In general, using an air-conditioner for heating is more efficient than a standard electric heater or radiator. But in our home, we keep the windows open all the time, in order to decrease the chance of infection. Is air conditioning (heat pump) still more energy-efficient than an electric heater? If not - what is the most efficient heating method in this situation? (Note: the outside temperatures are about 5-15 celsius).

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    I am puzzled by the use of "air conditioner" here. When I think of an air conditioner I think of a device used to cool. Maybe a translation issue? Does "air conditioner" refer to central heating as opposed to space heaters set around?
    – Willk
    Feb 5, 2022 at 17:34
  • @Willk my air conditioner has a "cooling mode" and a "heating mode". See for example here: efficientcoolheat.com/… Feb 5, 2022 at 17:39
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    Heat pump is the more general term for bi-directional (heat/cool) as opposed to "cool only" in common use.
    – Ecnerwal
    Feb 5, 2022 at 17:52
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    @manassehkatz-Moving2Codidact the first article I found re: "air conditioner" for heat pump was UK. Clearly my location radar is on the blink! Feb 6, 2022 at 1:46
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    I'm not sure if there's anything worse than electric resistance heat. If you want to feel warm with the windows open you need radiant heat, not forced air. I've seen people that don't pay for the boiler with their windows open all the time, because they also have little control over it and it's too hot.
    – Mazura
    Feb 6, 2022 at 4:16

3 Answers 3


A heat pump is still going to be more efficient than resistance heat - a good one at those outdoor temperatures will produce 4-7 times more heat from the same electrical input - even a poor one will manage 2-3 times the heat output.

So, whether or not the windows are open, a heat pump is more efficient than resistance heat. At those temperatures we don't even have to question the heat pump's technology. Below 0°C some older or less well designed heat pumps actually switch to resistance heat backup, while newer/better heat pumps can operate efficiently to -25°C.

However, there are much more efficient ways to ventilate than simply opening windows, specifically several versions of "heat recovery ventilators" which exchange air while transferring heat between the incoming and outgoing air streams.

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    Gotta agree with Ecnerwal. HRV is much more efficient, but there are equipment and installation costs. In the long run, it would certainly save tons of money and you'd get the ventilation you desire...but running electrical resistance heating in cold weather with windows is not a good idea. Note to POCO: lift the control rods on the reactor, we need more power (Sorry, couldn't resist). Feb 5, 2022 at 18:17
  • Have you considered a UV light in the ducts. I have them in both delivery and return ducts and keep the house closed, much lower energy bill that way. They are on 24/7 and the blower in the furnace is and always on 24/7.
    – Gil
    Feb 5, 2022 at 19:04
  • Thanks! I have never heard of heat recovery ventilators before. I have to check if they are available at my country. Feb 6, 2022 at 19:58
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    And if you live in an area with high humidity, then an ERV may even be better than an HRV.
    – Glen Yates
    Feb 7, 2022 at 16:46

Every heating option will be dreadfully inefficient since you are heating the great outdoors.

A heat pump will be more efficient than straight electric coil heating as long as it is operating in conditions where its COP is better than 1.0.

A heat pump will be more efficient than gas heat depending on a complex and arm-waving array of facts, depending on whether you are min-max-ing for operating cost or carbon emissions. Given that the best combined-cycle natural gas power plants are 50-60% efficient with a tailwind, and random vented gas furnaces are 70-90% efficient depending on design and drafting, you need to have a COP of 1.3 to 1.8 for them to be a net carbon gain vs burning gas locally.

Armed with COP and natural gas furnace effiency for your conditions at that time, you can number-crunch on cost of gas vs electricity, and energy extractable from each. Figure the number of BTUs, kWh or joules in a unit of gas x its cost x the furnace efficiency, vs the number of BTUs (3410) or joules (3.6 million) gotten from 1 kilowatt-hour x its cost x the COP of the heat pump.

Unfortunately I don't think Britain has a unit like the American BTU :)

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    'Unfortunately I don't think Britain has a unit like the American BTU' - that made me chuckle, BTU means British Thermal Unit!
    – NMF
    Feb 5, 2022 at 22:51
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    You might want to run your theories about infection control by some actual epidemiologists. Ventilation works.
    – Reid
    Feb 6, 2022 at 4:55
  • @Reid I won't fight. Removed. Feb 6, 2022 at 5:36
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    The standard unit of heat in most metric countries is simply the kilowatt hour. Feb 6, 2022 at 5:37
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    The metric unit of energy is the Joule not the calorie, and "calories per KWH" is not a unit it's a scale factor between two different unit systems, like "inches per light year" which would also be an "ungainly" number. In the UK energy from the utility companies is priced per KWH whether it is electrical or gas, a conversion factor is used to convert from the volume measured by the gas meter to the KWH value that goes into the bill.
    – Rodney
    Feb 6, 2022 at 5:51

We'll just pretend you never asked how to let a heat pump run with open windows 24/7, okay?

Ventilation is indeed a good way to reduce the risk of infection. But you don't need to ventilate all the time. You could simply measure the CO2 concentration, which correlates with the concentration of airborne aerosols particles. See "Exhaled CO2 as a COVID-19 Infection Risk Proxy for Different Indoor Environments and Activities" and "CO2 monitoring recommended to manage COVID-19 spread in schools and offices"

If the CO2 concentration gets above 1000ppm, you can cross-ventilate until it drops below 500ppm.

You could buy a CO2 traffic light, which blinks red above a certain threshold. I built mine with a Sensirion SCD30 sensor + ESP8266 microcontroller + NeoPixel LED ring. The corresponding code is open-source and available here.

CO2 traffic light

Even in a crowded classroom, it was enough to ventilate for 3 minutes every 20 minutes. With 3 people in a 70m² flat, we have to ventilate every 4 hours.

  • @ErelSegal-Halevi you should probably ask on a better site about ventilation parameters. These are well established in the public health community. CO₂ concentration is not one I'm aware of. AFAICT they tend to be expressed in exchanges per hour.
    – Reid
    Feb 7, 2022 at 17:20
  • @Reid: Sure, ventilation can be described with 1/h. This value is typically used as a rule of thumb in ventilation design (see here). And once you live or work in a building, it's kind of hard to calculate it without a blower door. A CO₂ sensor provides an easy and relatively reliable way to see how much fresh air is coming in. I'll add sources. Feb 7, 2022 at 18:08
  • @Reid: Another point I forgot : the risk of infection obviously depends on the occupancy, but air changes per hour doesn't depend on the occupancy. It's safer to be alone in a room with an air change rate of 2, than to be 4 people in a room with an air change rate of 12. Feb 8, 2022 at 8:50
  • downvoter: constructive criticism is welcome! Feb 8, 2022 at 8:50

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