What does emergency heat actually do?
It takes the heat pump out of the picture and lets the backup heat carry the whole house.
A heat pump warms the house by moving heat from the outdoor air inside, even on cold days. When that isn't enough, backup heat helps out. In many homes, that's a bank of electric heat strips in the indoor unit. They work like the coils in a toaster. In others, it's a gas furnace paired with the heat pump (a dual fuel system).
The name makes it sound more dramatic than it is. "Backup heat only" would have been a clearer label.
How is emergency heat different from auxiliary heat?
Auxiliary heat comes on automatically, and emergency heat is a setting you choose.
Aux heat kicks in when the heat pump needs help. Think very cold days, or right after you bump the setting up several degrees. On many systems it also runs during a defrost cycle.
With heat strips, the heat pump usually keeps running alongside them on aux. Emergency heat shuts it off completely. With a gas furnace, the heat pump normally shuts off while the furnace runs, on aux or emergency. The difference comes down to automatic versus manual.
Most thermostats show them as "Aux heat" and "Em heat."
When should I switch to emergency heat?
When the heat pump itself has stopped working and you need heat until it's fixed. That means an outdoor unit that's damaged, won't run or is iced over and not clearing.
We wouldn't nurse a struggling heat pump through a cold week. Running a damaged unit can make the damage worse.
Cold weather alone isn't a reason to switch. A working heat pump brings on backup heat when it needs help.
One more case comes up after ice storms. After a long power outage in cold weather, some manufacturers say to run emergency heat for a while before the heat pump restarts. That gives a small heater in the outdoor unit time to warm the compressor. Check your manual for how long.
Why does emergency heat cost more to run?
Because heat strips make heat from electricity, and a heat pump moves heat that's already outside. Strips get one unit of heat from each unit of electricity, and that's their ceiling. A heat pump gets more, often much more on a mild day.
Running on heat strips alone for days can raise your electric bill noticeably. With a gas furnace as backup, the difference depends on your gas and electric rates.
Is ice on my heat pump normal in winter?
A light coat of frost is normal. Heavy ice that won't clear isn't.
In cold, damp weather, frost builds up on the outdoor coil, and every so often the heat pump runs a defrost cycle to melt it. You may hear a whoosh and see a cloud rising off the unit. That's steam, not smoke.
Middle Tennessee gets hard freezes and the occasional ice storm. After freezing rain or heavy snow, some ice on the top grille can be normal and usually clears on its own. A gutter dripping onto the unit can build ice too, and then the gutter is part of the fix.
The ice to worry about covers the coil (the finned sides) or seals the whole unit, and it's still there after several hours. If you see that, switch to emergency heat and call.
Why is my auxiliary heat light always on?
Usually the heat pump can't keep up on its own, and the backup is covering for it. That's normal during defrost and on the coldest days, but not all afternoon on a mild one.
The cause might be low refrigerant from a leak, a failed outdoor fan or compressor, a stuck defrost control or the thermostat.
Heat pump thermostats need their own setup, including one for the valve that switches between heating and cooling (the reversing valve). If the light started staying on right after a new thermostat went in, the setup is the first suspect.
What can I safely check before I call?
The thermostat, the filter, the outdoor unit and the breakers, in the order we'd check them:
- The thermostat. Make sure it's on heat (not emergency heat left on by accident) and set above room temperature. If it takes batteries, swap them.
- The air filter. Turn the system off and pull it. If you can't see light through it, put in a new one the same size, and don't run the system without one. A heat pump that can't move air can't keep up.
- The outdoor unit. Clear away leaves, snow and anything stacked against it. Leave any ice alone.
- The breakers. There's often one for the outdoor unit and one for the indoor unit. If one has tripped and the panel is dry with no burning smell, reset it once. If it trips again, leave it off.
That's the homeowner list. Refrigerant, wiring and controls are licensed HVAC work.