Instant Water Heaters

[r2d2]

Hello d-i-y world.

Recently I've been contemplating getting rid of an old hot water heater/tank combination, and I wonder if I could replace it with an instantaneous heater.

This is for a flat with a kitchen, bathroom and shower. No central heating.

I'm guessing (!) that I could get one of these:

Or similar?

It looks like the installation would be simple, in fact the biggest job would be to get rid of the old unit.

Also, if anyone would like to suggest which search engine to use for uk.d-i-y nowadays, that'd be real handy.

Cheers!

[Tim+]

Just bear in mind that at 12kW it’s going to need a much meatier power supply than the old immersion heater.

Tim

[Martin Brown]

It needs a 10kW mains supply and the site you linked to seems remarkably cheap very amateurish layout and very light on crucial technical info. This site is better:

Google Groups is still there - though increasingly more painful to use :(

[Fredxx]

12kW, 48Amps

I guess 10mm2 cable should do the trick. I also expect you'll need an expansion vessel and a safety valve with pipework that can take scalding temperatures to a drain.

[Theo]

12kW. That's bigger than an electric shower (normally 8-10kW), but a lot less than a combi boiler.

The datasheet doesn't give any details of flow rate, which is a bit concerning, but I think it would take a long time to fill a bath.

It would be possible to calculate that...

100 litres 30 degreeC temperature rise (10C mains, 40C bath) 4.2 kJ/kg degC 12kW supply

4.2*100*30 = 12600 kJ = 12600 kWs

12600kWs/3600 = 3.5kWh 3.5kWh/12kW = 0.292 hours = 17.5 minutes

(during which time the bathwater will be cooling, so it'll take longer than that)

Theo

[Jeff Layman]

Agreed. The flow rate absence makes any statement about serving a shower and two sinks pointless. I have a small 3kW instant water heater in the garage, and although it's fine for that purpose, in winter it struggles a bit to provide decently warm water for washing at anything other than a slow rate.

Good calculation, but note that the technical info states the 12kW is for a 240V supply. The nominal UK voltage is 230, and the output for that is said to be 11kW, so you can can add a minute or so. Perhaps more to the point is that it's down to 10kW at 220V, and I'll bet most people will be taking a bath or shower in the morning or evening when there is most demand on the electrical supply and it's more likely to be nearer to 220V than 230V.

[Theo]

This is the point at which the difference between nominal voltage and the actual voltage matters. You may well find the voltage is in reality 240V and, given the resistance of the heater is constant, the power delivered is somewhat different between 230 and 240. While 220V is within EU spec, I'm not sure if it would be expected as normal in an UK installation.

YMMV, especially based on location, but here it's usually around 252V which is just about 230V+10%.

Theo

[Jeff Layman]

OK. I've just spent an hour trying to find a simple record of an actual measured mains voltage anywhere in the UK. No luck. Frequency, yes, but not voltage.

I'm about 50 metres from a substation, so hope my supply voltage is pretty stable. It's 240V at present. When SSE did some work on the substation, they brought in a temporary 500kW generator for 48hrs. After a few hours we noticed the lights were not as bright as they were normally, and varied a little; some then started going out. The TV was ok. I put a voltmeter on the mains, and it read 108V, occasionally dropping to 97V! The generator was making strange noises and at one stage some smoke appeared to be coming out of it. I contacted SSE, who came round in a few minutes and made some adjustments. It was fine after that. I assume the TV was ok as it would have a SMPS and probably work anywhere between 100 and 240V.

[Robin]

Your DNO might know.

They worry about falling below the statutory minimum when there's a big increase in demand. Traditionally they've run near the upper end of the statutory range to guard against that. And without much option given they didn't know what customers were getting. But they can now use smart meters to gather information on voltages and IIRC some at least are doing so to help plan/monitor their networks and to reduce voltages (which saves money).

[John Rumm]

No, this is basically an electric shower style heater, that can also feed 15mm pipework rather than a shower head. So no stored water or cylinder etc.

[SteveW]

It's always been 240 (with minor variation) here over the years. It was again yesterday, when I had occasion to check whether a faulty security light was getting power.

[John Rumm]

You could... but with some caveats; bath filling may take a very long time, and flow rates to the shower might be reduced from its current level of performance.

Yup much the same as installing an electric shower. Hefty radial circuit required. (Check what your main fuse is rated at - new properties will typically have 100A, but older ones only 60 or in some cases less. If yours is less than 60 you may have difficulty with a > 40A extra load on it.

groups.google.com will search. For general access, though news reader talking to a news server is better.

[Theo]

HomeOwnersHub actually isn't bad for searching...

[Animal]

The nominal is 230v, bt the actual target voltage is still 240. Mains doesn't sag to 220 under load, self adjusting taps stabilise it.

[Fredxx]

It's intended to be at mains pressure so not like a shower style heater.

I presume what you are saying is that there is insufficient water stored in the device such that any thermal expansion of water can be taken up internally.

[Brian Gaff]

I think this came up some months back and depending on usage of course, it could be very expensive to heat water directly, if, for example you wanted a bath. Also of course making the water different temperatures can be a bit more difficult. A friend of mine has begun to hate his as it seems to get full of limescale then the element fails. Immersion heaters seem to cope better with hard water, and still work when well encrusted. Brian

[Brian Gaff]

Maybe he would like a sauna then, rent that bit out to pay for the leccy. Brian

[SteveW]

Instantaeous water heaters don't store any water. Heaters which heat a small store of water (up to 30 litres) are permitted to use their feed pipe (minimum lengths and other rules apply) for their expansion.

[GB]

I reckon a decent size combi for a shower is around 25-30kw. This water heater is around half that.

In Germany, they use 3 phase instant water heaters, which do get up to much higher outputs, eg

[John Rumm]

Showers are at mains pressure.

I think you are confusing it with an unvented cylinder...

There is practically no water stored - it is an instantaneous heater - bit like a small electric combi.