This post is part of Pearl’s High-Performing Home series. In each article, we’ll detail important steps all homeowners should take when assessing and improving their home’s energy efficiency. In our previous installments we covered the energy savings and indoor air quality benefits of conducting a blower door test, air sealing your home, and installing proper insulation. We’re now kicking off all things heating and cooling, starting with the heat pump.
There are nearly 50,000 searches for “heat pump” in the United States alone — about 1.5 million if you count variations of the phrase. Per month. You don’t need to be a statistician to know that’s a lot of people looking for ways to upgrade their heating and cooling. But it also backs up what I’ve come to discover as an expert on building science and an educator: Heat pumps can be a confusing business. From how they work to the range of models on the market, to whether or not they perform well in all climates — let’s break down the mystery of the heat pump so you can better assess whether transitioning to this system is right for you.
How Heat Pumps Work
A heat pump is basically an air conditioning unit with a reversing valve that allows the cooling cycle to run backwards, making it a heating cycle. On the cooling side, it functions like most air conditioners, sucking in hot air from your house, running it over coils that contain very cold refrigerant. The refrigerant absorbs the heat, then passes through the compressor in the outdoor part of the unit, where it’s turned into a high-temperature gas. As the gas passes through the outdoor coils, the coils release the heat into the air outside your home. The gas then begins to cool back into a liquid. The cold refrigerant is sent back into your home along with cold, dry air, and the cycle starts again.
To heat your home, a heat pump does, well, just the opposite. The refrigerant in its coils absorbs heat from outside your home, is compressed and turned into a hot gas, and the heat from that gas is carried over the coils in the indoor unit, which then heats the air directly or through ductwork. For example, let’s say it’s 45 F outside your home, and the refrigerant in your heat pump is 30 F. Heat moves toward cold until a balance is reached. So the refrigerant will absorb the difference between 45 F and 30 F until it reaches 45 F. That difference is 15 F. The refrigerant will carry that energy (measured in BTUs) back into your home, where it heats up coils in the inside portion of the unit and passes that 15 F into your home directly or via ductwork.
Does that mean you’ll only raise the temperature in your home 15 F? Nope. Your heat pump is working continuously, which means if the temperature outside stays steady, it will keep adding 15 F worth of energy (again, in BTUs) each cycle until your home reaches the temperature you’ve set on your thermostat.
That long cycle time matters when it comes to comfort. Unlike a furnace, which has shorter cycles but produces much more heat per cycle, a heat pump warms the home more gradually, producing less heat per cycle. For many homeowners, this lower intensity is appealing — they’re not subject to big temperature fluctuations throughout the day or the space and can enjoy consistent temperature throughout the home. And, just like a car’s MPGs increase on the highway, a heat pump that’s continuously running uses less energy than a furnace that needs to stop and start over and over.
You’ve probably noticed that I made no mention of generating energy. That’s because a heat pump works simply by moving energy or heat from one place to another. There’s no combustion needed, like in a furnace, and that means no generation of carbon dioxide or other gasses, which can leak into your indoor air.
Heat Pump Energy Efficiency
Let’s get to the question that’s on the top of your list: Just how efficient are heat pumps? The Department of Energy estimates that modern heat pumps can reduce your electricity use for heating by 50 percent when compared to furnaces and baseboard heaters (basically any system that uses electric resistance to heat your home). Unlike, say, an LED light bulb, however, the energy efficiency of any one heat pump depends on a number of factors, first and foremost of which is how we think about energy efficiency to begin with.
If we think about efficiency in terms of energy input and energy output of a single unit, then heat pumps beat traditional heating methods, by a lot. Efficiency for heat pumps in cooling mode is measured just like air conditioners, with a seasonal energy-efficiency rating (SEER). The higher the SEER, the higher the efficiency. In 2023, the minimum SEER requirements for air-source heat pumps in the northern part of the United States will be 14, and 15 in the southern states, but models exist that run up to 33 (though most likely these top scores are derived in perfect lab conditions).
On the heating side, heat pump efficiency is measured with a heating seasonal performance factor (HSPF). Minimum HSPF will be 8.8 in 2023, and current high-efficiency models top out around 14.
Furnaces operate on a different rating system, but if we think about efficiency comparison in terms of how each type of unit utilizes the energy you put into it, we can get a good idea of relative performance. A properly installed air-source heat pump in moderate climates will move up to three times as much energy as it takes to power the unit into your home. A newish, high-efficiency furnace, on the other hand, will convert about 95 percent of the energy put into it into usable heat for your home. We’ll get into more detail on a comparison between heat pumps and furnaces in a later installment in this series, but for now it’s enough to say that for every unit of energy you put into a heat pump, you receive more energy back than you would if you put that same unit into a furnace.
But there’s more at play than just the unit itself. Several variables can influence just how much energy you save with a heat pump. First, let’s talk about supply and demand, or rather, capacity and load.
Heating/Cooling Capacity Versus Heating/Cooling Load
Every home has a level of heating and cooling it requires (the load), and every system has a level it can supply (the capacity). The trick is finding the balance point — the point at which the load and capacity are equal. When it comes to heat pumps, particularly air source heat pumps, that point is the outdoor temperature measured in degrees Fahrenheit. That’s because, as we know, a heat pump does not generate heat but merely moves it from one place to another.
For many homes in moderate climates, heat pump capacity can match most or all of the home’s heating load. But at lower temperatures, particularly below freezing, there’s not enough heat in the outside air to transfer into the home, which means the capacity of the heat pump diminish in comparison to the home’s load. Fortunately, that doesn’t mean heat pumps are off the table in colder climates. You have options!
Cold-climate heat pumps. Heat pump technology has advanced to the point where ultra-cold climates are no longer a barrier to their usage. Through the use of variable-speed compressors, inverters, and refrigerants with lower boiling points, cold-climate, air-source heat pumps can operate at far lower temperatures without losing efficiency.
Cold climate heat pumps are generally more expensive than traditional air source heat pumps, so if you’re looking to make a smaller investment, your contractor can install a backup heat source, such as a furnace or electric resistance heat strips, within the heat pump itself. You won’t achieve the same efficiency as running a heat pump alone, but you will be more comfortable in ultra-cold weather.
Stop air leaks in your home. You can also drop your balance point by taking care to air seal and insulate your home properly. We’ve discussed the importance of air sealing and insulation earlier in this series, but it cannot be overstated. No matter how efficient your heat pump, you will never achieve whole home efficiency if your heat is pouring out of your home through gaps in the attic and roof plane, through a non-weatherized crawlspace, or leak points around your outlets and ductwork. I highly recommend an energy audit with a blower door test before you even begin to think about installing a heat pump (or any HVAC system for that matter!).
Size appropriately. Which brings me to my last point on meeting your heating and cooling load with a heat pump: preparation and quality of installation. There’s a reason homeowners look to Pearl to connect with vetted contractors who perform certifiable work. Improper sizing or installation of a heat pump can tank your efficiency gains and cost you money. If you’re a building nerd like me, you’ll be tempted to DIY your load and capacity calculations with an online tool that spits out required BTUs based on square footage of the home. But, as we’ve seen, there’s more to capacity and load than just space. Air Conditioning Contractors of America (ACCA) has developed Manual J, the standard for determining heating load, which requires answers to 11 different variables about your home and outdoor environment before load can be established. Once you’ve completed your energy audit, I recommend working with a vetted Pearl Network Contractor who will take all the factors impacting load and capacity into account before recommending an appropriate unit for your home.
Types of Heat Pumps
So far we’ve focused primarily on air-source heat pumps, but even among air-source heat pumps, variations exist — and then there are those heat pumps that eschew air for other ways of heating and cooling your home.
Ductless mini-splits
A ductless mini-split is an air-source heat pump that, as you might have guessed, distributes heated or cooled air directly into the home, without the need for ductwork. While traditional air-source heat pumps use air handlers to distribute air, mini-splits mount on your wall and connect to the outside unit through a very small hole. Mini-splits generally do not have the air leakage problem common with ductwork. They also tend to carry higher SEER ratings than traditional air-source models. Depending on the size and layout of your home, your contractor may recommend placing a mini-split on each floor or in different rooms to ensure whole-home coverage.
Geothermal heat pumps
These heat pumps, with all their variations, deserve their own post (and will most likely get one in the near future). The basic gist: Rather than use air as the exchange point for heat, geothermal pumps (also known as ground source or water source) use, well, the ground. Unlike the air, the ground remains a constant temperature throughout the year. That temperature might vary based on location, but within that location, it’s pretty constant. That means that for part of the year, the ground is cooler than the air, and for part of the year, it’s warmer. A geothermal heat pump uses pipes buried underground that contain water or a mixture of water and antifreeze to absorb energy from the ground, compress it, and release it into the home as warm or cool air. Because of the consistent underground temperature, it’s easier to forecast and calculate energy savings using geothermal heat pumps than it is with an air-source model. That said, there are considerations you should take into account before deciding on a geothermal unit. You’ll need space to bury the pipes, and the right kind of soil with acceptable moisture levels. They tend to be more expensive than air-source pumps but can provide additional functionality, like hot water heating. As always, consulting with a contractor is the best way to make your decision.
Will I save money with a heat pump?
So, after all this, will a heat pump actually save you money on your energy bills? Going back to that DOE estimate of 50% reduction in electricity usage for heat, the short answer is yes because with a heat pump, every unit of electricity you use to heat your home goes farther than the same unit used by a furnace. Some studies have found that homeowners can save up to $1,000 annually. Here’s the big BUT. Just how much you save depends on a lot of factors: the size and layout of your home, the particular model you install, its cost, and SEER/HSPF rating; the range of temperatures in your location; and the degree to which your home is properly sealed and insulated. Starting with an energy audit and a consultation with an experienced HVAC or home performance professional can go a long way in helping you calculate your net savings.
There’s a lot more I could say about heat pumps, and we’ll be covering those topics in upcoming installations of this series, as well as how to heat and cool your house more efficiently even if you opt for a furnace and air conditioner. Until then, learn more about how you can start saving money on your energy right away by properly air sealing and insulating your home.
For more home performance recommendations, or to connect with a vetted home performance expert or contractor in your area, register for Pearl’s free, award-winning Green Door app.
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