In upstate New York where I live, we are in “heating mode mentality” from late September to early May. Perhaps it’s not surprising that I spend most of my time thinking about hydronic heating applications.
Still, the same physical properties that make water ideal as a conveyor belt for moving heat through a building also apply to its use in cooling.
Chilled water cooling has been used in larger buildings for decades. The 45° F to 50° F chilled water is produced by one or more large centrifugal chillers or absorption units. This water is usually conveyed to several air handlers where it passes through a coil to both cool and dehumidify the air stream of return air.
Today, there are several products on the market that effectively scale down this technology for use in residential or light commercial buildings. I’ve had several opportunities to design systems around this hardware in the last several years and have generally been pleased with the results.
Here are some of the benefits associated with small-scale chilled-water cooling:
• A range of products are now available to serve as the chiller. These include: ground-source, reversible water-to-water heat pumps; air-cooled chillers (e.g., air-to-water heat pumps); small gas-fired absorption cycle chillers that can dissipate heat to either outdoor air or an earth loop; and direct cooling from a lake or large pond in northern climates.
• It is easier to zone a cooling system using multiple small air handlers rather than installing multiple motorized dampers in a central duct system. Zoning can be done using zone circulators, or zone valves, in combination with a variable-speed, pressure-regulated circulator. The latter minimizes circulator power under partial load conditions. This is especially important in cooling applications because all such power adds to the cooling load.
• With a properly sized buffer tank, it’s possible to size a heat pump to the full design heating load of the building and not have it short-cycle during the cooling mode. This is common in situations where the design cooling load is significantly less than the design heating load.
Short-cycling was a problem in earlier generation water-to-air heat pumps, especially when used in northern climates with significant heating loads but small cooling loads. It led to the development of two-speed and even variable-speed compressors. This helps, but at the expense of complexity and higher cost.
Using a buffer tank to store chilled water allows a simple and less expensive single-stage chiller to be used. When properly configured, a buffer tank also can provide hydraulic separation between the chiller-side circulator and the distribution circulator. This is especially important if the latter is operated at variable speeds.
• Any concern over “frosting” of the cooling coil, which can occur with direct expansion coils operating at reduced air flow rates, is eliminated.
• The potential exists for using a radiant panel to handle the sensible portion of the cooling load and thus reducing the air flow rates to those required for latent cooling (e.g., moisture removal) and ventilation. This hybrid approach significantly lowers the distribution energy required to operate the system. It’s the primary driver behind rising interest in radiant cooling.
• In areas with time-of-use electrical rates, systems can be designed with chilled water storage. Ideally, the chiller would only operate during periods of low electrical rates to drop the tank temperature to where it could absorb the following day’s cooling load.
I designed and helped install a residential system using this approach back in 1991. It’s still operating. Adding storage also allows air-cooled chillers to operate at night when both electrical rates and outdoor temperatures are lower. The latter increases both the capacity and the energy efficiency ratio of air-cooled chillers.
• In applications where there’s a steady demand for domestic hot water, a water-to-water heat pump can serve double duty. The hot side of the heat pump supplies water at temperatures up to 145°, while the cold side supplies chilled water for cooling. Such applications can nearly double the effective coefficient of performance of a heat pump because both the heated water and chilled water are produced from the same electrical input energy.
Air-cooled condenser meets hydronics
Figure 1 shows a small-scale chilled water cooling system using zoned air handlers. Refrigerant lines run from the air-cooled condenser to a flat-plate heat exchanger inside the mechanical room. This heat exchanger is specifically designed to operate as the system’s evaporator, with refrigerant gas passing through one side and water through the other. Figure 2 shows an example of such a heat exchanger.
Manufacturers of such heat exchangers offer assistance and/or software to properly match the condenser’s performance to a specific heat exchanger model. This configuration keeps all water inside the building, thus eliminating the need to use antifreeze or drain external piping during cold weather.
The chilled water produced by the refrigerant-to-water flat-plate heat exchanger is routed to an insulated and vapor-sealed buffer tank. Only tanks with foam insulation should be considered for such applications. After the piping is in place, all connections should be sealed with expanding spray foam and wrapped with a suitable jacket. Any seams in the tank’s jacket should also be sealed with aluminum foil tape.
A temperature setpoint controller monitors tank temperature and operates the chiller as necessary to maintain it within a certain range, typically from a low of 42° to a high around 60°. Chilled water temperatures lower than 42° are not needed and only serve to lower the performance of the chiller. Water temperatures above 60° will compromise the moisture removal capability of the air handler coils.
The flow switch installed between the buffer tank and the heat exchanger is essential. It verifies flow in the loop as a prerequisite to operating the chiller. Without it, the flat-plate heat exchanger could quickly freeze and become damaged should there be a loss of flow.
The coolest water in the system will accumulate near the bottom of the buffer tank, thus the supply pipe to the distribution system comes out near the bottom of the buffer tank.
This distribution side of the system is a two-pipe direct return layout using zone valves and a variable-speed, pressure-regulated circulator. Notice that the zone valves are placed on the return side of the air handlers where the water temperature is slightly warmer. This helps minimize condensation, but does not eliminate the need to wrap the zone valve bodies with flexible foam rubber insulation. However, don’t wrap the valve’s actuator with insulation; this could cause condensation to form on the internal electrical components.
Don’t sweat it
All piping carrying chilled water must be insulated and vapor-sealed. Omit this detail and condensate damage to drywall surfaces will be quickly evident and lead to costly corrections. I prefer flexible foam rubber insulation with low vapor permeability. All joints and seams must be closed and glued. This insulation, also available in flexible sheets or tape, should be used to encase all portions of valves (other than the handle). Figure 3 shows some piping insulated with these materials.
The volutes of a circulator should also be insulated, but do not wrap the motor can.
Avoid installing circulators or valves directly above electrical components or in any area where an occasional drip of condensate would be a problem.
If you’re going to do chilled water cooling, you should also be planning for low temperature hydronic space heating. The system shown in Figure 4 does this using a modulating, “split-system” air-to-water heat pump. A refrigerant line set runs from the outdoor unit to the indoor unit. There is no water in the outdoor unit and thus no need for freeze protection. The indoor unit is equipped with its own internal circulator and expansion tank.
The distribution subsystems required for zoned heating and cooling have flow and head requirements beyond what can be supplied by the circulator in the indoor unit of the heat pump. The situation is handled by creating a relatively small and short primary loop, which has a flow and head requirement within the capacity of the heat pump’s circulator. This primary loop supplies two separate secondary circuits, one for heating and the other for cooling.
Each secondary circuit is hydraulically separated from the primary circuit by a set of closely spaced tees. This allows the flow rate and head produced by all three circulators in the system to be unaffected by the operation of the other circulators. Also, each secondary circuit uses a variable-speed, pressure-regulated circulator, configured for proportional differential pressure control in combination with zone valves. Reverse return piping is used to help balance flow through each zone.
However, balancing valves are still included to allow accurate setting of the flow through each zone. These balancing valves also can serve as isolation valves if necessary. The zone valves for the chilled water air handlers are again located on the outlet side of the air handler coil. The slightly warmer water temperature at this location reduces the possibility of condensation forming on the zone valves.
Domestic water is heated by the heat pump and controlled as a priority load. Whenever the thermostat in the domestic hot water tank calls for heating, the diverter valve is energized. Heated water is directed from the indoor unit through the coil in the indirect water heater. An electric heating element in the upper portion of the indirect tank provides any necessary temperature boost.
The ability of the heat pump to modulate its heating and cooling capacity down to about 25% of full capacity, combined with limited zoning, eliminates the need for a buffer tank.
Notice that a supplemental expansion tank is shown in this system (see Figure 4). This tank is needed because the expansion volume of the water in the system is greater than what the expansion tank inside the heat pump can accommodate. A standard expansion tank sizing calculation should be made to determine the total expansion tank volume required. The minimum volume of the supplemental expansion tank is then found by subtracting the volume of the internal tank from the total expansion tank volume requirement. The air pressure within the two expansion tanks should be adjusted to be equal.
The two tanks should also be mounted with their inlet connections at about the same height. This provides equal static pressure at both tanks. Another design option is to disconnect the internal expansion tank and size the supplemental expansion tank for the full expansion requirement of the system.
Firmly grounded
Figure 5 shows yet another configuration built around a water-to-water heat pump supplied from a closed-circuit earth loop.
This system is shown in cooling mode operation, with the heating-only portions of the system grayed out. The single-stage water-to-water heat pump is removing heat from the buffer tank, upgrading the temperature of that heat and dissipating most of it to the earth loop.
However, some of the higher temperature heat is transferred to the lower (cooler) portion of the domestic water heater through the “desuperheater” heat exchanger within the heat pump. This heat is truly “free,” since it would otherwise end up dissipated into the earth.
These systems are just examples of what’s possible using a variety of readily available components held together by modern design concepts. There are many more possible system configurations.
If you’re involved with hydronic heating in residential and light commercial buildings, I encourage you to take a good look at chilled water cooling. It’s a great way to deliver a complete comfort offering.
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