Instead of exhausting building heat generated during daily activity, a thermal-load-sharing system can direct that heat to pools, spas, and water heaters.Spring is here, and the cooling season is quickly approaching. Pools around the country that have been decommissioned during the winter are likely to stay that way well into June, unless some type of pool heating is implemented.
But heating open bodies of water with conventional HVAC heat sources can be a rather expensive undertaking, particularly in northern climates, forcing designers and owners to look for a relatively inexpensive heat source. Let’s look at the options.
Solar-thermal is the most energy efficient and renewable source for potable water and pool heating, but solar depends on cooperative weather. Cloudy and cool days can mean a cold pool, necessitating the need for backup heating sources much of the year.
Fossil fuel heating of potable water, pools, and spas is an old favorite. First cost is relatively low, but that comes at a higher price environmentally and monetarily as you move forward. In addition to high costs for propane and other fuels, safety issues are involved when fossil fuels are used as a heat source.
Electric-resistance heating uses raw electricity to warm heating elements over which the water passes, providing a clean and safe water-heating alternative. But it can be extremely expensive. Using the coefficient of performance (COP) rating system (used internationally) for heating equipment, electric heating has a COP of 1.0, meaning that 1 unit of heat is provided for each unit of electricity, a one-to-one ratio, or 100% efficient in the COP rating system.
Air-source heat pumps, designed for pool and potable-water heating, are environmentally friendly and pump outside air into a pool or hot-water tank. However, they too rely somewhat on cooperative weather conditions, i.e., air temperatures being warm enough to facilitate efficient heat extraction. Air-source heat-pump efficiencies are in the 3.0 COP (300% efficient) range.
For swimming pool and spa heating, the best scenario is attained with geothermal-sourced water-to-water heat pumps, pulling heat from a dependable, steady, and renewable energy source; the earth. Geothermal heat pumps can be about 5.0 COP (500% efficient).
Outside temperatures fluctuate with the changing seasons, but underground temperatures don’t change nearly as dramatically, thanks to the mass of the earth. Some 4 to 6 ft. below the ground, the temperature remains relatively constant year round (about 50 F to 75 F in the U.S.).
A geothermal-sourced water-to-water heat pump, which can work in tandem with a geothermal HVAC system, typically consists of water-sourced heat pump and a buried system of pipes called an earth loop, and/or a pump to send fluid to a reinjection (Class V thermal exchange process) well. This geothermal source can be shared between the building’s HVAC and water-heating systems.
Think of it like this: While providing power to run your building’s HVAC cooling system, you are also providing the energy to run computers, lighting, servers, copiers, and domestic water heating. Then the building’s HVAC system must use power to remove the heat created by all of these internal gains, on top of the occupant loads (one occupant presents a load of 1,200 BTU each hour). You pay for energy twice to remove this waste heat through the process of cooling your building. Why not channel that heat to where it’s needed?
Among the benefits that you can realize from a geothermal HVAC system is the ability to channel and use this waste heat energy. That’s because, unlike widely used cooling towers and air-sourced cooling equipment (those that have an outside condenser that discharges waste heat), geothermal systems discharge the heat through a liquid heat exchanger (such as with a chiller-cooling tower combination). The heat is entrained in the discharge water line. Most manufacturers of geothermal heat pumps even have a factory installed hot water generator available. This option gives you two extra connections, labeled DHW (Domestic Hot Water) “In” and “Out,” that may be connected to almost any hot-water tank.
There are thousands of geothermal heated pools around in the US. There is a good chance that the local YMCA, hotel, health club, or community pool near you already has geothermal sourced pool heating. Surprisingly, many of these still have air sourced cooling systems that could be converted to geothermal (and likely will be) during the normal course of HVAC equipment attrition and upgrade. When specifying a geothermal HVAC system, consider including a thermal-load-sharing system to make maximum use of building heat.
Jay Egg is a geothermal consultant, writer, and the owner of EggGeothermal, Kissimmee, FL. He has co-authored two textbooks on geothermal HVAC systems published by McGraw-Hill Professional. He can be reached at firstname.lastname@example.org.