ASHP installation cost

Cardew

Reed_Richards said:

@Cardew , you are very welcome to contribute and it's good to have a sceptic to counter enthusiasts like me.  Of course you don't have to own a heat pump to comment but I am in the position of being able to see how one works in a real domestic situation.

• People routinely over-size gas and oil boilers w.r.t. meeting the heating requirement of a house.  I swapped out an oil boiler with twice the rated power output of the heat pump that replaced it.  If over-sizing is perfectly fine for gas and oil what makes a heat pump different?
• The MCS guide does not give us a clue beyond two references to other documents.  Is the answer really so complicated it is incapable of being explained?
• So it's not "prevailing wisdom".  It might be true, it might not but without an explanation it cannot be classed as "wisdom".

@Cardew I would be grateful if you could use your background as a chartered electrical engineer to think of some plausible reasons why over-sizing a heat pump is wrong.  Because even if I was an installer and read the guides I would not be any the wiser.  

    

On a subject like this, scientists and engineers produce theoretical concepts which are computer modelled and then tested empirically. There is no reason for me to question their findings any more than I would question the science behind CoVid vaccinations.

As said previously, all of the manufacturers and regulatory bodies stress the importance of correct heat pump sizing. If oversizing was un-important why wouldn’t manufacturers push for larger units and hence more profit?   

With regard to your query why gas/oil boilers can be over-sized(which is also not recommended practice) but not Heat Pumps, below is a quote from the Government report I referenced earlier.

'As mentioned, heat pump sizing is affected by the calculated heat loss, which is sensitive to assumptions in  ventilation  and  U-values.  The  same  issues  apply  for  boiler  sizing,  but  the  cost and  efficiency implications of over-sizing boilers are lower.'

There is also this input on radiator sizing

'7.4Radiator SizingRadiator sizing analysis indicates that ‘star rating oversize factors’ as described in the Heat Emitter Guide (HEG) may be inadequately understood or ignored due to practical and aesthetic considerations of size and location'

Cardew

Reed_Richards said:

But @Cardew, surely you can see that there is no substance in this?  Of course it is important not to under-size a heat pump because then it won't keep you warm enough on cold days in winter.  It may well be significantly more incrementally expensive to over-size a heat pump than it is to over-size a gas or oil boiler (which, incidentally, has to be done to get you your hot water fast enough).  It might also lead to a reduction in efficiency but if that is true nobody is letting on why.  You say this is a finding of scientists and engineers but it looks to me as if everyone is just parroting the same thing without knowing what lies behind it.
       

I don’t agree with you; neither do any of the heat pump manufacturer’s technical manuals and guidance documents I have read. Why would they 'all parrot the same thing'; what would be their incentive? There are simplistic explanations on cycling e.g.  

'However, if you get a heat pump which is too big it will only need to run for a short period to heat water to the desired temperature and will switch itself off and on again regularly. This is not an efficient way to operate and will cost you more in energy bills.'

So in my book it is ‘prevailing wisdom’ that you should not over-size a heat pump; and I can see no reason to question that statement.

As you apparently disagree, why don’t you contact manufacturers/MCS and ask them to justify their conclusions.  

matelodave

C'mon chaps - this is getting a bit petty. I dont really think that arguing semantics really adds to the discusiion.

I don't honestly think that any of this is arguing about gross over or undersizing but you can always turn a larger unit down rather than discover that an undersized unit just wont do the job.

In the end I think we can all agree that the pump should be appropriately sized for the dwelling and heating load and in general it's probably better to err on the side of over spec than under spec if you can't get a unit that "exactly" maches the heating requirements.

Here is what the MCS guide has to say about it - 

3.4. Heat Pump Capacity

It is essential that a heat pump is selected to achieve the desired output for at least the design conditions, if not slightly more onerous conditions which may not be the same as the nominal rating of the unit.

Lack of understanding of the variation in the performance of heat pumps at different operating conditions has been identified as causing significant under-sizing in previous field trial reports. For instance, the standard rating condition for ASHPs used to be air at 7o C and water flow at 35o C.

 However, in practice they may be required to produce water at approximately 55o C with design ambient temperatures of circa -2o C. At these latter conditions the output of many heat pumps could be as little as 60% of the output at ‘standard’ (nominal) rating. For example a 13kW ASHP may provide 13kW of heat at 7o C and 35o C flow temperature but it is unlikely to do this at an air temperature of -2o C and water flow at 55o C.

The actual output could be as little as 7-9kW and hence, if relied upon for the design of the system, it will be vastly under-sized incurring the cost of expensive supplementary heating and/or lead to issues of poor comfort. 

Reed_Richards

I believe the correct MCS procedure to size a heat pump is to first determine what outside temperature is exceeded 99.6% of the time at your locale.  I think there must be look-up tables for this.  In my case that temperature is -3.7 C.  The heat loss of the house is then calculated at this external temperature.  The heat pump must be sized to meet this heat loss requirement when operating with the specified output water temperature and taking into account the fact that the heat pump will spend some of its time heating the hot water and defrosting itself.

Interestingly, I see that MCS launched a "new and improved" heat pump calculator a few months ago: https://mcscertified.com/mcs-launch-new-improved-heat-pump-calculator/   

shinytop

What about DHW?  If a large heat pump copes badly when it is oversized for heating, does it also struggle when asked to cope with very small DHW demand?  Does my ASHP cycle excessively/run inefficiently now, when it's only providing a little HW every day?  I get that it's a waste of money having an ASHP that's too big (like having a huge SUV to go shopping in) but is there really more to it?

 

Reed_Richards

The MCS guide https://mcscertified.com/wp-content/uploads/2020/07/Heat-Pump-Guide.pdf has this to say:

A consensus on how to adequately size the heat pump for domestic hot water production does not currently exist.

This just goes to show that an understanding of how to make best use of a heat pump in the UK is not complete.