Heat Pumps are one of those delightful technologies that do pretty much what they say on the tin. They transfer thermal energy (called “heat”) from one location to another.¹ Everyone has encountered heat pumps: your refrigerator is a one-way heat pump that transfers heat from the inside of the fridge to the air in your kitchen – they cool your food, warming your house a bit in the process. Similarly, an air conditioner is another one-way heat pump – it cools your house by pumping heat from the air indoors to the air outside.

Refrigerators and AC units are both heat pumps, but in modern parlance, we tend to reserve the use of the term “heat pump” for bidirectional systems – capable of both heating and cooling depending on which direction they are run. Modern home heat pumps are a lot like traditional ACs, but with the capability to run in reverse. As a result they can pump heat from inside of your house to the outside (cooling your house), or they can pump heat from outside your house to the inside (heating your house in the process). Sometimes, we also use the term “heat pump” for systems that only run in the heating direction (for example, a heat pump water heater). Heat pumps that only cool tend to be called air conditioners (or refrigerators when they are cooling your food rather than your air). But at the end of the day, refrigerators, ACs, home heat pumps, and heat pump water heaters are all examples of the same underlying heat pump principle. They consume electricity to move heat from one location to another.

And I care, why?

It turns out that if we want to continue to live on this planet, we really need to stop burning fossil fuels. One of the main uses of fossil fuels is home heating. In the Western United States this takes the form of the ubiquitous gas furnace – which burns natural gas in order to heat your home in the winter. Even if we succeed in converting all electrical generation capacity to non-fossil fuel means, the use of fossil fuels in home heating will continue to contribute unsustainable greenhouse gas emissions to our atmosphere.

If we can’t keep heating our homes by burning fossil fuels, what are the alternatives? If you live in very specific places, you could tap the Earth’s geothermal energy.² But this option is very expensive and extremely limited in where it can be applied. We could go back to burning trees (or other organic matter) for home heat, the way our ancestors did, but that requires a lot of work and raises significant air quality and fire safety concerns. We could design homes to use passive solar heating, which is great, but again, is limited to specific locations and new construction.

If we want a generally applicable, reasonably practical home heating method that doesn’t burn fossil fuels, we’re going to need to use electricity to heat our homes. There are two ways to do that. The naive way is electrical resistance heating: basically running a lot of electrical current through small wires causing them to heat up. This is very simple and very expensive. While electrical resistant heaters are technically 100% efficient (all of the electricity they consume is being turned into heat), turning high-quality energy like electricity directly into a low-quality energy like heat is rarely economically advantageous.

To demonstrate, look at the cost differences for electricity (a high-quality form of energy) and natural gas (a lower-quality form of energy). At the moment, electricity in Boulder, CO costs residential users somewhere in the range of $0.10 to $0.20 per kWh including taxes and fees. Meanwhile, natural gas costs roughly $1/therm. 1 therm is equivalent to 29 kWh, which means the price of gas is roughly $0.03 per kWh, or about one fifth the median electrical price of $0.15 per kWh. We could use electrical resistance heating to heat our homes, but we’d increase heating costs by roughly 5x.

Fortunately there’s the less naive way to heat with electricity – use a heat pump. While electrical resistance heating turns 100% of the electricity expended into heat, heat pumps do better – they provide more heat than the energy they consume.³ Instead of converting electricity directly into heat, heat pumps use electricity to move heat around. As a result, it is not uncommon to find heat pumps that provide “efficiencies” in the 200% to 400% range (e.g. that move 2x to 4x as much heat as the energy they consume). That goes a long ways to cutting into the cost advantage of natural gas, and puts us in the range of being able to heat homes using electricity for roughly the same cost as heating a home using fossil fuels (and in places where expensive fossil fuels like heating oil are used, heat pumps can be quite a bit cheaper).

Heat Pumps in Practice

This all sounds nice in theory, but what does it look like in practice? Can air source heat pumps handle places with cold winters? How can you harvest heat from sub-freezing outdoor air?

In 2020, we replaced the 20-year-old gas furnace (as well as an aging traditional central AC unit) in our 1600 square foot, 1999-construction home with a cold-weather rated Mitsubishi multi-zone Hyper-Heat heat pump. The installed system consisted of an MXZ-4C36NAHZ 36K BTU outdoor unit, an SVZ-KP18 18K BTU central air handler (to replace the old furnace) and two SLZ-KF09 9K BTU ceiling-mounted mini-split heads for the upstairs bedrooms. This system has a number of desirable qualities:

• It replaces the old furnace and AC unit with the SVZ-KP18 central air handler, allowing us to reuse the home’s existing single-zone duct-work to heat and cool the common areas of the home.
• It adds SLZ-KF09 individually-controllable mini-split heads to the upstairs bedrooms, allowing us to control the temperature in these rooms as separate zones. These units also do a far better job cooling the previously too-warm upstairs than the central duct-work ever did (pumping cold, heavy air through duct work from a basement to a second floor is hard to do).
• It is rated to operate down to an outdoor air temperature of -17 degrees Fahrenheit, which is about as cold as it ever gets in Boulder, CO.

This system cost us ~$21K to install in 2020 with equipment, tax, and labor. Of that, we got ~$1K back in various rebates from Boulder and the local power utility (Xcel). We had our system install by Save Home Heat, which is a local HVAC company that specializes in heat pumps and related technologies. At the time, they were one of the few local companies with experience in this space. In the past four years, however, many other companies have gotten comfortable working in this space. Like most things, the prices have also gone up – but the so have the available tax rebates thanks to federal and state efforts.

You can see the MXZ-4C36NAHZ outdoor compressor in the photo at the start of this post. It uses dual horizontally facing fans to move air through a heat exchanger and heat/cool compressed refrigerant in the process.

Here is the SVZ-KP18 central air unit. It sits pretty much where the old furnace used to sit and attaches to the existing duct work that ran to that location. One note on this – heat pumps heat air more slowly than a gas furnace, and as a result, duct work designed for gas furnaces is often somewhat undersized for a heat pump. In our case, it still works out fine, but in larger homes you may bump up against limits as to how well a heat pump will work with legacy duct work. In those cases, you can always turn to mini-split heads to augment the existing duct work. Which brings us to…

Here is one of the two SLZ-KF09 mini-split heads we installed to augment the legacy duct work in our upstairs bedrooms. These are ceiling-mounted units, which work well if you have plenty of workspace (like an attic) above your ceiling. Functionally, they work the same way the more common wall-mount units work, but aesthetically they have the advantage of blending into the room a bit better.

But does it work?

So how well does this system perform? Let’s start with the cooling capacity. Here’s the indoor and outdoor temperatures in a recent 24-hour period where the outdoor temperature peaked in the high 90s – one of our hotter summer days here in Boulder.

As you can see, the system does a reasonable job of holding both the downstairs (served by the central air handler and legacy duct work) and the upstairs (served by the mini splits) at around 75 degrees F (which is the temperature we leave the thermostats set to in the summer).

And how much energy does it take to do this? Well, here’s the snapshot of our home’s energy consumption (and solar production) on the day shown in the temperature graph above.

It’s a bit difficult to filter out all the other energy uses inherent in this chart, but compared against our base-load of ~1KW (the dip in the consumption graph around 6AM), you can see the heat pump is driving roughly another 1KW in additional electrical load throughout the day (the other spikes in the graph are cooking and the like). Not accounting for our solar capacity (which helps lower the cost), if we assume the heat pump runs for 20 hours of the day, that translates to a cooling cost of roughly $3 for one of the hottest days of the year. Not too shabby!

At the other end of the spectrum (and where folks tend to worry more about heat pumps in Colorado), let’s look at Boulder’s coldest week this past winter where we had multiple days of sub-zero temperatures.

Here you can see the system keeping the house above 60 degrees F, despite temperatures in the negative single digits. You can tell the system is operating near its limit on some of these days – it holds the house above 60F but struggles to hit the thermostat set point of 68. As soon as the outdoor temperature swings back up to positive temps, the system recovers quickly and heats the house back into the 70 degree range.

But what about energy usage? Heat pump energy usage is proportional to the difference between the indoor and outdoor temperature. That means that you generally use more energy to heat than to cool: very cold days like this have a 70+ degree difference between the indoor and outdoor temperatures, vs the hottest summer days where that difference is only 30 degrees. Here’s the energy production and consumption graph from January 13th, 2024 – the coldest day in the graph above.

Here you can see the heat pump driving ~2KW of consumption above our base load, pushing up toward 3KW at times. Couple this with the fact that there was snow on the ground (and our solar panels), and these sorts of days get a bit expensive. If we assume the heat pump consumed ~60KWh of energy throughout this sub-zero day, that translates to a cost of $9 to heat the home that day. But this is pretty much the worst possible case. A more typical winter day costs a half to a third of that. Using a heat pump generally means your electrical bills go up in the winter, but not an unmanageable amount.

One item to note – you can opt to install a heat pump with backup electric resistance coils that kick in in situations like this. We opted not to do that, in part because we like to live dangerously, and in part because of the added cost and hassle. As you can see, the system still meets our needs, even during the one sub-zero week Boulder gets each year. But if you prefer to have more buffer, you can always add the electric resistance coils to the main air handler as a backup (at the cost of higher energy usage on these very cold days).

Lessons Learned

Four years in, we’ve been very pleased with our heat pump install. Not only does it cool the house far better than the original AC unit ever did, but it has met our heating needs through three Colorado winters (including multiple sub-zero days). All without needing to burn any fossil fuels to operate (we use a mix of home solar and grid renewables for our electrical mix). We have learned a few lessons throughout this process:

• Because heat pumps heat more slowly than a gas furnace, it’s best to leave the heat set to a consistent value when it is very cold (single digits) outside, vs letting the thermostat drop 10+ degrees overnight as you might with a furnace. Heat pumps maintain temperature well, even when it’s very cold outside, but they don’t do a great job making up heating a house up once the temperature has dropped.
• Most of the winter, we can get away with just using the central air handler to heat the house. Since hot air rises, the upstairs ceiling units are less critical. But when it is very cold, it does help to turn on the ceiling units as well to take full advantage of the outdoor unit’s throughput to maximize heating capacity.
• In the summer, the upstairs ceiling units are extremely useful for cooling the home. Having cooling capacity at the top of the house where the cool air can then make its way downstairs is a lot more effective than trying to use the central air handler to pump cold air for the basement to the second floor.
• Heat pumps introduce as additional resiliency concern: you are now dependent on electricity to heat your home (although that also tends to be true of a furnace). We’re exploring adding a wood stove as an emergency backup heat source to help provide protections against extended power outages.

Heat pumps are a technology that provide both climate prevention (they help reduce fossil fuel usage) and adaptation (they help cool homes as temperatures rise). They also have the nice property that, at least in cold-weather regions, they will become more efficient as the climate warms. But even without additional warming, air-source heat pumps have reached the point where they are a feasible means of heating and cooling homes in most regions of the United States. If you’re looking to replace an existing fossil-fuel based HVAC system, they’re worth a look.