Gas boiler

[tabbypurr]

A metal bulb containing liquid expands when heated, pushing itself down the capillary tube and opening the switch.

NT

[alan_m]

What colour is the main burner flame? Mainly blue? Mainly yellow/orange?

[Tim+]

Lovely. How does that help Grumps? ;-)

Tim

[Roger Mills]

It might encourage him to do likewise and rely on the monitor rather than having the boiler serviced.

[Dave Plowman (News)]

Only if the boiler isn't room sealed - and it seems his is.

[Roger Mills]

You can still get CO from a room-sealed boiler if the casing seals are not intact.

[Dave Plowman (News)]

You can get an explosion if you have a gas leak too. Don't use any boiler with faults.

[Grumps]

Yep, room sealed boiler, and I also have a CO alarm.

[Grumps]

Mainly blue. Can't see the top of the flame though.

[John Rumm]

Which is why many modern designs run the case at negative pressure - so if there is a leak it draws air in, rather than allows combustion products out.

[Dave Plowman (News)]

When I had this problem, the flame on mine was a normal colour. Only very small.

[mechanic]

Can't see any mention of this on the Vaillant or Bosch sites?

[RobertL]

That can't be right can it? Surely if the pilot goes out then all gas must be stopped regardless of whether there is 240V or 0V on the valve.

Robert

[Roger Mills]

Indeed. AIUI, when the thermocouple is hot it holds the pilot valve open and 'enables' the main valve in some way. It clearly wouldn't be safe for the main burner to be discharging gas just because the valve is receiving a 240v demand unless the pilot is also on.

See

for sort of explanation

- American, but I guess their valves were similar to ours.

[Grumps]

Size matters ;) Mine was clearly too small and being sucked away from the thermocouple when the main burner started. Then the thermocouple cooled down after a while, and then the whole thing shut off. My flame is much bigger now after the gas man blew through it. It's still working two days later - touch wood.

[John Rumm]

Yup I would not expect it to pass gas on the main valve if the pilot is not lit - however the only bit I was not certain of what happens if you extinguish the pilot when the main valve is already lit...

Much would presumably depend on if the main burner will also keep the thermocouple hot enough to hold the valve open.

[Vir Campestris]

My old one was positive pressure - but fresh air. The burner was completely enclosed, and the only way out for combustion products was the flue - or against a pressure gradient. I never did work out why that was dangerous - but I was told it was.

Andy

[John Rumm]

On my Vaillant, the fan's induction side is connected to a duct that stops open ended inside the case. The case is open to the the outer part of the concentric flue - but the duct is not attached directly to the inlet. So the fan is actually drawing its combustion air from inside the case, and that in turn will pull air in through the balanced flue. I believe this type of design is fairly common these days.

[Fredxxx]

One advantage of the fan on the exhaust side is the boiler can be run safely with the cover off.

[mechanic]

The balanced flue connects to the outside world? How does that ensure under-pressure?