Heat Pumps and domestic hot water

(Index of other blogs)

Before I start, I hope nobody takes from this that we shouldn’t do domestic hot water (DHW) with HPs.  And as things evolve, I’m sure that we will do more of it, and do it better.  However, I was pondering the issue of mass adoption of HPs for smaller houses and how we tackle DHW in the immediate future.

Many years ago, when heat pump components were less developed, providing energy-efficient hot tap water (DHW) with a heat pump (HP) was a struggle.  However, it’s not difficult to add the feature since in its simplest form all you need is a diverter valve and a cylinder with large-area heat exchanger coil in it.  Even if the DHW might have had a COP as low as 2 (at that time), it seemed worth it.

It took me quite a few years to acknowledge that the losses associated with cylinders and distribution can be considerable.  We often hear claims that boilers are 90% efficient, or heat pumps (at 60°C) have a COP say 2.5, but if we look at the actual heat delivered to your bath or basin, then both of these figures could in practice be halved in a few cases, or possibly considerably worse. Ironically, frugal users have the lowest COP because the losses are a greater proportion of the total. (this doesn’t of course mean you save energy by using more water!!)

When I get the opportunity, I often run a hot tap in a house and see how long it takes for it to run-hot. Sometimes a whole basin is filled with cold water before the hot emerges.  We could in this scenario, get one basin of useful hot water for two basins draw-off from the cylinder.  This experiment demonstrates 50% efficiency, and effectively halves the boiler or HP efficiency.   The more that is used (in succession), the smaller the proportion of loss.

It’s not only the old houses with a 22mm main artery hot water draw off where the problem lies.       I am amazed how bad some new build is.  Surely pipes should take the most direct and shortest route…. Seemingly not.  Surely we would go for the smallest bore pipe that still gives adequate flow.  If we did, there would be a better range of pipe diameters on sale.   Excessively large bore pipes are a waste of water and heat, but arguably the heat is contained within the house, and is at least useful in cold weather.

For those of you thinking of lagging hot pipe runs… think it through. Sometimes the benefit of this is minimal.  e.g. after 30 mins most heat is lost regardless of insulation thickness.   Pipes around cylinders are however always hot, and these definitely need insulating well.

Pumped circulation loops are another thing.  This is a necessary evil for large places like hotels, and at least provide hot water immediately a tap is turned on.  I have limited experience of these… only bad ones.  I’m told this can be done well, nonetheless it’s potentially a big waste of energy so they need careful design and very good pipe insulation.

Loss due to pipe runs is one thing, but another is of course the heat loss from the cylinder, this heat is again arguably beneficial for much of the year. Even with no draw-off, a heat pump can spend a surprising amount of time simply topping up the cylinder.

Out there in the real world, I don’t actually know how bad these losses are, but feel they could be surprisingly bad in many cases, but hopefully many are acceptably good.  It is sort of bizarre that manufacturers optimise heat pumps to get the best COP, but on site, but some aspects need for energy-efficiency often seem overlooked.


Anyhow, with the thought in the back of my mind that DHW from a HP might be not as good as we often assume, I was pondering the issue of mass adoption of heat pumps.    One issue for smaller houses is the lack of cupboard space for a DHW cylinder.  They have not had one with the combi boiler, so why would they accept one with a heat pump?!

If cylinders are unacceptable, would it be a backward step to install a space-heating-only HP system and electric point-of-use DHW? (non-storage type that is). I don’t know, but there are some Advantages

  • No space lost due to a cylinder
  • Heat pump system extremely simple
  • considerably cheaper to install


  • you still need to install several instant point-of-use water heaters… more total electric supply required from distribution board and more cable runs.
  • Higher peak load on mains supply, but this would be spread by diversity

Ironically, those small houses that really won’t welcome a cylinder, could also have short pipe runs, so my argument about long pipe losses doesn’t hold here.   It’s actually larger houses where the long pipe runs can be very lossy.  Hopefully the installer thinks it through, and doesn’t (for example) run a hot pipe to a distant hand basin.

Some cylinder-related details to sort out

Another possible headache relating to heat pump DHW cylinders is the need for safety blending valves. And this is sometimes needed because of the required periodic legionella pasteurisation. These mixing valves usually allow some cold water to pass. This means the cylinder needs to be stored at a higher temperature to combat this cold ‘seepage’. Higher temperature means lower COP.  We need safety mixing valves that can shut off the cold completely.

The ‘thermal store’ type arrangement, where water heats instantly as it passes a store, can mitigate many of these problems, but it can have its own inherent losses. This type of storage arrangement could be the way things develop.  Who knows!

Phase-change storage could be a solution to the lack of space issue, but I feel there are a few challenges here. Namely that the phase-change often happens at a high heat-pump-unfriendly temperature, and conducting heat in and out can be an added problem.  Early days I feel.

Another element to this discussion is the quantity of water needed. If a house uses a lot of hot water, then the losses due to cylinders and pipes may be a relatively small proportion of the total, so HP-heated DHW would be advantageous.  It is ironic that frugal users score badly for overall efficiency.  If water use is low. Point-of-use (COP=1) might be the best option.

Whilst large houses surely use enough water to warrant heat pump heating, it is these that where the biggest challenges arise from large distances distributing water.  I feel that this is where research is needed.

I’m not really sure how I actually sit on this whole topic, but we seem to still be in the ‘finding out’ stage with heat pumps… finding out what is most viable and most practical, so we need to try things out on all fronts.   I can think of two small houses right now that could benefit from ASHPs feeding the existing radiators (with some size tweaking), and it would seem disproportionally expensive and complicated to add DHW to the install. One has an electric shower only.. no bath.    I can also think of older houses that really need the whole hot water distribution pipework renewing to save energy. This could be very disruptive and expensive.

On the other hand, I can think of more situations where a hot water cylinder is surely a worthwhile thing.  If nothing else, houses with any form of Solar heating… this needs storing somewhere!


Back to the actual heat pump, there is a thing called a de-superheater.  The concept has been around since heat pump started.  In essence it’s a heat-exchanger fitted to the compressor hot outlet pipe.   In simple terms, you can get say 10-15% of the heat at a higher temperature whilst also space heating or cooling.   Ecoforest offer this feature in their HTR version of GSHP.  I hope other manufacturers have this offering too.   In the past, I have felt it a case that manufacturers are driven by costs, and added manufacture cost are not welcome, but as a customer, this would be a feature I would like.  Systems with integral cylinders should be benefiting from relatively minor changes. I’m not aware of it, but maybe they are starting to do it already.    I am hoping that we see more developments like this.

For now though, I think more focus is needed on pipe distribution.   This should be basic good housekeeping.

There is a bit more about the basics of DHW here https://heatpumps.co.uk/types-of-heat-pump/domestic-hot-water-dhw/

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