The term “emergency heat” on a thermostat can be confusing and may sound like something you should turn on as soon as winter weather gets serious. During an ice storm, that is often the first question homeowners have when they see ice on the outdoor heat pump or notice the house is not warming up the way it should.
On most heat pump systems, emergency heat tells the thermostat to stop relying on the outdoor unit and use the backup heat source instead. In some homes, that backup is an electric heat strip, or a gas furnace if you have a dual-fuel system.
If your home is heating up and the outdoor unit is running normally, you usually do not need to make any changes. But if the outdoor unit is covered in ice, making unusual noises, not running, or the temperature inside keeps dropping, emergency heat may be the best temporary option.
Emergency heat protects your equipment in some situations and does nothing but raise your power bill in others, and the difference comes down to the performance of your outdoor unit.
In this article, we’ll walk you through when to use emergency heat during an ice storm, when to leave it off, and when to call an HVAC professional.
What Does Emergency Heat Actually Do?
Emergency heat is a backup heating mode, not a higher setting for your regular heat.
On a normal heat pump system, the outdoor unit is part of the heating process. Even in cold weather, the heat pump pulls heat from the outdoor air and moves it inside. During winter weather, it may also go through a defrost cycle to melt light frost or ice from the outdoor coil.
When you switch the thermostat to Emergency Heat, you are telling the system to stop using the outdoor heat pump and rely only on the backup heat source. If your home uses an all-electric system, that usually means electric heat strips inside the air handler, and in a dual-fuel system, it may mean the gas furnace takes over instead.
That is why emergency heat can be helpful during an ice storm if the outdoor unit is frozen, damaged, or struggling to run. It lets you heat the house without forcing the outdoor heat pump to keep working through unsafe conditions.
Emergency heat does not make the outdoor unit thaw faster, nor does it fix the problem that caused the unit to ice over. If your backup heat source is electric heat strips, it can also use much more power than normal heat pump operation.
For a standard furnace without a heat pump, this setting usually does not apply. The furnace is already the main heat source, so there is no outdoor heat pump to bypass.
In that case, ice-storm issues are more likely due to power loss, blocked venting, ignition problems, airflow restrictions, or a frozen condensate line on a high-efficiency furnace.
When You Should Turn On Emergency Heat During an Ice Storm
You should consider switching to emergency heat when the outdoor heat pump can no longer run safely or properly heat the home.
If you see a thin layer of frost on the outdoor unit, the system is designed to clear it automatically.
During an ice storm, though, the buildup can become much heavier than normal frost and may prevent the ice from clearing as it should. These are the situations where emergency heat is the right temporary call.
The outdoor unit is covered in thick ice that is not clearing. If a full defrost cycle has come and gone and the ice is still there, the unit can no longer pull heat from the outdoor air. Emergency heat shuts it down so it stops running against the ice.
The fan is not spinning, or the unit is grinding or rattling. Those sounds usually mean the fan blade is striking ice or the motor is straining against it. If you keep running the unit under those conditions, you risk damaging the motor, the blade, and the coil behind it.
A limb or storm debris has struck the unit. Ice brings branches down, and a bent fan or a cabinet that has shifted on its pad means the heat pump should stay off until someone inspects it.
The system keeps running, but the indoor temperature continues to drop. If the heat pump has run for hours and the house is still losing ground, it is no longer moving enough heat to keep up.
Ice has built up around the fan, coil, or cabinet. A unit that is physically blocked cannot move air across the coil, and running it that way puts stress on the motor and the compressor.
Your power has just come back after a long outage. Most manufacturers want you to run emergency heat for a while before the compressor starts again. There is more on how long to wait further down in this article.
A technician tells you to. If you call us during a storm, we may walk you through switching over the phone based on what you are seeing.
Once you turn the emergency heat on, treat it as a temporary setting. It is there to keep the house warm and protect the equipment until the ice clears or a technician can inspect the system.
Leave the outdoor unit alone while you wait, and switch the thermostat back to “heat” once the unit has thawed and is running normally again. The longer you run on electric heat strips, the more it shows up on your power bill.
When You Should Not Turn On Emergency Heat
Emergency heat is not something you need to turn on just because there is ice in the forecast. If your home is staying warm, the heat pump is running normally, and the outdoor unit only has a light layer of frost, leave the thermostat on its regular heat setting.
Most heat pumps are designed to handle some frost during cold, damp weather.
The system should go into a defrost cycle on its own, which temporarily melts light ice from the outdoor coil. You may notice steam coming from the outdoor unit or hear the system change sound as it shifts into defrost, and that can be normal.
You also do not need to switch to emergency heat just because your thermostat shows “aux heat” or “auxiliary heat.” Auxiliary heat can come on automatically when the system needs extra help, such as during very cold weather or when the thermostat is raised several degrees at once. Emergency heat is different because you are manually telling the system to stop using the outdoor heat pump and rely only on the backup source.
If you leave emergency heat on while the heat pump is still working, you unnecessarily increase your energy use. If your backup heat source is electric heat strips, the power draw can be much higher than normal heat pump operation. If your backup is a gas furnace in a dual-fuel system, you still do not want to run it unnecessarily when the heat pump is operating safely and keeping the house comfortable.
The simplest rule we recommend is that if the outdoor unit is not struggling and the indoor temperature is holding steady, you should leave emergency heat off.
How Long Should You Leave Emergency Heat On?
Emergency heat should be treated as a temporary setting rather than the normal way to heat your home through the winter.
If you turn it on during an ice storm because the outdoor heat pump is frozen, damaged, making noise, or not keeping up, leave it on long enough to keep the house warm while the outdoor unit is unsafe to run. That may mean a few hours, overnight, or until the storm passes and the ice around the unit has cleared.
Once the weather improves, check the outdoor unit from a safe spot. If the fan area is clear, the ice has melted, and the unit does not look blocked or damaged, you can switch the thermostat back to regular Heat and see how the system runs.
Give it some time to cycle normally. Heat pumps may still use auxiliary heat on their own when outdoor temperatures are low, but that is different from manually setting the system to Emergency Heat.
If the outdoor unit freezes again, makes unusual noises, will not start, or the indoor temperature begins dropping after you switch back, turn emergency heat back on and call an HVAC professional. At that point, the issue may be more than normal ice buildup.
The main reason not to leave emergency heat on longer than needed is cost. Electric heat strips can use two to three times as much electricity as normal heat pump operation for the same amount of heat. If you have a dual-fuel system, emergency heat may rely on the gas furnace instead, which makes sense in very cold weather, but it is still not something to run while the heat pump is working normally.
However, after a long power outage, most manufacturers want you to run emergency heat for a set period before the compressor restarts, even when nothing is wrong with the system.
The emergency heat setting is there to protect the equipment and keep the house warm during a problem, not to replace regular heat whenever the forecast looks bad. You should only use it until the outdoor unit can run safely again or until a technician has checked the system.
What to Do If the Power Goes Out?
Ice storms regularly take power out, and this is the part most homeowners have not planned for.
When the power goes out, your heat pump stops and so does its defrost cycle. If freezing rain keeps falling, the unit can ice over completely while you wait for the power to come back. There is not much you can do about that during the outage, but you should go out and look at the unit before you restart the system.
You should also be careful when restarting your heat pump after a power outage. Your compressor has a small crankcase heater that keeps refrigerant from settling into the compressor oil while the system sits idle, and an extended outage turns that off along with everything else. If the compressor starts cold, liquid refrigerant can get into it and cause serious damage.
Most manufacturers recommend switching the thermostat to EM HEAT first and allowing the compressor time to warm up before you let it start. The general guidance is that if the power was out for more than about half an hour, you should run emergency heat for roughly as long as the outage lasted, or a few hours if you are not sure.
Then you can switch back to Heat. Check your owner’s manual, because the exact recommendation varies by manufacturer.
Ductless mini-splits work differently. Most of them do not have an emergency heat setting at all and instead rely on auto restart. If a mini-split comes back confused after an outage, cycling the breaker will usually take care of it.
Call Air Today for Heat Pump Help in Greenville and the Upstate
Emergency heat is meant to help you get through a heating problem, not replace your normal heat every time winter weather shows up.
If your heat pump is still running normally and your home is staying warm, you usually do not need to turn it on. But if the outdoor unit is covered in ice, making unusual noises, not running, or the temperature inside keeps dropping, switching to emergency heat can protect the system and keep your home comfortable until the problem is resolved.
Air Today has been helping homeowners across the Upstate stay warm through all kinds of winter weather for over four decades. If your heat pump is iced over, your emergency heat is running longer than expected, or you are not sure your system is operating safely, our HVAC technicians can inspect your system, identify the cause, and help you restore dependable heat.
Contact us today or call (864) 295-0905 for heating service in Greenville or anywhere in the Upstate, and keep your home safe and warm during winter weather.