Heat Pump Heating

Heat Pump Compressor
Heat Pump Compressor

Most houses here in suburban California are built with natural gas forced air heating. If you have air conditioning, which was an option we did not really need 15 years ago when we moved into our Alameda home in the San Francisco bay area, you will also have a compressor for the cooling system, which otherwise shares the same forced air system. fifteen years ago there only seemed to be a few days of the year when we really felt air conditioning would be useful. In the last couple of years, we have needed it on a lot more days, and the trend seems to be suggesting there will be more days. Combine that with us working from home more, and that extra cooling became much more important.

Home heating is another area that we need to move quickly away from using fossil fuels, so when we started looking for options to add air conditioning, the idea of installing an air source heat pump based cooling and heating system, removing the gas furnace completely, was very appealing.


The air handler fitted into our attic, replacing the gas furnace directly, and the compressor unit was added to the back yard. When our house was built, the wiring and piping for air conditioning was included always. Our system is dual zone (upstairs and downstairs), and while we had a dual zone controller from the original system, it turned out to be incompatible with the Bosch system, so we needed a replacement controller board too.

The attic unit requires a 240V circuit, unlike the gas furnace that was there before which used a simple 110V circuit, but it was low enough current that our electrician was able to run the new wiring for it from the sub-panel just below, making it a much simpler install. Another advantage of the Bosch system is that we got to keep our existing Nest thermostats (originally, Green Energy Home Improvement, the company we hired for the install, was planning to install a Trane system, but that was going to require changing to their thermostats as well).

Outside, the special reversible compressor connected up to the existing piping. We needed to recover the wiring as we had previously taken that to provide the 240V circuit for the EV wall box (we ran a new circuit for that allowing us to upgrade the wiring there to support the full power of our JuiceBox EVSE), and he replaced the original electric box with a new one.

In Use

Energy Label

We haven’t needed heating yet, but the cooling system works well. We have had a few days now where the cooling system has kicked in.

The energy label suggests that this might not be the most efficient system, but in other places I looked online (e.g. Learn Metrics) 20 SEER (the rating that he told us the combination of units we have met) seems to be the best.

On the heating side, Learn Metrics suggests that HSPF 8.5 is a good number for a heat pump, and adds that there are heat pumps with a rating of 10 (like the Bosch unit we have).

So, although on this energy guide sticker it looks as though both units are at the lower end of the scale as drawn, it appears from information online that both are at or near the top of the efficiency expected at the moment. Obviously, if you can find one with a higher rating, it will be even more efficient.


If you are planning to switch from fossil fuel based heating, whether that is natural gas, propane or even oil, check whether there are any rebates available for doing so.

Here in the San Francisco bay area, BayRen offers rebates on heat pump central heating conversions. To qualify, you need to use an approved installer, and the will complete the paperwork and schedule the necessary inspections and tests. The rebate amounts to $1000, so it is a good incentive to consider a heat pump upgrade to your heating.

They also have rebates for other electrification options (electric heat pump clothing dryers, induction cooktops, heat pump water heaters and more). Local electricity utilities may also offer rebates for some or all of these items to encourage the switch to electric options.

How They Work

Rather than directly generating heat using a flame or resistive electric heating elements, heat pumps work by moving heat energy from one place to another. In the case of our system, the heat is either moved from inside the house out (when cooling) or from outside in, when heating. Much like your home refrigerator works, they make use of a refrigerant material that is heated and compressed. When heating your home, the refrigerant is pumped around the coils in the unit outside the home where the air heats it, and then it is compressed. That compression raises the temperature more and the resulting warm liquid is pumped through a heat exchanger, heating the air around it. In cooling mode, the operation is reversed. This allows the heat pump to move heat into or out of the house as needed. There is a more detailed description here.

Those familiar with electric vehicles will maybe have heard of EVs using heat pumps for heating the car in winter as well. Since they don’t generate heat, they are much more efficient. Under ideal conditions they can transfer several hundred percent more energy than they use (compared to a gas furnace which will likely be in the 90% efficiency range at best). Sounds too good to be true? Well, there are some downsides too – unlike the gas furnace which can heat near instantaneously, heat pumps work more slowly. In normal use, that’s not a problem and they can maintain the temperature in a house easily, even on cold winter days, but if the house is cold and you’re looking for that instant warmth, the heat pump may not deliver it. That said, we can feel the cooling effect within minutes of turning it on. We will have to wait for winter to tell how fast the heating is.

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