Pilot flame failure , new boiler?

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Adam Aglionby wrote:

    What info on the gas valve? You can still get many gas valves at >40 yrs of age. Get the Part number if possible and repost.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 28/08/2014 16:38, Adam Aglionby wrote:

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 increased.

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Does the pilot light appear to be the same as you remember it? If the jet is partially blocked, or the setting incorrect, they can 'blow' out when the main burners light.
--
*If a pig loses its voice, is it disgruntled? *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 28/08/2014 18:29, Dave Plowman (News) wrote:

+1
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.
--
Cheers,
Roger
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Adam Aglionby wrote on 28/08/2014 :

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.
--
Regards,
Harry (M1BYT) (L)
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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 circuit.
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[1], 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 clue.
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[2]), but it won't do any harm to check on availability of a replacement gas valve before being railroaded into a complete boiler replacement.
[1] 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).
[2] 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. :-(
--
J B Good

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.