Heat Pump Sizing

[Itrium]

The trade is full of warnings about the perils of undersizing or oversizing a heat pump. Symptoms vary from having a cold house to reducing life span from frequent stop-starts. In my mind, the only time a heat pump is the correct size is when it is supplying full power on the coldest day to give adequate warmth. At any other time, it will be running at reduced power… in other words, oversized.

At the low power end, the pump will be just ticking over at the start or end of the heating season, a fraction of the full rating. To me this raises all sorts of questions about sizing a heat pump. Surely a correctly sized pump is simply one that can cope with the coldest weather? A larger pump is just a waste of money for unused hardware. As for the low power end, there will be a minimum cycle time determined by the temperature rise, volume of water and heat pump power.

Besides scary stories from installers about wearing out or damaging heat pumps from short cycle times,  has anyone seen real data about running a heat pump at low loadings and is there an acceptable short cycle time?

[Adam2]

Hi, lots of factors to consider but with a suitably sized buffer tank you'll limit short cycling to some degree. We don't need a large kW unit so much for heating but for water re-heat which may for some be a determining factor in size of heat pump 

[SteamyTea]

When a heat pump is running at full power, it is because the temperature difference between inside and outside the house is at maximum.  This will affect the coefficient of performance, especially true for ASHPs.

It is usual to size a heating system to cover 99% of the time heating is needed.  This does not mean that during that 1% it supplies no heat, just that the CoP drops to parity i.e. you may as well have used a fan heater, or the internal temperature may be lower than the ideal temperature.

The trouble with with methodology is that there is no scale, so you may be 0.1°C below the ideal temperature, or 15°C below it.

So take where I live in Cornwall, I could set a minimum external temperature to -5°C and be fine.  Up on the East Coast of Scotland, probably have to set that minimum temperature to -12°C (bit of a guess there as I have no idea what the temperature profile actually is).

So apart from taking the possibility of frosting, in the case of ASHPs, into account, yes, set the size for the maximum load, or to the next available size up.

 

It is possible to model the expected temperatures if you already know the external temperature profile.

There are only two number that are needed, the mean temperature and the standard deviation (the spread of the temperature range).

Then it is just a matter of creating a dataset of the temperature range and plotting it.

This will create a chart similar to this:

From that, you can add up all the percentages below the whatever temperature you turn your heating on from.  These percentages are for the time the heating is on, not of total time i.e. 8760 hours in a year.

So taking 12°C as the temperature you need to start the heating season, this gives a total percentage of 93% of the time for the real data and 91% for the modelled.  Pretty close to the 99%.  So probably going up to the next size HP will cover it, especially as my data is using the min real temperatures, not the mean, which is higher at 9.2°C and a SD of 5.4°C.

The big frosting risk for an ASHP is between 0°C and 4°C, this is when water is most dense, so there is more to freeze.

Summing up the percentage of time for those ranges give: Real 31% and Modelled 29%.  By oversizing an ASHP (as opposed to a GSHP or EAHP), you get more running time before the defrost cycle needs to start.  This is worth bearing in mind when heating DHW because that is for a shorter time and at a greater temperature (generally close to the units maximum).  DHW is another reason to oversize a HP.

The most important thing to do though is a proper heat loss calculation for your house, that will establish the power needed at any external temperature, the W.ΔK^-1.  Without that calculation being done, it becomes guesswork, and that is never good.