Temporary use of electric 'instant' water heater

[Harry Bloomfield]

You'll be getting 45A as well if the voltage drop is anything notable.

An heater element is resistive - as the voltage decreases, so will the current consumption/wattage.

 

[BlueLoo]

An heater element is resistive - as the voltage decreases, so will the current consumption/wattage.

Wire is though. The longer the more so.

 

[Harry Bloomfield]

Wire is though. The longer the more so.

Eh?

 

[BlueLoo]

I was quite surprised at the voltage swing guarantees electric companies give.
Anything up to 260v.

 

[BlueLoo]

Eh?

Typo.

I was just really considering voltage swings on the input coupled with restistive heat generation and voltage drops on the cable.

 

[JohnW2]

An heater element is resistive - as the voltage decreases, so will the current consumption/wattage.

Exactly, hence my ...

Eh? Can you explain that?

Kind Regards, John

 

[JohnW2]

Wire is though. The longer the more so.

I think that probably warrants an even bigger "Eh?"

Kind Regards, John

 

[JohnW2]

Typo.

I was just really considering voltage swings on the input coupled with restistive heat generation and voltage drops on the cable.

How does that explain the current going up to 45A "if the voltage drop is anything notable" ??

Kind Regards, John

 

[JohnW2]

I was quite surprised at the voltage swing guarantees electric companies give. Anything up to 260v.

253V, actually ... and anything down to 216.2V

Kind Regards, John

 

[flameport]

From what you say, I guess they must work differently from that?

It's a safety feature. If the cold supply goes away, any hot is shut off completely to avoid scalding or worse.
Without that, hot water at whatever temperature it was supplied at would be provided from it, which could be a very high and dangerous temperature.

If you were expecting to just supply hot to the mixer then forget it - not going to work.
It's probably a total bust anyway, as an instant heater won't be able to provide water that's hot enough to mix with cold to give a suitable shower temperature.
See example specifications below - hot must be 12C more than the outlet temperature and suggested at 60C+, which isn't going to happen unless the flow from the hot heater is barely a trickle.
Other valves may vary, but these specs are pretty common.

 

[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

Thanks, that's a very interesting observation and, as so often seems to happen, makes me wonder to what extent theoretical arguments reflect, in practice, the real world.

Kind Regards, John

 

[JohnW2]

It's a safety feature. If the cold supply goes away, any hot is shut off completely to avoid scalding or worse.

Thanks, and many thanks for being one of the few people who is addressing and discussing the actual question I asked, rather than trying to give (in some cases confused and/or incorrect) advice about the wiring of a water heater which I definitely don't need!

What you say makes some sense, but ....

Without that, hot water at whatever temperature it was supplied at would be provided from it, which could be a very high and dangerous temperature.

If that's the case, then it indicates a mode of functionality different from what I had envisaged. I thought that what would cause the hot supply to be 'shut off' would be the outlet temp being above the 'set temp' (or a higher 'safety over-ride' figure), despite the mixer's best efforts to cool the output water by mixing it with cold water. However, you are indicating that such is not how they work

If you were expecting to just supply hot to the mixer then forget it - not going to work.

I certainly wasn't 'expecting' (or intending) to do that. As I said, I just 'got myself wondering' (about what would happen with no cold supply at all) as a result of a comment I'd just found myself writing!

It's probably a total bust anyway, as an instant heater won't be able to provide water that's hot enough to mix with cold to give a suitable shower temperature.
See example specifications below - hot must be 12C more than the outlet temperature and suggested at 60C+,

Are you not perhaps being a bit pessimistic? Desirable shower temps are said to be 35°-40° (and definitely not much above 40°), so the hot supply temp would not necessarily have to be much more than about 47° or so - and, furthermore ....

... which isn't going to happen unless the flow from the hot heater is barely a trickle.

I wonder how small this 'bare trickle' would actually be with a 10 kW heater producing water at about 47°. A 3 kW over-sink heater certainly provides far more than 'a trickle' of water at a temp at least as high as that. Don't forget that I'm not looking for something that would be desirable as a long-term thing but, merely a shower which 'works well enough to be 'usable' as a definitely temporary measure.

Other valves may vary, but these specs are pretty common.

You've done a lot better than I have - since I've more-or-less failed to find particularly useful technical specs for any mixer valves!

What I have noticed that Redring do a range of "Powerstream" water heaters (available as 9.5, 10.8 and 12 kW variants) which they suggest are suitable for use with a manual (not thermostatic) shower mixer - one of which they also supply. I suspecty you may well say that you don't regard a manual mixer as 'safe', but the manufacturer presumably must think they are 'safe enough' to be selling them?

Again, thanks for your continuing relevant input.

Kind Regards, John

 

[RandomGrinch]

which they suggest are suitable for use with a manual (not thermostatic) shower mixer - one of which they also supply

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???

 

[robinbanks]

Sack it all off and go green, fit a decent sized indirect cylinder with immersion and heat that for now. At some point down the line fit a heat pump and/or solar panels?

To the actual question, if you're looking at a decent unit like a Steibel Eltron DHC12 it's a fair old punch on the nose financially but it is self modulating and rated to run sink/shower. As it's thermostatic itself it wouldn't need a mixer per se but I get why you'd install one now if planning to install a boiler in future.

Should also be easily resaleable in future to recoup some of the cost but it's considerably more than just putting a cheapo electric shower and a cheapo sink water heater. Having an electric shower still installed can be a useful backup hot water source (use it to fill a washing up bowl) if the gas boiler fails for some reason, or if you have a setup where you generate and store hot water slowly (solar/immersion) and the HW is depleted

 

[BlueLoo]

253V, actually ... and anything down to 216.2V

Kind Regards, John

In the Uk.

207 here. (!)