiling space in which to run ductwork. Unit ventilators are best applied where individual space temperature control with large amounts of ventilation air is needed.

Fan-coil and unit ventilators often receive ventilation air from a penetration in the outer wall or from a central air handler. Units that have outdoor air ducted to them from an aperture in the building envelope are not suitable for tall buildings because constant changes in wind pressure cause variations in the amount of outdoor air admitted. In this situation, ventilation rates can also be affected by stack effect, wind direction, and speed. Also, freeze protection may be required in cold climates, because preheating outdoor air is not possible. Historically, fan-coils have often been used in residential construction because of their simple operation and low first cost, and because residential rooms were often ventilated by opening windows or by outer wall apertures rather than a central system. Operable windows can cause imbalances with a ducted ventilation air system; current standards have moved toward requiring specific ventilation airflow control in residences, therefore minimizing the ability to use noncentralized ventilation systems, even in residential applications.

Unlike fan-coils, unit ventilators can provide the entire volume of ventilation air that is required in many applications. The heating/cooling coils in unit ventilators therefore differ considerably from fan-coils. Coils in unit ventilators are much deeper, because the unit ventilator needs to be able to heat, cool, and dehumidify up to 100% ventilation air. Coil selection must be based on the temperature of the entering mixture of primary and recirculated air, and air leaving the coil must satisfy the room’s sensible and latent cooling and heating requirements. If variable occupancy levels regularly occur during system operation hours, such as often occurs with classrooms, sizing a single unit for the fully occupied outdoor air requirement could result in oversized cooling capacity during many hours of operating time. This could result in loss of humidity control. For variable-occupancy applications, demand control ventilation or pretreated outdoor air is recommended.

Some designers select fan-coil units and unit ventilators for nominal cooling at medium speed when a three-speed control switch is provided, to enable quieter operation in the space and add a safety factor (sensible capacity can be increased by operating at high speed). Sound power ratings are available from many manufacturers.

If using a horizontal overhead unit with ducted supply and return, fan capacity may be the factor that decides the unit’s size, not the coil’s capacity. Static pressure as little as 0.3 in. of water can significantly affect fan air volume and unit capacity.

If the unit is selected to provide full capacity at medium speed, the unit must also be able to handle the full volume of required ventilation air at that airflow. If cooling loads vary more than 20% during operation hours, it is highly likely this selection could result in oversized capacity operation and loss of humidity control. Current fan motor speed control options, such as electronically commutated motors (ECMs), and units designed to operate more quietly allow unit selection for the fan’s full-speed total capacity, and minimize the chance of oversizing.

Fan-coil and unit ventilator blower fans are driven by small motors. Fan-coil motors are generally shaded pole or capacitor start with inherent overload protection. Operating wattage of even the largest sizes rarely exceeds 300 W at high speed. Running current rarely exceeds 2.5 A. Unit ventilator motors are typically 1/2 hp or less. Operating power of even the largest sizes rarely exceeds 400 W at high speed. Almost all motors on units in the United States are 120 V, single-phase, 60 Hz current.

In planning the wiring circuit, follow all required codes. The preferred wiring method generally provides separate electrical circuits for fan-coil or unit ventilator units and does not connect them into the lighting circuit, or other power circuits. Separate wiring connections may be needed for condensate pumps.

Even when outdoor air is pretreated, a condensate removal system should be installed for fan-coil units and unit ventilators. Drain pans should be integral for all units. For floor-mounted units along the perimeter of the building, condensate piping can run from the drain pan to the exterior grade. Where drainage by gravity will not be sufficient, provide condensate pumps. Condensate drain lines should be properly sized and maintained to avoid clogging with dirt and other materials. Condensation may occur on the outside of drain piping, which requires that these pipes be insulated. Many building codes have outlawed systems without condensate drain piping because of the potential damage and possibility of mold growth in stagnant water accumulated in the drain pan.

Fan-coil and unit ventilator capacity is usually controlled by coil water flow, fan speed, or a combination of these. In addition, unit ventilators often are available with a face-and-bypass damper, which allows for another form of capacity control. For additional information, see the discussion on capacity control in the section on System Components and Configurations.

Fan-coil and unit ventilator systems require more maintenance than central all-air systems, and the work must be done in occupied areas. Units operating at low dew points require regular (multiple times per year) cleaning and flushing of condensate pans and drains to prevent overflow and microbial build-up. Coils should be cleaned at least once a year, and more often when they consistently are removing moisture. The physical restraints of in-room terminal units located high in rooms or concealed in ceilings, soffits, etc., can create challenges for proper coil cleaning. Water valves, controls, and dampers should also be checked yearly for proper calibration, operation, and needed repairs.

Filters are small and low-efficiency, and require frequent changing to maintain air volume. Cleaning frequency varies with the application. Units in apartments, hotels, and hospitals usually require more frequent filter service because of lint. Unit motors may require periodic lubrication. Motor failures are not common, but when they occur, the entire fan can be quickly replaced with minimal interruption in the conditioned space. More specialized motors with speed control devices may take several days to get a replacement, so large facilities should stock several spares for quick replacement to avoid significant down time for the unit. Chapters 20 and 28 provide more information on fan-coils and unit ventilators, respectively.

Passive beams are available in sections up to 10 ft long and 18 to 24 in. wide. They can be located above the ceiling with perforated panels below it, mounted into the frame of an acoustical tile ceiling, or mounted in the conditioned space. The perforated panels must have a minimum 50% free area and extend beyond either side of the beam for usually half of the unit’s width, so the convection current is not hindered. Also, care must be taken to not locate passive beams too close to window treatments, which can also hinder air movement around the beam.

Active beams are available in sections up to 10 ft long and 12 to 24 in. wide. They can be mounted into the frame of an acoustical tile ceiling or in the conditioned space.

Passive beams require a separate ventilation system to provide tempered and dehumidified air to the space. The ventilation air should be ducted to low-wall diffusers or in an underfloor distribution system so that the ventilation air does not disturb the convection currents in the conditioned space. Ventilation air can be ducted directly into the active beams. If more ventilation air is needed to meet the space requirements, the volume of air can be split by the active beams and high-induction diffusers. Care must be taken in selecting diffuser locations to coordinate well with the convective currents required by the chilled beams.

Chilled beams are selected based on the calculated heat gain for the space less the cooling effect of the primary ventilation air.

Chilled beams only require controls wiring. There is no fan or other electrical equipment to be wired.

Chilled beams are designed to operate dry, with few exceptions. In some active beams with vertical coils, a drain pan may be installed. However, as a rule, a separate ventilation system should be sized to handle the latent cooling load in the space, and the relative humidity should be closely monitored. If the primary ventilation system fails to properly control the space humidity, condensation may form on the beams and their housings. Dripping of this condensate could damage building materials and contents.

Chilled-water valves should have drain pans to contain any normal condensate that occurs, when the space is properly dehumidified. In case of loss of humidity control of the space, unpiped condensate drain pans will not be sufficient to avoid overflows and damage.

Capacity of the chilled beam is controlled by a two-way valve on the chilled-water pipe, which is wired to a room thermostat. There is typically one valve per zone (e.g., office, lecture hall). Beams should be piped directly in a reverse/return piping design. Beams are not typically piped in series.

Maintenance on chilled beams requires blowing off the coils on a regular basis. Because coils are located throughout occupied spaces, this requires some coordination with occupants and housekeeping personnel to minimize effects on furniture and space contents.

Consistent air movement with natural convection equipment is difficult to achieve. The designer should consult with the manufacturer to determine the proper spacing of chilled beams based on ceiling height, heat generation sources, occupant locations, and movement patterns. Prevention of cold air ponding on the floor, especially when heat sources vary, can be paramount in maintaining comfort.

Occupant comfort is affected by more than temperature and humidity. Noise levels and nonstagnant air are also important. Some spaces may not achieve sufficient air movement or background noise to allow occupant comfort through use of passive chilled beams. Additionally, insufficient filtration of the air may occur without sufficient air movement and filtration devices in the space.

When introducing pretreated ventilation air to a space, be careful not to interfere with convection currents while still complying with the requirements of ventilation effectiveness and efficiency.

Radiant panels are typically mounted on the ceiling near perimeter walls in a metal frame. Unlike finned-tube radiation, they do not limit furniture placement. Electric radiant panels are available from 250 to 750 W in standard single-phase voltages.

Radiant panels provide space heating. Ventilation air must be supplied by a central ventilation unit that can provide tempered, humidity-controlled air to the space.

Radiant panels are selected based on the calculated heat loss for the space.

Electric radiant panels are available in standard single-phase voltages. Panels are often prewired, including the ground wire, with lead wires housed in flexible metal conduit and connector for junction box mounting.

Panel capacity is usually controlled by coil water flow, or in the case of electric heat, capacity steps. Most radiant panels are controlled with a wall-mounted thermostat located in the space.

Because they have no moving parts, radiant panels require little maintenance. Water flow control valves require periodic verification that they are operating correctly.

Radiant-floor heating is located in the flooring or just below it with a heat-reflecting wrap. If the final floor finish is hardwood flooring, the radiant-floor piping can be installed in plywood tracks with a heat reflector, below the finished floor. Ensure that, as the final flooring is nailed down, the flexible tubing is not punctured. If the final floor is a ceramic tile or other surface requiring a poured concrete, the radiant floor can be laid out in the concrete, if hot-water systems are being used. Electric underfloor heating is also available, using a prefabricated mat that is applied on top of the subfloor and under with ceramic tile. Radiant-floor heating systems can also be mounted below the floor joists, with a heat reflector below the piping. This method is often used in renovations where removing existing flooring is not feasible.

Ventilation air must be supplied by a central ventilation unit that can provide tempered, humidity-controlled air to the space.

Spacing between rows of tubing that make up the radiant floor varies, depending on the heat loss of the space. Usually, the entire heat loss of the space is calculated and the tubing spaced accordingly. Another method is to place tubing closer together near the room’s perimeter and increase the spacing in the interior. This method is more time consuming, and the difference is only noticeable in large spaces. Supply water temperature in the tubing is determined based on the flooring materials’ resistance to heat flow; the temperature is higher for carpeting and a pad than for ceramic tile. The tubing is also available in different nominal diameters, the most common being 3/8 or 1/2 in.

Circulator pumps at the manifolds require power. Additional controls for zone control valves may be selected for either line voltage or low voltage.

Because radiant floors heat the mass of the floor, these systems are typically slow to respond to environmental changes. The circulator pumps start on a call for heat from a thermostat; however, rapid solar gains to a space with many windows could cause the space to overheat. Smart systems have been used to anticipate the needs of the space and overcome the thermal flywheel effects inherent to these types of systems. Smart systems anticipate the need for heating based on outdoor conditions and the conditions experienced in the recent past. They anticipate when set-point temperatures are about to be achieved and reduce heat generation, to minimize overshooting. They also learn patterns of operation that help overcome reasonably consistent daily solar gains.

The circulator pumps, valves, controls, and manifolds are the only components requiring maintenance. Once the tubing is laid out, it should be pressure-tested for leaks; once covered, it is extremely difficult and/or expensive to access.

The most critical design elements of a two-pipe system are the calculation of primary-air quantities and the final adjustment of the primary-air temperature reset schedule. All rooms require a minimum amount of heat and latent capacity from the primary-air supply during the intermediate season. Using the ratio of primary air to transmission per degree (A/T ratio) to maintain a constant relationship between the primary-air quantity and the heating requirements of each space fulfills this need. The A/T ratio determines the primary-air temperature and changeover point, and is fundamental to proper design and operation of a two-pipe system.

Figure 5. Capacity Ranges of In-Room Terminal Operating on Two-Pipe System

 Air-to-Transmission (A/T) Ratio.

 The A/T ratio is the ratio of the primary airflow to a given space divided by the transmission per degree of that space: A/T ratio = Primary air/Transmission per degree.

Spaces on a common primary-air zone must have approximately the same A/T ratios. The design base A/T ratio establishes the primary-air reheat schedule during intermediate seasons. Spaces with A/T ratios higher than the design base A/T ratio tend to be overcooled during light cooling loads at an outdoor temperature in the 70 to 90°F range, whereas spaces with an A/T ratio lower than design lack sufficient heat during the 40 to 60°F outdoor temperature range when primary air is warm for heating and secondary water is cold for cooling.

The minimum primary-air quantity that satisfies the requirements for ventilation, dehumidification, and both summer and winter cooling is used to calculate the minimum A/T ratio for each space. If the system operates with primary-air heating during cold weather, the heating capacity can also be the primary-air quantity determinant for two-pipe systems.

The design base A/T ratio is the highest A/T ratio obtained, and the primary airflow to each space is increased as required to obtain a uniform A/T ratio in all spaces. An alternative approach is to locate the space with the highest A/T ratio requirement by inspection, establish the design base A/T ratio, and obtain the primary airflow for all other spaces by multiplying this A/T ratio by the transmission per degree of all other spaces.

For each A/T ratio, there is a specific relationship between outdoor air temperature and temperature of the primary air that maintains the room at 72°F or more during conditions of minimum room cooling load. Figure 6 illustrates this variation based on an assumed minimum room load equivalent to 10°F times the transmission per degree. A primary-air temperature over 122°F at the unit is seldom used. The reheat schedule should be adjusted for hospital rooms or other applications where a higher minimum room temperature is desired, or where a space has no minimum cooling load.

Figure 6. Primary-Air Temperature Versus Outdoor Air Temperature

Deviation from the A/T ratio is sometimes permissible. A minimum A/T ratio equal to 0.7 of the maximum A/T is suitable, if the building is of massive construction with considerable heat storage effect (Carrier 1965). The heating performance when using warm primary air becomes less satisfactory than that for systems with a uniform A/T ratio. Therefore, systems designed for A/T ratio deviation should be suitable for changeover to warm secondary water for heating whenever the outdoor temperature falls below 40°F. A/T ratios should be more closely maintained on buildings with large glass areas or with curtain wall construction, or on systems with low changeover temperature.

Transition from summer operations to intermediate-season operation is done by gradually raising the primary-air temperature as the outdoor temperature falls, to keep rooms with small cooling loads from becoming too cold. The secondary water remains cold during both summer and intermediate seasons. Figure 7 illustrates the psychrometrics of summer operation near the changeover temperature. As the outdoor temperature drops further, the changeover temperature is reached. The secondary-water system can then be changed over to provide hot water for heating.

If the primary airflow is increased to some spaces to elevate the changeover temperature, the A/T ratio for the reheat zone is affected. Adjustments in primary-air quantities to other spaces on that zone will probably be necessary to establish a reasonably uniform ratio.

System changeover can take several hours and usually temporarily upsets room temperatures. Good design, therefore, includes provision for operating the system with either hot or cold secondary water over a range of 15 to 20°F below the changeover point. This range makes it possible to operate with warm air and cold secondary water when the outdoor temperature rises above the daytime changeover temperature. Changeover to hot water is limited to times of extreme or protracted cold weather.

Figure 7. Psychrometric Chart, Two-Pipe System, Off-Season Cooling

Optional hot- or cold-water operation below the changeover point is provided by increasing the primary-air reheater capacity to provide adequate heat at a colder outdoor temperature. Figure 8 shows temperature variation for a system operating with changeover, indicating the relative temperature of the primary air and secondary water throughout the year and the changeover temperature range. The solid arrows show the temperature variation when changing over from the summer to the winter cycle. The open arrows show the variation when going from the winter to the summer cycle.

Consider using nonchangeover systems to simplify operation for buildings with mild winter climates, or for south exposure zones of buildings with a large winter solar load. A nonchangeover system operates on an intermediate-season cycle throughout the heating season, with cold secondary water to the terminal unit coils and with warm primary air satisfying all the heating requirements. Typical temperature variation is shown in Figure 9.

Spaces may be heated during unoccupied hours by operating the primary-air system with 100% return air. This feature is necessary because nonchangeover design does not usually include the ability to heat the secondary water. In addition, cold secondary water must be available throughout the winter. Primary-air duct insulation and observance of close A/T ratios for all units are essential for proper heating during cold weather.

A two-pipe system can provide good temperature control most of the time, on all exposures during the heating and cooling seasons. Comfort and operating cost can be improved by zoning in the following ways:

Figure 8. Typical Changeover System Temperature Variation

Figure 9. Typical Nonchangeover System Variations

All spaces on the same primary-air zone must have the same A/T ratio. The minimum A/T ratios often are different for spaces on different solar exposures, thus requiring the primary-air quantities on some exposures to be increased if they are placed on a common zone with other exposures. The primary-air quantity to units serving spaces with less solar exposure can usually be reduced by using separate primary-air zones with different A/T ratios and reheat schedules. Primary-air quantity should never be reduced below minimum ventilation requirements.

The peak cooling load for the south exposure occurs during fall or winter when outdoor temperatures are lower. If shading patterns from adjacent buildings or obstructions are not present, primary-air zoning by solar exposure can reduce air quantities and unit coil sizes on the south. Units can be selected for peak capacity with cold primary air instead of reheated primary air. Primary-air zoning and solar compensators save operating cost on all solar exposures by reducing primary-air reheat and secondary-water refrigeration penalty.

Separate air and water zoning may save operating cost by allowing spaces with less solar exposure to operate on the winter cycle with warm secondary water at outdoor temperatures as high as 60°F during the heating season. Systems with a common secondary-water zone must operate with cold secondary water to cool heavier solar exposures. Primary airflow can be lower because of separate A/T ratios, resulting in reheat and refrigeration cost savings.

When room temperature rises, the thermostat must increase the output of the cold secondary coil (in summer) or decrease the output of the warm secondary coil (in winter). Changeover from cold to hot water in the unit coils requires changing the action of the room temperature control system. Room control for nonchangeover systems does not require the changeover action, unless it is required to provide gravity heating during shutdown.

Characteristics of two-pipe in-room terminal unit systems include the following:

Electric heat can be supplied with a two-pipe in-room terminal unit system by a central electric boiler and terminal coils, or by individual electric-resistance heating coils in the terminal units. One method uses small electric-resistance terminal heaters for intermediate-season heating and a two-pipe changeover chilled-water/hot-water system. The electric terminal heater heats when outdoor temperatures are above 40°F, so cooling can be kept available with chilled water in the chilled-water/hot-water system. System or zone reheating of primary air is greatly reduced or eliminated entirely. When the outdoor temperature falls below this point, the chilled-water/hot-water system is switched to hot water, providing greater heating capacity. Changeover is limited to a few times per season, and simultaneous heating/cooling capacity is available, except in extremely cold weather, when little, if any, cooling is needed. If electric-resistance terminal heaters are used, they should be prevented from operating whenever the secondary-water system is operated with hot water.

Another method is to size electric resistance terminal heaters for the peak winter heating load and operate the chilled-water system as a nonchangeover cooling-only system. This avoids the operating problem of chilled-water/hot-water system changeover. In fact, this method functions like a four-pipe system, and, in areas where the electric utility establishes a summer demand charge and has a low unit energy cost for high winter consumption, it may have a lower life-cycle cost than hydronic heating with fossil fuel. A variation, especially appropriate for well-insulated office buildings with induction units where cooling is needed in perimeter offices for almost all occupied hours because of internal heat gain, is to use electric heaters in the terminal unit during occupied hours and to provide heating during unoccupied hours by raising primary-air temperature on an outdoor reset schedule.

Zoning primary-air or secondary-water systems is not required with four-pipe systems. All terminal units can heat or cool at all times, as long as both hot and cold secondary pumps are operated and sources of heating and cooling are available.

The four-pipe terminal usually has two completely separated secondary-water coils: one receiving hot water and the other receiving cold water. The coils are operated in sequence by the same thermostat; they are never operated simultaneously. The unit receives either hot or cold water in varying amounts, or else no flow is present, as shown in Figure 10A. Adjustable, dead-band thermostats further reduce operating cost.

Figure 10B illustrates another unit and control configuration. A single secondary-water coil at the unit and three-way valves located at the inlet and outlet admit water from either the hot- or cold-water supply, as required, and divert it to the appropriate return pipe. This arrangement requires a special three-way modulating valve, originally developed for one form of the three-pipe system. It controls the hot or cold water selectively and proportionally, but does not mix the streams. The valve at the coil outlet is a two-position valve open to either the hot or cold water return, as required.

Figure 10. Fan-Coil Unit Control

Overall, the two-coil arrangement provides a superior four-pipe system. Operation of the induction and fan-coil unit controls is the same year-round.

Compared to the two-pipe system, the four-pipe air-and-water system has the following characteristics: