What is Auxiliary Heat? Auxiliary vs Emergency Heat

Auxiliary heat (aux heat) is an additional heating component to a central heating system using an air-source heat pump. Aux heat provides extra heat alongside the heat pump when the pump can’t draw enough heat from the air outside. This occurs when the weather drops into the mid-30s Fahrenheit – and definitely when it is below freezing.

But how can you tell if your heat pump also has aux heat? What is the difference, if any, between aux heat and emergency heat (em heat)? Is it possible to install aux heat when your system doesn’t have it?

Our comprehensive article below will provide answers for these questions and more in the following:

How a Heat Pump and Aux Heat Work

Heat pumps have two units—one inside your home and one outside. The inside unit contains a coil for standard heating and electric coils for aux heat. The outside unit, called the condensing unit or “heat pump” is needed for standard heating but not auxiliary or emergency heating.

Unlike a furnace, heat pumps do not heat by burning fuel, which makes them more energy efficient. They utilize a small amount of energy to circulate refrigerant and transfer heat from one location to another, making them useful as both a heating system for the winter (moving heat into the home) and an air conditioning system for the summer (sending heat out of the home).

However, the climate is a huge contributor to a heat pump’s effectiveness. They can function sufficiently in mild climates, but they cannot provide enough heat where temperatures typically drop below freezing (32F).

This is where aux heat comes in. Aux heat is a set of radiant heat strips that use electric resistance heating (ERH), which means that most of the electric energy used alongside your heat pump is converted into heat. Aux heat automatically turns on when the outside temperature drops and prevents the heat pump from drawing in enough heat to reach the set thermostat temperature. In other words, aux heat is a supplemental component to a heat pump; they work together to ensure your home is comfortably warm.

Aux heat requires a lot more energy to function. A heat pump alone reduces your heating by roughly 50% in comparison to ERH. So, you may not initially notice when your central heating system switches on the aux heat, but you will certainly notice when you see the expense of your utility bills.

ERH is controlled through your digital thermostat. This is why you see an “aux heat” light on it.

If your heat pump does not have aux heating, it is up to you to determine how to obtain backup heating.

Adding auxiliary heat.

Using a duel fuel system that includes a heat pump and a gas furnace.

Buying one or more space heaters – cheap upfront, but they use far more electricity than a heat pump, so operating costs are higher.

With or Without Aux Heating?

thermostat aux vs em heat

How do you know when your heat pump has aux heating? There are a few ways of figuring that out.

Here are two practical ways:

1. Climate type. Heat pumps without aux heat are typically sold in regions with milder winters, while heat pumps with aux heat are more commonly sold in regions with harsher winters. If your climate often falls below freezing, it is likely you have a heat pump with aux heat.

2. Looking at your thermostat. Aux heat appears as a setting on a wall thermostat. If you don’t see an aux heat enabling switch or an aux heat light on your thermostat, then your heating system doesn’t have it.

On the mechanical or equipment side, heat pumps with aux heat contain another section to their air handler. Most auxiliary heating equipment is electric resistance heating coils. A few are finned electric heating elements. Either way, they turn on when the heat pump can’t draw in enough heat from the outside to meet the thermostat’s set temperature.

Heat pump air handlers without au heat don’t have this equipment. Although, this doesn’t necessarily mean you can't have aux heat installed. Sometimes, heat pumps are manufactured to have aux heating as a potential “add-on” feature. This is usually the case for homeowners who experience milder winters so that they can pay for a cheaper heat pump upfront and determine later if it is worth spending extra money on aux heat.

Should You Have Aux Heating Installed?

If your central heating system allows for an “add-on” aux heat installation, then it will be in your best interest to install it if you currently live in a climate with colder-to-harsh winters.

Does your heat pump fail to keep your home warm on the coldest days? You could use auxiliary heat. In fact, if you experience this problem, your heat pump salesperson didn’t do their job.

Em Heat vs. Aux Heat

Aux heat is used as an automatic and supplemental heat source for your heat pump, but em heat takes over when your heat pump suffers mechanical breakdown.

Em heat acts as a safeguard during harsh winters. When em heat is turned on, it will simultaneously shut off your heat pump so that it won’t freeze over and provide your home enough heat so that you won’t need to evacuate and, hopefully, your water pipes won’t freeze and burst.

Em Heat vs Aux Heat

Unlike aux heat, you might have to manually switch your em heat on through your thermostat. This can be done by moving a switch or through the press of a button.

Em heating should not be used in excess. It should only be used when the outside temperature is below 32F or during an emergency—such as your heat pump malfunctioning. Why? Because like aux heat, em heat is a set of radiant heat strips that use ERH, requiring a large amount of electricity to work. Unlike aux heat, em heat is not used in conjunction with a heat pump to lower the amount of energy used. On the contrary. Electric resistance heat is very energy inefficient, considerably racking up your utility bills.

Aux and Em Heat FAQ

Here are questions we frequently get plus short, to-the-point answers.

How do I stop aux heat from turning on?

Here are two practical ways to turn off the aux heat or to prevent it from turning on:

1- Lowering the heat. It may be a bit chilly, but you will need to set your thermostat for 68F on a regular basis. By keeping the inside temperature lower, it will keep the heat pump from having to work hard. If the heat pump can more readily reach the set thermostat temperature, then it won’t need help from the aux heat strips.

For the same reason, you should program your thermostat to lower the inside temperature whenever you are sleeping or are spending an extended period of time away from home. It’s one thing if aux heat is needed once or twice per year when the weather is extremely cold. But if aux heat kicks on regularly, your heat pump is undersized. Some HVAC technician did not do a proper load calculation before choosing which size to install.

2- Your heat pump also has a “defrost” mode that works the radiant heat strips. This mode exists because the longer the condenser absorbs and transfers heat from the surrounding air, the colder it becomes. Think about it – as refrigerant evaporates in the outdoor unit, it takes heat with it and carries it indoors to heat your home.

Just as you feel chilly when water rapidly evaporates off your skin, so the outdoor unit gets really cold during this evaporative process.

At some point, the condenser begins to freeze. To prevent the condenser from becoming iced over, the system switches from heating to air conditioning, concentrating the heat in the outdoor condenser unit to “defrost” it. This process doesn’t last more than five minutes at a time, so you may never notice it, but the entire process can be avoided by maintaining a lower house temperature.

To deal with a frozen condenser unit, gradually increase the heat by, at most, 2 degrees at a time. If you raise the temperature any more in one sitting, the heat pump will quickly switch over to aux heat to reach the set thermostat temperature as quickly as possible.

Keep this in mind: Aux heat will come on if the heat pump cannot reach the new programmed temperature within fifteen minutes.

How do I turn off aux heat on a Honeywell thermostat?

In practice, the answer to this question for the Honeywell thermostat is the same as the answer above: you can turn off the aux heat by lowering the set thermostat temperature to 68-to-62F and gradually increasing your set thermostat temperature whenever you want your home to be warmer.

However, to control when the heat strips turn on, refer to this video, which goes through the steps you will need to take to reprogram your thermostat default settings. If you watch the video but still don’t quite understand how to navigate the thermostat controls, see this Honeywell user guide.

My aux heat runs a lot and even in milder weather. Is this a bad thing?

Yes it is. If your aux heat is on when the weather is above 35F and/or for longer periods of time than seems necessary, something is wrong with your heating system.

This could be the symptom of a number of things:

1-Your heat pump is breaking down, no longer being able to efficiently do its job without the help of the aux heat.

2-Someone sold you a heating system that does not effectively work in your home. This can be the case when your central heating system is too large or small for your home.

3-Your thermostat is malfunctioning and causing the aux heat to turn on excessively.

In any case, your best option is to contact your nearest and most reliable HVAC company to diagnose and repair the issue.

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