Crunch time for heat pump size

[PhilT]

12 hours ago, Dillsue said:

Would the 10% inefficiency represent an oversized heat pump not being able to modulate its output low enough to match a more usual winter heat demand and then start short cycling 24/7??

 

I appreciate that an oversized heat pump might perform better in cold weather if that's what manufacturers say, but cold weather only makes up a small proportion of the heating season. The rest of the time it's mild and an oversize heat pump could be short cycling for months. Do the manufacturers performance figures cover the situation where demand is less than the minimum modulated output?

The output ranges in that data table aren't exactly like for like, but it indicates that the larger unit is more efficient across the whole range of ambient temps, so a more cautious approach in selecting the 'next size up' should not significantly compromise efficiency and running costs.

[SimonD]

19 hours ago, PhilT said:

It would seem that for modern scroll compressors, for a given range of output, a larger compressor is more efficient than a smaller one across the entire ambient temperature range. This is the Ecodan data table. The Vallant Arotherm+ shows a similar pattern when comparing the 10kW and 12kW to the 7kW model. Up front cost is obviously a consideration.

11.2 "Mid range " vs 8.5 "Max" @ 35°C	In kW	In kW	Out kW	Out kW	COP	COP
	11.2 kW	8.5 kW	11.2 kW	8.5 kW	11.2 kW	8.5 kW	% dIff.
outside temp °C            -15	2.7	3.4	6.7	7.3	2.5	2.15	16%
-10	2.9	3.7	8.1	8.5	2.8	2.3	22%
-7	3.0	3.6	9	8.8	3.05	2.45	24%
2	2.4	3.0	9	9.7	3.75	3.2	17%
7	1.9	2.3	9	10.5	4.8	4.55	5%
12	1.4	2.3	9	10.7	6.25	4.6	36%
15	1.3	2.3	9	11.5	7.05	4.9	44%

 

I do think you have to be a bit careful making this assumption based entirely on catalogue figures and also just a couple of manufacturer details. For example, the Grant Aerona 290 is rather different:

 

@ air 7/water 35C

 

4kW effective power input = 0.82kW cop 4.99

6.5kW effective power input = 1.54kW cop 4.95

9kW effective power input= 1.90kW cop 4.93

12kW effective power input= 2.53kW cop 4.81

15.5kW effective power input= 3.42kW cop 4.62

 

This, I think is pretty much the inverse of the table you show?

[Beelbeebub]

The rating of a Heatpumps is a bit subjective anyway. 

 

Indint think you can generalise. 

 

Manufacturers make a series of differently sized machines, but the actual "rating" varies from manufacturer to manufacturer and market to market (due to climate variations) 

 

So a 6.5kw machine in the UK might only be 5kw in Sweden and 8kw in southern France. 

 

Some manufacturers quote the max power at 7C outside and others at -5C.

 

So the way manufacturer A's range falls might make the smaller units more efficent, whilst manufacturer B's range has the larger units more efficent.

 

As a rule of thumb the bigger the evaporator is, in relation to the load, the more efficent a HP can be and the physics of the rotary, scroll and swing compressors makes efficencies drop off markedly in low load regions. 

 

 

[PhilT]

4 hours ago, SimonD said:

 

I do think you have to be a bit careful making this assumption based entirely on catalogue figures and also just a couple of manufacturer details. For example, the Grant Aerona 290 is rather different:

 

@ air 7/water 35C

 

4kW effective power input = 0.82kW cop 4.99

6.5kW effective power input = 1.54kW cop 4.95

9kW effective power input= 1.90kW cop 4.93

12kW effective power input= 2.53kW cop 4.81

15.5kW effective power input= 3.42kW cop 4.62

 

This, I think is pretty much the inverse of the table you show?

The figures vary a lot by manufacturer for example, at 35C the 10kW Vaillant Arotherm Plus SCOP is 5.03 vs the 7kW model at 4.36 (MCS test results). But my calcs look at this in a different way, by comparing an 11.2kW heat pump running comfortably within it's so-called "mid-range" capacity (c. 9kW ball park) vs the equivalent 8.5kW running flat out, at different ambient temps. So an over-sized heat pump could be more efficient or, at worst, no less efficient than the "perfect size" (for want of a better description).

[Beelbeebub]

1 hour ago, PhilT said:

The figures vary a lot by manufacturer for example, at 35C the 10kW Vaillant Arotherm Plus SCOP is 5.03 vs the 7kW model at 4.36 (MCS test results). But my calcs look at this in a different way, by comparing an 11.2kW heat pump running comfortably within it's so-called "mid-range" capacity (c. 9kW ball park) vs the equivalent 8.5kW running flat out, at different ambient temps. So an over-sized heat pump could be more efficient or, at worst, no less efficient than the "perfect size" (for want of a better description).

Shouldn't the sizing include defrost and DHW as well though? 

 

If you heat demand on your coldest day is 7kw continuous, because you will spend some time defrosting (say 15% of the time) you need to up you actual output, then you'll be 2 or 3 hours heating hot water. So up again. 

 

So your 7kw house might need a 9kw or 10kw at -5C hear pump.  But it will only need that for a day or two a year. 

 

The rest of the time it'll be running at maybe 75% "full throttle" until you get to late spring or early autumn when your lower modulation limit becomes more important. 

 

The choice of HP will constrained by other factors like local availability and expertise, physical size, price, warranty, aesthetics etc. 

 

[sharpener]

1 hour ago, Beelbeebub said:

If you heat demand on your coldest day is 7kw continuous, because you will spend some time defrosting (say 15% of the time) you need to up you actual output

 

I thought the received wisdom was that defrosting is worst near zero but below that humidity falls off again so efficiency lost from that cause is not at the lowest OAT and so you don't have to account for it separately. 

 

Mind you my system is sized for Plymouth (-0.2C) so there is not a lot of difference, OTOH I have never noticed it defrosting either.

[Beelbeebub]

This is exactly the point.

 

The exact performance of any unit will vary by location even for nominally identical properties.

 

Each manufacturer will optimise for slightly different conditions. 

 

Model Y from brand X might be slightly better paper than Model A from Brand B, but in the exact conditions (even down to how thr occupier uses it) of a given install the latter might perform slightly better 

 

I think chasing the last few % of efficency by looking at the scant data given by most manufacturers is a bit of a losing battle. 

[JohnMo]

16 minutes ago, Beelbeebub said:

This is exactly the point.

 

The exact performance of any unit will vary by location even for nominally identical properties.

 

Each manufacturer will optimise for slightly different conditions. 

 

Model Y from brand X might be slightly better paper than Model A from Brand B, but in the exact conditions (even down to how thr occupier uses it) of a given install the latter might perform slightly better 

 

I think chasing the last few % of efficency by looking at the scant data given by most manufacturers is a bit of a losing battle. 

Yep. Best gains are by getting decent modulation, lengthy cycle times and running the minimum flow temp possible.  Everything is sh!te in sh!te out.  If you don't have plenty of water volume and good flow rates your stuffed anyway, and stand a chance of getting a decent CoP.