What is the 20 rule for heat pumps refers to several rules of thumb used by HVAC pros for sizing, temperature difference, and energy savings. Most common 20 degree rule for heat pumps states heat pump should be able to maintain indoor temperature within 20 degrees of outdoor temperature in cooling mode without supplemental heat, and temperature drop across evaporator coil should be 18 to 22 degrees Fahrenheit in cooling mode, meaning return air at 75 degrees should be supply air at 55 to 57 degrees, indicating proper operation and refrigerant charge, if difference is less than 15 degrees, heat pump may be low on refrigerant or have dirty coil. Second 20 rule is heat pump efficiency drops significantly below 20 degrees Fahrenheit outdoor temperature, where many air source heat pumps lose capacity and need auxiliary electric heat strips that cost 2 to 3 times more, so below 20 degrees, heat pump may need backup furnace in cold climate. Third 20 rule for energy savings states for every 1 degree you lower thermostat in heating mode, you save about 3 percent, so lowering 20 degrees when away saves significant, but more common is 7 to 10 degree setback saves 10 percent. Another 20 rule is heat pump should be sized so that balance point where it needs backup is around 20 to 30 degrees outdoors. Always consult Manual J for exact sizing rather than rules of thumb.
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