@papahuhu OK, filled cavity and 300mm insulation would sound about right. I think you are correct and they missed the fact that the attic insulation isnt consistent. Its a pretty easy job just to top up the insulation though.

Just to note that the design temp for the bedroom is 18C not 21C like the room below

@bontwoody yes, the design temp is 18. I just need to get them to fess up to what the surveyor has already admitted and remedy. There’s about 8 rads that need upsizing, I guess that’s why they are pushing back, but not megabucks. I elxpect them to do it though, I’m not paying for 2 lots of new rads when I already told them something was wrong at the time.

@papahuhu Hope you get somewhere with them. Sounds like you know what the problem is. Did you pay by credit card?

If you cant get anywhere, it might be possible to just replace one or two radiators and then move the others down into smaller rooms

@bontwoody Section 75 claims can be a useful tool; I had occasion to reclaim when my investment in a Wind Turbine project went belly up. Toodles.

@toodles Glad you got your money back Toodles. Unfortunately I didnt buy my wind farm shares with a CC 🤣

@bontwoody

Thanks for working through it, although I still don’t know why the heat pump is idle 50%

I intentionally paid £100 of it by card. 
It’s a bit more complicated by the fact that the installer is a sub. I paid the main contractor. The main contractor didn’t inform the sub of the correct insulation mods. So I’m piggy in the middle and why I need RECC in the first instance or the small claims court to mediate. 

@papahuhu Its really tricky to get these things remedied. Good luck.

For interest how big is your heat pump and your house?

@bontwoody 

Tell me about it!

it’s the 16kW gen6 1P R32 Samsung mono, 50L volumiser on return, upm4xl pump,  the heatable floor space is about 240m2, 25 rads. Significantly oversized, the latest survey calculated the heat loss as 11.6kW, my argument was that it was the oversizing causing the idling. Still 50 % idle at recent -7C OAT. Of course they didn’t agree. 

@papahuhu Well its certainly not helping! What is its minimum output at various flow temps. That should give you a clue. Oversizing of heat pumps is rife!

@bontwoody I’m at my math limit vs the data that is provided by Samsung, I can find max values from the heat tables but not min. But I’ve watched the controller it can be ticking along at 26C LWT then it stops dead and resets to 50C LWT, no modulation in between. I don’t know enough about the control logic to understand why.

life was so simple with a boiler.

@bontwoody 😟 Toodles.

Posted by: @papahuhu

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@old_scientist 

Thanks. How can it possibly detect frost if it’s not touching the coil please? It’s a good 5cm away from the fins.

It doesn't use that sensor directly to detect frosting - it uses an internal sensor that measures the refrigerant temperature leaving the EEV at or below -6C, combined with other control logic to trigger a defrost cycle. Part of the control logic does use the ambient outside temperature, so the sensor is required, but outside temperature is not the determining factor in detecting frosting.