What is emergency heat—and why do I need it?

As a follow-up to our recent blog “My A/C won’t keep my house cool enough on a 95-degree day,” we’re taking a look at how heat pumps use emergency heat to perform adequately during the winter. Emergency or auxiliary heat is a supplemental heating component installed in your heat pump to provide additional heat when outside temperatures fall below the balance point and when your HVAC system is in defrost mode. Supplemental auxiliary heat is standard practice in our area and is included in your heat pump to keep your home cozy warm and safe when the temperature drops. 

If you want to know WHY auxiliary heat—also referred to as a Kw heater, or electric heat strips—is necessary to help your heat pump keep you warm during the coldest winter days, this article will explain. 

The role of refrigerant

In a previous blog, we illustrated how an air conditioner works; it uses a refrigerant coil system to trap the heat inside of your home, and expel it outside using a condensing unit. Refrigerant is moved through four major components—compressor, condenser coil, expansion valve, evaporation coil—where pressure and temperature are changed. In the summer (cooling season), the refrigerant enters the compressor as a low-pressure gas where it is compressed and continues as a high-pressure gas into the condenser; it condenses to a liquid, and the heat is expelled into the outside air.

The reversal

Your heat pump uses refrigerant to remove heat from the air just like an air conditioner—only in reverse. During the winter, your heat pump utilizes a reversing valve which allows your heat pump to reverse the flow of refrigerant. A cold outdoor coil absorbs heat from the outside air. There is still heat in the air, theoretically, down to negative 412 degrees Farenheight. Colder refrigeration absorbs heat from outside and changes pressure; this changes the temperature to create a hotter indoor coil. Air crosses over the coils to provide warm air to your home. 

The balance point

Because this reversal uses refrigerant to heat your home when it’s cold outside, there is a temperature range to consider. Your heat pump works by removing heat from the air; consequently, they don’t heat as efficiently when outside temperatures fall below freezing. Specifically, a heat pump will start to lose efficiency when the temperature drops below about 40-degrees Farenheight outside; this is called the balance point. When the outside temperature drops below the balance point, there isn’t enough heat to pull from the air to heat your home. 

Now that we know heat pumps don’t work as efficiently at low temperatures, what is the solution? Below are three occasions when auxiliary heat is automatically engaged to keep you warm—and safe.

FIRST: Auxiliary heat when temperatures are below the balance point

When you see auxiliary heat, EM, or emergency heat on your thermostat, they mean the same thing. Here is how it works. If your thermostat is set to 70 degrees and it’s below the balance point outside; your heat pump cannot keep up. Your thermostat then drops to 68 degrees inside; this is when your auxiliary heat would automatically kick-in to maintain the temperature in your home. Electricity heats the electric coils enabling the colder air blowing across these coils to pick up additional heat. When the temperature outside rises, the auxiliary heat will automatically shut off.

SECOND: Auxiliary heat during the defrost cycle

Another reason the auxiliary heat strip will engage is to heat the supply air when your heat pump goes into its defrost cycle. If the outside air temperature is cold enough—long enough—the condenser coil outside can form a heavy frost or may even ice up. When this happens, your heat pump system is not working at peak performance. If your HVAC control board senses frost or ice build-up, the system goes into defrost mode: the system switches the reversing valve and turns on the air conditioner! Remember, in the air conditioning cycle, heat is being rejected out of the refrigerant lines (to bring cold air in during the summer). This heat is being used to melt the ice off of the condenser. While this is happening, cold air is being blown inside your home. The condensing unit now sends a signal to turn on the auxiliary heat to offset the cold air blowing into your home.

THIRD: Auxiliary heat as a safety measure

Heat pumps are popular here in central Virginia because we only see temperatures drop below freezing one percent of the time. During the winter, our 99-percent design temperature is the temperature in Virginia that stays above 17 degrees for 99 percent of all the hours in the year—based on a 30-year average. So the outdoor air in Virginia is going to be colder than 17 degrees for just one percent of the hours during the year. HVAC consultants will design your system—including auxiliary heat—to keep your house comfortable even when outside temperatures drop below 17 degrees. So whether it’s zero or ten degrees outside (this range is contractor specific), your heat pump is expertly designed to engage auxiliary heat to make sure your house stays warm enough to help you avoid any health-related risks due to an indoor climate that is too cold.

NOTE: Heat pumps are still an optimal solution because they are electric; they don’t require access to natural gas or propane. A furnace is something to consider (in a colder climate) because they do create heat, but you would need access to natural gas or propane. The benefits of a furnace are lower utility bills (if you have natural gas) and they contribute more humidity to the air than auxiliary heat—which is a dry heat. 

If you’re curious about auxiliary heat or have concerns that your auxiliary heat is coming on too often or staying on too long, schedule a check-up with a professional. Your HVAC system has many moving parts, and regular maintenance check-ups are the smartest and easiest way to keep your unit running optimally.

Photo by Matthew Henry on Unsplash