Successful Heat Pump Sizing - What About Cooling?
Even as we tend to have larger heating loads than cooling loads, we still need to consider sizing for both. PTCS requires sizing heat pumps to achieve a heating balance point of 30oF or less. If the heat pump sizing calculations suggest a larger cooling load than heating load, then achieving a heating balance point of 30oF or less will easily be achieved and may result in a slightly oversized heat pump for heating. However, in most cases, the calculated heating load will be larger than the calculated cooling load, and the heat pump will be appropriately sized for both. Since we are in a dry summer climate, generally there should be minimal concern about oversizing for cooling when sizing a heat pump for heating.
When sizing for cooling loads, the critical inputs are window orientation, solar heat gain coefficient, window shading, duct location, and duct condition.
Window Orientation – East and west-facing windows have the biggest impacts due to the low angle of the sun in the morning and evening during the cooling months.
Window Solar Heat Gain Coefficient (SHGC) – SHGC is the fraction of solar radiation admitted through a window -- either transmitted directly and/or absorbed, and subsequently released as heat inside a home compared to a single pane of glass. In the northwest, the SHGC usually aligns with the U-value of a window. Hence a window with a U-value of 0.4 would have SHGC of roughly 0.4.
Window Shading – Shading happens in many forms, including from trees, neighbor’s homes, exterior blinds, roof overhangs, screens, etc. If shading is not accounted for on east and west-facing windows in the heat gain calculation, the calculated cooling load might be significantly higher than the actual load. This could potentially suggest a larger unit be installed than needed. Manual J and most sizing programs have inputs to account for window shading.
Ducts – If the ducts are in the attic, the duct multiplier for cooling will be higher than for heating. If all of the ducts are in the attic, a reasonable cooling duct multiplier for well insulated and sealed ducts is 1.2, implying the cooling load calculated for the house needs to be increased by 20% to account for the duct gains in the attic. If not all of the ducts are in the attic, the cooling duct multiplier can be reduced. The multiplier should be increased if the ducts are unsealed or not insulated. The program recommends that you confirm that ducts in unconditioned spaces are connected, sealed, insulated, and supported. This will also keep the air temperature discharging from the diffusers at a nice low temperature, ensuring greater comfort levels.
