Back Boiler Thermo Coupler problems
Back Boiler Thermo Coupler problems

Back boilers are becoming a thing of the past, but a recent experience left me very cold and was very lucky to find myself a plumber before the cold snap finally kicks in.  Back boilers are a very old style boiler that sit behind the fire usually in the front room.  These boilers are workhorses, they fire up with a "wuummpphh" and heat pump hot water around the radiators and to the tank.  The massive downside to these boilers are the energy ratings associated with them.

Back boilers in the day were pretty much a direct replacement to solid fuel boilers.  Fitted in the chimney breast they were fantastic boilers, and worked for an awful long time.

Unfortunately with every boiler, if 1 part breaks then as a safety precaution the rest of the boiler does not function.  The beauty of the back boilers is the lack of PCBs, if there is a fault diagnosing it can be pretty quick and relatively cheap.

Common Faults with Central Heating Boilers

If your a bit chilli, and the central heating is on - the water is not hot, or the radiators are not warm enough, then turning up the heat on the boiler is usually all that is required.  However, if this can be a sign of the boiler needs a good service and a good clean. (a boiler service should be every 12 months!)

If your water is hot, but your radiators are cool at the top, then there is ait in the system which needs to be let out by using a special key that fits in the top of your radiator (called bleeding) - you need to do this for each radiator on the system - otherwise the air will just travel round your pipes making your boiler work much harder and increase your fuel costs.

One of the most common faults with any back boiler is the thermo coupler.  This little coil of metal is filled with a substance that acts as a complete circuit when heated. This coil is exposed to constant heat fluctuations and is usually the most faulted part of any boiler.

How do I know if my thermo coupler has blown?

If your heating has not kicked in, there may be a number of reasons.  But if your the unlucky few, it means that the boiler will not kick in when its supposed to.  With ours, the water was not warm and the radiators were not getting hot at all.  So quick look at the boiler showed that the pilot light had gone out.  I immediately thought it was related to the extremely high winds we were experiencing and began to remove the front of the fire to attempt to relight it.

The process is pretty simple, underneath the fire, the boiler has a manual gas valve (a push button) and an ignition button.  Pressing the gas valve button in, and repeatedly pressing the ignition will ignite the pilot light.  Once lit, after around 30 seconds slowly depress the gas button and if your lucky, the pilot light will stay lit.  This in turn will allow your central heating to kick in, and all is well.  Its also worth noting, that a thermo coupler does not work "sometimes", if its broke, then no amount of attempting to keep the pilot light lit will work.

So in 1 line: If the pilot will not stay lit - then your thermo coupler may need replacing

If your pilot light does not stay lit, then the boiler will not start, and a phone call to your local corgi registered plumber is required - where he/she will strip the offending boiler and 9 times out of 10 replace the thermo coupler.

Do they still fit back boilers?

I asked this today to our corgi qualified plumber, to which he replied with a little giggle in his voice "well there are boilers out there that do - but you need a drain".  Essentially a new back boiler is a combi boiler that fits into the same place as your current back boiler.  Unfortunately - as with all combi boilers you need a pipe that will take excess water away - and most houses will not have this - and installing it at such a low level will provide very little flow - causing more problems.  So he said "your best bet is to remove the old back boiler, and replace with a combi - in the loft / utility room.

But now, with all the safety regulations in place, its deemed that these once proud back boiler workhorses can be fitted no more due to them not being energy efficient.