Filling Bath using instantaneous Electric Water Heater

[mitch...]

My friend who lives in a flat (so cannot be gas connected) is pining for having baths instead of showers (a wimin thing maybe?) Problem is that their 125 litre electric (E7) boiler cannot fill a bath and so emptying till it runs cold then and boost switching it for another load are not really viable due to the time delay.

My question is are powerful (ie > 3kW) instantaneous electric water heaters suitable and viable (ignoring the eye-watering elec cost)?

Their supply has a 50A fuse which suggests ~10kW to play with, this relates to about 6 litres/min.

I've always been a bit puzzled why we don't see powerful instant elec water heaters much, it's either the powerful multipoint gas ones or, if electric, then slow immersion heater type boilers.

Perhaps the extortionate electricity cost would deter buyers?

[Jethro_uk]

I'm guessing the wiring required for *sustained* delivery of hot water over maybe 10-15 minutes is more chunky than you think ?

There might also be some sort of issue with needing more than one phase (since there's currently a thread on it ...) ???

Or, it's just habit. I learned long ago that "we've always done it this way" is the biggest single drag on progress - well ahead of economics or practicalities.

[John Rumm]

What capacity is the bath, and what temperature is the boiler/cylinder supplying the hot water at?

Since a 125L cylinder of water at 60 degrees, should be able to supply a moderate "bath full" when mixed down to temp with cold.

You get some inline ones that will do the job, so long as you don't mind the longer time to fill.

(IIRC Tim Watts posted previously about running such an arrangement at his place while renovating and before the CH was commissioned, and it worked well enough).

Probably because to get to the performance of even a low end combi would consume all of the electrical supply capacity for most properties.

If you want performance with electric heating, the go for an unvented mains pressure cylinder, and stick 3 immersions in it.

[Brian Gaff (Sofa]

What about bottled gas? Over in Spain they seem to use it to do this sort of thing using things that look a lot like those old Ascot mini water heaters on the wall with a mixer tap on the bottom. Brian

[Fredxx]

That was my thought. Is there any mileage in fitting a mixing valve to the hot water output to reduce/stabilise the temperature and run the tank at 90degsC?

If there is a cold feed then that would be the cold supply.

Or the suggestion above. Though that would make a good shower feed and fill a bath quickly!

[newshound]

Well there are plenty of simple instantaneous electric shower heaters from 6 kW upwards. That size can run off a 30A fuse/MCB via 4 mm^2.

Do they own or rent the flat? Unless it is *tiny* I would have thought there is no problem getting in a more normal sized domestic supply (80 or 100A) although obviously there is a cost.

[David]

This sounds like trying to fill a bath using an electric shower.

10.5 kW showers are readily available.

I can't recall ever having a modern 10.5 kW electric shower, but the older ones certainly wouldn't fill a standard bath with hot water in a realistic time frame.

You don't mention the size of the bath, but a quick search suggests that a standard 1700 * 750 bath holds about 220 litres so the hot water tank would presumably half fill that.

I don't think anyone yet has directly suggested fitting a larger hot water tank. I think John R more or less did.

I think the issue is that the hot water tank isn't big enough, and there isn't enough electricity supply to directly heat cold water fast enough.

The logical solution is to fit a bigger hot water tank or a smaller bath.

You need to be prepared for a fairly long wait for the larger tank to come back up to temperature, but again John R has made some suggestions there.

Cheers

Dave R

[Andy Burns]

Someone in another group has a "heat battery" I think stores off peak electricity into phase-change material, then heats water from it later, he seems happy with it, but do your own research ...

[ARW]

A different thought has occurred to me.

It's an E7 set up. Is the bottom element working or getting a full charge? Faulty/turned down stat etc.

I too would expect 125L of hot water to be plenty for a bath.

[RJH]

Anecdotally (from the 8kW shower I've got) instantaneous hot water won't work unless you've got a hugely powerful heater. The water will cool too quickly. Assuming this is a shower over a bath, try having a shower with the bath plug in . . .

As others have said, larger tank with bigger heaters.

[mitch...]

Thanks all for your ideas and advice. They are constrained by space so a larger tank is not possible (unless I put it in their lounge and call it steam punk art!). Yes both elements are pulling current. I'm going to investigate if I can fit higher power elements as, from memory, I think they're only 3kW and fitting bigger cable would be easy from their board.

[tabbypurr]

Increasing cylinder temp is likely to be your solution. 90 is excessive though, just try adding 5-10C & see how that goes first. You may run into the problem that recent immersions often trip out at not hot enough water temp. It also increases standing losses, significant if cylinder uninsulated.

If you went as high as 90C you would need to TMV it for safety, but something is going very odd if you need to raise it that much.

An instant inline water heater can certainly add to the heat delivered to the bath, though you probably won't need it once the cyl temp is increased. Worth calculating how much heat can be added in your fill time.

NT

[tabbypurr]

If there's one thing I can safely say it's that a 3kW limit is not the issue. Even 1kW would get you a bath full of scalding water per day, it just takes longer to get hot.

NT

[Cursitor Doom]

Where I live you have to improvise from time to time when there are fuel shortages. I had to directly heat a bath quite recently.. I can't recommend the method for anyone else to use unless they're really on the ball, but here's what I did just to get me by until the fuel tanker made a return appearance. I got one of those immersion heater elements they make for those large copper hot water cylinders. I manage to find a compact one that was

3kw but only 11" long; short enough to be able to dangle into the bath from a bamboo stick placed across the width of the bath and secured into place with zip ties. I ran some 15A cable to a nearby 3 pin outlet, filled the bath up and switched on. It's a good idea to chuck a thermometer into the bathwater so you can see how it's heating up rather than periodically sticking your elbow/foot/hand/penis into it, because it's a really bad idea to go anywhere near it while it's on. The bath concerned was larger than normal. I don't know the exact capacity of it, but it's bigger than average by some margin. It took about one hour and twenty minutes to get up to a nice, warm temperature. Provided you remember to switch off and isolate each time you want to go near it, you should be fine. Just make sure the element is submerged in the water at all times until you're ready to dive in. Top tip: remember to disconnect the power first!

[spuorgelgoog]

I'm not sure it would. It would run so slowly it would cool down before the bath was filled.

Anyway, it would hardly be instantaneous.

Owain

[newshound]

You can certainly get much more powerful elements with the standard

2 1/4 inch flange, a mate of mine uses 9kW ones that are designed for 3 phase but you can use them on a single phase with suitable cable and fusing.

[The Natural Philosopher]

most houses are main fused for 15kw

2x9KW is OTT

[John Rumm]

I sense some talking at cross purposes here. ISTM the OP described a storage system heated by immersion heaters. That not apparently being up to the job caused the query about if a instantaneous heater could be added to replace or augment the current setup.

[John Rumm]

I doubt its a power issue. It sounds like the thermostat on the immersion heaters is set to too low a temperature. Hence when you run the bath you need to use almost exclusively hot water to get a bath at the right temperature, and so only get the 100 ish litres of water.

If you increase the temperature of the stored hot water, then when running a bath you will need to add much more cold water to get the bath at a comfortable temperature. So the total volume of bath water will go up without changing the size of the cylinder.

If you read this:

It explains how you can work out what volume of stored hot water and at what temperature you need to store it.

[Fredxx]

I assumed anything over 50C ought to have a TMV. That was suggested in my post.

<snip>

An instant online heater will have no effect until the tank is empty and then won't have enough power to heat any flow of water unless 10kW or more.