Pumping Heat


Most people are familiar with heat pumps, though possibly not by that name. An ordinary refrigerator is technically a heat pump. So is air conditioning – it works by exactly the same principal. Heat pumps work not by creating heat, but by moving it around. By alternatively pressurizing, moving, and then depressurizing a fluid refrigerant chemical, heat is moved from place to place. The cool thing (hot thing?) is that the energy used to move the heat around can be far less than the amount of energy moved. It’s also cool that you can move heat from a cold environment to a warm environment, the opposite of the natural situation where heat always flows towards a colder environment.  So it’s kind of magical.

If you reverse the flow on an air conditioner, you can heat a space instead of cool it. It’s this function that is being used more and more in construction; air conditioning, which means cooling air to most people, has been common for decades, after all. To go back to the basic refrigerator example, it’s as if the controls have been reversed to warm up the space inside the box instead of cool it.




Most heat pump systems are ‘split’: there is a unit on the outside (see photo, about the size of a suitcase), which pumps heat into or out of the system to the atmosphere, and a unit on the inside, which conversely pumps heat into or out of the internal space of the building. Refrigerant piping links the inside and outside units. Water heating heat pumps can work similarly, though some aren’t split – like a refrigerator, everything is self-contained in one unit and they pump heat from the room they are in into the water.


With conventional heating, where gas or other fossil fuel is burned, the limit for efficiency is less than 100 percent, i.e., you can never achieve turning all of the chemical potential energy of the fuel into usable heat.  Traditional gas furnaces were perhaps 80 to 85% efficient. More modern furnaces, with sealed combustion chambers and condensing flues, approach 97% efficient. So little heat goes up the exhaust that it can be run in PVC pipe instead of a steel flue, and the exhaust gases are so cold that they won’t exhaust by convection but have to be blown out by a fan.

Similarly, electrical resistance heaters, where electricity is turned directly into heat, as in an electric stovetop, are nearly 100% efficient.

Heat pumps, however, can reach efficiencies of 300% to 400%: a heat pump using 100 watts of power can put out, for instance, 350 watts of heat. What’s not to like?

There are some limitations on heat pump usage, however.  The equipment is more expensive than conventional gas fired appliances, besides being more complicated and therefore more complex to control and more prone to break down. Recent technological advances notwithstanding, there are lower and upper limits on the air temperature that heat pumps can are compatible with – not a problem in California except in the upper reaches of the Sierra.

The cheap cost of natural gas versus high electric costs, and the low cost of gas fired equipment versus heat pumps, have kept them in limited use until recently. Now, however, interest in limiting the burning of fossil fuels and inexpensive solar panels are boosting their use.

The refrigerant fluids used in heat pumps have also been controversial. Refrigerants often leak out of systems and wind up, of course, in the atmosphere.  The common one used up till the eighties was Freon – phased out after it was discovered that it was destroying the ozone layer which screens solar radiation. Newer refrigerants don’t have this problem but are potent greenhouse gases – up to several thousand times more potent than the same amount of carbon dioxide. One of the more exciting technical developments lately is the use of carbon dioxide as a refrigerant in some high-tech equipment. It’s actually an even better refrigerant but requires higher pressure.  The important point, however, is to consider the impact of the use of fossil fuels.  Oil, coal and even ‘clean’ natural gas produce far more global warming that refrigerants, and a great deal of natural gas (with dozens of times more global warming potential) is leaked into the atmosphere, where it stays active for decades. Better to leave them in the ground.

Another reason heat pumps for space heating are taking off is that there are a variety of small, flexible systems which are good for ultra-insulated, well-sealed homes – a trend which becoming more common. You can’t get gas fired equipment that produce as little heat and cool as these homes need. The air handers can serve miniature duct systems, or be wall mounted, or ceiling mounted, or a combination, all served by one outside condenser unit.

Controls for these systems can be a bit cranky. The manufacturers apparently don’t like them to be tinkered with and don’t make it easy. There are some third-party thermostats which make them behave, however, like the Ecobee and Nest. And some installers have learned how to hack the controls or the control wiring.

One of the takeaways I got from a recent PG&E symposium on heat pumps was that heat pump water heaters work more efficiently when the incoming water is cold. They also need to have the water in the tank highly stratified, i.e., with a big temperature difference between the water at the top and the water at the bottom. A 50-gallon tank won’t have 50 gallons of hot water, therefore, but as long as the hot water production rate exceeds the usage, you’ll never run out.

Some research suggests the heat pump water heating systems combine the heat pump with solar thermal heating. And of course, a solar PV panel to run heat pumps makes even more sense, then you avoid to a large extent the fossil fuel burning that your electric vendor uses to make the power. Heat and light from the sun powering your lifestyle – that’s the sweet spot we want to promote.

Special thanks to the PG&E Energy Center, Dan Perunko and Rick Chitwood. More about our system design for Pacifica to follow!

2 Responses

  1. Hmm. Wondering about the need for highly stratified temperatures in a heat pump water heater. It would be “more” efficient of course, but is the loss in efficiency important if the difference is the result of solar hot water coming in at zero energy cost? I think not, but don’t have the calculations to prove it but a graph on the linked page suggests solar preheating is worth it. We have had a heat pump water heater for many years and have been very happy with it. Eventually we do run out of hot water if everyone wants to take a shower back to back and then we throw a load of laundry on top of that, but it never really gets cold either. Just sort of tepid. My memory from having a gas fired water heater is when the tank is out, its really cold. . . You might find this page interesting. https://www.onecommunityglobal.org/sustainable-water-heating/

  2. Robert Boles
    | Reply

    Dear Barbara and Gregory,
    My information about stratification comes from Dan Perenko of Balance Point Home Performance (www.builditzero.com), who taught the class at the PG&E Pacific Energy Center that I went to. I’m not crystal clear on the science, but I think it relates to temperature transfer being better with a higher delta T between the cold input and the hot output. Dan also noted the idea that heat pump water heaters worked better if the water source was cold, presumably for the same reason. I’m not sure this makes sense — I can see they might be more efficient but then you’re having to make up a higher temperature difference. Thanks for the intelligent questioning!

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