OK. I was concerned to find out where a slightly undersized Vaillant 7Kw might run out of puff as the weather got colder: the interaction of increasing heat demand and increasing defrosting as temperatures dropped could have been a problem for me.
Warning- involves quite a bit of interpolation/ reading of graphs on OEM, so treat results with caution.
My starting point was this Vaillant data sheet
My guess was that these figures were reasonable maximums but excluding defrosts. The data below is consistent with that guess, and provides some estimates for both. The alternative guess- that the stated outputs in the data sheet include defrost allowances- can’t be squared with the data.
To test the first part I found a V 7kW pump on OEM (#124) that seemed to be working hard in the 11 Jan cold snap. That pump was regularly showing 10.5kw steady output between defrosts at temperatures of c-1 (as low as I was interested in). For simplicity I assumed that actual peak output in favourable conditions was about 10% above the spec outputs at all OATs…
Now to estimate defrosts losses, comprising:
- heat draw per defrost (c0.5Kwh)
- heat output lost per defrost (c6min per defrost at stated output)
- ramp up after defrost back to full output (c10min, losing 0.2kwhr per cycle)
- ramp down as defrost approached (c2min, losing 0.15kwhr per cycle)
- defrost intervals, varying with OAT. My inspection suggested c40 mins at 1degC and below, steadily increasing as temps increase to 5deg (with a slight improvement at 0degC).
Put these together I got:
| OAT | Output kW | Defrost loss % | Output gain % | Net |
|---|---|---|---|---|
| 5 | 10 | -2% | 10% | 8% |
| 4 | 10 | -7% | 10% | 3% |
| 3 | 10 | -10% | 10% | 0% |
| 2 | 10 | -18% | 10% | -8% |
| 1 | 9.75 | -28% | 10% | -18% |
| 0 | 9.5 | -22% | 10% | -12% |
| -1 | 9.1 | -30% | 10% | -20% |
| -2 | 8.7 | -31% | 10% | -21% |
The key factor is defrost interval, not surprisingly- a small reduction in frequency at 0degC came from my observation that the interval increased from 40 mins to 50 mins at that OAT. I had no info on humidity but 11 Jan here in London was pretty damp.
Treat this with caution- other HPs would be different, and my observations and calculation may contain errors. But it gave me some confidence, for my various heat loss estimates, at what OAT I would need more heat from a backup with this model HP.