I fully agree with James that a HT heat pump will probably be no more costly to run than a LT heat pump when operating at the same LWT.

If the reason for installing a HT heat pump is because a LT heat pump would not be able to supply LWT at sufficiently high temperature to meet demand, then a HT heat pump is likely to be more expensive to run.

This is why improved insulation, larger heat emitters and other ways of lowering the required LWT should be the primary objective.

As I have stated repeatedly 'if you don't require the thermal energy in the first place, then it is not necessary to produce it'.

HT heat pumps are often installed as boiler replacements in systems designed to deliver a Delta T of about 20°C. So if you're paying to a HT unit, the expectation would be to run it hotter 

I don't think that ot is necessarily the case that you buy a HT pump for it's HT capability or that the expectation is to run a HT pump at ht. 

Manufacturers are tending to move from R410a (lt) to R32 (mt) to R290 (HT) which reduces the gwp of the refrigerant.  As the make this transition they are making other changes which are also desirable such as improving appearance and noise.  Furthermore many are abandoning the older models.  So if you want one of the quieter, more recent and less ugly models with a refrigerant that has lower gwp, you get HT capability as a 'bonus'.  You may or may not use it. 

The statement 'HT pumps are more costly to run' helps some manufacturers to shift older, noiser, uglier and sometimes inferior stock, but is not actually true, albeit that it is true that pumps run at a higher temperature are more expensive to run.  It's a subtle but vital difference.  Similarly touting the HT capability helps manufacturers 'sell' their latest and more expensive product.  The reasons to buy may be very different though.

It's questionable whether zoning saves money with a heat pump.  The saving in lost energy is quite small, and because you need a higher ft as a result of less emitter area the reduced efficiency can outweigh any savings. Heat geek have a worked example to prove this.  If your house is broadly cubic zoning is unlikely to save money, if it's long and thin and you can heat only one end then it might.

You are right to be concerned about over speccing, it appears to be baked into the industry.  If you can determine accurately your whole house loss eg from existing consumption, you can sense check the room by room calculations.  This should allow you to check which upgrades are necessary and which aren't.  

Regards

As for HT units, they are significantly more expensive to run than LT units. I’ll post some comparison data tomorrow.

Hi Mars,

I am running a Samsung 12Kw High Temperature Quiet heat pump in a 5 bedroom house which has a heat loss calculation of 10Kw at ambient -2C.

It just so happens I am tweaking the settings as I write this but I am in the position to give you some example running figures.

The heat curve (which seems to be the sweet spot for my property) is 35C flow temp at ambient +10C and +45C flow temp at ambient -2C. As I write this it is +7C outside and the pump in a steady state is pulling 1.4Kw (read from the electricity meter) to maintain a room temp of +21C. I am currently getting a daily COP of 4.3 based on the pumps own controller figures. (Glynn Hudson's analysis shows this maybe 30% lower than reality due to the sensor positions etc) Sadly I do not have heat meter equipment installed which I wish I had the forethought to do last year.

The vast majority of the time the pump works in the low temperature range however the advantage I think is a) it is capable of heating DHW to +55C autonomously, b) if the weather gets really bad it has the horsepower available (all be it more expensive) in reserve and c) vast majority of the time the pump is not working too hard so relatively quiet.

Running with flow temperatures of +50C and above, certainly with the Samsung, you are looking at COP's of 3.4 and less (as the flow temp increases to +70C) for an ambient temp +7C.

Right now the ASHP is running 15% cheaper than my oil boiler

Interesting to see how these figures compare with other HT and LT pumps. I guess the point is if HT pumps are run with temps as low as possible then they are possibly no more expensive but have the extra horse power when needed.

Regards

Obviously there will be detailed variations between manufacturers.

Posted by: @editor

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As for HT units, they are significantly more expensive to run than LT units. I’ll post some comparison data tomorrow.

 

I'm not sure that's strictly true.  What is true is that its more expensive to run a heat pump at a higher flow temp than at a lower flow temperature, but a HT pump and an LT pump run at the same temperature will be roughly the same cost (actually the HT one might be a bit cheaper because they tend to be more recent designs).

If @heacol is around, he's the best person to answer this because he has conducted a lot of tests and has data to that effect.