@guthrie There are 3 houses on the development: different designs and floor area. All have a large dining room at rear.

Interesting point is the 220m3; 250m3 and 280m3 property have same 11kW heat pump.  I have questioned that as my understanding is heat pump sizing is critical: it shouldn’t be too small/big.

@allyfish I asked builder for air leak test result; they said can’t find it. My understanding is they don’t need to test all houses on a development.  I also asked for SAP report which states air permeability is 4.64 at 50Pa.

We did find areas of missing insulation in main bathroom and a bedroom which have dormer type windows above them. Those ceilings have since been under-drawn with thermal plasterboard.  The infra-red survey is something I am considering.

@heacol I’m not sure the results from your Google search are correct.  At -2C and 45C water temperatures the pump output is 10.6kW. Therefore, at the design temp of -3C, output will be less than 10.6kW; and definitely not 11kW.  In a heat loss survey I had, I was advised that heat pump could only supply 96% of load. I believe regs say it must supply 100%.

 Bathroom is 10m3; 25m3 with calculated heat loss (by heat pump supplier figures) of 643W. The column rad was fitted by someone who did not appreciate deltaT.  The water going into the rad is about 39C and exits at 36C. Therefore average temp is 37.5C in rad. Design room temp is 22C. That gives deltaT of 15 which massively reduces the actual output of any rad.

It was the heat pump manufacturer/supplier who stated: “if we were to increase the size of the rad (bathroom) this would result in heat pump becoming overworked and running more defrost cycles, taking heat away from the primary circuit”.  Is that not a tacit admission that heat pump is under-sized?

There appears to have no joined up thinking on heat pump and rad spec.

The set back on the thermostats is only 1 degree overnight; if there’s a heat demand it’s running 24/7.

@dreg1 . Thanks, I echo @heacol comments. A couple of thoughts you said two things that appear to me contradictory, that the thermostat on the landing controls temperature and that you are using weather comp. I may be wrong but usually it’s one or the other. If you are using weather comp, then it’s tweaking the curve and getting that right to give you the temperature you want. The Thermostats then are simply thermometers telling the temp but not dictating it. I’m not sure what your heatmisers are doing either. I had one silly thought but worth checking, as a modern property I would expect you don’t have chimneys, if not good, if you do are they insulated when not in use? Was thinking of the draughts you were getting.

@heacol I’m not sure the results from your Google search are correct.  At -2C and 45C water temperatures the pump output is 10.6kW. Therefore, at the design temp of -3C, output will be less than 10.6kW; and definitely not 11kW. It’s the RASM 4VNE.  In a heat loss survey I had, I was advised that heat pump could only supply 96% of load. I believe regs say it must supply 100%.

 Bathroom is 10m3; 25m3 with calculated heat loss (by heat pump supplier figures) of 643W. The column rad was fitted by someone who did not appreciate deltaT.  The water going into the rad is about 39C and exits at 36C. Therefore average temp is 37.5C in rad. Design room temp is 22C. That gives deltaT of 15 which massively reduces the actual output of any rad.

It was the heat pump manufacturer/supplier who stated: “if we were to increase the size of the rad (bathroom) this would result in heat pump becoming overworked and running more defrost cycles, taking heat away from the primary circuit”.  Is that not a tacit admission that heat pump is under-sized?

There appears to have no joined up thinking on heat pump and rad spec.

The set back on the thermostats is only 1 degree overnight; if there’s a heat demand it’s running 24/7.

@terry1812 Thanks for that. My understanding is that the weather comp is dictating the output of the heat pump. I did have a heat pump service engineer tell me that I should not have any thermostats at all. As I have mentioned, I do not think the system has been designed particularly well.  On the landing, where the thermostat is for the radiators, there's a rad with a TRV.  That's bad design and I'm arranging for that to be corrected.  We have a chimney in the lounge for a wood burner which is built into the wall [contemporary style]. I bung insulation in chimney when not in use.

Hi @dreg1  11, Kw or 10.6 is neither here nor there, it is considered to supply 100% of the load and will with ease, if your system is controlled properly, your 1-degree setback could effectively reduce the size of the unit to 9 Kw, or even less as when the setback kicks in, the heat pump switches off until the room thermostat calls for heat again, this couls take 2-3 hours,  there for a capacity reduction of 13-14%. Your heat loss calculation is probably 15-20% greater than actual.

I think you answered you own question about your radiator in the bathroom.

The bathroom 643W is included within the 10.3 total load so icreasing the bathroom radiator, will have no effect on the heat pump.

Unfortunatly the design of the system is ver poor and not suited to a low temperature heat pump. I am sorry to say but your heat pump will only work correctly with a correctly designed system.

Hi @heacol Thank you for that. Would you be able to elaborate on why the system is very poor and not suited to a low temperature heat pump, please?

@dreg1 I am a cynic and believe that only the actual house which is tested will pass the building regs. The other will have considerably less care taken in build and will have draught faults. You already know the builder is sloppy that insulation was missed. You own air link test will cost about £300 and the engineer will keep it running while you find where the faults are.

Read the ashp basics turn up ALL stats to above the room temp you want. Remove ufh manifold controllers balance the flow for heat in each room. Get an extra radiator in the bathroom. Your system has been designed for gas and the guy working on it is still learning. The heat pump probably provides enough heat for the whole house but each room needs enough radiators, no amount of balancing will warm your bathroom without enough emission.

Thick curtains work wonders with lots of glass. https://protonsforbreakfast.wordpress.com/2024/12/18/the-u-value-of-curtains/ The curtains can double your insulation in effect.

Hi @dreg1 I was wondering if you’ve made any progress in resolving your issue. 

Posted by: @terry1812

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Hi @dreg1 I was wondering if you’ve made any progress in resolving your issue. 

Hi

I am waiting to hear from the developer about what they propose to do next.  They said they might put a higher output rad in bathroom.

That’s good gives you more capacity. It made me think, have you checked what your total radiator capacity is, because regardless of the heat pump, if the radiators can’t give out the required 10.3kw you aren’t going to get to temperature. That goes for the UFH too, but I’ve no idea how you assess the capacity of UFH. @judith was spot on. Have you been keeping warm. I know it’s started to warm up, but we have had some very cold days.