I bought a Heat Pump

Reed_Richards

shinytop said:

I don't agree with the comment about COP for the reason I explained above.

I don't agree with the comment I think you think I'm making either !!  I was very careful to talk about heating 'the flow' and not the house.  When my ASHP detects a big delta between the actual and target flow (e.g after the heating has been off for a while) it definitely works harder until the target it reached. I can see this from my monitoring data. It then drops back to maintain that flow.  According to Mitsubishi's data, working harder reduces the COP.  This is nothing to do with the room temperature. 

I have moved this comment from another thread so @shinytop (if they wish) and I can continue a highly technical discussion without taking that thread way off-topic.  My response:   

If there is a big delta between the actual and target flow temperature then the heat pump ought to be working at a better COP precisely because the water temperature is low.  It may be putting a lot of power into heating the water to the set temperature but that does not necessarily mean a reduced COP; superficially it does not mean that at all.  If Mitsubishi say to the contrary then I don't know the reason.  And if, say, you are 10 degrees below the set water temperature, is the COP reduced by comparison with what it would be maintaining that particular temperature or reduced by comparison with maintaining the set temperature?

QrizB

Reed_Richards said:

It may be putting a lot of power into heating the water to the set temperature but that does not necessarily mean a reduced COP; superficially it does not mean that at all.  If Mitsubishi say to the contrary then I don't know the reason.

On this specific point, Mitsubishi's published data shows how COP varies with load. See section 5.2 of https://www.mitsubishi-les.info/database/servicemanual/files/201803_ATW_DATABOOK.pdf

Eg. for PUHZ-W85VHA2, WT 45C, AT 2C:

• Nominal (and max) output 8.5kW, COP 2.61
• Mid output 5.6kW, COP 3.10;
• Min output 3.2kW, COP 2..86.
  

Verdigris

ASHPs are all efficiency rated against  an ambient/flow temperature of 7/35 degrees, respectively, similarly to cars being rated at 80kph/56mph for fuel efficiency. QrizB's figures show clearly that heat pumps are optimised at mid range, just as a car is optimised at something like 3000 rpm (for petrol, lower for diesel) as that is most likely the range the machine will operate at, most of the time, in real life conditions.

Reed_Richards

Wow, thanks @QrizB, that's great research.  Lets look at the figures now for WT at lower temperatures. AT at 2 C.

WT = 35 C

• Max output 8.5 kW, COP 3.15 - that's a better COP than the best case for WT = 45 C

WT=40 C

• Max output 8.5 kW, COP 2.89 - that's a worse COP than the best case (Mid) for WT = 45 C although marginally better than Min and better than Nominal

So if a Mitsubishi heat pump starts from cold and works really hard ("Max") to get the WT up to 45 C it will be working at a better COP for as long as the WT is less than about 35 C but if it continues at Max output until it gets the WT right up to the target 45 C it will be operating at a worse COP between about 35 C and 45 C (assuming that it can operate at Mid output once it gets to 45 C).  Of course it could "decelerate" as it approaches 45 C to achieve better economy at the expense of a slower response. 

QrizB

I would hope that all manufacturers have that sort of detailed information on the performance of their heat pumps, but so far I've only found it published by Mitsubishi.

Reed_Richards

Yes, that has got to be worth several plus points in favour of Mitsubishi when choosing a heat pump.

Reed_Richards

Verdigris said:

ASHPs are all efficiency rated against  an ambient/flow temperature of 7/35 degrees, respectively, similarly to cars being rated at 80kph/56mph for fuel efficiency. QrizB's figures show clearly that heat pumps are optimised at mid range, just as a car is optimised at something like 3000 rpm (for petrol, lower for diesel) as that is most likely the range the machine will operate at, most of the time, in real life conditions.

QrizB said:

Eg. for PUHZ-W85VHA2, WT 45C, AT 2C:

• Nominal (and max) output 8.5kW, COP 2.61
• Mid output 5.6kW, COP 3.10;
• Min output 3.2kW, COP 2..86.
  

If the first figure is the heat output and this example is a nominally 8.5 kW heat pump then it will operate most efficiently when the load is in the vicinity of 5.6 kW.  You particularly want good efficiency when it's cold out, 2 C as in this example.  Does this mean that we want to oversize our heat pumps by more than the MCS calculation would recommend in order to reduce the running costs?  I think it probably does.     

Verdigris

Does this mean that we want to oversize our heat pumps by more than the MCS calculation would recommend in order to reduce the running costs?  I think it probably does.

It does look, on the face of it, that it wouldn't hurt, as long as the capital cost of a larger model isn't too much greater. I guess the labour would be the same, so it ought to be relatively trivial.

I'm thinking of attacking this the other way round. Get a heat pump sized for the house, as is, and then externally insulating it afterwards, to reduce the demand.

Cardew

Reed_Richards said:

 Does this mean that we want to oversize our heat pumps by more than the MCS calculation would recommend in order to reduce the running costs?  I think it probably does.     

That seems to contradict the technical advice on heat pump sizing. If you Google 'heat pump sizing' it would appear that this statement sums up the various contributions.

'Overall, an undersized heat pump is better than an oversized one. An undersized heat pump will take longer to change the temperature. But, the unit won't be as loud and will last longer. An oversized heat pump will turn on and off more frequently, which will use more electricity.'

Verdigris

'Overall, an undersized heat pump is better than an oversized one. An undersized heat pump will take longer to change the temperature. But, the unit won't be as loud and will last longer. An oversized heat pump will turn on and off more frequently, which will use more electricity.'

That seems reasonably logical but I think may only apply to relatively simple machines. I'd have thought a unit capable of modulating output to match demand would obviate the stop/start behaviour, as long as it wasn't grossly oversized, say, not more than 20%.