How much hot water do I need?

[Adsibob]

My plumber is telling me I need to allow 100L per person per day (implying 400L tank), whereas my architect is telling me I should go for 300L because plumbers always over-specify.

 

Our water usage pattern is as follows:

The way things will be in the immediate future:

• we run one bath for both kids to share every evening - the bath we will have requires approx 125L of (blended water); 
• we each shower for about 12-15 minutes in a fairly good rain shower, usually in the evening but occasionally in the morning. No idea of water usage for that.
• once or twice a week, I run a large bath (250L of blended water required), instead of the daily shower.

 

In the future, things for us adults will stay the same but I imagine the kids will eventually want to bathe separately, so that will be an extra 125L per day of blended water and when they are teenagers I'm assuming showering will replace bathing.

 

So what indirect unvented tank should I go for: 300L or 400L? Struggling to understand how much I should plan to heat on an everyday basis and how often I should use the "extra hour" feature. For example, would a 300L tank suffice as long as for my once or twice weekly giant bath I used the extra hour feature. How does washing up impact the above? We mostly use a dishwasher, but there are still large pans to do in the sink etc.

[SteamyTea]

6 minutes ago, Adsibob said:

we each shower for about 12-15 minutes in a fairly good rain shower, usually in the evening but occasionally in the morning. No idea of water usage for that.

Measure it, only a jug and a clock.  If you want to get posh, take the temperature as well.

 

How are you going to heat your water, system boiler, ASHP?

[Adsibob]

17 minutes ago, SteamyTea said:

Measure it, only a jug and a clock.  If you want to get posh, take the temperature as well.

 

How are you going to heat your water, system boiler, ASHP?

Unfortunately, I can't actually measure it as the system isn't installed yet. We are completely re-doing our house. Part of that includes upgrading our water supply with Thames Water. They've told us peak performance with the 32mm pipe that they are upgrading us to (which will also be upgraded all along to our house) is going to be 1.3 litres per second. I find it hard to believe we will actually get anywhere near that as 78L per minute sounds a lot and as they haven't told us this in writing I'm assuming it is going to be much closer to 30L a minute. The rain shower we'll be installing is rated at 24 l/min at 3 bar. Hopefully we will have 3 bar most of the time (still debating whether to install an accumulator). So assuming the shower uses all 24L of it's capacity, that 288L of showering water in 12 minutes. But how much of that is hot?

 

The system is a gas boiler.

[IanR]

1 hour ago, Adsibob said:

My plumber is telling me I need to allow 100L per person per day (implying 400L tank), whereas my architect is telling me I should go for 300L because plumbers always over-specify.

100l per person per day seems high for hot water usage. I thought 50l per person was considered high usage. But, perhaps that's a rule of thumb he uses for sizing cylinders. After all you're not going to heat the water once a day and then use it all, your heating system will be topping it up though out the day.

 

Should you not be sizing for the capacity of the house, rather than your particular usage? Also, falling in line with the 2022 regs seems a good idea, ie. sizing the UVC for a low temp heating system, avoiding having to refit later.

 

If 4 adults is the capacity of the house (3 Bed??), and a low temp heating system is allowed for, then your plumber's 400l tank sounds about right. Maybe you could go a bit smaller, but if there's space, bigger is better - just don't run it hotter than you need to.

Edited September 28, 2021 by IanR

[hendriQ]

Good luck @Adsibob, this sounds tricky. I always find that if you ask two plumbers the same question, you will get three different answers!

[Iceverge]

As @IanR said just get the biggest one you practically can. You'll have lower losses and the boiler will be less prone to cycling. 

 

We have a 300l immersion heated to 65-70deg overnight ( cheap electricity)  with very similar usage patterns to yourself and it's fine with no daytime topup. You have the option for day heat also so i reckon anything north of 200l will be fine. 

[Adsibob]

58 minutes ago, Iceverge said:

As @IanR said just get the biggest one you practically can. You'll have lower losses and the boiler will be less prone to cycling. 

 

We have a 300l immersion heated to 65-70deg overnight ( cheap electricity)  with very similar usage patterns to yourself and it's fine with no daytime topup. You have the option for day heat also so i reckon anything north of 200l will be fine. 

So this is another area of confusion for me. I was chatting to the plumber about the setup and asked him what temp to set the UWC to and he said 50C. I thought that must be wrong (and googling I now see that anything below 60C risks legionella). Maybe I misunderstood him but he seemed to be implying that if I had it at 60C, let alone 65C, it would risk scalding and googling that he seems to be right. So now I'm thoroughly confused. 

Again, this is just an area where everyone seems to have their own opinion/view.

 

I also don't understand why bigger cylinder implies lower losses. Looking at Telford Tempest's figures for heat loss, is actually suggesting the opposite in respect of losses:

Heat loss for a 300L tank: 2.32kwh/24hrs

Heat loss for a 400L tank: 2.58kwh/24hrs

 

And I also don't understand why you say a smaller cylinder is more prone to cause the boiler to short cycle.

 

 

[SteamyTea]

18 minutes ago, Adsibob said:

I was chatting to the plumber about the setup and asked him what temp to set the UWC to and he said 50C. I thought that must be wrong (and googling I now see that anything below 60C risks legionella).

Most UVC don't mix the water in the cylinder with the water you use.  Basically just a pipe coilled up that the cold, mains, chlorinated water passes though while picking up energy from the heated water in the cylinder.

Non of us, who have looked, have yet found a proven case of legionella disease caused by DHW, anywhere in the world.  It is a much more complex organism with a very strange lifestyle/life cycle.

18 minutes ago, Adsibob said:

I also don't understand why bigger cylinder implies lower losses

I think the context was that a larger cylinder can store water at a lower temperature for the same energy storage.

18 minutes ago, Adsibob said:

Heat loss for a 300L tank: 2.32kwh/24hrs

Heat loss for a 400L tank: 2.58kwh/24hrs

This again has to be taken in context.  If you loose an extra 0.26 kWh (not kwh) for an extra 200 litres stored, then as a fraction, the losses are lower.

There is an odd methodology used in the heat loss calculation of DHW, so take it with a pinch of salt anyway.

Edited September 28, 2021 by SteamyTea

[SteamyTea]

4 hours ago, Adsibob said:

Unfortunately, I can't actually measure it as the system isn't installed yet.

Are you staying in a half finished project without water at the moment?

If not, measure what you use where you are currently bathing.  Your bathing habits are not going to change much.

[Adsibob]

44 minutes ago, SteamyTea said:

Are you staying in a half finished project without water at the moment?

If not, measure what you use where you are currently bathing.  Your bathing habits are not going to change much.

We are staying in an old rental property down the road. The showers aren't great and we miss more powerful showers. The baths at our proper house are bigger than the ones here, so it's really chalk and cheese. But either way, this still hasn't answered my question as to what formula one uses to convert hot water in the cylinder to blended bathing water. I appreciate it depends on the temperature of the cold water, but to get a rough idea, how many 125L showers and 250L baths would I get out of a 300L cylinder? 

[Adsibob]

51 minutes ago, SteamyTea said:

Most UVC don't mix the water in the cylinder with the water you use.  Basically just a pipe coilled up that the cold, mains, chlorinated water passes though while picking up energy from the heated water in the cylinder.

I'm not sure this is right. Maybe you aren't expressing yourself clearly, but clearly the water that is in the hot water cylinder does end up coming out of the shower (at least that's what this says: https://www.plumbnation.co.uk/blog/benefits-unvented-hot-water-cylinders/). So I rather there was zero risk of legionella in it.

[ProDave]

4 minutes ago, Adsibob said:

I'm not sure this is right. Maybe you aren't expressing yourself clearly, but clearly the water that is in the hot water cylinder does end up coming out of the shower (at least that's what this says: https://www.plumbnation.co.uk/blog/benefits-unvented-hot-water-cylinders/). So I rather there was zero risk of legionella in it.

I think @SteamyTea must have been confused on this occasion and was thinking of a thermal store not a UVC.

 

But a UVC fed from treated mains water does not pose a legionairs risk even without high temperature cycles.

[SteamyTea]

4 minutes ago, ProDave said:

I think @SteamyTea must have been confused on this occasion and was thinking of a thermal store not a UVC.

Yes I was, been a long night at work.

 

I cant even work out how much water is needed to cool stored water down to an acceptable shower and bath temperature tonight.

 

Time for bed.

Edited September 28, 2021 by SteamyTea

[Adsibob]

So shall I get a 300L cylinder or strengthen my loft floor to have a 400L one. Strengthening the floor might not actually be necessary - SE is looking into it (at his usual snail's pace).

[SuperJohnG]

If you have a gas boiler and can heat water easily to 60+ degrees then 300l will be adequate as you blend down to whatever is acceptable to you which is usually around 48-50 degrees. Which will give you more than 300l water. 

 

If you had an ASHP then 400l would be better suited as yiu would want to store it at thst 48 degrees. 

 

If you want to cover all bases and space and cost are not an issue then go 400l. 

[dpmiller]

and if you're intending to have lots of longish showers, consider adding a drain heat exchanger (WWHR) to the mix.

[Iceverge]

16 hours ago, Adsibob said:

We are staying in an old rental property down the road. The showers aren't great and we miss more powerful showers. The baths at our proper house are bigger than the ones here, so it's really chalk and cheese. But either way, this still hasn't answered my question as to what formula one uses to convert hot water in the cylinder to blended bathing water. I appreciate it depends on the temperature of the cold water, but to get a rough idea, how many 125L showers and 250L baths would I get out of a 300L cylinder? 

 

The sums aren't too involved. Lets look at the heat loss from the cylinder first. 

 

 I reckon that the average temperature  of an UVC is 30 deg when you stop getting useful hot water out of the tap. It'll obviously be higher at the top of the tank and lower and the bottom. 

 

Formula for heat energy stored in water is  ( temp difference before and after) X  (4.2) X ( kg of water) = Energy in kJ. 

 

Take stored water at 70 deg and a 300l ( also 300kg) tank. 

70-30 = (40 deg difference X 4.2 X 300kg ) = 50400kJ 

Divide by 3600 for kWh = 14 kWh of energy stored. 

 

Now if you have a 400l tank at 60 deg

60- 30 deg =30deg temp difference

30 X 4.2 X 400L = 50400kJ = 14 kWh.

Exactly the same useful stored energy. 

 

 

Take the two cylinders at a surface area of 2.82m2 and 3.35m2

 

Multiply an assumed u value of 0.5 for both X ( temp difference to room at 20deg) * Surface area = heat loss in Watts. 

 

for the 300l  0.5X50x2.93 = 73 Watts

 

for the 400l 0.5X40X3.44 =69 Watts 

 

Less energy lost by the larger tank as the decreased temperature more than compensates for the increased surface area. 

 

 

 

Now if you want a 130l bath at 40 deg. Your incoming water temp is 10 deg. Effectively you need to get the energy equivalent of heating 130l X 30 deg out of your tank or

 

130l X 30deg x 4.2 /3600 or 4.55 kWh of energy. As your 300l tank at 70 Deg stores 14kWh you could get 3 Baths  before you had used all the hot water. 

 

 

However if you opt for the 400l tank and turn it up to 70 Deg you have 18.7kWh or 4 baths. 

 

In short. with the bigger tank you have the choice of storing more energy or having less energy losses for the equivalent energy stored.  

 

 

 

 

 

 

 

 

 

 

[Iceverge]

 

 

 

 

I've boiled (lol) it down to the above rough formula. 

[Adsibob]

@Iceverge thanks for doing those sums, much appreciated. A couple of assumptions I didn't quite follow:

Why would the stored temperature in a vertical tank be 70C? Isn't that going to cause scalding? I thought recommendation was 60 to 65 but even that is on the high side and could potentially scald a toddler.

Why would the stored temperature in a horizontal tank be lower? 

57 minutes ago, Iceverge said:

Your incoming water temp is 10 deg. Effectively you need to get the energy equivalent of heating 130l X 30 deg out of your tank

I'm not sure i follow this. I appreciate you are trying to calculate the "energy equivalent", but I think you are you assuming that the energy required will be the same as the energy required to convert 10C water to 40C water. But in actual fact, won't the system mix X litres of hot water with Y litres of 10C water to make 130L and the issue here is we don't know what that ratio is? Or maybe we are talking about the same thing in different ways.

 

There are a couple of calculators here that appear to answer my question: https://www.engineeringtoolbox.com/mixing-fluids-temperature-mass-d_1785.html

 

If I am using the calculator correctly, the ratio of COLD 10C water to HOT 60C water required to make 40C water is 10 to 15

 

So for a 130L bath at 40C one would need 52L of cold water and 78L of 60C water.

 

If we increase the incoming hot water temperature to 65C, the ratio falls to 10 to 12, meaning that one would need 

 

59.091 litres of cold and 70.909 litres of 65C water.

[Iceverge]

2 minutes ago, Adsibob said:

Why would the stored temperature in a vertical tank be 70C? Isn't that going to cause scalding?

 

You should have a thermostatic mixing valve (TMV) to prevent this in any case if you are planning on storing water much above 50deg. Like you say scalding isn't nice.

 

3 minutes ago, Adsibob said:

Why would the stored temperature in a horizontal tank be lower? 

You've lost me. I was assuming a standard vertical tank throughout. 

 

15 minutes ago, Adsibob said:

I appreciate you are trying to calculate the "energy equivalent", but I think you are you assuming that the energy required will be the same as the energy required to convert 10C water to 40C water.

 

 

It is. 

 

There was 130 Litres sitting in the pipe outside your house at 10 deg and now there's magically 130 litres at 40 deg in your bathtub. 

 

 

[Nickfromwales]

22 hours ago, SteamyTea said:

I cant even work out how much water is needed to cool stored water down to an acceptable shower and bath temperature tonight.

 

Time for bed.

 

23 hours ago, SteamyTea said:

If you loose an extra 0.26 kWh

How do you loosen a unit of energy?

 

 

 

 

That'll teach you to pick me up when I'm head-fried and put the wrong kWh into a kW sentence 

[Adam2]

On 29/09/2021 at 15:30, Iceverge said:

 

 

 

 

I've boiled (lol) it down to the above rough formula. 

 

Thanks this is incredibly useful

 

But it does worry me ? as implies 2 x 10 min showers at 12L/min would be roughly max we could get from a 400L UVC at 48 degrees. I guess we would probably use a bit below 40 - though my wife for some reason likes hot water torture! With an ASHP you wouldn't be getting much replenishment either in that scenario. I imagine (can check COP) that it make sense to heat the ASHP to say 55 in the morning (maybe using E7 or E10) to compensate if we find we're running short sometimes..

 

[Iceverge]

I cannot guarantee the formula is correct but it seems to be ballpark and i’m delighted someone found it useful.

 

You would need a 25kW combi boiler to keep up with that flow rate. A heat pump at say 8.33kW ( for easy sums)  output will buy you a meaningful amount of extra hot water. It would reduce your effective draw on the stored energy in the tank by 1/3 to 8l/min giving you 30 minutes of continuous showering. 

 

If you were to space them out over a period of an hour you could have 4 x 10 minute showers, In a situation of exceptionally high demand,like with guests, you could have the tank at a higher temp or engage a couple of immersions to give you even more capacity.

Edited October 21, 2021 by Iceverge

[Stones]

We have a 300l UVC heated by a 8.3kW ASHP, water heated to and stored at 48C, legionella cycle every 3 weeks to take it over 60C.

 

Similar pattern of use, 2 long showers in the morning, 2 in the evening. We can comfortably run a deep bath and run one shower from the tank before it runs cool. Showers flow at 16l/m.

 

We have never run out of hot water.

 

Your main challenges are recovery time, i.e. how long to heat the UVC up - with an ASHP it will take longer and therefore a bigger (300l) UVC makes sense. Previous experience from a rented property with a 200l UVC was it always ran cold after two showers, and being direct electric took ages to recover.

 

Second challenge is when you choose to heat the UVC - if you only heat during the night, e.g. E7, then ideally you'd be looking to heat all your water then and need to store enough until the next heating cycle, and therefore a bigger (400l+) UVC.  Top it up through the day and you can reduce the size.

 

And just to add another variable, are you planning solar thermal or PV?

 

 

 

 

[SteamyTea]

10 minutes ago, Stones said:

Showers flow at 16l/m

Isn't that about 60% greater than building regs suggest?