If you need to fit a new thermocouple
The "copper bit" to which ahmed refers is called a thermocouple. His "front cover" is the inside front cover not the white enameled outer cover - take that off first.
On the front of the boiler you should now see a metal plate (the front cover) just over one foot wide (320 mm) and about 7.5 inches (190 mm) high with a small window in the top right hand corner, through which the pilot light is visible when it is alight.
The plate is secured by four screws. You'll need a fairly large cross-point screwdriver (bit size is around PZ3 if you are using an electric screwdriver). Remove the front plate.
The thermocouple has a "junction end" (ahmed's "pointy bit" that "looks like a small pen") that is held in the pilot flame and connected via a long copper wire to the "button end" that is inserted into the gas control unit at the back of the boiler. The junction end is held in place by a brass gland (looks like a nut around the copper wire - 11 mm spanner). The button end is held by a similar 10mm gland into the gas control unit.
You can buy a replacement thermocouple from a plumber's merchant. I got mine from the Parts Centre at Plumb Centre. It was just over £5. The part is made by Honeywell marketed as TRADELINE Q309A 2788. You may, however, be able to get a 'universal' thermocouple that will do the same job. The Honeywell part has these specifications on the packaging which may be important if you are trying to find an equivalent part:
900 mm
45 s
900 mm is the length of the thermocouple from the tip of the junction end to the button when the copper wire is straightened out (however, the old part that I took off was shorter - only 750 mm)
45 s - I don't know what this refers to, so it may or may not be important.
To fit the new thermocouple you'll need to unwind it (it will come wound in a coil) and then bend it carefully to follow the same route as the old one around the outside of the boiler (inside the outer cover).
Two things are important when fitting the new thermocouple:
1. Don't bend the copper wire around too sharp an angle (no sharper than a one inch radius.
2. Don't do up the gland at the button end too tightly - no more than a quarter turn beyond finger tight.
On one of the universal thermocouples I looked at, it read, "GAS APPLIANCES (SAFETY) REGULATION 1995 This thermocouple is currently exempted from requiring a fittings certificate."
I take that to mean that you are not required by law to have a qualified gas plumber to fit the thermocouple for you.
This is an easy job if you can just use a screwdriver and a spanner! I know almost nothing about how gas boilers work and I managed it easily. My boiler has now been working fine for a day since I did the job.
hedles
November 2007