Heat pump backup heat and lockout temperatures

Aux heat (auxiliary heat) is a backup heat source that the thermostat turns on automatically to help the heat pump: when it cannot keep up, during defrost, or below a set outdoor temperature. Emergency heat is a mode you select by hand that shuts the heat pump off and heats with the backup only. Lockouts are thermostat settings that block the compressor or the backup from running above or below set outdoor temperatures.

The backup is usually electric resistance elements (in the ductwork or as baseboards) or, in a dual fuel system, a furnace or boiler.^{[Source 2]}

Aux heat

Aux heat runs alongside or instead of the heat pump in a few situations:

• The heat pump cannot keep up. Below the thermal balance point, heat pump output alone is less than the house needs.^{[Source 2]}
• Defrost. NRCan notes that "supplementary heating may also be required when the heat pump is defrosting."^{[Source 2]} Lennox's user guide says "the thermostat may engage auxiliary heat during the defrost cycle."^{[Source 3]}
• Large setpoint increases. DOE notes that "many heat pump controls are incorrectly wired to automatically turn on the auxiliary strip heat if the indoor thermostat is changed by more than 3 degrees F," and gives the example that if it is 50°F outside and you raise the thermostat from 66°F to 70°F, "the strip heat should never activate."^{[Source 1]}

Seeing "aux" on the thermostat in cold weather or during defrost is expected. Seeing it often in mild weather is a sign the controls may be set up wrong.

Emergency heat

Emergency heat is for when the heat pump has a problem. Rheem's owner's manual calls it "a back-up heating mode to be used only if there is a suspected problem with the outdoor unit," in which "the outdoor unit will be locked off."^{[Source 4]} Lennox: "In EMERGENCY HEAT mode, all heating demand is satisfied by auxiliary heat and heat pump operation is locked out."^{[Source 3]}

With electric heat strips, running on emergency heat means paying for resistance heat, which delivers about 3,400 Btu per kWh, the same as the energy in the electricity.^{[Source 1]} A heat pump running normally delivers more heat per kWh. Leaving emergency heat on by mistake raises the electric bill.

One manufacturer-specific use: Lennox's guide says that after a power outage lasting several hours, use emergency heat for a "six-hour compressor crankcase warm-up period" before returning to normal heat.^{[Source 3]} Check your own system's manual for this.

Lockout settings

Heat pump thermostats that use outdoor temperature commonly have two lockout settings. The names differ by brand. Two examples:

What it does	ecobee name[Source 5]	Resideo T6 Pro name and range[Source 6]
Stops the compressor below an outdoor temperature	Compressor Min Outdoor Temperature	ISU 355, Compressor Lockout / Balance Point: Off, or 5°F to 60°F in 5°F steps
Stops the backup above an outdoor temperature	Aux Heat Max Outdoor Temperature	ISU 356, Outdoor Lockout Backup Heat: Off, or 5°F to 65°F in 5°F steps

ecobee describes them as setting "the lowest outdoor temperature at which the compressor operates" and preventing "auxiliary heat from running above a specific outdoor temperature."^{[Source 5]} On the Resideo T6 Pro, these settings need a wired outdoor sensor on some models.^{[Source 6]}

What values to use

There is no single correct setting. It depends on the heat pump's capacity at low temperatures, the house's heat loss, the type of backup and, for dual fuel, energy prices.

• All-electric with heat strips. The goal is to let the heat pump run as cold as it usefully can and keep the strips off in mild weather. The Bonneville Power Administration's PTCS support sheet for ecobee thermostats, written for its heat pump program in the Pacific Northwest, tells technicians to set Compressor Min Temperature to "DISABLED" or 5°F or lower, and Aux Heat Max Outdoor Temperature to 35°F or lower.^{[Source 7]} Those values come from one program and region.
• Dual fuel. The compressor lockout sets the switchover to the furnace, based on the balance point and the economic comparison. See Dual fuel and hybrid heat pump systems.
• Cold-climate heat pumps. A compressor lockout set high (for example 35°F or 40°F) on a cold-climate unit cuts off much of the heat pump's useful range. Compare the setting with the manufacturer's capacity data.

To estimate a lockout temperature for a specific model with your weather and electricity rates, heatpump.blue is one tool for that.

NRCan also recommends that electric backup be set "2°C to 3°C below the heat pump heating temperature set-point" so the heat pump runs first.^{[Source 2]}

Codes require backup heat controls

Energy codes require controls that keep resistance backup from running when the heat pump can meet the load. The DOE and New Buildings Institute inspection checklist lists a backup heat lockout under IECC R403.1.2.^{[Source 8]} DOE calls improper control of heat strips "one of the most commonly violated installation practices."^{[Source 1]}

Setbacks and aux heat

A deep thermostat setback can trigger aux heat during recovery, which uses resistance heat. DOE says that with heat pumps in heating mode, "setting back its thermostat can cause the unit to operate inefficiently," and that some thermostats designed for heat pumps "use special algorithms to minimize the use of backup electric resistance heat."^{[Source 9]} See Heat pump thermostat settings and setbacks.

Related

• Aux heat (auxiliary heat) and Lockout temperature (glossary)
• Heat pump not heating if the system is on aux or emergency heat and you are not sure why