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Hot Water Cylinder sizing and Mixing

The biggest change I encountered when moving to a heat pump is being constrained by the limits of a hot water cylinder.

For all my adult life I had owned a combi boiler.  All I have ever known is that when you turn on the hot tap, an endless supply of hot water came out.

With a water cylinder, water is no longer endless as the hot water needs to be preheated before use.

Gas boilers can produce 20-30kW of of heat and can heat hot water very quickly.

But a heat pump cannot create instant and unlimited hot water like a combi boiler can.

So when you use up all the stored hot water in the cylinder with a heat pump, you would have to wait until the heat pump heated more water ready for use.

How long to heat the water?

I have talked about hot water heating times in some of my previous heat pump blog posts, but I think it is worth reiterating again here.

How long this heating process take depends on a few things, but primarily how big your heat pump is (its kW output), how much water you want to heat up and to what temperature.

So the smaller the heat pump the longer it takes to heat the water.

Time in minutes to heat water (from 10C to 50C) using various power output sizes

3kW 5kW 7kW 10kW 12kW 14kW 16kW
50L 47 28 20 14 12 10 9
100L 94 56 40 28 24 20 18
150L 140 84 60 42 36 30 27
200L 187 112 80 56 48 40 36
250L 233 140 100 70 60 50 45
300L 280 168 120 84 72 60 54

As you can see from the chart, with a 5kW heat pump it would take 56 minutes to heat 100 litres of water from 10C (cold) to 50C.

With a 12kW heat pump that would only take 20 minutes for the same amount of water.

So even a large heat pump isn’t instant.  There is still a length of time required.

You only have to imagine sticking a 24kW or 32kW combi boiler on the end of the chart to see how we’ve become accustomed to instant water.

It would take less than 9 minutes for a 32kW boiler to heat the 100L of water.  Compared to the 94 minutes a 3kW immersion would take.

This is a great online heating calculator that would put your own numbers in to.

What Size Cylinder?

The usual ‘rule of thumb’ I was told for sizing a water cylinder for the home is somewhere around 40 to 45 litres of water per day per person.

MCS have a hot water cylinder selection document on their website that is worth a look at.

From my own personal experience over the last 2 years, I find that 45L per person recommendation to be way too low.

We are a house of 4 with 2 adults and 2 kids at secondary school.

4 x the recommended 45L would be 180L in total.

I can tell you without doubt, that would not see us through 4 showers one after the other, let alone a whole day.

The 4 showers scenario is a common occurrence in our house after work, school, gym, football practice etc etc.

Calculating how much hot water you might need

Since getting the water cylinder it has really made us think about how precious hot water is, especially as:

  • a) water takes a long time to reheat and
  • b) how much energy it takes produce the heat

You absolutely take all that for granted with a combi boiler.

It has also made me realise how much water we use each day.

For example, this might be a worst case evening when we all get home from work/school/gym.

  • 60L – 10 min shower at 6 litre/min
  • 60L – 10 min shower at 6 litre/min
  • 60L – 10 min shower at 6 litre/min
  • 60L – 10 min shower at 6 litre/min
  • 12.5L – Wash the pots in the sink
  • 30L – 12 hours of heat loss from the cylinder
  • 282.5L – TOTAL

I know our shower uses 6 litre/min as it came with a restrictor valve.  These simple little discs come in various sizes to restrict down to different flow rates.

Before we put one of these in, the shower was going through 10 litres a minute, so well worth looking at those.

On the Mixergy cylinder, I can also confirm this by looking at the usage graphs in the app and the online mixergy portal.

You could also find out how much your outlets are using by putting a bucket or something with capacity markings under your shower/tap and crank the water up to full whack.  Time it for 1 minute and measure how much at the end.  This will give you an idea of each outlets usage.

I absolutely recommend doing this if you’re thinking of moving to a cylinder, be that direct electric (for off peak tariffs or PV) or with a heat pump.

It is far better to be armed with this information beforehand than finding out you’ve had the wrong sized cylinder installed.

At the moment only the wife and I have a shower before work in the morning but I don’t think it will be long before the kids decide to do this too, so the above 282.5L total could soon be required twice a day.  8 showers a day?

That would be 565L of water in total based on the above (2 x 282.5L).

So you can see why I think that the 45L x 4 = 180L recommendation falls well short of the mark.

Real world family of four here, not just numbers on a spreadsheet.

Those of us with small 5kW heat pumps and high hot water demand really feel the pain of slow recharges.

So at the moment, I am currently doing two full recharges a day.

One overnight on a cheaper tariff and then another in the afternoon when everyone at work/school, hopefully on PV power, but more likely from batteries (from overnight and/or PV).

So that when we get up and when we get home from work/school we’ve got a full cylinder waiting.

I currently have a 250L Mixergy cylinder but my daily requirements is heading towards 2 x 282.5L (565L) of water.

How can that work?

Storing water hotter than required and mixing it down

In most cases you only need around 38C water for a shower and only a bit over 40C for washing pots in the sink.

You will scald your hands anywhere close to, or above 50C.

But that doesn’t mean you can’t store water in the cylinder at hot temperatures.

You just have to make sure you have mixing valves on individual outlets where required so water to taps are not too hot, especially if you have kids in the house.

Personally, I have a thermostatic mixing valve (TMV) in front of our cylinder so I can set what temperature I want ALL the water that comes into the house to be.

So the cylinder might be full of 60C water, but it only comes out at 45C to all taps, or whatever temperature I sent it to, because of the mixing valve.

It has been 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 the taps to be, to make the contents of the cylinder last longer.

The lightbulb moment came thanks to this mixing calculator I found.… 2

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

I think the chart helps highlight the advantages.

Update: An amazingly helpful follower on Twitter (steve_dvo) came up with the formula.

(MixTemp – ColdTemp) / (HotTemp – ColdTemp)

e.g. if ColdTemp = 10, HotTemp = 60, MixTemp = 41

(41 – 10) / (60 – 10) = 0.62 [i.e. 62%]

Finally cylinder size x 1.62

So 150L x 1.62 = 207L of usable water at mixed temp

So feel free to create your own spreadsheets etc.

Usable Litres at Mixed Temp
Cylinder Hot Temp Cold Temp Mixed Temp % Hot % Cold 150L 200L 250L 300L
42 10 41 97% 3% 155 206 258 309
43 10 41 94% 6% 159 212 265 318
44 10 41 91% 9% 163 218 272 326
45 10 41 89% 11% 167 223 279 334
46 10 41 86% 14% 171 228 285 342
47 10 41 84% 16% 174 232 291 349
48 10 41 82% 18% 178 237 296 355
49 10 41 79% 21% 181 241 301 362
50 10 41 78% 23% 184 245 306 368
51 10 41 76% 24% 186 249 311 373
52 10 41 74% 26% 189 252 316 379
53 10 41 72% 28% 192 256 320 384
54 10 41 70% 30% 194 259 324 389
55 10 41 69% 31% 197 262 328 393
56 10 41 67% 33% 199 265 332 398
57 10 41 66% 34% 201 268 335 402
58 10 41 65% 35% 203 271 339 406
59 10 41 63% 37% 205 273 342 410
60 10 41 62% 38% 207 276 345 414

I chose 10C cold temp as that is what I saw in December and January coming into the house from the mains and chose 41C mixed as that seems an acceptable temperature for showers and doing the pots in the sink as described in an earlier section.

Down the left of the table 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 if we go back to my 282.5L requirement I could get 285L of usable 41C water if I heated the whole of my 250L cylinder to 46C.  So gaining 35.5L of usable water.

If I was to heat the whole 250L to 55C I would get 328L of usable water at 41C.  Gaining 78L.

The Mixergy is great in that you do get almost all 100% at the stored temp due to its tech. 

Note: it is very unlikely you will get every litre of a conventional (non-Mixergy) cylinder usable.  There will usually be a small portion of cold water below the coil at the bottom of the cylinder.  So bear that in mind with your calculations.

Why not just heat all the cylinder to 60C?

Honestly, this could be a very valid approach, especially if you only want to heat the whole tank once a day and even more so if you’re on a cheap overnight tariff or any tariff with lower/cheaper elements at certain times of the day.

If you heated all your cylinder to 60C it would have 345L of usable 41C water.  Gaining almost 100L of usable water.

The only downside of heating to very high temperatures with a heat pump is the efficiency really drops off.  And some heat pumps can’t even get this high.

You can read my detailed investigation into hot water performance with a heat pump here:

Mixergy Heat Pump Performance

But with certain electricity tariffs having cheaper and sometimes free times, heating the cylinder as hot as you can get and to hell with efficiency could well be the cheapest route to hot water.

A basic TL;DR summary from the article, COP (performance/efficiency) takes a big tumble when you start heating past 45C.

You lose about 0.4 COP heating from 45C up to 50C alone.

But if you’re heating the water overnight at a quarter of the price on a cheap overnight tariff, does it matter?

Another big performance tip from the article is to try and start your hot water runs with the cylinder as empty as possible.  Personally, i’m using the mixing calculator to find out what is the lowest mixed temp I can get away to satisfy the house demand, which in turn leaves us with an pretty empty cylinder ahead of the next hot water run.

Trying to eek out as much performance as possible.

What about Legionella?

Legionella is a bacteria that grows in water and if consumed or inhaled can cause a pneumonia like illness known as Legionnaires Disease.

The amount of cases in the UK of Legionnaires disease is very very low.

There are various ‘rules’ or ‘recommendations’ about legionella and stored water.

In the first revision of this article a number of people replied to me about needing to “keep stored water at a minimum of 55C” or “all water must be stored at 60C”.

Which simply isn’t true.

Heat Geek has a fantastic in-depth article on Legionella that I recommend everyone reads.

Hot Water Temperature – Scalding and Legionella


I personally take the advice within the article and don’t worry too much about Legionella as I currently store my water at 48C and turn the tank over twice a day.

There is a legionella cleansing cycle available in the Mixergy app, but I have that set at just 51C and this runs at least once every 2 weeks.

Legionella – What is it and why does it matter?

Obviously though, these are my circumstances.  If you don’t use much hot water and it sits there for a long time, then do consider taking the appropriate precautions.

Summary and Closing Thoughts

My current approach is two big hot water heat ups to around 48C each day.  This gives us enough usable water in our 250L cylinder to cope with those 4 showers in a row events.

I do two runs to 100% because I want to avoid small “top ups”, which is primarily for performance reasons.

But if you only have physical space for a small cylinder in your house you may have no choice than to do top ups after each shower.  Cylinders can be as small as 120L or 150L.

By typing this article out many things have clicked into place for me.

Like most things in this renewable journey, I didn’t know any of this before I got the cylinder.

So I hope the info helps you in some way, especially if you’re looking to size your own cylinder.

A lot depends on how much water you use of course.  For us, between Sept 2023 and Jan 2024 hot water made up 33% of the heat we created with the heat pump.  But for some households, hot water might only be 10%.

From what I know now, I’d advise going for as big a a cylinder as you  have the physical space for.  Taking into account the mixing abilities talked about in the article.

Another reason is you can also use the water cylinder as a thermal battery, giving you the ability to store cheap/free energy in there.   Or even get paid to fill it up when the Agile tariff goes negative.

In those circumstances you’d want the largest store you could get.

I guess what the main gist from the article is to consider your hot water use and potential cylinder requirements before making the move to a cylinder, not afterwards.

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