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GSHPs are typically sized based on “tons” of cooling. A ton of cooling is equal to extracting 12,000 BTU/hr of heat from a space. For example, a 5-ton heat pump will be capable of extracting 60,000 BTU/hr of heat from a space. That doesn’t necessarily mean it supplies 60,000 BTU/hr of heat to a space, though. The heat delivered to the space is dependent on the temperature of the fluid from the ground loop – the higher the temperature, the more heat that can be delivered. Thus, a 5-ton GSHP might deliver 53,000 BTU/hr or 65,000 BTU/hr depending on the temperature of the incoming fluid. Understanding the local ground temperature is vital to ensuring the GSHP can deliver adequate heating and cooling to a structure. In heating-dominated climates, GSHPs are typically sized based on the amount of space heating needed for the building. In cooling-dominated areas, the cooling needs would dictate sizing.
A heating system should be sized to meet the heating demand of a building on the coldest day of the year (or, the cooling demand on the hottest day if sized based on cooling). During the coldest day, the GSHP should be running continuously. An oversized GSHP will increase initial costs and operate less efficiently, as the unit will cycle on and off for much of the heating season. A properly sized heating appliance will have long runs with steady state efficiency. As described in step 5, if planning to make energy efficiency improvements to a building envelope (for instance, adding insulation in an attic, replacing windows, or sealing air leaks), these should be completed before buying the heat pump to reduce the building’s heating load, while allowing the purchase of a smaller, less-expensive heat pump.
The heating demand of a building is a complex calculation that takes into account the size of the building, the amount of insulation, the number of windows and doors, and the local environment. There are several ways to calculate this heating demand, but these complex calculations are typically performed using computer models. Energy-raters or heat pump installers often can perform a heating load calculation. Both of these professionals should use a robust calculation method or software, such as the Air Conditioning Contractors of America Manual J, to size a heating appliance – methods much more accurate than following typical “rulesof-thumb” that were often used in the past. To estimate the heating demand, calculate the heat loss of the buildings being considered by estimating the inslaution level (R-value) of the walls, floor, roof, and windows and the indoor and outdoor temperatures. Heat loss through the building envelope is calculated based on the R-value of each component and the difference in temperature across the envelope. The heat loss from the envelope is equal to the change in temperature divided by the R-value of the envelope (then multiplied by the area). For example, a wall with R-21 fiberglass batt insulation on a day that is 10°F outside with an interior temperature of 70°F will lose 3 BTU/hr of heat energy through each square foot of the wall (not counting windows that usually have an R-value of 3 or 4.) Each component of a building has a different R-value, and structural members, like studs, have a lower R-value than the rest of the wall. The sum of all the heat loss across the varying components of the envelope will give you the conductive heat loss of the home. If the building is leaky with cold air entering through windows and doors, then the convective heat loss needs to be added to all of the conduction losses in order to determine heating demand. The overall heat loss from the building at the coldest outside temperature for your area should be used to determine the size of the heat pump.
Roth, K., J. Dieckmann, and J. Brodrick. (2009) “Heat pumps for cold climates.” ASHRAE Journal.