@sandman1600 

heat loss is heat loss. Yours is not anywhere near 14. My house is 263m2, built 150 years ago and I’ve the 11.2 ecodan. Insulation is ok, done what I’ve can, but no floors are insulated. 

yesterday in Sheffield was 1-3 degrees and I needed 8.5kw ave over 24 hours 

the flow rate you have is 17, that’s typically what is with the 5 or 8.5 unit. With a DT of 3 or 4, you’re not using much power at all.  The 14 normally needs 40l/min.  They have set that up for you, so they know this. 

Posted by: @sandman1600

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I've attached some of their calculations from the documents they sent over. Does any of this explain why they specced a 14Kw..?

 

The total at the bottom of the 'heat loss calculations' column is why they specced 14kW.  I note that the design FT is 45, which begs the question - why was it operating at 55?  The good news is that they didn't saddle you with a buffer tank (although if they used the Mitsubishi pre-plumbed cylinder that has an integral Low loss header - it might be worth posting a picture of the cylinder and surrounding pipework so commentators can take this into account.)

Unfortunately these loss calculations look scientific but actually they can be complete nonsense (GIGO applies!).  I sometimes think that their purpose is to protect the installer, not the householder (they have followed the rules so are nearly bullet proof).  I had 2 full three hour surveys done on my house, one I paid £300 for.  Both came out at 16kW.  The actual measured figure is 7kW, which is what the more 'switched-on' installers worked to.  The difference is accounted for by

• surveyors ignored fabric upgrades that I told them about but they couldn't see - this appears to be quite common and there are examples, on the videos on this forum, of installers defending the practice.
• the calculations counted room to room heat loss but ignored room to room heat gain, this double counting some 'losses'
• calculations assumed an ACH *(air changes per hour) of 2-3.  I dont have a measured value for ACH but anecdotally installers that have actually measured it on houses say its never more than 1 and usually about 0.5.  if I assume 0.5 and dial in the correct fabric I get to 7kW matching the actual facts.

I would try, if you can, to avoid getting too stressed about this.  Do you have a COP readout somewhere (sorry I dont know the Ecodan control unit particularly well, ive read the manual but thats it).  This might give you an early indication of whether the changes you are making are helping. 

Whatever else you do I would definitely keep reducing the WC curve until the house is only just warm enough.  That's the essential.  Once this is settled you can look at the graphs and see what to do next.  Batch heating during cheap rate feels like it may be a good option in your case; although Im not fully familiar with it I know that the Miitsibushi controller is pretty feature rich, so hopefully its possible to set this up so you don't have to touch it (if its necessary).  You never know, it could end up very cheap if you can do this. 

But first things first, get the FT optimised and see how it behaves!

@jamespa 

Yep, in Mel cloud you’ll see energy consumed and delivered. What was yesterday? 

@sandman1600 

PS.  I just checked your EPC, Its F rated with the potential for D.  Its interesting to note that much of the fabric is 'assumed' ie the EPC surveyor couldn't be bothered to find out. 

Im guessing that you have done a fair few of the suggested improvements, but as they are invisible they have been ignored by the HP surveyor and/or the HP surveyor made different assumptions re the fabric.  Just a guess!

@jamespa 

Yeah that EPC was done a few years before we moved in. Since then we've had cavity wall insulation throughout, insulated the suspended floors, we've got low energy lighting and room thermostats and then solar panels are being fitted next week so we're at least an E now I would hope.

The energy stats from the app are here for yesterday. Thanks @davidnolan22 for pointing that out.

So I'm betting if we go back to the installers about the oversizing, they're just going to point at the heat loss report. Stats can prove anything..!

We're with Octopus so we are planning to switch to the time of day tariff but if we've set everything up and got it working with WC curves I'm not even sure how we force the ASHP to work overtime during cheap times. I'll take a look into that though.

The graph for the last couple of hours is jumping around again. Is this what cycling looks like?

@davidnolan22 That's crazy. So yeah, we're clearly oversized. Really frustrating. 

I'm concerned if we push for a smaller HP, something like the 8.5 might be more borderline. If we can't trust their report, I don't know what to reliably look for.

I suspect they'll resist changing the pump anyway so I suspect we're stuck with it. 

Can you do it for one day. That Cop is a problem

yes, the pump can’t deliver the heat at that low temp. The emitters / secondary flow is not big enough and can’t pump can’t modulate low enough. It will turn off 2 degrees above the set temp.

@davidnolan22 Yeah I didn't think the COP looked good. That's for one day... yesterday. It was worse the day before. I'm hoping it's going to come down as we reduce the WC curve but it's not good.

@sandman1600 

there is no chance your unit used 200 kw in a day…..

what does your octopus app say?

@davidnolan22 Ah, I've just noticed the report was for a year. 🤦🏻‍♂️ The start date defaults to 2024. 

So here is the correct usage graph... sorry.

Posted by: @sandman1600

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@davidnolan22 That's crazy. So yeah, we're clearly oversized. Really frustrating. 

I'm concerned if we push for a smaller HP, something like the 8.5 might be more borderline. If we can't trust their report, I don't know what to reliably look for.

I suspect they'll resist changing the pump anyway so I suspect we're stuck with it. 

If you are going to have the discussion with the installers then I have some thoughts:

• it may be worth waiting until you have more data and have the arguments together, which you are welcome to test here.
• it may also be worth waiting until you are clear what you want them to do
• did you tell them about the fabric upgrades relative to the EPC, where they cant reasonably be inferred by inspection. 
• it might be worth asking for the calculations and/or fabric assumptions underpinning the heat loss report

Any or all of these could affect the likelihood of success.  No need to rush.  As you say their first response will be to push back (and of course tell you that you were wrong to touch anything), so knowing you are on solid ground before you start is worthwhile.

Its also worth reassessing once you have sorted out the FT.

That said a COP of 2 in the graph above falls below the minimum requirement for the BUS grant and the value they claim you will get.  Of course their claim had plenty of get out clauses! 

FWIW I got a (claimed) COP of 3.8 on 14/3/2025.  Every indication I have says that I can more or less trust this figure.  My FT @ -2 is 42, IAT about 20.5.  Below are the OAT and FT profiles for that day.  During the night you can see periodic defrosts and also that the FT is a bit lower than might be expected, because Im using Noise reduction mode at night.  Daytime its steady as you see, apart from the spike for the DHW.  Unless you OAT profile differs radically you should expect a similar COP once your WC curve is sorted (heat pumps don't differ that much) and if not we need to look for why.  Melcloud is your friend here as it gives you the monitoring needed to see whats going on.

PS If you could post a picture of the DHW tank and surrounding plumbing we can check if anything is amiss here!

PPS Not sure the graph you showed of FT is cycling, it looks more like some kind of hunting behaviour (ie its trying to find the right level but oscillating because the control loop is forcing it too hard - a known challenge with control loops).  Probably worth posting a few more of these particularly a plot of 24hrs - the current conditions are useful for working out when the onset of cycling is because the OAT varies so much.  If there is an energy use plot that would also be great as it would confirm whether its cycling or hunting.

@sandman1600 

Some good advice from James. 

On those charts there is good news and bad news. The good news it it only took 70Kw to heat your house to a nice temp all day.  I was quite cold in Sheffield yesterday, what was it like near you? But that's less that 3kw an hour.  

The bad news, how much energy you used to get that.  for comparrison. My house produced 182Kw and used 42 Kw for a COP of 4.3.  My fixed rate tariff is 19p, so £8 to heat it, I'm happy with that. my 260m2 house largely to 20 degrees for 24hours for a pint and a packet of peanuts

I've seen those charts a lot on Mel cloud, I think that is cycling and turning off. When you're asking for 35 degrees or so, your emitter output is below the minimum output of the machine, it can't get rid of the heat.  Get James's view, but might be worth running it hotter on a fixed flow, say 45 degrees, this will increase the emitter output and run it for 10-12 hours a day.  at 45 degrees it should still give a COP of well over 3. But will so many competing views here, you might confuse yourself.

But, I agree, I think you need to take some time to really get a flavour of what's going on, and I suppose it depends on the relationship you have with your installer. If its turning off a lot it might not be getting consistent heat in the floor and hence why you feel the rooms drop a bit.  Its not that you can't heat the house at lower temps, but your pump and emitter relationship might struggle to deliver them. 

But, loads of assumptions there and I think you're in for a bit of a journey here.  I've had some difficult times over the last 18 months, but I think I'm getting there now.