What is backup heat on a heat pump?

Heat pumps are amazingly efficient at turning electricity into heat. In moderately cold weather, a heat pump is roughly 250% to 300% more efficient than an electric space heater.

Air-source heat pumps work by scavenging heat from the outdoor air and moving that heat inside. The outdoor air temperature is a key factor in the efficiency of this process: as it gets colder outside, a heat pump produces less heat. If it gets too cold, a heat pump might not be able to keep up with a home’s heat loss, causing the home to slowly cool down.

This doesn’t mean that heat pumps aren’t a viable option for cold climates. There are several options to ensure that your house stays toasty warm even on the coldest days of the year.

That’s when backup heat, which is also known as auxillary heat, supplemental heat or emergency heat, comes into play. It’s used when the primary heating source isn’t able to provide enough heat to meet the indoor temperature set by the thermostat.

Most residential heat pumps will have a backup heat source. There are several options, and choosing one for your home means balancing the pros and cons of each system. This guide will walk you through the choices and hopefully make things a little more clear.

Myth: the need for backup heat means that heat pumps aren’t good in the cold

First of all, let’s clear up a myth. The inclusion of backup heat is a normal design for a heat pump system. It doesn’t mean that heat pumps don’t work in the cold.

Instead, it’s better to think of backup heat as a way to minimize the cost for a homeowner. It does this by allowing the HVAC installer to choose a heat pump size that is optimized for the average seasonal temperatures rather than occasional temperature extremes.

For example, if you wanted a heat pump that never needs backup heat, you could accomplish that by installing a heat pump that is much larger than is needed for your average winter. A big heat pump will also lose efficiency in the cold, but with higher heat output to begin with, it will have more operating margin to still generate enough heat to keep your house warm on the coldest days.

However, an oversized heat pump isn’t a good system design because it will tend to run for short cycles in both winter and summer. A heat pump is more efficient when it runs for long periods of time: in fact, 2-stage and variable speed heat pumps are designed for long cycles or even continuous operation. Short cycles also allow less time for a heat pump or air conditioner to dehumidify the air in summer, further reducing comfort.

This means that even though backup heat can be less efficient than a heat pump and require additional hardware expense, the overall cost can be lower than installing an oversized heat pump. A correctly sized heat pump will also give you better comfort than an oversized system.

When does a heat pump need backup heat?

As the temperature outside drops, a house will lose heat at a faster rate. Unfortunately, the performance of an air-source heat pump also drops the colder it gets. Heating performance is often measured using as the coefficient of performance (COP). Higher COP numbers are better, and a COP of 1 is equal to an electric resistance appliance, such as a space heater, toaster, or hair dryer.

A home’s increasing need for heat and a heat pump’s drop in efficiency (measured as COP) as the temperature drops is illustrated in this graph:

As it gets colder, a house will eventually lose heat faster than a heat pump can supply it. That’s when you’ll need backup heat. The temperature at which your thermostat switches over to backup heat is called the balance point.

This graph is just an example: every house has a different rate of heat loss, so your HVAC installer will need to perform a load calculation (ie. energy analysis) on your home to determine the ideal balance point for your heat pump.

What is the typical balance point for a heat pump system?

The ideal balance point for a heat pump is significantly affected by the home’s rate of heat loss in cold weather. The more well-insulated a home is, the lower you can set the balance point.

As an example, imagine that you have a net zero house¹, which is a home designed to generate all of its energy needs with solar panels. Net zero homes have excellent insulation so they only need minimal heating and cooling. For a net zero home, a heat pump system can have a much lower balance point because its rate of heat loss is so low.

For an average home, the balance point might be set somewhere between 25°F and 32°F, while a well-insulated home with a cold climate heat pump might have a balance around 15°F or even lower. Again, it all depends on the results of the load calculation that your HVAC installer performs.

What are different types of backup heat?

For backup heating that switches on automatically below the balance point, these are the typical systems you’ll find:

• Electric resistance “strip” heating
• A gas or propane furnace
• Boiler (ie. radiator heating)
• Gas fireplace

However, a backup heating system technically doesn’t need to be connected to your thermostat and switch over automatically below the balance point temperature. It can simply be any heat source in your home that you switch on when it starts to get cold. Here are some examples of systems you could use:

• Electric space or baseboard heaters
• Wood or gas fireplace
• Wood pellet stove

The advantage of a manual system is that it has a low upfront cost: you can buy a single space heater for around $30 at a hardware store. Even cheap space heaters have thermostats built-in, so they can automatically maintain the temperature you want. A main disadvantage is that you may need several heaters for your home to warm all the rooms you want. You also have to be careful about placement, because a space heater that’s too close to a thermostat sensor can cause your heat pump to shut off prematurely.

However, this could be a good option, especially if you have a smaller home.

What is a heat strip?

A common option for heat pump backup heating is a heat strip. It’s not shaped exactly like a strip – it’s a short segment of electric coil. It works the same way that a toaster does: electricity passes through, causing the coil to heat up.

If you have a ducted heat pump, the heat strip will be installed along side the indoor heat exchanger. Here’s what one looks like:

This particular heat strip is rated for 5.4 kW. In contrast, a 120-volt space heater will have a maximum output of only 1.5 kW. If your house needs a larger heat strip, you’re only limited by the capacity of your electric panel. Heat strips for homes will have a maximum output of about 25 kW.

Ductless mini-split heat pumps can also have heat strips as backup. In this case, the heating element is embedded in the indoor unit.

Heat strips have a low hardware cost, but high operating cost due to low efficiency (which is discussed further below). Also, because of the high amperage heat strips may need, they may require extra electric wiring expense.

Using a furnace as backup heat

Another option is to have a furnace as backup heat. This is the setup in my own home: my heat pump replaced a central air conditioner, and I retained an existing gas furnace as the backup heat for the heat pump.

For me, this was a simple option because the gas furnace was still in pretty good condition, and the existing central air conditioner meant that the heat pump installation required few modifications to my forced air system. I just needed a new outdoor unit and indoor heat exchanger.

If you are in a similar situation where you are upgrading from any type of furnace – electric, gas or propane – to a heat pump, this could be a good backup option for you, especially if you will only need backup heat occasionally. My furnace runs for perhaps a couple weeks a year, so even though it’s less efficient than a new furnace, its operating cost and environmental impact is low. And because it’s needed for only a couple weeks a year, the wear-and-tear on the system is minimal. This means that it might last another decade or even two.

Efficiency of backup heat versus heat pump

If you have a cold climate heat pump, the worst efficiency you can expect at 5°F is a COP of 1.75. That’s because the cold climate designation is a Department of Energy specification, so any heat pump with that label will meet this minimum requirement.

Remember that any type of electric resistance heating has a COP of 1, which means that a cold climate heat pump will be at least 175% more efficient even in extremely cold weather.

Conventional heat pumps are roughly 30% less efficient than a cold climate model, though models can vary significantly in efficiency. In any case, this means that it will almost always be more cost efficient to operate your heat pump than backup electric heat.

However, if your heat pump can’t keep up on your home, you may need to rely on backup heat to stay warm.

Cost of backup heat when using a gas furnace

The cost equation is tricker if you’re using a natural gas or propane furnace as your backup heat. That’s because you need to understand the efficiency and fuel cost of the furnace and then compare that to the efficiency and cost of your heat pump at the balance point temperature. Depending on these variables, it could be less expensive to operate a fossil fuel furnace than a heat pump on cold days, or even at milder temperatures if your gas prices are low.

Electric Dwelling created a simple calculator to help you compare the operating cost of furnace to a heat pump. It’s designed to only give you a rough idea of whether it would be more economical to switch to a heat pump from a furnace, so it’s not a substitute for a comprehensive analysis from an experienced HVAC company. However, it is useful for giving you a very quick way to get a ballpark estimate using only a few inputs.

Adjusting backup heat in your thermostat

The balance point is set at the thermostat, which will be labelled differently by different manufacturers. For example, my Ecobee thermostat labels it as “Aux heat max outdoor”:

In contrast, the Google Nest thermostat labels it simply as “Heat Pump Balance”, which is certainly clearer. The Nest also has options for Max Comfort and Max Savings, which will either maximize or minimum its use of backup heat to maintain your referred temperature².

Geothermal heat pumps don’t need backup heat

One type of heat pump that doesn’t need backup heat is ground-source, also known as geothermal.

As opposed to extracting heat from the air, a geothermal heat pump gets its heat from the ground underneath a house. Accomplishes this with loops of polyethylene pipe buried around 6 feet for horizontal installations or up to about 150 feet for vertical installations.

Unlike air-source heat pumps that are affected by the air temperature, the temperature of the ground doesn’t change very much from winter to summer.

Because of this, the efficiency of a ground-source heat pump doesn’t change very much seasonally, so there is never a need for backup heat on extremely cold days. The ground temperature stays relatively constant throughout the year.

Bottom line: most air-source heat pumps will have backup heat

Unless you live in a mild climate, your air-source heat pump will probably be installed with a backup heat source. That’s a normal design, and it doesn’t mean that your heat pump can’t work in the cold. Provisioning backup heat in a heat pump system is actually about cost optimization, because it lets your HVAC installer choose a heat pump size that runs efficiently in both the summer and winter.

Talk to your installer about the best backup heat option for you. It could be relatively simple heat strips. Or, if you have a furnace already, you might be able to keep your old furnace for occasional backup use.

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