Reality check, know less than nothing about central heating.
14 year old Gloworm combi ,hot water and heating fail to do anything.
Installer and maintainer of last few years now retired.
New plumber diagnoses PCB failure and replaces PCB.
Just after leaves , HW and heating fail again.
Plumber returns, pilot light is going out, says it may need relit couple of times until boiler gets back to heat as been off for a few weeks.
Pilot going out continues over few days.
Plumber returns, says spent over an hour on phone with Gloworm service and most parts now obselete, but can change thermocouple for generic part.
Thermocouple fitted appears to still be extinguishing pilot light on heating demand.
Plumber says boiler requiring replacement.
My (very) limited understanding puts the pilot light and its thermocouple and valve as seperate sub system which would be generic and replaceable without junking boiler.
Or is it the thermocouple controlled supply valve that is an unobtainable part?
It may well be that most economic repair is to replace boiler but just like to check. Especially after some less than stellar experiences with `rated people`.
Thanks for any advice.
The pilot light would source its gas from the gas-valve.
I would suggest the pilot jet needs looking at as well as the area
surrounding the flame. Perhaps the gas rate for the pilot needs to be
I see no reason why the whole boiler needs replacing. Most plumbers
haven't got a clue. Replacing a boiler is easy money for them.
Do you have the boiler make and model? There are many installation and
service instructions that can be found online for most boilers.
Make sure that the pilot flame is a decent size, and that the
thermocouple sits fully within the flame. Check the pilot jet first, and
blow compressed air through it if possible. *Don't* poke anything metal
in it. Also, there *may* be a separate pressure regulator for the pilot,
enabling you to feed it with a bit more gas. If it lights and stays lit
when the main burner isn't running, there can't be all that much wrong
with it - certainly not enough to scrap the boiler.
The pilot light, when lit, heat up the thermocouple. The thermocouple
thus generates a tiny current which operates a solenoid. The solenoid
latches a valve, which retains the button you have to press, to allow
gas through to the pilot jet.
Pilot goes out, thermocouple cools, stop generating current, nothing to
retain the pilot valve open, so it closes to shut the gas off.
Assuming the thermocouple is correctly located in the flame and
generating enough current, your likely problem is a fault on the fine
winding of the solenoid, which is failing to adequately hold the valve
open. They can be replaced, but a gas fitter would be unlikely to know
how - he would replace the entire gas valve, if one can be sourced.
On Fri, 29 Aug 2014 22:40:11 +0100, Harry Bloomfield
The solenoid winding uses very thick wire on account the thermocouple
only generates either 15 or 75 mV afaicr so needs a low resistance
winding to generate sufficient ampre turns (the thermocouple can
generate several hundred mA, perhaps even an amp into a short
circuit). The loop resistance of the mini pyro co-axial cable is quite
low specifically to meet the needs of such a low voltage high current
Even so, the ampre turns used to hold the solenoid latched in is well
below that required to operate the solenoid but this is not a problem
since most of this work is done by the operator pressing the solenoid
into the valve open position reducing the reluctance of the magnetic
circuit allowing the low value ofavailable ampre turns to generate a
sufficiently strong holding field.
I think the brass shim, normally used by conventional solenoids, may
even be omitted to maximise the latching effect from the lower than
usual magnetic force available in this case.
High resistance faults can develop in the solenoid circuit, possibly
at the thermocouple connector or, more usually, in the solenoid
itself. Re-making the connection might fix the problem but, more
likely, assuming the correct generic replacement thermocouple had been
used, it will be a fault in the solenoid. Since this is part of the
gas control valve, an item that cannot be dismantled and repaired on
site due to safety regs, it will need to be replaced with a new one.
Replacement gas valves are usually readily available for virtually
all models of boiler since they can be readily swapped out on site to
effect a safe repair. You just need to identify the gas valve part
number and track down a suitable supplier to verify availability and
pricing before you either call the gas engineer back in to finish the
repair or call on the services of another gas fitter with more of a
I'd be surprised to find that a 14 year old CH boiler can't be
repaired with a replacement gas valve (replacement gas valves for my
30 year old Ideal Mexico Super CF 100 floor standing boiler are still
readily available), but it won't do any harm to check on
availability of a replacement gas valve before being railroaded into a
complete boiler replacement.
 I've seen references to both 15mV and 75mV thermocouples so
there's a possibility that the fitter used the wrong generic part
(either too low a voltage or not enough amperage).
 I've just noticed a couple of invoices for boiler repairs tucked
into the boiler installation and servicing guide that we were supplied
with when the boiler was first installed some 30 years back.
The oldest was a bill for a service including a new Honeywell gas
valve and thermocouple back in November 1998 for a total of £105. The
later bill, dated Dec 2012 was for supplying and installing a new
thermocouple which cost me 45 quid. All in all, the total repair costs
on the boiler have amounted to 150 quid over a 30/31 year period.
Aside from replacing the pump and isolator valves about a decade or so
back (and the inevitable spend on Fernox corrosion inhibitor), it's
not cost me very much to keep the system running.
The only items I'm seriously thinking of replacing now are the two
way Honeywell motorised diverter valve and, rather optional in my
circumstances, a new Varta Memtec 3/V 150 backup battery for the
Potterton 2000 controller.
ISTR the price for the diverter valve being in the region of 70 quid.
After some 30 years of service, it owes me nothing so it's not so much
the cost as the inconvenience of draining down and refilling the
system to effect the repair. Even this is an overdue opportunity to
flush and replenish the system with fresh corrosion inhibitor so it
looks like I'm going to have to get up off my big fat arse and do some
long overdue maintenance RSN. :-(
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