What are heat strips?

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If you have an electric heat pump, during the course of your fall preventative maintenance 

you may have overheard the technician say that he tested the “heat strips” and wondered what they were talking about? Well wonder no more!

“Electric heat strips”, “auxiliary heat strips”, or just “heat strips” for short, are a supplemental or secondary heat source that is installed inside the air handler to help a heat pump’s heating  output when the outdoor temperature gets really low. But before I can really explain what heat strips are and how they work, let’s first take a moment to go over a heat pumps operation.

When an air conditioner is running in A/C mode, it is taking heat from inside the house, where we don’t want it, and moving it outside where we don’t care about it.  This is how the air conditioner cools us, by transferring heat or heat transfer.

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Well a heat pump is just an air conditioner that can run in reverse.  which means when it runs in heat mode, it takes heat from the outside air and moves it inside to heat your home, again through heat transfer.

However, because there is less heat in cold air, if it gets really cold outside, like when outdoor temperatures drop below 40 degrees, the system needs a little bit of help – enter the heat strips or auxiliary heat.

The heat strips are an electric heating element; very similar to the heating element in an electric oven; that come on to help a heat pump produce heat even when the temperature gets really cold outside.  The difference is, you don’t generally manually turn them on (see below).

It’s worth noting that only heat pumps have heat strips.  Gas furnaces with air conditioning use a combustible fuel source (either natural gas or propane) as their only heat source and don’t need supplemental heat.

During heating operation, the electric heat strips can come on for three different reasons.  I’ll go over each of them below:

Run Time

This is by far the most common way that heat strips come on.  Most thermostats have programming in them that will automatically turn on the electric heat strips if the system runs for more than say 15 minutes without raising the temperature at the thermostat more than a half of a degree.  

If this happens, the thermostat will send a signal to the air handler to turn on the heat strips.  Depending on your thermostat, you may see either a red light come on, or an LCD display show “AUX” or “AUX HEAT”.  Many heat pump thermostats show both. The “aux” means auxiliary as the heat strips are designed to be a second stage of heat

Demand

The second most common time that the heat strips will come on is when there is a “demand” for them.  This means that someone has set the temperature more than three degrees above the thermostat setting.  

Three degrees isn’t an absolute though.  Different thermostats may have different balance points that may or may not be able to be changed.  

Even if your thermostat will allow you to modify this range, I would recommend not adjusting it.  Three degrees gives a good balance between comfort and energy savings as the heat strips are expensive to run since they are designed only as supplemental heating source, not a primary one. 

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Defrost

Now we get to the one that confuses most people who are unfamiliar with heat pumps – defrost mode.  Yes, occasionally we have to de-ice the heat pump!  

When the system is running in heat mode, the condenser outside gets colder as it absorbs heat from the air around it.  

After long periods of running, or if it runs in particularly humid conditions, it will get so cold it begins to get frost on it.  This frost will eventually turn to ice, and when it does it will significantly reduce the amount of warm air that the heat pump can produce.

To keep the condenser from turning into a solid block of ice, the system has a way to thaw itself out – defrost mode.  Once the condenser outside reaches a certain temperature, the system will switch into air conditioning mode!

You’re probably wondering why, if it’s already cold outside we would want to be running in air conditioning?  Well the reason is because shifting from heating to cooling mode forces all the hot refrigerant that is providing our primary source of heat to the outdoor condenser to melt the ice.

To keep us from feeling a sudden rush of very cold air from the system running in A/C, the system turns on the heat strips to balance the temperature.  The defrost mode generally lasts less than five minutes, so most of us never realize this is happening.

Emergency Heat

Unlike the previous three examples, this is something that you can turn on yourself, however, like the name says, it should only be used for emergencies.  When you turn the thermostat to the “Emergency Heat” setting, what happens is you lock out the outdoor unit, so ONLY the electric heat strips and the blower fan come on.  

This setting is only used for emergencies, such as if the compressor were to fail and you had to have heat until a repair can be made.  The electric heat strips are expensive to run, and can cause your energy bills to be much higher than normal. Don’t turn them on unless you know that you need them! 

When the heat pump is working correctly, you’ll probably never notice the heat strips turning on and off.  This is what we want.

Now that you know what heat strips are, hopefully you’ll have a better understanding of how they work and when they come on.  

However, if you start noticing cooler air coming out of your air vents, or the “AUX HEAT” begins to show up more frequently on the thermostat, there may be a problem with your heat, and a repair may be necessary.  Just give Carolina Comfort a call and we can assess the problem, and offer solutions to get you heating again in no time!

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Phone: (803) 794-5526

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