Air Source Heat Pump

We recently installed an air source heat pump to heat our house. If you heat yours with electricity from the grid, and if the structure isn’t divided into many small rooms, then a heat pump will cut your power consumption so dramatically that the whole $2500 installation pays for itself in two years. And power consumption equals environmental footprint.

It’s quite a fascinating technology, and friendly to the environment too as long as you don’t rupture a pipe and release circulation fluid. You know a fridge? An air source heat pump makes your house into a fridge turned inside out.

Heat is movement among molecules. At absolute zero, molecules are still and there is no heat. What a fridge does is it removes heat from the inside of the cabinet and deposits it outside, in effect heating your kitchen ever so slightly at the expense of the food-spoiling microbes’ comfort inside. A heat pump attempts to refrigerate the outside world, depositing the harvested heat inside your house. Whether the temperature’s freezing outside is irrelevant: as long as it’s not absolute zero out there (which is rare even in Stockholm, Sweden), you can get all the heat you need.

In the summer, you can shift the thing into reverse mode and use it as an air conditioner. I’m in awe!



  1. #1 MRW
    March 30, 2009

    “Whether the temperature’s freezing outside is irrelevant”

    That’s not quite true. Yes, there is always *some* heat to be extracted, but heat pumps become inefficient in anything but a mild climate. In cold climates, they need to be combined with traditional resistive electric heating (the combination is still a pretty efficient heating strategy).

  2. #2 RyanG
    March 30, 2009


    How quickly does yours work? Mine is slow enough that below about -15 it can’t keep up with heat loss, and needs a wood furnace for backup.

  3. #3 Martin R
    March 30, 2009

    MRW: Point taken. What I meant was that the freezing point of water, though hugely important to the weather, is irrelevant to the machine’s operation.

    Ryan G: Don’t know yet as we’ve only had the machine for a few days with no lower temperature than a handful negative centigrade. I guess it depends on the building’s insulation.

  4. #4 tenine
    March 30, 2009

    I understand that 32 degrees does make a difference in heat pump operations. If you get below that temperature the unit will have to occasionally use backup resistance heating as the coil is defrosted, reducing efficiency somewhat. It still better than resistance heating, but might be inferior to some sort of gas or oil heat if you live in an area where it regularly gets below the freezing point of water.

  5. #5 Jason Young
    March 30, 2009

    Yes heat pumps are pretty cool, I have one for my own home. But the blanket statement “heat pump will cut your power consumption so dramatically” should be made with the caveat that heat pumps are not one-size-fits-all solutions. In fact their efficiency (and their environmentally-friendliness) depend on the outside temperature. They really work best in places with mild winters, you would not want one in Montana! If the heat pump can’t run at a decent efficiency (say the outside temperature is 0F) then it kicks in the “emergency heat” which is essentially heating your house with a giant toaster. That said, if a heat pump is appropriate for your location, I thoroughly endorse them as an efficient and cool technology.

    A funny story: heat pumps put air into your house at-or-close-to the desired temperature, so in the winter you don’t feel *hot* air coming from the vents though it’s working correctly. One time my brother’s wife discovered this and said “Why are we running this thing if hot air isn’t coming out?” and turned it off in the middle of winter. They woke up pretty cold that night… Haha, good times!

  6. #6 Don
    March 30, 2009

    I live in Nova Scotia and this is my first winter with a heat pump–I have it installed with my hot air furnace for my back up heat—My house is 1100sqft plus full basement–the furnace has to kick in to help when the temp falls below 10c(14f) NOW THE COST –Electric bill Dec & Jan was 480.00 (I have an electric hot water heater. This is 300 above last year- My Feb March bill will be 430.00– The oil used for my furnace is 150L(43 us gal)Dec to end of March —This is a big savings –I was paying well over 2000 for oil before–I know the oil price is down now –but it will be up again
    Keep in mind that even with electric back up heat your worse case is 100% efficient the heat pump at -10c is still 200%

  7. #7 Martin R
    March 30, 2009

    might be inferior to some sort of gas or oil heat
    Inferior in terms of total price? I think we should look at the CO2 footprint as well. The electricity we buy off the grid is from dammed rivers up north.

  8. #8 tenine
    March 30, 2009

    I meant in terms of btus. In terms of carbon footprint I’m sure hydropower is better. As I assume electricity is moderately priced if its from hydro you’re probably better off on cost as well.

  9. #9 Art
    March 30, 2009

    Yes, there is energy available right down to zero kelvin but it gets much more difficult to extract it as temperatures fall.

    Most air sources heat pump system start to struggle when temperatures get below much 20F and start to lose the fight around 10F. This is about ten degrees lower than it used to be and the systems and insulation are still improving. This may be a slight up side to global warming.

    One way around this limitation is to use a different source. Earth sourced heat pumps work well in even the coldest northern states. Benefiting from the averaging of summer highs and winter lows.

  10. #10 Thinker
    March 31, 2009

    Another useful system is a heat pump which extracts heat out of the air exiting the building and returns it to the heating system. My house is fitted with this (distributed through radiators, with resistive heating to support during the colder months.)

    Anything energy-consuming in the house (TV, PC, stove, yours truly, etc.) creates heat, as does incoming sunlight. Instead of just sending that energy out with the ventilation air, you re-use it.

    Indeed, during the warmer months (in my case roughly May-October), that heat exchanger is sufficient for all my heating and hot water needs. The house then essentially acts as a solar collector.

  11. #11 Martin R
    March 31, 2009

    Cool! Our house hasn’t really got a ventilation system. When it was built in 1972, energy was cheap, so there were no insulating strips in the windows. They all acted as ventilators. Then came the Oil Crisis, strips were put into place, and when we moved in the windows immediately misted up. I recently removed the lower-edge strips from certain windows to allow a modicum of air to enter.

  12. #12 Thinker
    March 31, 2009

    Certainly, those houses were built prior to the oil crisis, when no one thought of the heating cost. (I used to live in the same area, just a few hundred meters from your house.)

    Regarding insulating the windows: When you put the strips in, make sure they stop air flow at the innermost frame of the window, so that the space between the panes in the double-glazing frame is still ventilated with air from the outside. Then you can usually avoid the misting up, while still minimizing energy loss.

    More long term, it may actually be worth the investment to change to modern, energy-efficient triple-glazed windows, and to check how your roof is insulated. That is typically where you can find the “negawatts”!

    This all reminds me of the old saying that a homeowner’s life has two really happy days: the one when he moves in, and the one when he moves out…

  13. #13 samarkeolog
    April 1, 2009

    Did you actively choose not to use those “deep earth” heating systems (the “vent”-style ones?), that release heat from underground, or simply choose the environmentally-friendly system?

    (I mean, was the deep earth heating available but prohibitively expensive, or was it not an option? I’ve only ever seen the deep earth system in schools and other bigger buildings, public works, etc., but I don’t know much about the science, so I don’t know whether it does/doesn’t work/pay in smaller sites.)

  14. #14 Martin R
    April 1, 2009

    Drilling for a geothermal heat pump is expensive, which means that it takes longer to recoup the investment. When we asked around about the heat pump, only an air-based one was mentioned. It seems to be the standard thing in the area.

  15. #15 kai
    April 1, 2009

    My parents had a geothermal heater in their house and it indeed did pay for itself in a couple of years. But, as it was installed it had a bug: The heater incorporated a fan which made a certain amount of noise, not noticeable in the daytime, but somewhat annoying when going to sleep. So, once when my family and I visited my parents, I closed the door between the guestroom, where we slept, and the hallway, where the heater was located. Now, the sensor for the thermostat was located in the the guestroom, obviously to keep it at a suitable temperature. But, having closed the door, air circulation between hallway and guestroom was cut off and the temperature in the guestroom lowered a bit over the night. No problem for us, but the thermostat decided that more heating was needed, which made no effect on the sensor, so more heating, etc. In the morning I stepped out into the hallway, which by that time was a toasty 310 K or so.

  16. #16 Brian Einstein Lassiter
    April 2, 2009

    Just a quick correction to the common misconception that, “At absolute zero, molecules are still and there is no heat.”

    According to Quantum Mechanics, even at absolute zero, atoms have what is known as the “zero-point energy”. So there is always motion, it is just at a minimum at absolute zero.

  17. #17 Anthony
    June 29, 2009

    I had a Daikin Altherma air to water source heat pump installed last year to heat my hot water and underfloor, i was amazed when we had all of the snow which came to me as a suprise. I was worried about the operation of the system but it performed really well during those extremely cold months.
    I can say that now i am not burning oil and my bills have been reduced i am extremely happy.

  18. #18 P West
    February 14, 2011

    Theres no doubt that heat pumps are more efficient when it comes to saving costs compared to the other options out there like gasfurnaces. I found some good reviews of heat pumps online that I thought were quite helpful!

  19. #19 jane woodward
    December 18, 2011

    Hi, we have just had a daiken altherma heat pump installed, and not been told how we can have continuous heat on a low temperature for the winter, we have a 14kw pump and a 300 litre tank inside, please help as we would like this to run efficiently, and have constant hot wter and heat on a low budget, we need simple instructions ,please,

  20. #20 Martin R
    December 18, 2011

    I’m sorry, but I don’t have an air-to-water model like yours. Mine is an air-to-air thing that heats indoor air but does not provide hot water.

  21. #21 Domestic & General Insulation Ltd
    September 7, 2012

    There are a few different varieties of air source heat pumps, as has been mentioned by some of the other comments.

    Did you consider a ground source pump before the installation? Won’t work if you don’t have a garden to bury it in mind, but if you do it can be just as efficient.
