A Chilling Thought
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.
However, 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 chiller in many of these systems may be a large centrifugal machine, or an absorption unit that turns out water in the range of 40 to 50 degrees F. 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.
Why not apply this technology to smaller buildings? It certainly can be done, but before looking at schematics, let’s look at the reasons that small-scale, chilled-water cooling makes sense:
1. A range of products are now available to serve as the chiller. These include:
- Ground-source, reversible water-to-water heat
- Air-cooled chillers (e.g., air-to-water heat
- Small gas-fired absorption cycle chillers that can dissipate heat to
either outdoor air or an earth loop.
- Direct cooling from a lake or large pond in Northern climates.
2. It is much easier to zone a cooling system using multiple small air handlers rather than installing multiple motorized dampers in a central duct system. Zoning could be done using zone circulators or with zone valves in combination with a variable-speed, pressure-regulated circulator.
The latter option offers the same percentage savings in electrical energy as when it is used in a zoned heating system. It also eliminates the need for a differential-pressure bypass valve in the system. Last but not least, any electrical power supplied to operate one or more circulators in a hydronic cooling system adds a corresponding amount to the cooling load. It only makes sense to minimize pumping energy in such applications.
3. With a properly sized buffer tank, it’s possible to size a ground-source heat pump to the full design heating load of the building and not have it short-cycle during the cooling mode. Such short-cycling has been a definite problem in earlier generation water-to-air heat pumps, especially when used in Northern climates with minimal cooling loads. It led to the development of two-speed and variable-speed compressors, which helps, but comes at the expense of complexity and higher cost.
Using a buffer tank allows a simple and less expensive single-stage chiller to be used. It also provides hydraulic separation between the chiller-side circulator and the distribution circulator. This is especially important if the latter is operated at variable speeds.
4. Any concern for “frosting” the cooling coil, which can occur with direct expansion coils operating at reduced air flow rates, is eliminated.
5. 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 reason underlying increased worldwide interest in radiant cooling.
6. 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.
7. 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 degrees F, 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.
The chilled water produced by the 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. All seams in the tank’s jacket also should 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 40 degrees F to a high around 55 degrees F. Chilled water temperatures lower than 40 degrees F are not needed and only serve to reduce the performance of the chiller. Water temperatures above 55 degrees F 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 this loop as a prerequisite to operating the chiller. Without it, the flat-plate heat exchanger could be quickly frozen and damaged should there be a loss of flow.
This distribution side of the system is a home-run layout using zone valves and a variable-speed, pressure-regulated circulator. The coolest water in the system will also be the most dense and settle to near the bottom of the buffer tank. Thus, the supply pipe to the distribution system comes out near the bottom of the buffer tank.
We’ve had good results using so-called “hi-velocity” air handlers equipped with higher-static pressure blowers to deliver cooling air flow to the building. A typical approach equips each air handler with a discharge trunk duct, to which several 2-inch diameter, flexible, pre-insulated “mini ducts” are attached. The latter are easy to route through the framing systems of small buildings and do a good job of sound attenuation.
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.
All piping carrying chilled water must be insulated and vapor-sealed. If you omit this detail, condensate damage to drywall surfaces will be quickly evident and lead to costly corrections. I prefer using flexible foam rubber insulation with a low vapor permeability. All joints and seams must be closed and glued. This same insulation in the form of flexible sheets or tape should be used to encase all portions of valves (other than the handle).
The volutes of the circulator should also be insulated, but do not wrap the motor can. Chilled water cooling is one application where three-piece or two-piece circulators - as differentiated from wet-rotor circulators - have an advantage. The coupling assembly between the volute and motor helps prevent condensation formation on the latter.
As a rule, don’t install circulators or valves directly above electrical components or in any area where an occasional drip of condensate would be a problem.
Radiant ceiling, wall or floor panels can be used with chilled water cooling, but only in combination with a reliable dewpoint controller that ensures their surface temperature remains above the dewpoint temperature of the room by at least three or four degrees. The dewpoint controller operates a mixing valve that regulates supply water temperature to the panels.
The rear side of the gypsum panels have CNC-milled grooves that hold very small-diameter PEX tubing. Panels are then mounted to the ceiling framing and the small tubing circuits are connected to manifold piping using shape-memory PEX fittings. These panels are manufactured specifically for each installation, with panel shape and tubing placement adjustments that accommodate ceiling penetrations for lighting, ducting or other equipment.
We’ve been involved with several residential hydronic cooling projects over the years. I’m pleased with the results and certainly recommend it as a unique offering that complements what you provide for comfort heating. Stay cool.