heating question

[vinny electric]

is it ok to wire a heating system with .75 flex as it is protected by a three amp fuse all answers welcome

 

[telectrix]

i can see the need for a 2 pole FCU on the boiler as the damn rubbishy things are always failing ( the boilers and ancillary bits that is ) and taking out RCDs.

 

[Dunc]

Steve not having done a CH system for a few years is the 3 pole isolator a manufacterers requirement??

Only if the FCU supplying the central heating system is located away from the boiler. If the FCU is beside the boiler then no additional isolators are required.

 

[telectrix]

steve. this switched L you are talking about. it will be isolated when the L&N to the boiler are isolated anyway, so what's the point?

 

[malcolmsanford]

Only if the FCU supplying the central heating system is located away from the boiler. If the FCU is beside the boiler then no additional isolators are required.

I was not woried about the isolation part dunc, just why the 3 pole part?

 

[telectrix]

perhaps it's because these modern boilers are so big and heavy, it takes 3 immigrant poles to carry in and mount it. then , as they are on less than minimum wage, he wants to isolate all 3 of them before lunch break.

 

[Dunc]

S/L is looped in the boiler for a combi / loop removed for system boiler(insert sw-wire in S/L)

I really think you're confusing some people here with your replys as they aren't overly clear. Normally when wiring a combi you will have a supply to the boiler (permanent live, neutral and earth). To satisfy Part L of Building Regs you would normally wire a 4 core flex (or 5 core if the on signal is volt free) to a room thermostat of your choosing. Isolation for mechanical maintenance would then be via the FCU which is located beside the boiler.

When wiring a S or Y Plan the main supply for the central heating system would often be located in the airing cupboard (beside the cylinder) as this is often the easiest and most efficient (from a cabling point of view) place to locate it. You will then (normally) require 4 or 5 cores sent to the boiler from the main wiring centre which would normally be located elsewhere in the property. The 4 cores will cover you for permanent live, switch live (on signal to boiler), neutral, earth and in some instances a 5th core is required to operate the pump (where the boiler has a pump overrun fitted). Due to the boiler being located remote from the point of isolation a isolator for mechanical maintenance is required. This would normally be a 3 pole isolator which would isolate PL, SL and N. In the instances where a pump overrun exists the core feeding the pump obviously does not need a switch in line with it as isolating the PL and SL supply to the boiler will automatically de energise the pump if the system is wired correctly.

There are however some exceptions which mostly exist in electric, oil fired boilers and old gas boilers. Often oil fired and old gas fired boilers will only require 3 cores (SL,N+E) as the boiler doesn't have a flue fan or pump overrun facility. Also, electric boilers will often have volt free switching so an additional core (6 cores) will be required. There are still more exceptions to the above but i hope i've covered most bases and everything makes sense.

 

[Dunc]

Also, as a side note, the isolator beside the boiler is a requirement of the plumbing regulations as far as i know. Don't think there is a specific electrical regulation that states that isolation is required at the boiler itself however it is good practice unless the isolator in the airing cupboard is lockable. I stand to be proved wrong but that is my opinion without getting the BRB out and spending hours flicking through it which i'm not about to do on a Sunday!

 

[telectrix]

what makes sense to me is to fit the isolator next to the boiler. i was always taught to fit isolators within reach of whatever they fed. e.g. cooker switch within 2m of cooker. i would never fit a cooker switch in an airing cupboard so would not do it for a boiler unless that was the location of the boiler.

 

[Dunc]

what makes sense to me is to fit the isolator next to the boiler. i was always taught to fit isolators within reach of whatever they fed. e.g. cooker switch within 2m of cooker. i would never fit a cooker switch in an airing cupboard so would not do it for a boiler unless that was the location of the boiler.

So what about the motorised valves, pump and other controls? If you were working on them then the isolator would not be 'within reach' if it were just at the boiler. Horses for courses i suppose but i've always wired it as i've explained above as it saves on having two wiring centres when you consider how many cores a modern boiler can require.

 

[malcolmsanford]

There is nothing in the regs regarding the position of the isolation, but if it is placed remotely then as per reg 537.2.1.5 it must be able to be secured in an open position either by lock or removal of handle. As a normal domestic FCU will not give you this then it is always considered to be best fitted locally.

As for the Isolation part a double pole isolation as a FCU would give, for the main supply into the boiler would give you adherence to regulation 537.2.1.1 for a TT system and if it was a TN system then a single pole device would be more than adequate.

As this would kill the boiler circuit including any control circuits, I can't see why you would want to also isolate a S/L

 

[Dunc]

As this would kill the boiler circuit including any control circuits, I can't see why you would want to also isolate a S/L

But in the instance where the boiler is wired to a S or Y plan where the main supply (a non lockable FCU) is located in the airing cupboard then 3 pole isolation is required at the boiler as the SL in to the boiler will still be energised. Is this making sense?

 

[steve.b]

As Dunc so rightly said. Take this scenario!---Boiler fitted on wall to nearest GAS/WATER supply (say outhouse) no electrics in sight(convenient for gas fitter) all controls ie: valve/cylinder stat/pump fitted in cylinder cupboard.Room stat self explanatory,hard wired or wireless.
Logical power supply located------------------You guest it IMMERSION HEATER SUPPLY,now for the dreaded 3pole isolator fitted at the side of the boiler,to isolate 1/switch wire 2/permanent feed 3/neutral.
WHY PERMANENT SUPPLY????? I Hear you scream, most modern boilers have digital displays or self diagnostics and require it. Just to hop back a post or two, 7core 0.75 from cyll cupboard to boiler you can now fit the controller at the side of boiler hence 4core from controller to 3pole isolator to boiler.
PS. Look for isolation for maintainence !

 

[telectrix]

afraid this discussion is now going way above my head as my knowledge of heating systems is confined to chucking another log on the wood burning stove.

 

[malcolmsanford]

I have my boiler in the garage and the motrized valve and the pump is in my airing cupboard. Are you saying that you would not still fit a 3 pole switch on it if you made the FCU safe, by removing the fuse in a domestic situation.?

 

[Dunc]

I have my boiler in the garage and the motrized valve and the pump is in my airing cupboard. Are you saying that you would not still fit a 3 pole switch on it if you made the FCU safe, by removing the fuse in a domestic situation.?

But removing a fuse from a FCU is not locking it off. I'm probably being pedantic here but there is nothing to stop someone bunging a fuse back in it and switching it on while someone is working on the boiler. Like i said a few posts back the isolation switch at the boiler is a requirement of the heating regs similar to how a boiler in a loft must have adequate lighting and access which includes solid floors leading to and around the boiler. If nothing else there is often a section in the boiler's installation manual that states that isolation must be present at the boiler itself. We therefore have to apply to this rule as there is a reg that states that equipment must be installed as per the manufacturers instructions.