Hi @dreg1 , I’m fairly new to this too. You’ll probably get some really helpful replies. A couple of questions, you referred to a primary circuit, which might imply you have one or more zones in the house? Do your radiators have TRV’s on them and are they all turned to max, ie allowing full flow through. Your house will have had an EPC I assume, does the heat loss on that agree with the 10.3kw you mentioned above? Do you know if your system has a buffer on it and how many pumps it has? Are you running it by thermostat or by the weather dependent curve. Aside from the bathroom do you have other radiators or is everything else  UFH. You did mention that you had to run it overnight. With heat pumps I think the idea is that they are low heat systems and should be run 7/24, esp when it’s colder. Although your 48c  flow temp is pretty high. If you can provide answers to the questions above, someone with knowledge will probably be able to give you some useful input. 

That does sound like a very high heat loss but then your house is bigger than our one, which is 2 storey 4 bedroom, maybe half the floor area.  We have a 12kW heat pump which works find down to -8 so far.

Have you had the insulation checked and how drafty does it get when windy?  To me, if you are struggling especially when it is windy, that sounds like greater losses somewhere due to drafts.  Do the other properties with similar sized pumps have similarly huge dining rooms?

I think your issue here may be infiltration, especially as you say the heating performance deteriorates when the 3 storey building is subject to windy conditions. Has the house had a pressurisation permeability test to Building Regulations Part L? The required criterion is 8 m3 /(h·m2 ) @ 50Pa.

For a 280m2 house your permissible leakage rate at 50Pa is 2,240m3/h. The leakage rate under normal atmospheric conditions would be expected to be much lower, but strong winds create draughts and infiltration/exfiltration through the fabric of the property.

I would also consider getting an experienced surveyor in to thermal image the property, while the weather is still quite cold. That will show any thermal bridging and leaks, especially around obvious areas like windows and doors, but heat loss at electrical socket outlets and skirting edges can also show up very well with thermal imaging.

You could arrange our own whole house leakage rate test if you can't find the builder's information, or you have questions over the accuracy of it. Both the above might seem expensive, but so is the cost of heating a leaky house for the next decade or two.

Posted by: @dreg1

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Hi All

Any advice or opinion appreciated.

Bathroom, it it is a collum verticle radiator, then standard deration factors do not apply, a further 15-20% needs to be deducted for low remperature opperation. However, if you assume that the bathroom, at 22 deg C will have a heat load of 60-80 w/m2, your bathroom is approximatly 10m2, you need 600-800 watts, but you are only supplying (acccording to your calculation) 410 watts, it is going to be cold by design.

Installing a larger radiator will make no difference to the ability of the heat pump to supply the heat, the bathroom is already in the 10.3 Kw heat loss calculation.

Your majour problem appears to be the way you are opperating the heating in your house. It appears you are zoning it, with on-off thermostats which form an intermittent heating patern and I verry much doubt the heat emmitter design has taken this in to account. Therefor you are expecting the emmiters to do significantly more that they were designed to do, and as the heat pump is opperating intermitintly, the off time does not count, therefor you need a significantly larger heat supply (heat pump) to catch up when a thermostat switches on, hens the correct assunoption in the currect scenario, that the heat pump is significantly underscized.

This it the trap most heating engineers fall in to as a boiler is usually 3-4 times (a 30-40Kw boiler would have been installed on your property), hence it has plenty of capacity to catch up, whilst a heat pump, scized close to the demand does not and falls behind, resulting in a cold house. The only way you are going to get a reasonable result is to open zone your entire property and let the heat pump do it's job. (test it by turning all your thermostats to 24Deg + and you will see things significantly inprove, your bathroom may even get warmer). 75-90Kw per day with your configgeration ans house scize, is not bad  😊 

@heacol , your footer says you are Director at Heacol & Head of Domestic Heat Pump Design Net Zero British Gas. I don’t know if you can help, cheeky intervention on this chat stream. I have a Daikin Altherma 3 8kw, installed by British Gas. It’s working fine but I have a few issues. But the thing I’m trying to resolve in my head is the capacity of the heat pump when working in pure weather dependent mode. My heat pump seems to operate with 3 priorities, which in order of precedence are, deliver Target LWT, Get to target delta T, which for radiators is 10, Get flow rate to 7L/m. Now if that is correct then once all 3 targets have been achieved the pump is happy . My design temp is -1.6c to 21c with a heat loss of 7.3kw. Depending on how long it takes to get those 3 in line, once it does, my modelling says the output drops from its maximum to 4.88kw, given 45c flow 35c return, therefore delta T of 10 and a flow rate of 7. There appears to be nothing to push the flow rate up further to increase output because in weather comp mode it’s not chasing a temperature. Can you help?

Secondly I’m feeling puzzled about heat loss. So I have a heat loss of 7.3kw, so at an outside temp of -1.6 I need that full output to get me to 21c. Partway through ( ignoring the issue I raised above for a moment) some of that 7.3kw will be pushing me towards the 21c and some covering my then loss of heat because I’m warmer inside than outside. However and here is the question, heat is coming in at 7.3kw all the time, but how much heat is going out, because the heat loss doesn’t seem to be flowing out instantly? I saw a report that said the leakiest houses lose about 1c in about 2 hours. But at 7.3kw one is putting in more than 1c. So at some point isn’t the heat going in too much, or does it suggest the heat loss is overstated? Thanks.

@terry1812 Sorry, I did not realise it was there, Admin chainged it a while ago, it has been corrected.

@heacol , so does that mean you can’t comment?

Posted by: @terry1812

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@heacol , so does that mean you can’t comment?

@heacol , sorry how do I send a pm, I know I ought to know, but I don’t. Thanks.

@terry1812 Go to "My Dasboard" Top right and click on the messages icon, I have sent you one.

@terry1812 There are 2 UFH manifolds downstairs. Main supplies hall, study, kitchen, utility. The other supplies a large dining room.  Each downstairs room has a Heatmiser thermostat. Radiators upstairs, TRVs fully open. One thermostat on landing controls temperature. There’s a 60L buffer tank and 2 Grundfos pumps. Weather compensation set to control heat pump water output temperature. I appreciate the heat pump should run 24/7.