Seeing “aux heat” light up on the thermostat in the first cold week is unsettling if nobody explained it. Clement Boston Heat Pumps sets up the heat pump aux heat on every system we install so it runs when the house needs it and stays off when it doesn’t, and we record those settings at startup.
What aux heat is
Aux heat is the backup heat source that switches on alongside the heat pump. In an all-electric system it’s a set of resistance strips inside the air handler; in a dual fuel system it’s the furnace.
Aux heat and emergency heat
| Aux heat | Emergency heat | |
|---|---|---|
| Who turns it on | The thermostat, automatically | You, manually |
| Heat pump running? | Yes, both run | No, backup only |
| When it’s right | Defrost, cold snaps, recovery from setback | The outdoor unit can’t run |
| Running cost | Partly heat pump, partly backup | All backup |
Why every system has backup heat
A cold-climate heat pump sized for your town’s design night still needs help in two situations: the defrost cycle, and the rare nights colder than the design temperature.
Backup also covers a failure in the middle of a cold snap. Claims that backup heat is never needed don’t hold for a Massachusetts house; the goal is to need it for as few hours as possible.
When aux heat should come on
These are normal, and short:
- Defrost: the heat pump runs in reverse for a few minutes to melt frost off the outdoor coil, and the strips keep the air at the registers warm.
- Cold nights below the balance point: the outdoor temperature where the house loses heat faster than the heat pump can deliver it.
- Recovery from a deep setback: a jump of several degrees in the morning can call the strips to help.
- Startup after a power outage while the system catches up; see heat pumps and power outages.
When it shouldn’t, and what that means
If aux heat runs for long stretches on a 35°F day, something is set or working wrong. The usual causes:
- No outdoor lockout, or one set too high, so the strips can run in mild weather.
- Thermostat staging that calls for backup after a short timer instead of letting the heat pump catch up.
- Low airflow, from a clogged filter or undersized return, which makes the heat pump underperform.
- A refrigerant problem that cuts heat output.
- An undersized system that can’t keep up at temperatures it should handle.
The first two are settings. The last three need a service call; see heat pump repair.
What it costs to run
Electric strips deliver one unit of heat per unit of electricity. A cold-climate heat pump delivers two to three on an average winter day. So an hour on strips costs two to three times what the same heat costs from the heat pump.
That’s why aux heat shows up on the electric bill. The winter heat pump rate lowers the price of every kilowatt-hour, but it can’t turn resistance heat into an efficient one.
Strip size and the panel
Strips are sized in kilowatts, and big ones draw heavy current. Choosing the smallest strip that covers the gap between the heat pump’s output and the house’s load at design keeps the circuit, and sometimes the panel upgrade, smaller.
The balance point and the lockout
The balance point is the outdoor temperature at which the heat pump’s output equals the house’s heat loss. It’s different for every house, because it depends on the load and the equipment, not a universal number.
How we set the lockout
From the load calculation and the equipment’s listed capacity, we find where the heat pump stops covering the load. The strips are locked out above that temperature, with a margin, and the setting goes on the startup record. On dual fuel, the same logic sets the changeover temperature to the furnace.
When the backup is a furnace
On a dual fuel system the furnace, gas or oil, is the backup, and the control decides which one runs. It should never run both at once through the same coil, because furnace heat over the heat pump coil raises its pressures.
Choosing the changeover
The control switches to the furnace below a set outdoor temperature. Set it from the balance point for comfort, or from fuel and electric prices for running cost; on oil, the heat pump usually stays cheaper well below freezing. Mass Save’s partial-home rebate asks for that integrated control. See heat pump vs oil heat.
Defrost: normal frost versus a problem
In damp weather near freezing, frost forms on the outdoor coil. Every so often the unit reverses for a few minutes to melt it; you may see steam and hear a whoosh.
Not urgent
- Light frost that clears every cycle
- Steam rising from the unit during defrost
- A few minutes of aux heat during defrost
Call for service
- Solid ice that builds over the coil and doesn’t clear
- Aux heat running for hours on mild days
- The outdoor fan not turning while the unit hums
The thermostat matters
Aux heat that runs too often can trace back to a thermostat that doesn’t know it’s running a heat pump. The right one stages the compressor, holds back the strips and reads an outdoor sensor. See heat pump thermostats.
If your aux heat runs more than it should, call us or send the form; we’ll check the settings first, then the system.