Optimising a heat pump

70sbudgie

Apologies @Reed_Richards I have just seen your post and had missed it. 

NedS

70sbudgie said:

Reed_Richards said:

I think you must have blocked me, @70sbudgie, because you're not paying attention to the comments I make.  To repeat, an 8.5 kW unit is insufficient for an 8.4 kW heat loss.  

Apologies if I have blocked you,  it isn't something I have consciously done. On the other hand, I can't see where you have previously said an 8.5kW unit is insufficient, so I must have done/ missed something.

I have read NedS 's comment about not undersizing, but also got the impression that you disagreed

6) Conventional wisdom is don't oversize your heat pump.  It's rarely - 5 C outside but a lot of the year it could be relatively mild out so your heat pump only needs to operate at low power.  Heat pumps don't have the modulation range of a gas boiler and the modulation range can vary a lot from one model to another.  If your heat pump can't modulate down to the low output required for milder weather if will be forced to make the water hotter and cycle and that will reduce your efficiency.  It could be cycling for months for the sake of not working so hard on a few really cold days so I think Ned is wrong on this point

I am trying to understand where the threshold is that would be over / undersizing.

It's probably helpful to understand how/why people end up with oversized heat pumps. Mostly it is as a result of an overly inflated estimate or calculation of the property's heat loss, most commonly caused by over-estimating the air changes per hour, which I think MCS estimate at 1-2 changes per hour which may be accurate for properties with open fire places and chimneys, and/or old windows and doors, but it may be more like 0.5-0.75 air changes per hour for modern uPVC double glazed properties.

If the heat loss figure is accurate, and supported by real world energy usage data, then slight over-sizing by selecting the next model up in the range is fine. E.g, you have a heat loss of 8.4kW, the 8.5kW unit can't quite meet that heat loss so you go with the next model up in the range. Where that would result in gross over-sizing is if you later discovered that your heat loss was really 4kW and the 8.4kW was a gross overestimate and you're now stuck with a 12kW unit.