"Heavy duty" thermocouples

[Dave Liquorice]

Hi,

Do such things exist? The local community centre has a gas boiler that eats thermocouples, a new one will last not much more than a year.

Unfortunately the maintenance is under contract via the council and I suspect they just bung a (cheap?) generic one in each time it fails.

Calling them out everytime it fails is a PITA, it doesn't help that that they have yet to appear to respond to the call out made 10 days ago and repeated calls since... If I knew that a "heavy duty" thermocouple existed I might be able to beat 'em over the head to fit one and have a sensible life (5 to 10 years is sensible in my book).

What causes a thermocouple to fail anyway? Is the load presented by the gas valve important? Does the thermocouple need to match to the gas valve in anyway?

[Roger Mills (aka Set Square)]

Don't know about heavy duty ones - but a bog-standard one should last far longer than a year. I wonder whether the pilot flame has been set too high, and is making the T/C hotter than necessary, reducing its life? There's probably a separate little governor for adjusting the pilot.

[John]

The generic universals can be quite crap. I used to carry as stock Honeywell Q309A in the Tradeline blister wraps and found these fitted "most" of the boilers I came across. A couple of "decent" universals sufficed for the odd appliance which wouldn't take the 309s

[Colin Stamp]

Or it could be the opposite - the pilot flame is too weedy, so the thermocouple isn't getting quite hot enough, and the system is marginal as a result.

Cheers,

Colin.

[Tim S]

They burn out - quite literally. Sometimes, it's that they are wrongly positioned, by design error or installation error, and they get exposed to the main burner flame, which is comparatively vicious compared to the pilot flame.

I grew up with a potterton boiler that ate thermocouples too. Think my Dad managed to bend the end slightly to keep it further away from the main burner.

A boiler shouldn't really be having one per year, but some make/models are prone to it.

Tim

[R.P.McMurphy]

useful thread. What is the best position for the thermocouple then?

Cheers

Steve

[Dave Plowman (News)]

My RS80 has had one replacement in near 30 years. ;-)

[Roger Mills (aka Set Square)]

It needs to be in the pilot flame but *not* in the main burner flame - and the pilot needs to be adjusted to be as small as possible, consistent with heating the T/C and not blowing out too easily.

[Andrew Gabriel]

...and reaching high enough into the main burner to be sure of lighting the gas flow very quickly (or you get explosive main burner ignition).

The installation/maintenance instructions usually have a picture giving the exact positioning of the thermocouple and size/height of the pilot flame.