Combi boiler wiring question.

[Stephen H]

Now There is a new condensing combi boiler being fitted in the kitchen along with two heating zones.

I have two wall mounted programmable thermostats, one for downstairs radiators and one for upstairs radiators.

The two zone valves are in the airing cupboard along with the main supply for the whole heating system. (this is taken from the immersion FCU and hence on its own MCB at the Consumer unit as the HW cylinder is soon to be redundant)

I am running 5 core cable from the airing cupboard wiring centre to the boiler in the kitchen. The wires are obviously earth, permanent live, permanent neutral and a pair of volt free switch contact wires (from the two zone valves) to control the CH part of the condensing combi boiler.

There is a fused connection (FCU) switch in the airing cupboard that will turn off all power to the boiler, zone valves and the two programmable thermostats.

Now my question is do I need to put in a fused connection switch adjacent to the boiler with a neon indicator in the kitchen, or is the FCU in the airing cupboard sufficient even though the engineer will have to go upstairs to isolate the power?

Regards,

Stephen

[A.Lee]

I'd be tempted to put one in, just so it is easier to service in the future, the cost and time to fit it is minimal.

[Stephen H]

That was my initial thought too, but the boiler is going in between a kitchen window and a kitchen internal corner. There is simply no room at either side of the boiler to fit a FCU.

I'm certainly not putting a FCU under the boiler as that is considered bad practice in case the boiler leaks.

My only option is to put one on the wall above the boiler next to the flue or to put it on an adjacent wall.

Also this is 5 core flex, so that means I'd have to cut the brown and blue wire to put via the double pole switch and allow the earth and 4th and 5th core to continue on towards the boiler in broken as theres no wiring choc block in a FCU.

[John Rumm]

You want the whole system to be isolated from one place. Otherwise there is a danger that a maintainer could see an adjacent FCU and assume that will make the boiler safe to work on without realising that there is additional control equipment still powered.

Hence given where you are starting from, it may be better to leave the isolation in the airing cupboard, and don't have a local switch (you can use a flex outlet if that makes things easier / neater), unless you can arrange things such that there is no possibility of the control wiring introducing a switched mains feed to the boiler.

[Dave Liquorice]

Those switched lives could become live if the system demands heat and it's not been switched off upstairs. Single point of isolation is much safer.

You also only need to bring one switched live down from the zone valves as either valve being open means that the boiler should be on.

I'd use a flex outlet to the boiler possibly with a lable on it saying where the DP isolation switch is.

[A.Lee]

It's a combi, so will only need a permanent 230v L/N/E supply, and a volt free switching circuit.

Yes, some use 230v, but the majority have got a LV/volt free switching circuit.

Alan.

[Dave Liquorice]

Without volts how does the boiler detect the state of the valves?

Presumably the "volt free" switching circuit is just a "make to fire" loop? So you know have "volt free" and mains in the same cable. Cable fault (nail, screw WHY through cable, rodents) and you get mains on the "volt free" wires and zap your boiler...

[Heliotrope Smith]

"Dave Liquorice" wrote in news: [e-mail]:

Volt free switching (usually thermostat/programmer) means the switch does not produce any voltage but transfers the voltage produced by the boiler to switch itself.

Combi boilers usually have a link so that the heating would be switched without a room thermostat thus transfering voltage from one terminal to another. Take out the link and wire in a room stat (volt free) and this would act in the same way as a light switch.

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[geoff]

In message , Heliotrope Smith writes

Or, put another way, the common of the relay inside the timer isn't connected to the live driving the timer