Fitting an undersink instantaneous water heater

[Richard]

I want to fit undersink instantaneous water heaters, just to provide hot water for the sink.

I'm on a budget but I was looking initially at the Redring RP1. That gives 9.5KW - current 41.3A. I don't know if it's somewhat overpowered or not, considering all I want to do is obtain hot water for a sink.

One heater would be for the upstairs wash basin, the other for the kitchen sink.

Question: Can I feed one or both of these heaters from the cable that is supplying my electric shower? The width of the cable to the elecric shower shower is 16mm. (Not sure of the cable amperage, under 41A I think.) Possibly then there would need to be a cable upgrade. And possibly a special connection to the mains (i.e. not through the regular distribtion box).

(Max current through the regular disribution box seems to be 32A. That would come out at 7.36KW. I don't think normal distribtion boxes allow much more than 32A)

Alternatively, (and probably the answer is YES to all):

- do I need to take dedicated cables to these undersink heaters?

- with a special distribution box?

- with a BIG cable?

I suspect folks are going to say - don't use the shower cable - even it is really big. True?

TIA.

[Richard]

On 12/05/2010 14:15, Richard wrote: ...

Actually, if I'm smart, I can put a heater in between the top and bottom sinks. Then I just need one cable and one heater.

[Dave Osborne]

No.

With a suitable arrangement for achieving appropriate fusing at the consumer unit and appropriate isolation at the heater.

With suitable cables.

True.

[Man at B&Q]

Cue dribble.

In more ways than one, given my experience of such things.

MBQ

[Richard]

What's the problem exactly?

[Richard]

Okay, Lets say that I need to connect a cable that is capable of handling 41A (10mm cable I think) what equipment would I need at the mains end?

I believe that there is a need to draw current directly from the electricity meter. That would require some kind of disribution unit with it's own short circuit protection.

[David Hansen]

On Wed, 12 May 2010 14:15:06 +0100 someone who may be Richard wrote this:-

The more quickly one wants to heat up water the more power must be put in. A 3kW immersion heater will warm up a bathfull of water, slowly, but to warm up a sinkfull of water quickly means putting a lot of heat into it quickly. The heater doesn't sound overpowered,

12 kW models are available, for example

With some storage power consumption can be reduced, for example

You will need a cable of a similar size to each heater.

The cable sounds like it is 10 square millimetre (the area of the conductors)

The typical domestic consumer unit was for a long time designed to take a maximum of one 45A fuse and a variety of other smaller fuses. That is good to remember.

The maximum demand in final circuits to loads like this is the full load, so individual circuits to each are necessary. However, there can be an allowance for diversity at the consumer unit and mains. Both under sink units and the shower are unlikely to all be operating at the same time for long periods. The Wiring Regulations say that you can assume 100% of the largest + 100% of the next largest + 25% of the next largest for instantaneous water heaters. Assuming they all draw 41.3A that's 92.9A. Not much to spare on the typical domestic consumer unit, but not a problem with suitably sized supplies and distribution boards.

I think that in a domestic setting the regulations are too restrictive. A suitably qualified electrical engineer may look at the individual circumstances and decide greater diversity is appropriate. Am I going to tell you that you can do the same in your house? Not on your nelly. I have not seen the installation or done calculations about it.

[Richard]

Thanks, that's useful stuff.

Having considered a bit more, what I think is this:

I've looked and I have an MEMERA 21 disribution board. That is good for

100A (80A ELCB).

I think I ought to be able to end up with a solution that allows me to connect an undersink (or otherwise) instantaneous water heater to this distribution board. I might have to shift wiring to the left to make space for the circuit breaker for this heater. But that's no problem as long as I switch of all the mains switches on the distribution board and I'm very careful.

Whether I could install a 9.5KW heater is at the moment uncertain. But here is my distribution board setup:

TOP ROW

Immersion Down Up Freeezer heater Lights Lights

16A 5A 5A 5A

BOTTOM ROW

Boiler Sockets Socket Cooker Shower Up Down

16A 32A 32A 32A 32A

I don't have an electric cooker.

Okay:

- Can I get a 45A CB for this dstribution board?

- Could I then wire up a 9.5KW heater for the sink?

- Do I really need a 9.5KW heater, just for the kitchen sink?

I'm really trying to use the current distribution board to feed an instantaneous undersink water heater. Much easier to handle.

[John Rumm]

Don't assume that because a small heater may look cheap to buy, its going to remain cheap to install. Often the install costs can dwarf the unit cost.

9.5kW is adequate for a sink - it will deliver about 3 - 5 litres per minute of hot water depending on the temperature of the cold water.

Does the house not have hot water otherwise?

The cable could probably be reused if the shower were being removed. However it can't feed both at once.

sounds like 10mm^2 cable. The current carrying capacity f which should comfortably exceed 41A in all but the harshest of installation conditions.

There is no reason why it can't be through a regular consumer unit. However that does depend on where you are starting from - i.e. what type of CU you have, how much spare capacity it has etc.

How did you draw that conclusion?

While old wylex re-wireable fuse CUs may have difficulty with circuits over 30A, a modern consumer unit has no such limitation.

Alternatively, (and probably the answer is YES to all):

Not necessarily.

Depending on the distance, and how the cable is to be installed you will likely need 6 or 10mm^2 T&E. Either of which probably qualify as being "big" (and both can be bastards to work with in confined spaces!)

Depends on if you plan on leaving it connected to the shower!

[Richard]

The answer to this is yes.

But, what type? Type B, type 1, type 2 - whatever.

[Richard]

The mcbs in my distribution board are like this:

is what it says on my 32A mcbs:

MEM M6 Type 1

32A 240/415v 51QEB

I guess I'm looking for this kind of mcb, but 45A.

[John Rumm]

For this application you ought to be looking for a Type B these days... (similar but now quite the same response curve as a type 1)

[Richard]

's a Type 1.

I hope it fits my MEMERA 21 distribution board.

Will it be okay?

[John Rumm]

>> It's a Type 1.

Should be ok if it fits...

[ARWadsworth]

> It's a Type 1.

That looks about right.

Not a bad buy at £6.99. But it is not NEW it is USED.

Adam

[ARWadsworth]

You may not be able to get a B type to fit the Memara21 CU. The bottom screw is offset to the right on the originals and on the retrofit new MCBs it is offset to the left.

It will work if there are a few spare slots.

Adam