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. https://spiraxsarco.com/resources-and-design-tools/calculators/water-mixing/water-mixing?sc_lang=en-GB…
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.