Hot Water Cylinder Size?

[ashthekid]

What would be the recommended size of the cylinder for a 6 bedroom, 6 bathroom property. Specifically with 4 showers, 2 bathtubs, 10 sinks. 
I would prefer to oversize it to be on the safe side but am curious to see what people suggest.

[Conor]

It's the number of people that matter, not the size of the house. 2 people, 20 people??? Plus usage patterns. We're building a 5bed, 4 bathroom, 2 utility house, but as there are just two adults and one child, 200l is enough for us.

Edited May 28, 2021 by Conor

[Andehh]

I'm assuming 500L for 6 bed 3 bathroom! Anything bigger becomes hard work to source I think. 

[Dan F]

I'm not sure that you can really go off house size or number of occupants, it comes down to various things that you haven't detailed:

 

- What are you using to heat hot water?

- What is the concurrent/subsequent shower requirement?

- How long is a typical shower and what flow rate are you expecting?

 

[SteamyTea]

This is an interesting problem and needs to be split into a number of parts.

The first one is what is your DHW heating type i.e. gas, oil, electric, heat pump, solar.  What are the expected power outputs of each.

The second is what your estimated DHW usage is, I think the 'standard' is 150 litres a day per person, which personally I find quite high, especially if the stored water is at 65°C.

The third is the 'diversity', which is really just knowing how many showers or baths would run concurrently, and how much time between consecutive baths or showers.  This will give you 'time to recharge' the cylinder depending on the heat source power and temperatures..

The fourth is 'what you expect'.  Do you like long showers, say 20 minutes, daily baths for everyone (would be my choice if my water and waste was not over 8 quid a tonne), or are you like many on here that take a 3 minute shower?

Only you can answer these questions.

 

Technically, you can fit two systems, of different sizes and power outputs, say one to do the four most used bathroom, and another for the little used ones, or split it 50/50.

[PeterW]

Limitation here will be flow and pressure not just stored water. No point having 600 litres of hot water if your showers are a bare dribble ... 

[PeterW]

22 hours ago, ashthekid said:

What would be the recommended size of the cylinder for a 6 bedroom, 6 bathroom property. Specifically with 4 showers, 2 bathtubs, 10 sinks. 
I would prefer to oversize it to be on the safe side but am curious to see what people suggest.

just to add to this, rough sizing is here 

 

 

I would be going for tandem 300 litre tanks with a switch over valve to take it from 300 to 600 capacity assuming you have the space and appropriate heat source 

[ashthekid]

I’m not sure we would have room for dual cylinders to be honest but I do like the idea of that. 
ASHP heating of DHW and UFH with addition of Solar PV with the Eddi diverter to provide any unused electricity to the hot water cylinder directly. 
Occupancy wise there will be 4 adults and one child. Mainly showers only to be honest, only baths for the child but those are only small shallow baths so not your normal adult bath usage.
The plumber has suggested a 300l which is now sounding like an inadequate size after what has been said on here. Perhaps a 500l would be sufficient capacity to safeguard myself, knowing that on the odd occasion demand would be higher and somebody would just have to wait a little bit before having a shower. 

[SteamyTea]

1 minute ago, ashthekid said:

ASHP heating of DHW

There is a risk that you set the temperature too high to overcome the capacity limitations.  This will cause the built in resistance heater to kick in, and your CoP will plummet.

One way would be to fit the two main bath bathrooms with additional inline heaters, these will only start when the DHW temperature goes below a set point, say 38°C.  Most of the time they will not be needed.

The trouble with one, large cylinder, is that you ae always heating a large cylinder.  A large cylinder will have temperature gradient, with the base being close to the mains temperature and the top close to the temperature that you set the ASHP to (say 48°C).  This will give a mean temperature somewhere between the two.

You can get around this by circulating the water in the cylinder with a pump, though I do not know anyone that has done this.  That way, you can get close to the full capacity.

 

On the plus side, a larger cylinder has lower percentage losses than than a smaller one (volume to surface area is better), but a well insulated system, in a well insulated cupboard, means these losses are not that significant.

 

Strange no one has mentioned Sunamp.

[ashthekid]

That Sunamp setup looks very interesting and a perfect solution. Essentially like a combo boiler with instant hot water with no cylinder and looks like you don’t lose any water pressure which is a bonus.
Does anyone have any experience using a Sunamp system? Looks like it works well with Solar PV. What’s not to like. 
Is it expensive?

 

Could we have a Sunamp system alongside a backup 300l cylinder perhaps for when we have people staying over knowing that water demands will increase temporarily?

Edited May 29, 2021 by ashthekid

[jack]

1 hour ago, ashthekid said:

That Sunamp setup looks very interesting and a perfect solution.

 

Search the site for "sunamp". There are a LOT of discussions about it!

[PeterW]

2 hours ago, ashthekid said:

Essentially like a combo boiler with instant hot water with no cylinder and looks like you don’t lose any water pressure which is a bonus.

At least twice the price of a similar standard UVC, nothing like a combi boiler as it only produces hot water at 58°C until it is exhausted and I real idea of the state of the unit in terms of charge. 
 

2 hours ago, ashthekid said:

Looks like it works well with Solar PV

So does a pair of immersions for a lot less money. 
 

2 hours ago, ashthekid said:

Could we have a Sunamp system alongside a backup 300l cylinder perhaps for when we have people staying over knowing that water demands will increase temporarily?

Why not have a very simple setup and a pair of cylinders ..?  A pair of 300 UVC will take up the same floor space as a Sunamp and a UVC. Telford will also custom make any size you want  too, so if you want it tall and slim they will do that too. 

[PeterW]

8 hours ago, ashthekid said:

The plumber has suggested a 300l which is now sounding like an inadequate size after what has been said on here.

Ask him how he calculated it ... 

[Dan F]

Not everyone is a fan of them and they don't work for baths, but WWHRS units (e.g. showersave) can make a 300L give you the same number of showers as a 400L+ tank and will also save energy.

 

Downside is they are around £400 each, and if all four showers get equal use and aren't near each other, you could need up to four of them. 

[Nick Laslett]

2 hours ago, Dan F said:

Not everyone is a fan of them and they don't work for baths, but WWHRS units (e.g. showersave) can make a 300L give you the same number of showers as a 400L+ tank and will also save energy.

 

Downside is they are around £400 each, and if all four showers get equal use and aren't near each other, you could need up to four of them. 

Thanks @Dan F, never heard of WWHRS, but looks like a very simple solution for a common problem. 
 

I was surprised that it had not really been mentioned much on the forum. But I did a search and there are very few posts on this nifty solution. For a new build I can’t see what the catch is?

Edited May 29, 2021 by Nick Laslett

[Nickfromwales]

4 hours ago, PeterW said:

At least twice the price of a similar standard UVC

3x   

[Nickfromwales]

13 hours ago, ashthekid said:

Perhaps a 500l would be sufficient capacity to safeguard myself, knowing that on the odd occasion demand would be higher and somebody would just have to wait a little bit before having a shower. 

500L is deffo the minimum you’ll need if a heat pump is the heat source. Get one from Trevor@cylinders2go and mention the forum for preferential prices. He can arrange up to 4x 3kW immersions alongside the HP coil. 
Have it routinely set to 55oC for daily driving, but arrange the immersions ( +1 / +2 / +3 ) for guest boost when you expect higher DHW use. 
Heating the same 500L cylinder to 75/80oC will massively uplift its useful DHW sustain and tick all the boxes. 
A tandem UVC arrangement is far less favourable option imho, and bloody expensive by comparison. 

[SteamyTea]

2 minutes ago, Nickfromwales said:

Have it routinely set to 55oC

Starts to kill the ASHO CoP at  that temperature.

[Nickfromwales]

Just now, SteamyTea said:

Starts to kill the ASHO CoP at  that temperature.

Yup. I know. What temp do you suggest instead ? 

[SteamyTea]

Just now, Nickfromwales said:

Yup. I know. What temp do you suggest instead ? 

48°C

And ditch the 'disinfectant cycle'

Edited May 29, 2021 by SteamyTea

[Nickfromwales]

1 minute ago, SteamyTea said:

48°C

And ditch the 'disinfectant cycle'

Forgetteth’yea the above mention of PV and diversion?

Summertime DHW via PV top up for at least 6 months of the year, and then who’s gives a Foxtrot about the CoP in the winter? The OP states they need a practical solution for lumps of DHW, so a sensible approach is not to dip under 55oC as that would push them to a bigger cylinder. 
Your turn  

[SteamyTea]

7 minutes ago, Nickfromwales said:

Forgetteth’yea the above mention of PV and diversion?

Summertime DHW via PV top up for at least 6 months of the year, and then who’s gives a Foxtrot about the CoP in the winter? The OP states they need a practical solution for lumps of DHW, so a sensible approach is not to dip under 55oC as that would push them to a bigger cylinder. 
Your turn  

The OP will learn to use less DHW once it has run out a few times, or the bill comes in.

If we are to take climate change seriously, then taking a shorter shower i.e. turn the (expletive deleted)ing water off when you soap up, is a very small price to pay.

[Nickfromwales]

Not yet a practical answer to the OP’s question. 
Your reply is a bit ‘wider afield’ and better off in a dedicated thread. 
The OP mentions a HP ( a well established energy multiplier ) and PV pus diversion ( so harnessing an energy source that promotes responsibility, at least as much is reasonably expected from a domestic residential setting aka Joe Publique ) so best get them apples and apples in a line me old China mug ?. 
Saving the planet is one thing, but not getting clubbed to death by a blunt instrument because the water ran cold is ‘slightly’ more pressing.

6 bed 6 bath. And a thread titled “Hot water cylinder size?”  
Back on track please.  

[SteamyTea]

6 minutes ago, Nickfromwales said:

6 bed 6 bath. And a thread titled “Hot water cylinder size?”  

Why I suggested splitting the system into two, one for main use and the other of guest use.

The space issue is an interesting one, we hear it a lot.  Not sure what the answer is, maybe a smaller 'litre' like the US gallon compared to the proper Imperial one is the answer.

 

[Nickfromwales]

1 minute ago, SteamyTea said:

Why I suggested splitting the system into two, one for main use and the other of guest use.

The space issue is an interesting one, we hear it a lot.  Not sure what the answer is, maybe a smaller 'litre' like the US gallon compared to the proper Imperial one is the answer.

 

After 3 pints I may fall for that one. 
However, the cost of splitting the system is huge and the losses expanded, so the swords remain crossed my pastie-loving opponent.