Heatpump turning on and off - is this normal?

…and what type of flooring do you have?

Who? :slight_smile:

I have an 8.5kW Ecodan in a detached house, quite exposed and about 1000ft up. Its entirely heated with radiators and even with temps dropping to -6C over the last week I’ve only needed to run with a flow temp of 45C to maintain 19-20C. It feels like you’re trying to run with a flow temp way above what it needs to be. My design point is -5.25C ambient with a flow temperature of 45C and heat loss of 8.2kW - if you have an MCS approved install you should have that information somewhere. If you haven’t then you’re within you’re rights to complain (I’m a former UKAS Technical Assessor for MCS :)). The chart is my ASHP performance since start of December, I should say that I think the heat meter is over-reading by 15-20%, which means my actual CoP over the period is probably a bit under 3, but given the average ambient has been floating around zero, that’s about right. You really need to have a weather compensation curve to keep the bills down though as others have said.


Nice to see degree day in there.

The hot water loop is maintaining setpoint of 48DegC and later in your tests, 40C so the hot water loop itself is working as expected.

The return temperature is pretty high in your graph, over 40C while the mitsubishi system diagram I just found shows a return temp of 35C. Seems like there’s a lot of heat that isnt getting into the zones for some reason. I am not a hydronics guy and could be way off but if there’s any balancing valves you could fiddle with those. Also check for air in the system. Try closing two zones and see if the one zone can at least cycle at setpoint.

Your heatpump seems to be working ok but not very efficiently. Weather compensation is the way to go and pretty much all modern units (and gas boilers too ) can be set up that way. The real problem would appear to be that the heat is not getting into your house effectivly and this is having a knock on effect on the heatpump. have you checked the surface temperature of the floor. have you got thick carpets ?. You may want to check out a company called Heat Geeks who train heatpump engineers and who will be able to give you a list of advisors in your area who can check out your system properly. They will check out the entire house as a whole system. The heatpump cannot be looked at in isolation. There is not necessarily much wrong but the setup needs to be good.


Maybe - would to help you to have some more hard data?
If the ecoDan data is not easier to digest - then maybe get yourself the emonPi system from the shop here: and then start to log things like the following (others will have other ideas)

Evidence is the UFH able to ‘dump’ the heat coming in via the heated water? - what delays?

  • floor temperature - the warmest point on the UFH heated floors
  • flow temperature
  • ----- then can can graph the Delta between those two.

As the HP fires and pauses - what kind of delays are there before hot water water causes the floor to start heating.

Evidence: is the floor temperature not high enough above air temp, to cause the room to warm quickly - ie your UFH plumbed area is not big enough

  • monitor room air temperature
  • – then plot delta between that and floor temperature
  • will be similar to above - except when eg - initial warm up: what is the delta when the floor is barely warm and the air not started warming yet;;; versus later when the floor doesn;t get any warmer.

Hi, Here is a picture of the manifolds for the UFH.

Hi Tim, We mostly have sandstone flooring with some rooms engineered wood. One of the reasons that we chose the sandstone was because it is supposed to work well with UFH

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Hi Rachel, Thanks for this. Here is our usage over the last month. It looks as though our COP is somewhere between 2 and 3 depending on the weather outside. It has definitely improved since I turned the flow temp down from 48 to 40. You can see the difference between 15th and 17th December - the energy consumed is much lower but the energy delivered is almost the same:

Hi, thanks for the response. I can look into getting some more monitoring equipment. We strongly suspect that a lot of the issue is that there are simply not enough pipes in the underfloor heating and they are spaced too far apart.

Since turning down our flow temperature (currently at 35degrees) we are getting lots of spikes where the heat rises up to 60!

It seems to me like the return temperature is too high and not enough of the heat is getting absorbed before it flows back round? In this case should there be a way that we can slow down the flow of the water through the pipes…?

Hi, thanks for the reply and for pointing me towards heat geeks. It looks like there is an ‘elite’ heat geek in the local area so I have dropped them an email now :slight_smile: I think someone who can come and look at the house and give some expert independent advice is exactly what we need!

I agree in principle though. I don’t think our heatpump or anything else is ‘broken’, it’s just not been set up properly at all. It’s becoming clear that this kind of installation can’t just be done and then forgotten about and it needs tuning for each particular house, and the customer also needs some education in how it works. This is really something that installers should be making customers aware of and is the polar opposite of what we were sold, which was that it would ‘just work’ and we wouldn’t have to touch anything…

We did get lots of calculations done to size the heatpump. Unfortunately all of our work was arranged by a woefully incompetent project management company who got one of their mates to install the UFH, whilst another large company did the actual heatpump. A lot of the difficulties we are having is that the heatpump company tell is the problem is with the UFH setup and the UFH installer tells us the problem is with the heatpump… :roll_eyes:

This is our EPC. Since this has been done we have insulated under the floor which I believe will have dragged us up a little further. The EPC states the house is currently classed as C

I’m not sure what a MVHR is? We do have a bunch of certificates but none of them say that. I know that we have not had an air tightness test done though

We don’t have a buffer tank I don’t think. Sorry, not sure what DHW is?

These spikes up to 60C might be for DHW cycles (Domestic Hot Water). How is the heat pump configured for heating the water? You can probably change it to heat to a lower temperature, and less often. Many people only heat their cyclinders up once or twice a day, depending how big it is and how much is used. If your heat pump can be convinced to spend less time heating water, it will put more heat in the house.

The “dT” between flow and return is very tight, and does suggest that heat being produced isn’t being dissipated by the emitters. There’s probably two pumps in your system, with a low-loss header in between. It’s possible that the flow on the heat pump side is too high, or the UFH side it too low.

It will be very interesting to see what your experience with the heat geek guy is and what the solution is. like you I am in a heavily restored old house. unlike you my system seems to work fine so i guess I got lucky with the installer. Your epc looks similar. I have UFH everywhere including upstairs, except in the kitchen which has radiators. I also have heat recovery ventilation system (MVHR) which works really well in a house which is not airtight but I guess is just pretty good. The MVHR does have a slight unanticipated side effect in that it is so good at evening out the temperatures across the house that having thermostats in all the rooms is largely redundant as they have little effect… The whole house is at a constant 19c. I think I may have set the flow rates a bit high. Yes the heat pump power consumption gets a bit high when its really cold but is compensated for by low running costs when its more normal. (we had -9c outside for the whole weekend and it took it in its stride).

Ah yes this makes sense as the hot water was extremely hot yesterday. I think it heats up to 60 every now again for the legionella prevention. At the moment the hot water runs constantly (its set to 46 which we find comfortable for showers/baths). We should certainly look at turning this off overnight at the very least

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Hi @bazmod,
heatpump user of 7 years here, but certainly no expert. We have an old Cornish stone barn which we retrofitted with improved floor, wall and loft insulation plus underfloor throughout. I guess we got lucky with our installer, ours works very efficiently.

Looking at your manifold you have a mixing valve with a thermostat on the input side - our system has the same. This is designed to prevent overheating of the UFH damaging your floors, but is usually unnecessary in a heat pump system where the flow temperatures are much lower. I would check the temperature before and after this valve - you don’t want the heat pump spending lots of energy to heat up the water just for the mixing valve to have to cool it down. The heat pump will run much more efficiently at lower flow temperatures, as you’ve already discovered.

You want to give the heat pump as large a surface area as possible to transfer heat into the house. Every time a zone thermostat turns off you reduce that surface area so it’s best to control the temperature from the heat pump itself rather than room thermostats as much as possible.

If you haven’t watched the heat geek channel on you tube they have lots of great info there, will be interested to hear the verdict of your heat geek approved engineer.


We could look into getting a MVHR installed although I’m guessing this will be pretty expensive! The house is very open plan with a large open mezzanine overlooking the main living space, so quite a lot of the heat does tend to end up upstairs. It might make quite a big difference if we could redistribute this rather than it just slowly seeping out through the roof. Hopefully I will get a response from the Heat Geek person

Hi, thank you and yes hopefully we can get a heat geek person out to assess. If we do I will post back here and let everyone know what they said :slight_smile:

That’s a very good point about the mixing valve. We actually noticed this ourselves and got the plumber to disengage it last year for exactly that reason, so it should be letting all of the heat through.

There’s a setting on the EcoDan for how much the water temperature can drop before the heat pump heats it up again. Given that the temperature sensor is at the bottom of the tank, it’s totally fine to make this setting a high as possible (I think 15 degrees?). It’s quite possible for the temperature at the bottom of the tank to be below 30 after a shower, yet the top of the tank is still hot enough for another shower. Prohibiting it overnight would also save it heating up the cylinder needlessly, when it’s air outside is colder.

You can also set how often the legionella cycle runs, and monthly is probably sufficient. There’s an argument for not needing it at all; I don’t bother with it myself, but you have to look at the usage and risks for your property.