Temporary use of electric 'instant' water heater

[JohnW2]

I may come to regret this, since, knowing this forum, I suspect that I may well attract comments/advice/'warnings'/whatever which are not actually answers to my question - but, nevertheless ...

... I am working on a house in which the only hot water is that from a couple of ancient 'over-sink' instant electric heaters, either or both of which may be on their last legs.

The situation will eventually be addressed, almost certainly by installing a combi boiler in the kitchen, but that could be many months, or longer, down the road, so I am considering temporary measures.

I'm therefore considering temporarily installing a 'plumbed-in' electric instant water heater in the kitchen, probably of around 10 kW. That would presumably work adequately for the taps in sinks, basins and baths (even if fair slow flow into a bath!), but I'm wondering if anyone knows whether it would also 'work' reasonably with mixer shower?

The bathrooms are immediately above the kitchen, and no water (hot or cold) is needed other than in those places, so pipe lengths will be relatively short. Use would only be pretty 'occasional', so the excessive cost of electrically heating the water would not be a major issue (and it would be no different from what is happening already, anyway)

In case Mr Jobsworth is around, there is already an (unused) 40A shower circuit present, which was installed 'just in case' an electric shower was ever needed (which I doubt it ever will), so I could use that circuit for a water heater - so that, since what I'm suggesting would require neither a 'new circuit' nor any electrical work in a bathroom, it presumably would not be notifiable, at least as far as Part P is concerned.

So, the question is really simply that of whether (and 'how well') such a water heater would satisfactorily run a mixer shower. Since I'm talking about a power similar to that of an electric shower, I would have thought it would 'work', but I'd be interested to hear other people's views (and, obviously, other people's experiences of similar arrangements, if there are any such people!) .. and please remember that I really am talking only of a fairly 'temporary' arrangement.

Kind Regards, John

 

[BlueLoo]

That's a high load for a long time which is not good for cable heat.
These things are not designed to be running for long periods. Or if they are, the cabling needs to designed to accommodate the resistance heat gain.

Ignoring the backflow, pressure, flow rates etc. involved in shower plumbing.

 

[flameport]

It could work, but the flow into the mixer for both hot and cold would have to be significantly restricted, and that is likely to cause problems in locating one which can work properly at very low flow rates.
If both shower and taps are needed, easier to just install a cheap electric shower as well as the water heater with an interlock so they can't be used simultaneously.

A 10kW instantaneous heater will be useless for a bath.

 

[JohnW2]

That's a high load for a long time which is not good for cable heat. ... These things are not designed to be running for long periods.

As I said, I suspected/feared that people would probably answer questions other than the one I asked

Who said anything about 'long periods'? I would not be contemplating filling a bath which such an arrangement, and wouldn't need to if a shower could be got to work to at least a usable extent - and the duration of water heating and the amount of water heated would then be no different from the situation with an electric shower of the same rating.

Or if they are, the cabling needs to designed to accommodate the resistance heat gain.

The way in which we size cables does not take duration of current flow into account - or, I suppose more correctly, assumes that the current will flow for an indefinite period of time. Hence, if a certain cable is, per standard calculations/tabulations deemed to be adequate to carry, say, 40A, it is deemed to be able to carry that current for an indefinite period of time - in other words, the current-carrying-capacity figures we work with are ones relating to contemptuous/indefinite flow of the current concerned..

You seem to be talking about a situation in which one loads a cable to beyond its "(continuous) current-carrying-capacity" - which is something which one really shouldn't do, even if the current is probably not going to be flowing for very long, and would not be compliant with regs.

Kind Regards, John

 

[Harry Bloomfield]

I'm therefore considering temporarily installing a 'plumbed-in' electric instant water heater in the kitchen, probably of around 10 kW. That would presumably work adequately for the taps in sinks, basins and baths (even if fair slow flow into a bath!),

The bath filling, is simply not practical, because it would take far too long. The water would cool down too much, whilst you were waiting for it to fill.

 

[Diver Fred]

It's not just the 40A shower circuit provision in the consumer unit but also the cable installed - is on the surface or buried, The CSA of it should be 6mm at least.

 

[BlueLoo]

As I said, I suspected/feared that people would probably answer questions other than the one I asked

Who said anything about 'long periods'? I would not be contemplating filling a bath which such an arrangement, and wouldn't need to if a shower could be got to work to at least a usable extent - and the duration of water heating and the amount of water heated would then be no different from the situation with an electric shower of the same rating.

The way in which we size cables does not take duration of current flow into account - or, I suppose more correctly, assumes that the current will flow for an indefinite period of time. Hence, if a certain cable is, per standard calculations/tabulations deemed to be adequate to carry, say, 40A, it is deemed to be able to carry that current for an indefinite period of time - in other words, the current-carrying-capacity figures we work with are ones relating to contemptuous/indefinite flow of the current concerned..

You seem to be talking about a situation in which one loads a cable to beyond its "(continuous) current-carrying-capacity" - which is something which one really shouldn't do, even if the current is probably not going to be flowing for very long, and would not be compliant with regs.

Kind Regards, John

My girls take fooking ages in the shower!

Well, yea, but, hoo, the regs are a deep and not always crystal clear dive!

I got my code flippers wet recently when revamping my en suite and noticing the shower cable is 6mm2, not 10mm2.

The kneejerk is that it should be 10mm but then a rummage behind the regs suggest this is only a maybe.

Much depends on time of installation (codes change) and form of installation. (How).

In my case, i found nothing to suggest 6mm was unacceptable (although not the norm these days).

However, i did find very intriguing studies done by the commitees on heat gain and current load in twin earth. The results were suprising in some respects.

My take away was that cable current capacity is heavily governed by heat loss/gain.

Loss from the installation but gain from the load and resistance.

An old house with old thin cables, running long distances pulling high current in enclosed spaces might be a recipe for problems.

Filling a bath on a cold night with a electric water heater isn't going to be quick. If the voltage drop is high then the heat gain is going to get troublesome.

I wouldn't do it tbh unless i ran conservative, well placed cables.

 

[JohnW2]

It could work, but the flow into the mixer for both hot and cold would have to be significantly restricted, and that is likely to cause problems in locating one which can work properly at very low flow rates.

Thanks for your thoughts.

Is it not more aa question of how low a hot water temp the mixer can handle, rather than anything to do with pressures? As I see it, if the water heater were of the same power than one would have as an electric shower, then hot water would be delivered, at whatever flow rate, to the mixer at about the same temp as the electric show would output, hence, at least in theory,the mixer would not use any cold water at all if it were set to that, or a higher, temp?

In fact, that makes me wonder - what would happen if I supplied the mixer only with hot water (at whatever temp and flow rate the heater provided)? Again, in theory, I would have expected that to work similarly to the equivalent electric shower.

If both shower and taps are needed, easier to just install a cheap electric shower as well as the water heater with an interlock so they can't be used simultaneously.

That's obviously a possibility, albeit an additional expense (particularly for 'temporary' measure). However, the most attractive aspect of what I am suggesting is that it would enable the bathroom plumbing to be 'finalised', such that all that would have to happen subsequently would be the swapping of the water heater for a combi (in the same location)

A 10kW instantaneous heater will be useless for a bath.

I wouldn't consider trying to fill a bath with such a heater. It presumably would 'work', but painfully slowly.

Kind Regards, John

 

[JohnW2]

It's not just the 40A shower circuit provision in the consumer unit but also the cable installed - is on the surface or buried, The CSA of it should be 6mm at least.

Thanks - but, again, not the question I asked. I am perfectly happy with the electrical aspects of what I am considering.

The cable installed has a CSA deemed to be able, with the installation method utilised, to carry 40A continuously.

Kind Regards, John

 

[BlueLoo]

10kw is 43a@230v?

 

[Diver Fred]

Thanks - but, again, not the question I asked. I am perfectly happy with the electrical aspects of what I am considering.

The cable installed has a CSA deemed to be able, with the installation method utilised, to carry 40A continuously.

Kind Regards, John

As you hadn't offered that information in the first place many of us assumed, as we now discover, incorrectly that the cable size to be fitted or already installed of the wrong size.

 

[JohnW2]

My girls take fooking ages in the shower!

So did mine, in their time. However, no teenage girls are going to be anywhere near this shower during the duration of the temporary arrangement I am proposing.

The kneejerk is that it should be 10mm but then a rummage behind the regs suggest this is only a maybe. .... In my case, i found nothing to suggest 6mm was unacceptable (although not the norm these days). .... My take away was that cable current capacity is heavily governed by heat loss/gain. ... Loss from the installation but gain from the load and resistance. ......

As I've just written, I am perfectly happy with the electrical aspects of what I am proposing, and am not asking about that.

The comments I'm getting (which were expected!) seem to be overlooking the fact that the current-carrying-capacities, for a cable of given CSA installed with a particular installation method, we work with relate to 'continuous ratings' - in other words, if a cable satisfied the requirements for a 2-mimute shower, it would be deemed to still be OK if the shower lasted for 2 hours, or even 2 days.

Kind Regards, John

 

[flameport]

what would happen if I supplied the mixer only with hot water

If it's a decent one, you get no water out of it at all.

 

[BlueLoo]

So did mine, in their time. However, no teenage girls are going to be anywhere near this shower during the duration of the temporary arrangement I am proposing.

As I've just written, I am perfectly happy with the electrical aspects of what I am proposing, and am not asking about that.

The comments I'm getting (which were expected!) seem to be overlooking the fact that the current-carrying-capacities, for a cable of given CSA installed with a particular installation method, we work with relate to 'continuous ratings' - in other words, if a cable satisfied the requirements for a 2-mimute shower, it would be deemed to still be OK if the shower lasted for 2 hours, or even 2 days.

Kind Regards, John

Including the MCB?
A potential continuous 45A load on the 40A breaker has to be pushing up on the curves, no?

 

[JohnW2]

10kw is 43a@230v?

Yes, roughly that. However, the "10 kW" advertised power will almost inevitably relate to 240V, in which case the current at 230V would be about 39.9A.

Having said that, even if it were 43A, I would personally not lose a moment's sleep over that.

Furthermore, I said "40A", but it might actually be a 45A or 50A MCB (I'm a long way from the property at present). All I can remember is that it was installed with the thought that a 10.5 kW shower might possibly be installed in the future. Also, whilst I know that the installation method is Method C (I've seen the entirety of the cable run), I can't remember for sure whether it's 6mm² or 10mm² cable (I think probably the former) - but even 6mm² (Method C) has a CCC of 47A.

However, yet again, I did NOT ask about electrical aspects of what I proposed.

Kind Regards, John