Temporary use of electric 'instant' water heater

[JohnW2]

I may be completely making this up, but I seem to recall reading that instantaneous water heaters were unsuitable for thermostatic showers.
It was something to do with back pressure, the heater being triggered by a pressure sensor and the thermostatic mixer attempting to modulate or cut off completely, the on/off heater - something that a gas combi can deal with, but may cause issues constantly switching a 10kW load???

I would certainly doubt that you are 'making it up' because, although it's not something that had occurred to me, what you say is broadly consistent with what flameport has written, and also Redring's apparent view that their Powerstream instant water heaters can only be used with manual mixers.

As for the mechanism/reason, I'm not sure that it can be exactly as you suggest, since thermostatic mixers certainly work with (were originally designed for) stored hot water, so cannot be reliant on any control or modulation of the water heating. On the face of it, my (probably naive) way of looking at is that if one looks at whatever is heating the water as a 'black box' which heats water whenever flow through it happens,then the mixer ought not to know (or care) whether there is a combi or an electric instant heater within that box - but it seems that is probably not the case!!

As you will have seen, flameport has posted a specification which explicitly says that there is a 'safety feature' which "ensures safe shut down within 2 seconds if either hot or cold supplied fail" (albeit I don't really understand why shut down is really required if the hot supply fails), but that is based on a minimum difference (12°) between hot supply and outlet temp. However, as I wrote to him, I'm not convinced that is necessarily unattainable with an instant heater, since it only requires there to be a 'useable' flow rate at a temp of about 47° or above.

The other thing I haven't mentioned is that Nasi Lamak (above) is not the first person who has told me that they are successfully running a mixer shower from an instant heater -but what I don't know (and will ask Nasi Lamak) is whether they have thermostatic or 'manual' mixers. If using a manual mixer would 'solve the problem', to the extent of resulting in something 'usable', that would be fine for my (temporary) needs.

Kind Regards, John

 

[JohnW2]

I have exactly this setup in my garden room installed by the builder. Instant water heater under the basin supplying both the basin taps and a mixer shower. It works really well. So we’ll in fact that it set me wondering why people fit expensive electric showers

Per what I've just written to RandomGrinch, is your mixer shower a thermostatic or 'manual' one?

Kind Regards, John

 

[Harry Bloomfield]

I would certainly doubt that you are 'making it up' because, although it's not something that had occurred to me, what you say is broadly consistent with what flameport has written, and also Redring's apparent view that their Powerstream instant water heaters can only be used with manual mixers.

The heater compartment, is unsuited to being placed under any water pressure, so the outlet has to be completely open to atmosphere. In other words, the tap has to be on the input to the water heater. Whether that is true of your instant water heater, best check the specs.

 

[JohnW2]

The heater compartment, is unsuited to being placed under any water pressure, so the outlet has to be completely open to atmosphere. In other words, the tap has to be on the input to the water heater.

That's true of 'over-sink' instant heaters, but that's not what I'm talking about .........

Whether that is true of your instant water heater, best check the specs.

I'm talking about instant heaters specifically designed to be 'hard-plumbed' to sink/basin taps (including [manual] mixer taps) and, in some cases,showers - so they obviously must be happy with the 'tap' (flow control) on the 'output', and what that means in terms of the pressure in the 'heating compartment'.

Kind Regards, John

 

[RandomGrinch]

were originally designed for) stored hot water, so cannot be reliant on any control or modulation of the water heating

Perhaps I was using the wrong expression.
The thermostatic mixer adjusts (modulates!) the flow of hot water into the mixer - it doesn't have any external control of the source.

When we shower my 4 year old, the set temperature is low, with a low flow. The combi regularly cuts in and out.
I could imagine under certain circumstances, switching an instantaneous water heater like that, may cause issues?

 

[Harry Bloomfield]

That's true of 'over-sink' instant heaters, but that's not what I'm talking about .........

OK....

I'm talking about instant heaters specifically designed to be 'hard-plumbed' to sink/basin taps (including [manual] mixer taps) and, in some cases,showers - so they obviously must be happy with the 'tap' (flow control) on the 'output', and what that means in terms of the pressure in the 'heating compartment'.

The open vented ones, are the only ones I've come across.

 

[JohnW2]

Perhaps I was using the wrong expression. The thermostatic mixer adjusts (modulates!) the flow of hot water into the mixer - it doesn't have any external control of the source.

Indeed. You seemed to be implying that the operation of gthe mixing could in some way 'control' the on/off and/.or 'modulation' of the source heater (whatever it may be).

When we shower my 4 year old, the set temperature is low, with a low flow. The combi regularly cuts in and out. I could imagine under certain circumstances, switching an instantaneous water heater like that, may cause issues?

That's surely what the instant heater would do - it is, for example, user to 'turning on and off' in response to connected taps being turned on/off, so presumably would just do the same is a mixer were 'turning on and off' it's 'call for hot water'?

Am I missing something?

I've just asked the same question in the plumbing forum, and will be interested to see what they have to say. As I've told them, I cannot really see the difference between a ~10 kW instant heater and a ~10 kW shower (provided the shower mixer will work with the former) - so I wonder if those who are suggesting that the performance of the former would be 'miserable' are perhaps the same people who would also say that the performance of a ~10 kW electric shower was also 'miserable'?

Kind Regards, John

 

[JohnW2]

OK.... The open vented ones, are the only ones I've come across.

Fair enough but, as I said, I'm not talking about them but, rather, about ones which are definitely not 'open vented'.

Kind Regards, John

 

[SUNRAY]

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.

Really?
While replacing our bathroom, as in replacing the walls we basically had a bath and a shower heater and had no problems running a bath.

 

[SUNRAY]

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

The simple thing to do is source a heater from abroad where it is common to use a single electric heater for the whole property. This will require the use of an expansion vessel.

 

[Harry Bloomfield]

Really?
While replacing our bathroom, as in replacing the walls we basically had a bath and a shower heater and had no problems running a bath.

I didn't say it was impossible, simply impractical, because it takes so long.

 

[JohnW2]

The simple thing to do is source a heater from abroad where it is common to use a single electric heater for the whole property.

What is the difference between a water heater 'from abroad' and one 'from here'?

12 kW ones are readily available in the UK, and I don't really think that anything much more powerful than that would be (electrically) practical, particularly with the house's current v"60A supply" (which is currently 'under negotiation ).

Kind Regards, John

 

[SUNRAY]

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

Something which happens on a regular basis when DIYers replace the 8 or 9KW unit with 10.5KW and the OCPD is nowhere near their thoughts.
One of my rentals had a 10.5KW heater on 4mm² and 32A fuse with no apparent failure, overheating damage yes not no failure.

 

[SUNRAY]

What is the difference between a water heater 'from abroad' and one 'from here'?

12 kW ones are readily available in the UK, and I don't really think that anything much more powerful than that would be (electrically) practical, particularly with the house's current v"60A supply" (which is currently 'under negotiation ).

Kind Regards, John

I truly don't know what the difference is, but what I do know Is staying with relatives in Berlin, way back in 80's there were 11 of us in the house and in the morning there was simply no feeling of lacking hot water in 3 bath/shower rooms and kitchen. The heater was bigger than a typical shower unit by maybe a factor of 3 and being permanently pressurised by mains water the pressure vessel was bigger. They occupied the rear half of a kitchen base unit, so a chunky bit of kit.
I have no idea what power but the pair of overhead wires feeding 2 properties looked smaller than our typical feed to one property

 

[JohnW2]

I truly don't know what the difference is, ......

As far as 'instant water heating' is concerned, obviously all that matters is the power. As I said, 12 kW are readily available in the UK, and even 15 kW would be pushing things a bit for the current supply of the house in question. However .....

.... but what I do know Is staying with relatives in Berlin, way back in 80's there were 11 of us in the house and in the morning there was simply no feeling of lacking hot water in 3 bath/shower rooms and kitchen. The heater was bigger than a typical shower unit by maybe a factor of 3 and being permanently pressurised by mains water the pressure vessel was bigger. They occupied the rear half of a kitchen base unit, so a chunky bit of kit.

... how substantial was this thing you're calling a 'pressure vessel'? It it were storing a substantial amount of heated water, that would obviously be immediately available to taps/showers with no need for real time heating, and the 'instant heating'would only come into play when all that store of hot water had been used up. One of the over-sink heaters I have at the house has a small (I think 2-3 litres) storage tank ('open-vented', not pressured) - so, again, that much hot water become immediately available when the water supply to the heater is turned on, before real-time heating takes over.

I have no idea what power but the pair of overhead wires feeding 2 properties looked smaller than our typical feed to one property

Was that single phase or 3-phase (or even ';2-phase')? If single-phase and you're talking something like 16mm, then that (and perhaps also the service fuse!) could probably happily carry at least 150A or so (so maybe around 35 kW) without coming to any harm - so I suppose it mind depend upon the extent to which people in Germany were happy to 'overload' cables and supplies back then

Kind Regards, John