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New house. Underfloor heating. First signs of winter 2020. Heat pump starts showing errors and stops working. Provider and repairman need 4 months to figure out that the heat pump must be replaced with a new one. In between I must check the heat pump every day just so that it can not-work properly (only lets the floor be heated with 'raw electricity' (? not an engineer) if the error sign is not blinking & the error sign started blinking at least once per day).

How can I calculate (at least approximately) how much more electricity did we have to use when the heat pump was not working?

i.e. What is the difference between electricity used for keeping a house warm between when a heat pump is working and when it is not (and the house is warmed by raw electricity alone)?

I obviously need this information so I can know how much of the electricity costs I can demand the heat pump provider to return to me because of their slow repair and unwillingness to replace the broken machine sooner. I have received quite a large electricity bill (750€) that cover the whole period (+ some more) but can not determine how big part of that is due to the broken heat pump... So far the provider has only offered me 1 free yearly service, but since the heat pump always warns that 'if not working you may get large electricity costs' and it did not work for several months I believe this gesture is too little too late.

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    Would need to compare a working heat pump to a non working pump for the same house and time period to get an exact cost difference. Can compare different heating seasons over a few years to get a good estimate or use the companies estimates on savings you were expected to have to see if a free year is good enough. Some local media stations have consumer report/watchdog sections if company does not come though for you.
    – crip659
    Jul 2, 2021 at 13:35
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    @crip659 need degree-days temperature data as not all cold seasons are the same... nature can vary +\- 10%
    – Solar Mike
    Jul 2, 2021 at 13:38
  • It would depend on the specifics of the heat pump in question, and the temperatures. For certain types of air-source heat pumps, -5°C is "too cold" and backup would be expected - for others, -25°C is a workable temperature. For ground-source or water-source heat pumps, outside air temperature does not matter. At air-air extreme operating temperatures, the heat pump might use 75% as much power as resistance heat. For ground-source or water-source or air-air at moderate temperatures, 25-33% is more typical.
    – Ecnerwal
    Jul 3, 2021 at 2:21

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Heat pumps cannot operate below a certain temperature. This is normal. That temperature varies by the model and technology of the heat pump.

As such, heat pump installations include an alternate heating source, called "Emergency Heat", which is used when the heat pump cannot operate. It can be any source, such as natural gas or electric resistance heating. For instance a natural gas furnace that is convection and does not require electricity is an excellent choice, since it works in power outages.

However, in all-electric homes, they generally use electric resistance heating, which is very expensive to operate. This will be necessary in very cold days, but it will also operate if the heat pump is inoperative due to a malfunction.

If you allowed a heat pump to be broken all winter, then yes, you should expect to have a hefty electric bill. That is the electric resistive heating.

Your best bet to determine how much of that was the resistive heating is to compare this year's electric bills to last year's.

However that does not mean the heat pump supplier is responsible for those bills. It is your job to maintain the heat pump, not theirs. You can hire someone to help you, but that does not transfer all responsibility to them. After all, if those were the rules, no one would offer their services as a maintainer, would they?

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  • My best guess would be a HP uses about 1/3 the power of electrical resistance heating. Next: If it's such a new build, are there any warranty options to explore? It would be a push to get them to cover excessive power costs, but it might be a possibility. Hopefully they at least covered the cost of the new HP.
    – George Anderson
    Jul 2, 2021 at 18:35
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If you measured the electricity use for the floor with a watt-meter then you would know. If you did not use anything else electric in the house then you would know. But you could be running the cooking stove etc so much that half the bill is cooking or only 10% is cooking etc.

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