BoulderDash

Do you think that a larger pump would help with this issue? The installer dismissed this possibility by saying that a bigger pump would do exactly the same. Is an 8kW pump for a house with 176m2 floor area in the right ball park? How do you go about calculating the right pump size? Sorry, lots of questions...

Given that it’s likely to be predominantly 0-5C and moist during the winters where I live in SE England, does anyone have any suggestions as to how I can get a flow temp >25C in these conditions? I had thought that the impact of defrosting on ASHP performance had largely been overcome, but if this is the issue with my system, then it seems significantly inferior to a conventional oil-fired boiler. At the moment I seem to be spending about twice as much on energy for less heat..

Thanks ProDave, that’s interesting. Unfortunately, I don’t have the data to answer your question yet, as the outside temp hasn’t really gone below 0 where I live yet this winter. I do have the weather curve set-up pretty much how you suggested though. Is there anything else I should consider here, or do I just have to accept that this is an inherent flaw in the way ASHPs perform?

I moved into a 4-bed dormer bungalow (176m2) last summer that had just been gutted, extended and completely renovated, including ASHP for the DHW and UFH. The ASHP is a Nibe F2040-8 (8kW). The UFH is in screed downstairs and on aluminium trays in the suspended floor upstairs (both with 200mm pipe spacing). There is no buffer tank, although there is a volumizer unit. Room temps are set to 18C for bedrooms and 20C for the kitchen/lounge etc (no night-time setback). The UFH downstairs can maintain target room temps with a flow temp of 25-35C. However, the upstairs seems to need a higher flow temp of ~45C to maintain target room temps. The problem with this is that when the outside temp falls below 5C, the ASHP continually defrosts and is unable to deliver a flow temperature >30C (i.e. the opposite of the intended weather effect compensation!). I have attached some screenshots of plots from my Nibe Uplink – the first is when the outdoor temp was ~10C, which shows the flow and return temps oscillating around the target flow temp (which is what I would expect to see) and the other shows the same plot when the outdoor temp is ~2C. In the latter plot, the pump is defrosting so frequently that the flow temp never achieves its target, resulting in high electricity consumption (>£200 p/m), a really noisy pump, angry neighbours, and room temps <18C upstairs. The installer is refusing to investigate this problem and Nibe will only interact with installers. Q: is the impact of the defrosting cycles what you would expect for an ASHP at <10C? If not, any thoughts on the underlying cause – insufficient pump size? Lack of a buffer tank in the design? Anything else you can think of that might cause this…? Looking forward to your expert input and happy to provide any additional details that might be helpful.

Hello BuildHub community - I moved into a dormer bungalow late last year that has had a recent major renovation including installation of an ASHP and UFH. There seems to be lots of great contributors on here and masses of useful information, so looking forward to exchanging ideas and tapping into your expertise.