Can You Find the Hidden Heat Pump Thermostat Setting?

Did you know that there is one hidden setting on the heat pump thermostat that can have a bigger energy impact than adding R-10 continuous insulation to your home? When improperly configured – as it often is – the result is a tremendous waste of money on unnecessarily high utility bills.

Most heat pumps, especially those used in new construction, use two sources of heat. One is the highly efficient heat pump itself. The other is an electric resistance element, sometimes referred to as “auxiliary” or “emergency heat.” This heating element is 300 – 400% less efficient than the heat pump itself and is intended to only be used sparingly by the system to meet loads during the coldest hours of the year.

To understand this, there are 2 settings to be aware of:

• Outdoor temperature lockout – A user-set temperature threshold, below which the heat pump will not operate and 100% of heat comes from the inefficient auxiliary element.

• Indoor temperature droop – A user setting which will automatically turn on the auxiliary heat if the indoor temperature droop is greater than this threshold below the setpoint. This is typically 1-3°F.

It is up to the installing contractor to properly configure these settings at the time of installation. However, most contractors do not correctly configure the settings – a step that takes less than 5 minutes. Many don’t even know that these settings exist, and they leave them set at the manufacturer defaults.

Some contractors may “overreact” and set the ambient temperature lockout well above what is needed, either due to experiences with much older heat pumps, or simply out of fear that there will be a customer callback. Adjusting this setting to 45°F could cost a customer hundreds of dollars a year in excess energy bills. Historically, some heat pumps could not operate below certain ambient temperatures without risk of damage, however most modern equipment can safely operate below 5°F or even lower. This information can be found in the manufacturer's published product data.

The reality is that it is safe to adjust the lockout temperature to a lower value or disable it altogether, as some manufacturers do by default. The secondary, indoor temperature droop will still engage the auxiliary heat if the setpoint isn’t met, or the equipment has been running longer than a set amount of time. This will keep your customers happy and comfortable, while maximizing the efficiency benefits of the heat pump.

Ultimately, deciding on the optimal configuration requires a brief review of the heat pump data sheet and the thermostat installation guide – something that should take your contractor a few minutes but save your customer thousands of dollars over the life of the equipment.