I remain very doubtful about the utility of such appliances if used for wet central heating which most are.

They combine the worst points of gas central heating, leaks, single point of failure, frost damage, losses from pipes with the high unit cost of electricity.

Electric wet central heating CAN be viable but only with thermal storage in a large hot water tank, heated of peak, and NOT via an “on demand” electric boiler.

A modern and super insulated homes should not need wet central heating.

If however the customer is determined that they want one, then I agree that a dedicated switch fuse is far preferable to a long hour 50 amp load on a domestic consumer unit.

I would select a 100 amp switch fuse, fitted with a 63 amp fuse. Double check that the boiler requires only a SINGLE supply, some require several smaller supplies, one for each element.

Not a good replacement for a gas boiler, okay for supplying wet under floor heating, but gas would be cheaper.

The customers have an old oil boiler and no mains gas supply. They have signed on the dotted line with the supplier and installer ref the new all electric “boiler”. They seem concerned about the green aspect and availability of fossil fuels. I did not get into any debate about that issue. Yes, a 60 Amp supply seems like a good thing.

Z. 

12000 watts divided by 240 volts = 50 amps, so you cannot use a standard circuit out of the Onsite Guide.

Using a 50 amp MCB and 10 mm twin and earth isn’t impossible, but a bit tight.

I’d ring my electrical wholesaler and ask if they will supply a cut length of 16 mm twin and earth, if they don’t then it’s probably going to be 16 mm SWA or insulated and sheathed singles, because they will cut those to length.

So having selected the cable based on availability, I’d then do the calculation based on using a B63 MCB.

But of course you do need to check the manufacture‘s installation instructions.

You could then do the sums again for comparison using a B50 MCB and 10 mm, but I’d err on the side of caution with this one as the boiler will struggle and run for hours on end.

I would be uneasy about use of a 50 amp MCB, hence my earlier suggestion to use a 100 amp switchfuse fitted with a 63 amp fuse.

I consider a long hour hour load of 55 amps to be entirely possible, due to manufacturing tolerances and supply voltage variations.

Small and brief overloads by say an electric shower do not worry me unduly, but long hour overloads do.

If an electric shower has a calculated loading of 40 amps, I would use a 40 amp MCB with a clean conscience, despite the fact that manufacturing tolerances might mean an actual loading of 42 amps at nominal voltage, and 44 amps with a 252 volt supply.

But for a long hour load a much more cautious approach is prudent. Also, is the stated 12 kw at 240 volts, or at 230 volts.

Hi Zoom,

At risk of jumping off track a little, here is my anecdotal evidence base on my experience of electrical boilers. I feel it ties in with other recent discussions about the current trend for going all electric.

I tend to agree with the view that electric boilers should be installed with caution. I find it terrible that people are being frightened into making these decisions without the full knowledge of the impact it will have on their wallet.

First, a bit of background to the construction of the property:

The house was a small, converted brick barn attached to another barns/cottage. 

It consisted of a very small upstairs bedroom, a small cosy living room, small hallway and stairs. The kitchen was in a 3 metre by 5 metre conservatory which was unbearable in the summer and cold in the winter.

The home had wet electric fed radiator heating and a single shower. Although it had been lived in for years by an old lady, the place was refurbished by the new owner/landlord in 2019 so the boiler was new.

Heat-loss wise, the windows were double glazed but old. The same goes for the conservatory. There was little scope to add much insulation as the ceiling was vaulted in the bedroom. The walls were single skin 9” brick. The property was a little draughty due to the conservatory.

She, who must be obeyed, likes a toasty warm house, and, it was.

The manual said the boiler was 14.4kW. There was no gas supply.

I was testing a data logger in February this year, this coincided with the coldest period of the winter when the temperature didn’t get above zero.

The kWh usage for the boiler between 10th Feb to 15th Feb was 672.3 kWh.

The total bill for electricity, for one year was £5k.

I have attached the stats from Powerpoint Pro below:

When installing the circuit into an existing consumer unit two spare ways are required, so the boiler supply MCB can be installed on the end of the row of MCBs with a void way to provide an air gap between it and the adjoining MCB to prevent the MCB overheating, as shown in the manufacturers instructions and as I did when I installed the supply to one of these boilers earlier this year.

You also need a 3 amp supply for the controls which I did by running a SFCU off the boiler supply so the boiler isolator isolates both supplies to the boiler, again as per the instructions.

Not the prettiest of consumer units it is not one I installed and I would not have put the transformer where it is, the last two RCBOs on the peak supply busbar are the immersion heater and electric flow boiler with blanks between them and the adjoining devices as air gaps following both the consumer unit and boiler manufacturers instructions. 

There's a spare busbar tucked underneath,  which was the original I replaced because it was  too short to space the MCBs out.  

This is a bit of problem , as if not careful installation (to an old distribution board) could cause fairly regular imbalances , particularly when tasked with starting from cold/with cold water.

I know the manufacturers have gained CE marking , but I think we need some clear guidance form the IET now on this.

I know some comments don,t like wet system electric boilers , and if you have a home pre 2000 , gas is much cheaper from the heat perspective , however that may change. My home is fully electric and not very thermally efficient so i know a little about how the 4 months dec to march can feel.

Installation wise , you cant really use the din rail on a distribution board , so should be a separate switch near distribution board (and it perhaps should also feed the water pump) , however 40amp may not be enough , and finding a 50 amp is not easy , the manufacturer should have recommended unit to use , so perhaps give them a call. High amp fuse is an interesting choice , and car chargers (if they do get to home charging) will need something similar. 

Basically I think the IET need to have a discussion , I think we need something that is for new build , and something for retro fit, and we need to nudge the elctrical components people quickly , before we see high amp single spurs , causing affects on distribution boards , that could be affected , or even if we start having scenarios  where we affect the 100A distribution fuse.

In theory an electric wet system (and thermal losses of the building are key whether gas or electric ) , might not be too bad 150w water pump included on top of 12000w , providing you have thermostatic control , and keep it low 18oC. The real problem will be homeowners who run central heating at 22-24 , in a thermally inefficient home , and any external temperature below 5oC  , or if your tasking the system with hot water provision. With a quality gas boiler at £1400 the electric replacement should be around £500 , and if your really going electric , you have to ask the gas board to disconnect you (and remove the meter) to not pay further standing charges , actually removing your gas pipe to your property is lot more expensive and difficult.

In a well thought new build home an electric wet system should not be problem , one new 3 bed house i have looked at , was thermally efficient and 9kw would probebely do it as you can attain much more stable temperatures , so for new homes a wet system may well work.

I used some top of the range German fire clay inner radiators , and they have worked very well and i have run various experiments on efficency , from timing , to temperature set , and for me (and my thermally inefficent house) i try and set them in winter to back ground low heat and get by , but in a minus 5oC couple of days , things were not really as i would have liked , but most minus oC events are ok .

In bigger picture use of electricty for electric heating , for low income households needs looking at , and for the many 3 , 4 bed houses built before 2000 , who go to full electric heating , thermostatic control systems are a must.

The other thing is that its not just about keeping people warm , older houses may suffer slight damp and heating in some ways can be seen as good maintenance practice.          

Andy65: 
 

Hi Zoom,

At risk of jumping off track a little, here is my anecdotal evidence base on my experience of electrical boilers. I feel it ties in with other recent discussions about the current trend for going all electric.

I tend to agree with the view that electric boilers should be installed with caution. I find it terrible that people are being frightened into making these decisions without the full knowledge of the impact it will have on their wallet.

 

First, a bit of background to the construction of the property:

 

The house was a small, converted brick barn attached to another barns/cottage. 

It consisted of a very small upstairs bedroom, a small cosy living room, small hallway and stairs. The kitchen was in a 3 metre by 5 metre conservatory which was unbearable in the summer and cold in the winter.

The home had wet electric fed radiator heating and a single shower. Although it had been lived in for years by an old lady, the place was refurbished by the new owner/landlord in 2019 so the boiler was new.

Heat-loss wise, the windows were double glazed but old. The same goes for the conservatory. There was little scope to add much insulation as the ceiling was vaulted in the bedroom. The walls were single skin 9” brick. The property was a little draughty due to the conservatory.

 

She, who must be obeyed, likes a toasty warm house, and, it was.

 

The manual said the boiler was 14.4kW. There was no gas supply.

 

I was testing a data logger in February this year, this coincided with the coldest period of the winter when the temperature didn’t get above zero.

The kWh usage for the boiler between 10th Feb to 15th Feb was 672.3 kWh.

The total bill for electricity, for one year was £5k.

I have attached the stats from Powerpoint Pro below:

Thanks very much Andy1 and Andy 2 and all others. Very helpful.

Z.