Heat pump or not!

We have a summer house (50 sqm) on an island in the Gothenburg archipelago in Sweden. We don’t live in the house during the six coldest winter months. To avoid mould, we have basic heating these months (+12 degrees Celsius). The house is heated by electric elements with a thermostat. All other electrical equipment is switched off except for a fan which ventilates the house. We have wall vents in the bedroom and living room. The fan sucks air via the bathroom’s dry toilet. The house has an open floor plan.
Our dilemma right now is that the electricity price will double 4-5 times as of this winter. I am in the process of investigating whether it pays to install an air-to-air heat pump or not. This will only be switched on during these six months when the rest of the time there is no need for heating.

I have tried to obtain data on a number of pumps on my own. The only thing delivered is data for SCOP. I need data on COP (at low indoor temperatures). The dealers say they cannot supply this type of data, even though the pumps can be set from +10-35 degrees indoor temperature. I then turned directly to the suppliers, i.a. large Japanese supplier which give the same answer. They claim they have no data because they are not the ones measuring the COP of the pumps!!

While searching the Internet, I found your site. It seems that several of you have a high level of knowledge regarding heat pumps. I am therefore making an attempt to get some help in my investigation. With any luck, someone has a similar application.

When it comes to this type of pump, I have understood that it is important to keep a constant flow in the pump. The lower the flow, the higher the efficiency. After all, defrosting is a problem that must be minimized.
Perhaps it is not smart to set a fixed indoor temp of, for example, +10 Celsius, but let the pump vary the inside temperature depending on the outside temperature in order to maintain a constant low flow. As long as the indoor temp does not go below + 5 C, there is no risk of freezing. If the efficiency is much higher, it doesn’t matter if the indoor temperature is a little higher at certain times.

I am attaching data of last year’s power consumption & outdoor temp (daily average value) for the current months.

October, 496 kWh, 11.5 Celsius
November, 417, 7C
December, 894, 2C
January, 603, 4C
February, 502, 4C
March, 552, 4C

Jan P

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Hi @JanP

Welcome to the OpenEnergyMonitor forum!

Here’s the post that gives you the information you are after:

If you are really deeply interested there is this chapter which shows the performance data in detail for a range of Mitsubishi heat pumps:


Of course other manufacturer’s heatpumps will behave differently, but hopefully that will give you a flavour.

BTW your Summer house is as big as my actual house :slight_smile:


I’ve no experience with air-to-air heat pumps but there’s a poster called Derek M on another forum who has one and knows a lot generally. It might be worth you registering and asking, or simply digging around his past posts: Comparing COP (coefficient of performance) – Air Source Heap Pumps (ASHPs) – Renewable Heating Hub Forums

Generally, I’m surprised there aren’t COP data tables by indoor and outdoor temperatures in the data books for the pumps on the manufacturer websites. Mitsubishi offer those for download for their air-to-water pumps, but I just found the data book for their PEH air-to-air range and no, it doesn’t include similar COP tables. Disappointing!

to prevent mold you be better to reduce your moisture then trying to heat, as 12c not very warm and a good temperature for mold to grow . if it well insulated a dehumidifier set at 50rh would probably reduce your mold as and it will also heat your house to 12c in such a mild temperature

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Thanks for the feedback! In my case, it applies to an air/air heat pump. In the attached link they show the COP for air/water heat pump, if I interpret the material correctly.


Jan P

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Thank`s for the reply! The same answer as for My Forest, applies to the air/air heat pump. I wrote in my post that a large Japanese supplier… It was Mitsubisch Electric that I was in contact with and who gave me an answer that they have no data for air/air heat pumps regarding COP at low (10-12 Celsius) indoor temperatures. The reason would be that they do not carry out the measurements themselves.


Jan P

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Thanks for the reply!
Interesting thought, something I missed in my thoughts about how I can safely keep electricity consumption down without damaging the interior and the house. My current solution is based on me bringing in cold air from the outside via the vent using the fan in the bathroom. Cold air is much drier than warm air, which can bind a lot of moisture. This is the theory I have worked with until today and which has been successful (even if the temperature is low). Never any tendency to mold in the textiles. However, a lot of electricity is needed.

I can mention that my first thought was to use solar panels on the roof. Everyone writes about how efficient solar panels are in the media. Two problems with solar panels. It is at night that the energy is needed as it is the coldest outside. Also counted on a facility with a back-up battery to take advantage of the solar energy during the day to use at night. Was not profitable. However, the main reason why I abandoned solar panels was the fact that here in Sweden the sun is very low in the middle of the day during the winter months. There is a very good website where you can choose where you live and set the time of day that is interesting to you. In my case, the sun is only about 10 degrees above the horizon. Solar panels on the roof in winter do not work in Sweden in winter for heating houses. In summer, it works perfectly well when the heating requirement is zero. However, you can heat the hot water.

Something to think about perhaps. In Sweden, you always hear that solar panels on the roof must face south. It is true that you get the most effect towards the south. However, the electricity price today in Sweden is highest in the morning and afternoon. In my case, with a summer house that stands in an east-west alignment (the long sides) it turned out that even though you get 15-20 percent less power from the solar panels, the end result is the same. A bit of an aha experience.

Back to your suggestion. I will investigate how much power loss = heat an absorption dehumidifier gives off and what power is needed to run it.

Thanks again for the tip and have a great day!


Jan P

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This is a really good point.

@JanP do you measure the internal humidity? What is the external humidity like during this timeframe?

They don’t work well heating houses anywhere in the winter.

Yes, and charge your car, run the cooker, run the lawn mower, charge the house battery, and run the air con - because it can be hot in the summer.

A fair bit as the latent heat in the water vapour is put back into the air. Mine uses around 300W, I bought it because the operating temperature can go much lower than with a refrigeration type dehumidifier.

I haven’t taken any measurements. Relative humidity is higher in winter compared to summer. It is highest in the spring when the sun warms the sea in my case (the house is on an island with sea all around). My theory is based on the lower the temperature, the less moisture=water the air contains. The humidity is higher in winter, but the temperature is much lower. According to the diagram below, 20 degree air can “contain” almost 4 times (18g / 5g) more water than 0 degree air.


Jan P

I have contacted a company called Wood’s. They are the market leader for all types of dehumidifiers. Wood’s started its operations in Canada in 1956. They have moved their operations to Sweden due to EU legislation regarding gas content requirements, etc. for this type of products. Will be interesting to see what feedback I get from them.

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I’d agree that a dehumidifier is a better option. I used one in a leaky garden shed for years. Bit smaller than your house though!

Only problem is that the usual compressor based ones don’t work terribly well when the temperature is low. There is an alternative sort that use a desiccant wheel that works better at low temperature I am told. But uses more energy, as they have a heater in them, but a bit of heat is no bad thing in your case.

Make sure you get one that has a humidistat in it. Otherwise it just runs continuously whether or not the air is damp.

With such low power usage (only 50 to 200W) solar power is entirely possible (unless snow cover affects it) throughout the winter and you could have it such that it only turns on when there is sun.

Or use a dehumidifier in conjunction with a lower level of background heating maybe from PV electric heating.

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My conclusions after being in contact with a large number of companies in industries regarding products for energy management and after interviewing many people who have today installed heat pumps, solar cells and similar products are: 1) If you live in an old house, restore and put the money on insulation and new windows. These measures last a lifetime. Today’s products last maybe 10 years with maintained performance. The performance and sound from them gets worse as time goes by. 2) For those who have the opportunity, build new.
It should be added that this mainly applies to houses in northern Europe and countries with a similar climate.
With this I say thank you and end my time on this forum.

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@JanP “Today’s products last maybe 10 years with maintained performance.” that sort of true if they are using the newer gases as the gases break down and run at much higher pressure … as you have thoroughly flush the system and replace the gas every 7 -10 years. but if you buy one using the older gases that run at lower pressure and the gas does not need to be fully replaced only topped up those ones last basically forever or until the pump gives out.

but yes one should always insulate better first – as suggestion down load a copy of rem-rate - not sure about the newer ones but download an older version you will get 90 day of usage before you get timed out …
but you can use that program to determine the best course of action to get the best saving at the lowest costs.-- personally windows are neither here nor there if you got the money sure if not you can buy window winterizing kits ( plastic film) and a roll of paint tape will solve most issues of poor quality windows . for a couple bucks per window per year

I am looking at a different solution that may or may not be available to you. Here there are a few tariffs that give free hour(s) of power in off-peak times, and so I intend to store Hot Water during this period and then use it to heat radiators through the day. i.e. Free heat

This will have its own challenges and hence why I have found this system to help with this project.