Can Immersion heater be supplied by mains plug?

[Bodgedbuild]

My heating and hot water are by Air Source Heat Pump but I have had external electrical supply problems (ongoing for years!),I have also had a failure of the heat pump programmer in the past. What I am wondering is I could put a mains plug on one of the two redundant immersion heaters that are fitted to the tank? it's a hot system so doesn't need extra anti-salmonella heating, they are just fitted but not wired up.
It's 3,000 watts so I could potentially either plug it into a socket or run a heavy extension lead from my generator.
Thanks in advance.

 

[winston1]

It is not conventional but I don’t think there is a reg against it. Use heat resistant cable.

 

[ericmark]

In 1954 when dad's house was built it was standard to have the immersion heater supplied from a 15 amp socket, so the plumber could change an immersion heater without needing an electrician on site.

However a 15 amp plug does not have a fuse in, the over load protection was from a 15 amp fuse at the fuse box, the 13 amp plug has a fuse so produces heat, and needs to be in free air to keep cool, so when the 15 amp plug and socket started to become rare, we did not move to using a 13A plug and socket as people tend to put cloths in the airing cupboard so there is very little cooling and it runs for an extended time.

So with that in mind a plug and socket without a fuse with the fuse or trip else where no problem, be it 15 amp or 16 amp does not really matter, supplied maybe from a fused connection unit (FCU) which can cool as fixed to the wall, but would not recommend the use of a 13 amp plug simply as all fuses get hot, that is how they work.

 

[JohnW2]

It is not conventional but I don’t think there is a reg against it. Use heat resistant cable.

As you say, although it is unconventional and probably fairly undesirable, there is not an explicit regulation against it. There is, however, a recommendation ('guidance') in Appendix 15 of BS7671 that immersions should not be supplied by ring final circuits. That seems pretty sensible, since an immersion is one of the very few domestic loads likely to draw 3kW for appreciable periods of time ...

Also, even with washing machines and tumble dryers, the plugs have been reported to sometimes overheat, and they generally represent less of a continuous load than does an immersion.

 

[Bodgedbuild]

Thanks guys, I think I will stick a heat resistant wire on it but leave it coiled up. I could then put a plug on it if we either have a prolonged power cut or if the heat pump fails. For the sake of about £30 I could put a 1kw element in to replace the standard one?

 

[AndyPRK]

see what your generator can supply.

Maybe a 2200W immersion heater maybe a better crompromise

 

[Bodgedbuild]

see what your generator can supply.

Maybe a 2200W immersion heater maybe a better crompromise

That's a possibility? It might only ever get used once a year and they have replaced a lot of the overhead lines into my village recently so I'm hoping the supply becomes more reliable. One of the immersions is quite close to the top of the cylinder so hopefully it would heat some of the water up quickly.

 

[ericmark]

Yes the Irish have that cracked with the Wilson system where it heats the tank from the top down, rest of UK never seem to have adopted the system.

 

[JohnW2]

Yes the Irish have that cracked with the Wilson system where it heats the tank from the top down, rest of UK never seem to have adopted the system.

Heating the cylinder 'from the top down' is presumably only generally feasible (to permit heating of all the water in the cylinder - e.g. for a bath) if the thermostat is lower down the cylinder, rather than being integral with the immersion?

The usual (English!) idea when one has two immersions (each with integral thermostats), one 'top' and the other 'bottom', is that the top one can be used to significantly heat only a relatively small amount of water at the top of the cylinder, since its thermostat will operate when the water 'at the top' is up to temp, even if most of the water lower down in the cylinder is relatively cold.

Kind Regards, John

 

[AndyPRK]

Modern unvented cylinders tend to have two heaters due to the size.
Maybe there is a required heatup time that needs to be met’

 

[sparkwright]

Sooner or later, it seems all 13 amp plug tops (yes, plug tops, that's what they were and are called in the trade) eventually burn out when fitted to immersions. Fact. Just the way it is. So use of a plug top (yes, a whole plug top) on an immersion should be very, very occasional, and I personally wouldn't even leave the house.

Something about a 13 amp plug top (or plug if you don't know what that is) that doesn't like 3kW loads.

 

[AndyPRK]

Be the insulation on the pins

 

[chivers67]

It's close to the limit on the fuse and will no doubt get hot for sure. Odd that the last two days we've have few Immersion topics that some of us should no doubt brush up on (me included not being a Domestic)

 

[JohnW2]

Modern unvented cylinders tend to have two heaters due to the size. Maybe there is a required heatup time that needs to be met’

Maybe. I have no personal experience of such things.

Are those heaters at appreciably different heights (i.e. one much closer to the top than the other and, if so, does each one have an integral thermostat? If so, as I said, the upper one will 'turn off' as soon as the water at the top of the cylinder is up to temp, even if most of the water in the cylinder (lower down) is still fairly cold.

Kind Regards, John

 

[AndyPRK]

Yes there own thermostat.
The bottom one would continue heating.