Instant hot water boiler for shower..??

[Bohobee]

Hi - have an outbuilding going up in my garden, after some deliberation with how to get hot water etc it now has it's own dedicated electric circuit to avoid any issues with overloading the current ring.... so I thought my issues were resolved and I could just get an instant hot water boiler to service the shower and 2 basins (not at the same time) ...however I just seem to have landed myself in another minefield of problems!! From what I am reading now an instant water heater cannot be used in conjunciton with a thermostatic shower... (and you try finding me a non-thermostatic one! )
Everything is now set up - plumbing in place shower bought, taps installed etc... just waiting on the boiler now!!
So can I please get help on my options.....
A) the ideal for me is that I CAN still go with an instant boiler.... is there any that anyone can recommend which WILL work as intended?? or has anyone had any success with them? I don't understand how they can be advertised as being used with a shower if they can't!! ?

B) 2nd best I can see so as not to waste all the work that has gone into everything so far is to get a hot water storage tank... not ideal as it will be wall mounted so can't be massive, plus I Was hoping for instant hot water out there as I don't have it in the house and would be useful to have, plus for the occasional use in the outbuilding don't want to be heating water when it's not in use.

c) i suppose I could get an electric shower and a small instant heater for the taps? This is a pain as it means swapping the shower which it may be too late to return, also means big holes in my freshly fitted cladding and means diverting the wiring (which is already all in place)
Is there a D ??????????????

Many thanks for any help/advice.
B

 

[muggles]

Can you give more details of the available power supply please? Maximum current draw on that circuit, single or 3 phase?

 

[CBW]

You can still get non thermostatic mixer showers (manual mixer). Which water heater do you refer to?

 

[Johntheo5]

Have you a link to your proposed heater. Assuming it's 9kw heater then that that will heat 5.16lpm from 15c to 40c from a15c mai s temp of 15c a electric shower switche's the power off at 48c or 3.9lpm .if your heater doesn't switch off until say 65c your thermostatic shower will mix 2.9LPM of HW at say 60c with 2.3 LPM of cold water to give you 5.2LPM OF 40C of mixed water so it should work as long as you keep a certain flowrate through the shower.
A hot tap will have to flow a minimum of 2.9 LPM to prevent the temperature rising g above 60c. You might have to install TMV. Except that you can set the heater to 40c the tap will then have to flow 3.9LPM to avoid exceeding 48c.

 

[Bohobee]

@Johntheo5 @CBW - ok - heater wise I was looking at something like this:
Electric water heater or Water heater
the only manual mixer showers I seem to be able to find are the type that attach to bath taps... can you send me a link to another type? I would ideally like to use the shower riser and heads I already have.. but if attaching them to the thermostatic mixer is going to cause problems maybe I can attach to a different type of mixer? but I don't know how to search what I need... i only seem to get thermostatic or bath mixer options.
Thanks!

 

[Bohobee]

Can you give more details of the available power supply please? Maximum current draw on that circuit, single or 3 phase?

erm... I'm sorry I don't know - all i know is I've had the electric board out to upgrade my supply to my house as it is an old property it was only a 60A supply but has been upgraded to 100A. The electrician is then running a big cable underground direct from where the electric comes into the house, round the side of the house and straight to the new building where there is then a distrbution board with circuit breakers..... hopefully that means more to you than me! the only things running off it will be a small heater (1kw) a fridge and a few lights/ tv plus whatever hot water option I go for.
Thanks!

 

[muggles]

erm... I'm sorry I don't know - all i know is I've had the electric board out to upgrade my supply to my house as it is an old property it was only a 60A supply but has been upgraded to 100A. The electrician is then running a big cable underground direct from where the electric comes into the house, round the side of the house and straight to the new building where there is then a distrbution board with circuit breakers..... hopefully that means more to you than me! the only things running off it will be a small heater (1kw) a fridge and a few lights/ tv plus whatever hot water option I go for.
Thanks!

OK you need to find this out before you can specify a water heater, as you don't want one that's too powerful for the supply being installed. Ask your electrician for this info.

 

[Johntheo5]

The referenced heater is 9.5kw with a minimum flow switch probably set to 3/3.5LPM and two thermal cut outs, one, operating at 57c, the other, manual reset, at 90c.
Be aware that you will only get a flowrate of 3.9LPM in winter at 40c from mains at 5c and 5.4LPM in the summer from mains at 15c.
The minimum flowrates from a hot tap at 57c are 2.6/3.2LPM winter/summer. It says not suitable for a thermostatic shower but I really can't see why not if the flowrate is above a minimum

9.5kw will draw a current of 41.3A so probably 10mm2 cable required, your electrician will advise.

 

[CBW]

ScrewfixToolstationVictoria plumbing (although I don’t recommend the latter company)

 

[oldbutnotdead]

Do you have an electric shower in the main house? If yes, having a similar item in the outbuilding wouldn't be ideal.
Your easiest solution will be a small unvented cylinder with immersion heater. Yes you lose the instant hot water bonus but you would at least get a decent shower out of the thing.
There are 3kw instant water heaters that would work for a handbasin if that's the sort of instant hot water you need but they are inadequate for a kitchen sink (far too slow)

 

[Bohobee]

Do you have an electric shower in the main house? If yes, having a similar item in the outbuilding wouldn't be ideal.
Your easiest solution will be a small unvented cylinder with immersion heater. Yes you lose the instant hot water bonus but you would at least get a decent shower out of the thing.
There are 3kw instant water heaters that would work for a handbasin if that's the sort of instant hot water you need but they are inadequate for a kitchen sink (far too slow)

No - just a cylinder in the main house which is partly why I thought it would be a bonus to have instant water outside for those times when we run out of water in the house and have to wait for half an hour to have a shower!
However I am realising that to get a decent shower I may just have to go for the same out there...
I am wondering about something like this if I go for the storage type...Water Storage heater

 

[oldbutnotdead]

50 litres is a bit marginal for a decent shower. I'd also be a bit wary of buying a large pressurised cylinder from a random Chinese supplier based in East Germany...if the immersion heater thermostat fails closed it'll make a right mess, pressurised cylinders can be dangerous.
The house cylinder, is it heated by the gas boiler? If yes and it's modern (foamed insulation jacket) then set dhw to be on during waking hours, it'll cut that delay right down.

 

[Johntheo5]

Better to buy one from a reputable manufacturer.
A 50L, effective volume 39L will provide a mixed vol of 58.5L at 40C in winter, 6 min shower at 10LPM, and will provide 70.2L in the summer, a 7 min shower.
Recovery time, it will take 1hr 15mins to reheat to 60C in winter and 1hr in the summer with a 2kw heating element.

 

[Bohobee]

@muggles it is a 20A cable running to the building

@Johntheo5 I forgot to say that at this stage unfortunately cost is a huge factor as I have run out of money! so as far as I can see a cylinder from main manufacturer is into about £300.... this is why I as hoping to just get away with the instant boiler at around £100 and this one from germany wasn't too much more....
@oldbutnotdead no gas in the house either - everything is electric :/
@CBW thank you for those, so you reckon if I was to swap the thermostatic mixer for something like that it would work better on an instant boiler?

Thank you all for your help!

 

[oldbutnotdead]

No such thing as a 20A cable, the current carrying capacity depends on cross section area, length, installation method and load type.
If the max current permitted is 20A you can forget all flash heaters except the small 3kw washbasin ones mentioned.
EDIT If £200 is a critical amount in your outbuilding build then you've been over optimistic with your budget
By now there is probably £10,000 or more invested in the thing, the phrase 'spoil the ship for a hap'orth of tar' springs to mind.
Is this going to be a revenue generating space or just a guest facility? If the former then a shoddy install will come back to bite you, if the latter the guests will have to cope with a trip to the house for a shower until you can afford to finish it properly.