Why are FCU used to supply boilers?

[ericmark]

I want my boiler to work even if there is a power failure, OK mine will, I have a 5 kW inverter and 6.4 kWh of battery and solar to keep it running. But most people don't have this, so in a power cut to have a small generator outside and an extension lead going to the boiler would make sense.

However, most boilers seem to have a FCU not a plug and socket, so hard to plug into a generator in an emergency without the use of a widow maker, also need a locking off device designed for that make of FCU, to lock the switch off. (The fuse only supplies line, so neutral still connected so technically not isolated) Where if using a 13 amp plug, where these

are universal for all 13 amp plugs.

The only reason I can think of, was some regulations back, we did not need to protect FCU with an RCD, however buried cables in wall rule has in the main change that.

Asking on Plumbing section as I am sure most electrics would like me not know why the FCU is used, other than it says so in some boiler manuals, likely written before the rules on RCD's changed. So other than that's the way we have always done it, why would one use a FCU to supply a boiler? As to being able to unplug locking sockets would stop that

so why are FCU used to supply boilers?

 

[Jurassicspark]

Nothing wrong with having the boiler plugged in to the socket.

However, having it supplied from a FCU is much easier and it remains powered on. If its plugged in from the socket, numpties will unplug it and use their extension leads, drills and so on and might forget to plug it back in.

Its easier to switch off too, instead of unplugging and plugging in, it saves the plumbers from killing themselves just by pressing a switch (double pole isolation) when working anything related to the electrical side on the system.

 

[Murdochcat]

It’s a gas safe urban myth that a FCU should be used IIRC

 

[oldbuffer]

From memory, any socket used should be unswitched. This is to ensure that the plug must be pulled out disconnecting both live and neutral.

 

[CountryFan]

also need a locking off device designed for that make of FCU, to lock the switch off. (The fuse only supplies line, so neutral still connected so technically not isolated)

They should be double pole switches, therefore both L and N isolated when turning off.

 

[ericmark]

They should be double pole switches, therefore both L and N isolated when turning off.

Good point, how does one know if a FCU switch is single or double pole. It is clear when unplugging that both poles, in fact even the PEN has been isolated. Or is the loss of PEN the reason for the FCU, so one can't disconnect the earth, but seems unlikely.

 

[CountryFan]

Good point, how does one know if a FCU switch is single or double pole.

Probably can’t tell just by looking and tbh I wouldn’t know. Perhaps continuity on a dead test?

 

[ChrisJP]

I have changed the fused connection unit supplying my oil boiler to a double pole switched socket and ordinary 13 amp plug. When I suffered a 3 day power outage after the January storm it was simple matter to unplug the boiler feed plug from the socket and plug it into my generator - which I had bought anticipating power cuts from our rush to net zero. The generator also supplied some lighting in the house using old fashioned lead lights and extension leads. The generator made all the difference between the power cut being tolerable and intolerable.

 

[CountryFan]

Maybe it’s just what manufacturers ask? This is from Vaillant instructions from 2006, they say 2 options available.

 

[ericmark]

This is my point @ChrisJP I can't see why it is made so hard to transfer the power supply to a generator.

Maybe it’s just what manufacturers ask? This is from Vaillant instructions from 2006, they say 2 options available.

Clearly rather old and outdated it has not been the IEE for some years, now IET, so is that still valid, BS7671:2001 when that was printed, we have had BS7671:2008 and BS7671:2018 since then. And it does give the option of plug and socket.