Given two identical heat pumps, one installed well and the other installed poorly (with a buffer tank, for example), and both operating at a fixed set point temperature of 45°C in the same outdoor conditions, would they consume the same amount of electricity?

@editor Great question!

This entire thread/discussion is really helpful for me to learn and make decisions - I have to get this installation as 'right' as possible within my financial limitations. 

And as @heacol points out system design can have a big impact on running costs. I'm shoe-string productions. 

Posted by: @editor

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Derek-m, thank you for your reply, which highlights, to a degree, the point I’m trying to make. Perhaps I’m struggling to articulate the dilemma I have in my mind. Essentially, I am not asking which system is more efficient in terms of overall performance, heat output or comfort. My question focuses solely on electricity consumption under specific conditions. 

Given two identical heat pumps, one installed well and the other installed poorly (with a buffer tank, for example), and both operating at a fixed set point temperature of 45°C in the same outdoor conditions, would they consume the same amount of electricity?

If both systems are running continuously at 45°C, would the poorly installed system with a buffer tank end up consuming more electricity compared to the well installed one? If anything, the better installed system might cycle more under these set point temperatures. But if neither cycles, and the outdoor conditions are cold enough to keep the heat pumps running to continuously satisfy the flow temperature, would the running costs be the same?

As @derek-m says you need to unbundle 'poorly installed'.  

• WC disabled is worth 10-20% (less at lower fixed flow temperatures, more at higher fixed flow temperatures)
• A poorly adjusted 4 port buffer tank/llh is worth 10-15% (it is possible to run a buffer tank in a way which reduces this penalty, but it seems many are poorly adjusted)
• A PHE wrongly plumbed is also worth 10-15%
• Running at 45C with WC vs running at 50C with WC is worth 10-15%

by 'worth' I mean 'reduces efficiency by'/'increases electricity consumption by' (for roughly the same house temperatures).

Adding these together its not difficult to see how a poor installation can consume 40-70% more electricity than a good installation.

Actually you can, to first order, reduce all of them back to 'flow temperature at output of heat pump'.  For each 1C increase in the flow temperature at the output of the heat pump, very roughly speaking your electricity consumption will increase by 2-3% (assuming you achieve the same house temperature).  The penalty for no WC is an average increase in FT over the season, a poorly adjusted buffer tank will require you to increase the FT at the o/p of the heat pump by about 5C to achieve the same FT at the radiators, a wrongly plumbed PHE ditto, and of course the final example is what it is.

These are all very approximate and ignore some engineering subtleties, but are good enough to understand whats going on in many, if not most, cases.  Better still they are directly a function of the underlying thermodynamics, so are likely to apply for most heat pumps.

Amongst the engineering subtleties they ignore are any penalty for cycling, which is not well quantified so far as I am aware (I would be absolutely delighted to be corrected).  However no evidence has been published on this forum, on Buildhub, or in manufacturers manuals that I have read (I have read a fair few) to suggests that the effect of cycling, which will always occur at some OAT, is likely to dominate the above. 

Incidentally much of the early aversion to 'short cycling' appears to have been based on compressor wear.  With inverter drives many, if not all, compressors will have slow starts which will mitigate this quite significantly.  There is actually a plausible thermodynamic argument that short cycling is better (in the sense of more efficient) than long cycling.  Perhaps this is why heat pump controllers frequently set the default min cycling period to 10 minutes, which presumably suggests that the manufacturers consider this a reasonable operating point. 

Equally there is a very plausible argument, backed up to some extent by reported experiments, that when the OAT gets sufficiently high and thus cycling becomes inevitable, 'batch heating' is a cost effective way to operate.  The argument is this.  If you batch heat (as opposed to heating continuously) you are forced to operate at a higher flow temperature and thus lower efficiency to achieve the same energy delivery.  However at a high OAT the amount of energy you need to deliver is anyway low, so the fixed elements of the heat pump consumption (principally the water pump) start to take over.  Couple this with the fact that 'continuous' heating at high OAT is in fact cycling, and it becomes quite plausible that batch hating is no less or even more efficient (because at least some of the fixed elements of consumption are turned off during the non heating periods).  Thus, either end of the heating season, returning to the 'old' pattern of morning and evening heating can make sense.  Even more so of course if you can benefit from ToU tarifs.

Hope that helps.

I'm following this thread with great interest as I am in the same boat as it were. When the energy prices went mad I set the boiler to run 24 hours at 40C and mostly maintained a temp of 19C so I felt that a HP could work for us. Now Octopus didn't have installers in East Devon and the three SW companies I contacted last year had such wildly different estimates I could make no sense of them. One installer was keen to come around but his insistence on a buffer tank made me wary, and then I developed optic shingles with Post Herpetic Neuralgia and I had to shelve the project for the rest of the year.

Starting again this year I contacted Ultimate Renewables. This discussion prompted me to reread @heacol's "How to install a heat pump with best efficiency" that they sent me ,and with, all the reading in this forum, I feel I understand it better now. So for now I await news from Ultimate Renewables that they have an installer for me and, in the meantime, I have British Gas coming around later this week to see what they propose. Interestingly their estimate of savings, which are mostly due to removing the gas meter, will probably be wiped out by the service charge. I can't understand why the service charges are 3 or more times those of a gas boiler, can anyone explain?

@jancold The service charges are in my opinion  extortionate. Most of the tasks are simple enough that a competent DIYer can do them. Even easier if you have anti frost valves rather than antifreeze. 

I’ve always done it myself and never had a problem. It depends on how much you value the warranty I guess. 

Posted by: @jancold

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. I can't understand why the service charges are 3 or more times those of a gas boiler, can anyone explain?

Does the 'service charge' include the annual uvc inspection?  

That might explain part of it.  Otherwise it's probably a matter of price vs cost in a sellers market!

@jamespa my UVC inspection was £42 inc VAT so I can't believe that is a major factor in the increased service costs!

Posted by: @jancold

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I can't understand why the service charges are 3 or more times those of a gas boiler, can anyone explain?

The issue with heat pump servicing is that it's usually tied into the warranty. No annual servicing could see the heat pump warranty voided, so installers are charging a lot on the back of this because the heat pump unit is so expensive to replace.

Our manufacturer (Global Energy Services) charge £216 (this price is now 2-3 years old) and we're not having the heat pump serviced because we do it ourselves. Here's what the £216 buys you:

One of our qualified engineers will come to your house and ensure the system is running safely and efficiently. They will service the system and carryout number of tasks including:

@editor Daikin charged me £345 for the annual service and maintenance. contract. Seems that printing money comes in many flavours … Toodles.

Posted by: @toodles

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@editor Daikin charged me £345 for the annual service and maintenance. contract. Seems that printing money comes in many flavours … Toodles.

Sellers market!

Posted by: @editor

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Our manufacturer (Global Energy Services) charge £216 (this price is now 2-3 years old) and we're not having the heat pump serviced because we do it ourselves. Here's what the £216 buys you:

This is yet another reason why Octopus are such a good deal, their annual service charge for a Daikin is £108 -this is what it includes

Posted by: @bontwoody

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@lucia

I would try and ascertain your own heat loss figure before they do, to get a good comparison to ensure you dont get lumbered with an oversized unit. There are several different ways to arrive at it without doing a room by room survey. This is a good article to start. https://energy-stats.uk/what-size-heat-pump/

Be very wary of quotes that insist you need low loss header/buffer vessel, even challenge why a volumiser is needed.

I've just used the simple formula on the Protons For Breakfast site based on recorded annual gas consumption - it comes out at 5.612kw

Thus as I suspected, the (verbal only) Octopus assessment of a 4kw Daikin is too small. Let's see if the written report is any different... 

@lucia For us, it a shame that OE could not accommodate us in the early days of their heat pump installation service.I think that they are genuinely trying to offer a fair service to GB Domestic.

I am not sure that the 50 degree C. rating is such a good idea but then, our own installer was working on everything running well with 50 degrees rather than our actual ~35 degrees which seems to work very well for us. Regards, Toodles.