Understanding and Evaluating Heat Pumps


Heat pumps are a bit of magic. But you use one in your home every day – your refrigerator. So if you can accept that your fridge works, somehow making 0 degree air from the 70 degree air in your home, plus some compressor action, then you should be able to accept that heat pumps can produce 95 degree air to heat your home while it’s 25 degrees outside.

I’m not going to subject you to a full lesson on thermodynamics and the refrigeration cycle. Instead, I just want you to accept the fact that heat pumps work on the same principles as refrigerators and air conditioners – they move energy from one place to another. In the process, they make one side of the system hotter and the other side colder.

Heat pumps, refrigerators and air conditioners all work the same way – they move heat from one place to another, amplifying the effect using a compressor and refrigerants.

Intuitively, you already know some things about these systems. You know that when it gets really hot outside, the air conditioner struggles to keep up. It’s the same as when it’s cold out and the heat pump doesn’t crank out as much heat as it does when it’s mild. Why? Because there’s simply not as much heat available outdoors when it’s 15 degrees as when it’s 45 degrees. So rule 1 of heat pumps is:

  • A heat pump produces less heat the colder it is outside

How much less? If you look at the Wikipedia article on heat pumps, a typical heat pump drops from 100% of its rated output at 47F to 60% at 17F.

The second part of the equation that you already know is:

  • when it’s colder outside it takes more energy to heat your home. (Duh).

We now have two obvious, but vital, pieces of information: 1) a heat pump can’t produce as much heat when it’s cold and 2) your home needs more heat when it’s cold. When you combine these two into a graph, you get the heat pump balance point – the temperature at which a heat pump can exactly satisfy the heating requirements of your home.

Graph of heat pump balance point from Elite Software

The graph to the right was produced by software from Elite Software.

What this means to you

Homes require different amount of energy to keep warm based on many factors: insulation levels, air tightness, home size and indoor and outdoor temperatures among others. These are all wrapped up into a single factor called the “heating load.” This is the blue line on the graph. It goes up as the temperature drops. You have some control over the amount of energy needed to keep your home warm. Adding more insulation and air-sealing reduces the amount of energy needed, so the blue line doesn’t increase as quickly as it gets colder out.

When you do that, you change the equation in your favor. Now the green and blue lines will intersect at lower temperatures meaning the heat pump can keep your house warm when it’s colder out.

  • Weatherizing your home allows a heat pump to keep your home warm at colder temperatures

The green line on the graph is determined by the heat pump. Larger capacity heat pumps put out more heat. That raises the green line which again moves the intersection of the two lines to a lower temperature.

  • Increasing the size of a heat pump allows it to heat your home at colder temperatures

Both of these facts are intuitively obvious. You install a bigger heat pump to provide more heat. You insulate your home so that it uses less heat.

In “real life”, there are practical limits to how well you can insulate and how large a heat pump you can install. At some point, you’re going to be stuck with what you have. In most homes, the balance point (the intersection of the green and blue lines) is somewhere between 35F and 45F. This is why heat pumps have such a bad reputation. What happens when it’s colder than the balance point? The heat pump runs and runs and your house just keeps getting colder!

Backup heat

In order to supply heat when it gets too cold out, heat pump systems almost always have some form of backup heat. In the simplest case, the backup heat is supplied by “electric heat strips.” This is just a giant version of your electric toaster sitting inside the air handler. In other cases, the heat pump will be built into a furnace, often powered by natural gas or propane. The backup heat is controlled by a special two stage thermostat that determines when to run the heat  pump and when to switch on the backup heat.

Why not use the backup heat all the time?

Here’s where things get really interesting. A heat pump is very efficient at producing heat compared with toasters. Higher efficiency means lower utility bills. The measurement of efficiency for heat pumps is called its “COP,” or coefficient of performance. For consumer heat pumps, there’s also a rating called the “HSPF” or heating seasonal performance factor. This number is supposed to give you an idea of the system efficiency over the course of the entire heating season.

The COP is more understandable, so let’s look at that for a second.

Toasters (electric heat strips) are (essentially) 100% efficient at converting electricity into heat. This is a COP of 1.0. Heat pumps typically have COP ratings of 2 to 4, depending upon the outdoor temperature and design of the unit. So the heat pump puts out two to four times as heat for a given amount of electrical energy. Or, put another way, it’s two to four times as efficient so your electric bills are one half to one quarter as much as they would be if you heated with toasters.

Side note: Toasters, portable radiators, space heaters, electric heat strips, baseboard electric heaters, ceramic heaters and even those “Amish electric fireplaces” are identical – they convert electrical energy into heat with a COP of 1.0. Don’t believe the hype. They’re identical. Only the packaging is different. Heat pumps work on different physics so they’re much more efficient.

Now that you understand COP and see that heat pumps are two to four times as efficient as electric backup heat, you can see why you don’t want to run the backup heat all the time! Would you rather have a $100 heating bill or a $300 bill?

How about gas backup heat?

It’s much more difficult to compare the cost of heating with gas vs. heating with a heat pump. Gas and electricity prices are different in every town. And you have to take into account the efficiencies of the gas furnace and the heat pump. There are also safety and environmental considerations. But let’s assume for this discussion that running a gas furnace is more expensive than running the heat pump. Otherwise, there would be no point in using a heat pump, you’d just use gas all the time!

It’s all about the balance point

All of this tells us that you want to run the heat pump down to as low temperatures as possible before you switch over to more expensive backup heat. Unfortunately, this requires diddling with thermostat settings that usually are hidden from consumers or not even available in some thermostats. Why not just use the “default” settings? Because doing so can double or triple your heating bills.

A tale of outrageous heating bills…

I was called in to help diagnose high electric bills in a townhouse. My client paid top dollar for a high-end heat pump and his heating bills went through the roof. For a modest sized home, he was paying $500 a month using a super-efficient heat pump.

It turned out that the installer set the thermostat so that the backup electric heat would come on any time it was below 40F outside. The system went from running at a COP of 3.5 to about 1.5 (the combination of electric backup and heat pump). In our area, it’s below 40F most of the winter, so all winter, the backup heat was running, leading to excessive heating bills.

By tweaking the thermostat, we could reduce the temperature for backup heat from 40F down to 30F or even lower. The heat pump was plenty capable of supplying all the heat that was needed down to about 25F, but to be conservative, we set the switch-over to 30F. This little 10 degree change was enough to reduce the use of backup heat by about half, shaving hundreds of dollars off his winter heating bills.


Reduce the backup heat switch-over temperature to lower your electric bills

Optimizing heat pumps to reduce your electric bills

Let’s put all this together:

  1. More insulation and a tighter home lets a heat pump work at lower temperatures
  2. A larger capacity heat pump lets the heat pump work at lower temperatures
  3. Backup heat costs more to run than a heat pump. Electric backup is two to four times as expensive.
  4. Reducing the temperature at which backup heat comes on can dramatically reduce your electric bills.

Action items:

  1. Add insulation and weatherize your home. This makes it more comfortable and energy efficient year round
  2. When choosing a heat pump for a cold climate, err on the side of a larger heat pump. An undersized heat pump will cost you vastly more to run because it will rely more on expensive backup heat.
  3. Learn when the backup heat comes on so you can see how much it’s running. If it comes on when the heat pump is doing fine by itself, look into changing settings so that the backup doesn’t come on until a lower temperature.

Some more details

Most of you can stop reading and just implement the action items listed above. But you may want to know some of the additional technical details that come into play

Sizing for air conditioning

When you size a gas furnace, you’re not worried about how it affects air conditioning because they’re two separate systems. So almost every furnace has more than ample capacity to heat you home. With a heat pump, the same unit supplies you with cool air during the summer, so when you select a certain sized heat pump, you’re also selecting the size of the air conditioner. This is one of the most important reasons that installers don’t just install the largest heat pump available.

During the summer, especially in humid climates, much of the reason that you use an air conditioner is to remove humidity from the air. The longer the air conditioner runs, the more it dehumidifies the air. If the air conditioner is too large, then it runs for short blasts which cools the air but doesn’t properly dehumidify. This leads to uncomfortable conditions in the house, where the air is cold and clammy. It also leads to wide temperature swings.

In humid climates, get the smallest air conditioner that does the job. This provides the most comfortable, even cooling and best dehumidification.

To make matters worse, in my area of the country (eastern Pennsylvania), we get many days that are mild but humid. This is even tougher for over-sized air conditioners. They might only run for a few minutes an hour to keep the temperature comfortable, but yuck, the humidity is awful! To make matters worse, the high humidity can lead to mold problems.

Catch-22 – air conditioning effectiveness vs. heating efficiency

So, I’ve just told you two contradictory things. First, you want a large capacity heat pump in order to minimize the use of backup heat and greatly reduce your winter heating bills. Second, you want a small capacity air conditioner in order to get the best dehumidification and comfort in the summer. What can you do?

As they say, “it’s complicated.” You have to carefully balance these conflicting needs. And there are things you can do to improve the situation.

  • Make sure you have lots of insulation and your home is properly air sealed.

The better your home is weatherized, the less you need heating and air conditioning. You get fewer drafts and your home is more comfortable. So weatherization is pretty much always the first thing to do. In addition, a tighter home will be less humid in the summer because during the summer the outside air carries the moisture into the house. Reducing outside air infiltration means less humidity and greater comfort. That also makes air conditioning less important.

  • Check your ducts

I’ve written a LOT about leaky ducts so I’m not going to repeat it here. Just know, leaky ducts lead to wasted energy and uncomfortable conditions. That means dry air in the winter and humid air in the summer. And almost all ducts are really leaky (30% on average). Do yourself a favor – make sure your duct system is properly sealed.

  • Listen to your system

You’ve heard of the “horse whisperer” and probably the “dog whisperer.” I try to be the “house whisperer.” You learn a lot by listening to your house, especially your heat pump and air conditioner.

During the summer: make notes about how often the system runs. Keep a notebook with the date, time of day, how sunny it is, the indoor and outdoor temperatures, and the amount of time the system runs vs. the time it is off. Also note how comfortable you feel. This one set of measurements gives you more information about your system than your HVAC contractor can tell you with all their calculations and measurements.

The thing this tells you is how much of your system’s capacity is being used. For example, suppose it’s a sunny afternoon in July. It’s 90 degrees outside and your system runs 20 minutes every hour and is off the other 40 minutes. This says your system is only using one-third of its capacity to cool your home under warm conditions. Unless you live in a really hot climate, where it’s over 100 degrees and humid for long periods, your system is much too large.

On the other hand, if it’s 85 degrees and humid and your system is running constantly and you’re still not comfortable, then you’ve got a problem. Either you’ve got really bad duct leaks, your home is poorly insulated and air sealed, or your system is not working properly. Regardless, you’re probably wasting a lot of money. Time to get it serviced.

During the winter: Keep the same notebook and also log when your backup heat is coming on. You can usually tell this because most thermostats have a blue light that comes on to tell you that the backup heat is on.

Your best bet is to monitor the system at a few temperatures. Try 45 degrees, 40 degrees and 35 degrees. If your system is properly sized and configured, your backup heat should almost never come on at these temperatures and you should be able to get a good idea of the system’s capacity.

For example, suppose the system runs for 20 minutes per hour at 45 degrees, 30 minutes per hour at 40 degrees and the backup heat comes on at 35 degrees so the system only runs 15 minutes per hour (because it’s now heating with heat pump plus backup heat).

This tells me that the backup heat is coming on too soon. If the heat pump is only operating 30 minutes per hour at 40 degrees, it could probably handle having the backup heat not come on until below 30 degrees. On the other hand, if the heat pump struggles to keep up at 40 degrees (maybe running 50 minutes per hour) then it’s likely undersized or there’s other problems (just like when it couldn’t keep up with a modest air conditioning requirmement).

Putting the information together:

So you’ve got your notebook and you know that your system is properly functioning. The ducts aren’t leaky and you’ve got plenty of insulation. Now you want to upgrade your heat pump/air conditioning unit. How do you put it all together?

  • Compare the heating and cooling notes under typical summer/winter conditions

For example, where I live, I’d look at how the air conditioner runs when it’s 85-90 degrees and the heat pump when it’s 25-30 degrees. This covers about 90 percent of the conditions the system needs to run.

How much does the system run under those conditions? Does the air conditioner run much more or less than the heat pump? Does the heat pump need lots of backup heat to keep up with the heating demands? Or are things well balanced?

Typically, if you’re in a “heating dominated climate”, you’ll find that your air conditioner won’t run much and the heat pump will run too much. If you’re in a balanced climate, then the system will likewise run about the same in winter and summer. In my climate, I like heat pumps to be able to keep up down to 25-30 degrees, otherwise the backup heat runs too much. The problem is, this will lead to air conditioning that runs too little. What to do?

Two-stage heat pumps to the rescue

Higher end heat pumps (and many mini-split heat pumps) have compressors that can run at different speeds. This allows the system to vary its capacity as needed, giving more efficiency and comfort during mild temperatures and the capacity needed for extreme temperatures. Or, if you live in a climate like mine, it allows the air conditioning to run on low capacity and the heating to run on high capacity, providing the best of both worlds.

This is why I always recommend that people install multi-stage heat pumps. Over the course of the year, they’ll save hundreds of dollars and they’ll make your home much more comfortable year round. So even though it might cost a couple thousand dollars more to install, it’s well worth it.

The key thing with a two stage heat pump is to size it large enough to satisfy most of your heating needs if you live in a heating dominated climate. If you size it too small, then you may as well have saved your money and bought a single stage system. But how to decide how large?

Remember the notebooks? Refer back to them. How big is your current system? Was it big enough to handle your winter heating needs without turning on the backup heat? If so, you should probably keep the same sized system. But if it couldn’t keep up with the heating needs at 40 degrees, then you probably need a system at least one size larger. Just don’t go crazy or the system may still be oversized for air conditioning even at the low setting. It’s all a balance.

Install a two-stage or variable speed heat pump if at all possible. It will make your home more comfortable and energy efficient year round.

Other references

Carrier variable speed heat pump – Infinity heat pump with Greenspeed intelligence

Department of energy – Energy savers: Air source heat pumps

Energy Star – Air source heat pumps and air conditioners

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98 thoughts on “Understanding and Evaluating Heat Pumps

  1. Just stumbled over your site while helping my daughter and son-in-law with their first house. Lots of great stuff.

    One thing I’ve found about heat pumps around here (eastern TN), is the operation of the thermostat is a bit different. Instead of a fixed cut-over temperature that kicks on the backup heat, the thermostat on the wall does it when the inside temperature drops 2 degrees below set-point. Method works well, but some folks are surprised how bad their heating bill is, if they constantly ‘fiddle’ with the thermostat. Turning it up and down throughout the day, they often trigger that two degree offset and the backup heaters kick on, even when it’s as warm as 40F outside.

    So when adjusting setting with this type, I caution them to only move a degree or so at a time (gitial thermostat) and let the heat pump take its time to warm the house.

    • Great reminder, thanks for posting this. This is exactly why most HVAC installers try to “train” home-owners to set the thermostat to a comfortable point then don’t mess with it.

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