Heatpump turning on and off - is this normal?

Hi, We have an ecodan heatpump that powers underfloor heating and have had endless problems with it keeping the house warm. The underfloor heating is set at 48 degrees, however on cold days the house never gets up to the temperature we have set (19 degrees inside). Looking at the graphs we get from the melcloud app it seems that the heatpump constantly turns on and off and the flow temperature is extremely erratic, even though the thermostats are never reaching the desired temperature. I don’t think I’ve ever see it maintain the flow temperature of 48degrees for more than about 45 minutes.

I was wondering if anyone has any experience of this or can tell us if this is normal behaviour for the system. I have added an image showing the erratic flow temperature. This image was taken when the temperature in the house was around 16 degrees (so well below the desired 19 it was set at)

Hi Barry, welcome to the forum.

The first dip looks likes the dT has gotten too small, and the heat pump has stopped for a little rest before starting up again. I see this quite often with mine, so I don’t think it’s necessarily a problem.

The second dip you’re seeing here is a ‘defrost’ where the heat pump runs in reverse to melt the ice that’s built up on the outside unit - very much normal operation, and likely to happen every hour or so, depending on the weather.

Reducing the flow temperature might help reduce the number of pauses and defrosts it needs to do, though the house is already struggling to get warm. Have you got the heat pump running 24/7, or does it turn off at night? Are there any valves or zones in the system?

Hi Tim, thank you very much for the reply and that is very helpful info.

We leave the heatpump on overnight (24/7) as this is what we were told to do by the installers. To be honest if we didn’t I don’t think the house would ever get back up to temperature again the next day! Our heating bill is certainly pretty astronomical during the winter, although I imagine that’s true for everyone at the moment.

We have 3 thermostats in the house that control the temperature in different areas, however there is only one ‘zone’ on the melcloud app. Sorry, I’m not quite sure what you mean by valves?

Are these set to the same temperature all the time, or do they have a lower temperature overnight?
If the house is too cold in the morning, then perhaps try increasing the night time temperature.

Hi, We used to reduce them a bit at night but now we just leave them set to 19 degrees 24/7.

Just to clarify would you say it is normal for the hourly flow temperature to bounce around constantly? My understanding with the heatpump was that it would maintain the temperature but its like we have our heating constantly being turned on and off every half hour or so, which seems like a very inefficient way of working.

Also, what would be a normal cost expected for a heatpump to heat a medium sized detatched house? At the moment the heatpump alone is using around 60kWh per day so its costing us £20+ per day to run (the weather is very cold at the moment to be fair). This cost seems really high and we still have a cold house! Our house is (in theory) extremely well insulated too

That sounds high even to my untrained eye. I don’t have a Heatpump and you’ll soon discover that my feelings are there are a lot of folk like you who have been let down by installers who just want to fit and forget (and it makes my blood boil). HPs can be very efficient but that requires on correct install and tuning to suit your circumstances.

There is a public dashboard of other installs https://heatpumpmonitor.org

Search this site, there is lots of discussions on improving efficiency , but I suspect you need to improve your monitoring to find the answers you need. Depending on what is fitted However, at that current running cost, the investment in monitoring equipment will pay for itself quickly!

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thanks I’ve had a look to see if I can find some posts that will help but will continue researching it. At the moment its not so much the efficiency of the system that is the worry, the problem is more that it simply isn’t heating the house despite costing a fortune to run.

Unfortunately our experience of installers (actually all trades) has been abysmal and very much as you describe. Our installers just tell us “sorry but we’re too busy at the moment to help” :roll_eyes:

I have a medium-sized detached house and that is using around 35kWh per day – a more useful metric to consider is how much heat your system is generating – I get around 70kWh a day, which seems to be enough despite a couple of cold spots. My graphs are also variable, sometimes quite even and consistent and how I think they should look, and sometimes very jagged and hard to interpret.

It is necessary to understand your house’s heating system is configured and how it is controlled in order to predict how the HP will run. The HP only knows and reacts to the flow and return temps, and the external temp (giving the target flow temp) and possibly the internal temp if you use the Mitsubishi wireless controller. What do the three thermostats turn on and off and what consequential effects do they have? Only one can have the effect of turning the whole HP heating mode on and off.

A basic suggestion is to raise the thermostats to let the system run as much on its own algorithms as possible, and adjust the flow temps if it actually then gets the house to the temperature you want. But if none are reaching temp then that should be happening anyway. I don’t have UFH but, as TimBones says, is 48° too high a flow temp? If the slab can’t absorb quickly enough then the return flow may become too high and the HP reacts to that by stopping.

You might find some more useful pointers on other forums that are more oriented towards general users rather than the energy monitors on here (try buildhub or renewableheatinghub)!

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Try posting on the heat pump group on Facebook. There are Mitsu experts on that.

48C seems ways too high for UFH. Even at these temps, ours is 30C max.
Are you using weather compensation? You should.

Hi, thanks for the advice and I will defintely check out those other forums.

None of the thermostats in our house currently ever get up to temperature so I don’t think any of them are shutting off the heatpump - it is working maxed out all day. It is starting to seem to me that there is no ‘problem’ as such with our system, but perhaps it has been under-specced for the size of our house. I wonder if we need some additional radiators installing in certain areas to help the underfloor heating out?

Hi, thank you for the response. I don’t have a facebook account but I will take a look on there and see if there is anything to help.

Sorry for my ignorance, but what is weather compensation and how would I turn that on? When we were sold this system by the installers we were told we wouldn’t need to touch it and it would just work…!

I can certainly try turning down the flow temperature. Perhaps I’ll try turning it down to around 42 and see what happens there

Someone will correct me if I am wrong,

Did you get new radiators when the system was fitted?

Do the radiators get uniformally warm?

Are there TRVs fitted?

Hi, we don’t have any radiators at all, we have UFH throughout. It is very hard to tell the temperature because the floor never really gets hot, and there is a huge delay between the heating coming on and the time it takes for that heat to work its way up through the floor.

Do you have a picture of the UFH manifolds?

Have you ever had a boiler or is this a new build?

This is normal, though the optimal scenario is for it to be running constantly at low power. There are various factors due to installation, configuration and weather that can cause “cycling”.

This does seem a bit on the high side, especially for a house with good insulation and UFH for it to not be warm enough. My older detached house with radiators is using 40 kWh per day.

What model or size of EcoDan do you have? There are various videos and guides on how to set up weather compensation, though it it won’t make a difference when it’s freezing. I’d suggest trying a flow of 40C at -5C outside and flow of 25C at +15C outside.

Oh, and is there a “buffer tank” in the system?

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weather compensation varies the flow temperature based on the outside temperature. It usually works in conjunction with the heat curve (mentioned above).
I have a vaillant ASHP so afraid I cannot help with the Ecodan config.

Thanks for this suggestion. I changed the flow temperature to 40 for the last day or so and it seems to have made a difference. The system is now staying ‘on’ much more consistently, and generally the graph only starts wobbling about once every few hours (rather than every half an hour). It has also reduced the energy consumption from ~60kWh to ~45kWh which is a massive saving and seems much more reasonable!

The issue though is still that the system isn’t capable of keeping the house warm and the house temperature just slowly drops throughout the day, even though the heatpump is working flat out. Once it drops to around 16-17 inside we are using fan heaters for an hour or so to take the chill off but as soon as we turn them off the temperature in the house starts to slowly drop again. With that said it doesn’t seem all that different at a flow of 40 than it was at 48, so the system certainly seems more efficient at this setting.

The weather is due to warm up massively from today (from -3 up to around 10) so I will try setting the flow to around 30 and see how we get on with that. Generally though our system seems to already work very well when the weather is mild, it’s once it gets to around freezing point that it just kind of falls to pieces

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I’ll grab a picture of the manifolds later and post them here.

Our house is effectively a new build. It is an old house but we completely gutted it and started again. We have a new roof, new doors and windows, new floors and ceilings and have added external insulation on the whole house. In theory it should be very eco-friendly!


thank you for this, I will look into setting up a weather compensation curve on the system. I guess this isn’t going to make much difference in the extreme cold but it should bring down our bills further in the autumn/spring which is definitely appreciated! :slight_smile:

Did you get a new EPC?

Did you get a heat loss survey done to size the UFH?

Do you have MVHR and did you get an air tightness test done?

Is there a buffer tank? What do you do for DHW?