Hot Water Cylinder Mixing

Another DHW rambling thread.

It’s almost 2 years since I’ve owned my Mixergy cylinder and I “think” I’ve only just cracked the whole gift of mixing. ie, heating the whole cylinder hotter than you actually need the supplied usable water to taps to make it last longer.

The lightbulb moment came thanks to this mixing calculator I found.…

I took the liberty of putting some results into a spreadsheet (see attached) so I could understand it myself.

I chose 10C cold temp as that is what i’m seeing now in December and chose 41C mixed as that seems an acceptable temp for showers and doing the pots in the sink.

Down the left are various stored temps in the cylinder and on the right you’ll see the amount of usable litres at mixed temp for various cylinder sizes.

So I can use this now to work out how much usable water I need in a full cylinder at worst case. ie, we all come home from a muddy walk and we need 4 x 10 min showers straight after each other, with no recharging.

  • 60L - 10 min Shower 6 litre/min
  • 60L - 10 min Shower 6 litre/min
  • 60L - 10 min Shower 6 litre/min
  • 60L - 10 min Shower 6 litre/min
  • 12.5L - Pots in sink
  • 12.5L - Pots in sink
  • 30L - 12 hour losses
  • 295L - TOTAL

So if I could use all of my 250L Mixergy to accomodate all that, i’d need to raise that to around 48C stored temp for 296L usable at 41C.

The Mixergy is great in that you do get almost all 100% at the stored temp due to its tech. If you have a standard cylinder and coil then you’d need to account for the natural mixing as you get towards the bottom of the cylinder.

Now as this is coming from a heat pump angle and primarily performance, we know that COP takes a big tumble when you start going past 45C. You lose about 0.4 COP heating from 45C up to 50C alone.

I probably need to dig around my data and see the COP losses heating beyond 50C.

So if you can fit one, for performance, are you better with a 250L cylinder that you’ll only heat to and store water at 45C than a 200L cylinder heated and stored at 60C for the same amount of usable mixed temp water?

Hope that makes sense and i’ve got it right.

For the sake of 30cm extra height (200L to 250L), it seems a no brainer if you can? Doesn’t it?

My Mixergy skews the way I think about water cylinders, so keen hear from those with normal cylinders if this makes sense.

I prefer two DHW runs a day; 1am and 1pm ready for morning showers and then everyone coming home from school / work etc. I prefer this as you will also getter better heat pump performance letting the cylinder empty down than doing constant small top ups.

So this mixing process for me is understanding what target temp can I choose that is “just enough” to cover what I need whilst at the same time give me the best heat pump performance. It’s a fine line. It’s almost like “range anxiety” for water

Obviously, if you don’t care too much about performance and you want tons of usable hot water, just go as hot at the heat pump will allow. You might be on an off peak tariff, so price trumps performance.

Note: I have an adjustable thermostatic mixer valve on the hot outlet of my tank. This allows me to choose the output temperature of the hot water to all taps/showers(around 41c). So you can have the whole tank hotter whilst still allow cooler outlet water. Ie really hot from PV in summer.

If any of this thread is way wrong, please let me know. Always keen on learning.

I hope to include all this in a new energy-stats blog post about DHW and heat pumps in the new year.


The logic is all sound. I would advise:

… increase the required storage temperature to account for the offset between the inlet and outlet temperatures that often occurs…twice in the case that you also have thermostatic mixers on the shower/bath/sink etc…and prevent the hot water pipework from ever being pasteurised.

These whole house thermostatic mixers were popular down under years ago. You’re better off with one at each critical (shower/maybe bath) point of use - and none all but a reasonable storage temperature for the sinks.

You may find that you “run cool” later if you don’t have that central mixing valve in place as the water to the final mixer at the shower is then hot enough for it to maintain setpoint.

Perhaps the other thing to note is that if you do have the 4* showers because dogs winter etc then this can take long enough that you can reheat a decent amount of water in that time period (assuming that you’re using a coil nearer the base of a cylinder rather than stirring it all through a plate)

May the flow rate be reduced for the hot water cycle? Or does low and slow not work so well for DHW?

It would, but some ASHP controllers (like my Samsung) are not set up to limit heat input rate on DHW. The ASHP inlet temp is whatever leaves the hot tank coil, so if you have a smaller than optimum coil area in the tank the ASHP can rapidly wind itself up to give a high LWT (and thus poor CoP) :face_with_diagonal_mouth:.

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Low flow rate is not the same as low heat output (ie low compressor mode).

I have flow rate at 100% on my runs but still how lower heat output.

Watts heat output is just a calculation of mass flow.

heat transfer (W) = specific heat (J/kg.K) x flow rate (kg/s) x DT (K)

Purely for DHW cylinder, is there evidence to suggest whether a lower flow rate offers the heat more/less time to transfer? I’m assuming there needs to be a balance between getting the heat round the circuit and allowing it to be extracted from the coil into the water. Are heat pumps intelligently adjusting the flow rate to maximise the DT or just pumping at a default flow rate?

If there is no urgency to complete the DHW cycle in a limited time, (as the house heat loss is not an issue during set back hours) can I save electricity and/or energy by configuring the DHW cycle to run at a lower flow rate and lower temperature.

EDIT: Googling the subject and coming across “dt” (elapsed time) more than “DT” (temperature) :man_facepalming:

Did find this: 008-012-pme0917_Siggy.pdf (

I haven’t checked the documentation for my heat pump yet. Perhaps it includes information on whether the flow rate is dynamically adjusted based on the Delta T and/or whether an installer/homeowner may set limits/targets.

Looking at the flow rate, I am clearly focusing on the wrong aspect with respect of reducing electricity consumption. Am I correct in thinking it’s the compressor consuming the most electricity, rather than the pump?

The high peaks in flow rate do not appear to have a significant impact on electricity consumption, unless the measurement frequency or smoothing disguise the impact. The power hovers at just over 1kW for the last 30 minutes of the DHW cycle.

I am interested in whether reducing the aggressiveness of the increase in flow temperature in the first twenty minutes of the DHW cycle could save electricity, at the expense of it taking longer to heat the tank to the desired temperature.

With regards to your original post though, I’m a fan of only heating what I’m about to use. The heating higher to make it last longer thing feels like a compromise. I would rather have a smaller tank (than the size MCS suggests that ignores actual occupancy) to minimise having to heat more than we normally consume.

That looks like a neat solution.

My approach uses a separate 140 litre tank that is heated to 80 deg C then a heat exchanger coil to transfer heat to the DHW tank.

The problem with DHW analysis is that the process is unsteady-state - things like physical properties (both inside and outside the tank coil) and heat transfer coefficients (in the ASHP and in the tank) are changing as the temperatures increase, making the maths a bit trickier.

The slightly better htc at a higher tubeside flow is probably cancelled by the extra pumping power, so net no difference.

Personally I’ve found that I can reduce DHW operating cost by heating the tank is a couple of short (15 minute) bursts, with 15 min rest between. This seems to decrease the maximum ASHP temperature reached in each burst (hence better CoP), I assume because the tank temperature gets more time to equalise (through convection) during the rest period, so gets to the setpoint temperature quicker.

Whatever, you are right about relative power requirements of ASHP and circulating pump - your compressor probably consumes 20 times the power of the pump.


Unless your doing it on a cheaper rate, i have 0.09 p/kWh at night and then its 0.30 p/kWh during the day.

I can now run my heat pump upto 75 degrees flow, i used to store the DHW around 50, but now taking it to 60-65.

I also have 450l store, than can be part of my system or not depending on my HA automation’s. Again heating this to 50, with my old HP on the cheaper rate, is rubbish for my SCOP but gave me 2 to 2 1/2 run on my heating at 32 degrees flow.

With the new HP, i could take this to 70 degrees, need to work at how much that would cost me on the cheaper rate, how long it would run my heating for before depletion and then the heat pump takes over.

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Will be having our old cylinder replaced with a custom one from Newark next month. Will be interesting to compare how much better the significantly larger coil surface area makes on COP. It is twenty litres larger though so sadly won’t be suitable for a perfect comparison.

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