The thing that most often go wrong with gas appliances and central heating
Today, this is a very short list. I'll add to it as I think of more things
that go wrong regularly.
This is one half of a safety device. It is the sensor that sits
in the pilot light flame and delivers an electrical current when hot, holding a
solenoid valve open, allowing gas to flow. If the pilot goes out the
thermocouple cools, the electricity stops and the gas gets shut off. Thing is,
these thermocouples seem to degrade over time, producing less and less
electricity. The boiler will spontaneously shut down, and be hard to light
A new thermocouple is about £5 to £10, plus, say £50 or so to fit and test
that it is working correctly and safely. Any decent gas engineer will have a
selection of 'universal' thermocouples in the van.
The flue fan:
These fail on modern boilers at about two year intervals. The boiler will simply
fail to start, and will sound different from usual, because the fan isn't
New fans vary from about £80 to £150, and take about an hour to diagnose.
Then there will be a trip to the spares shop the get a new one and another hour
or so to fit it.
Radiators getting hot when heating is turned off:
I'm getting quite a few questions about this at the moment. There are two
completely different ways of piping up and controlling a heating system, and
both ways can suffer from this for different reasons. Both will take a bit of
explaining but here goes...
1) The old type of heating system has a boiler that is kept hot all the time
'hot water' is selected on the programmer. Hot water from the boiler rises and
travels by natural convection up to the hot water cylinder and keeps it warm
too. When the heating is turned on as well, a pump starts and draws water from
the already hot boiler and pumps it around the radiators. When the house is hot
enough, or the heating is turned off, the pump simply stops running. But natural
convection can make hot boiler water circulate around the upstairs rads when the
pump is NOT on, so a "check valve" is inserted into the pipe feeding
the upstairs radiators. This check valve is held closed with a weight or light
spring and stops the natural convection occurring, but the pump is able to
overcome the resistance when it starts running. Upstairs radiators get hot when
the heating is turned of if this check valve fails or has debris lodged in it.
2) The modern heating system has both a room thermostat and a hot water
cylinder thermostat. These are temperature-sensitive switches that turn on the
boiler when heat is required by either the hot tank or the living space. When
the room 'stat calls for heat the boiler fires up and a 'diverter valve' directs
the hot water to the radiators. When the cylinder 'stat calls for heat the same
thing happens but the diverter valve directs hot water to the hot water tank.
When both 'stats are calling for heat the diverter assumes a 'mid position'
allowing hot water to flow to both circuits at once. When a diverter (or
'mid-position valve' as it is sometimes called) fails or gets some debris stuck
in it, hot water intended for the hot water circuit can leak past into the
radiator circuit, making the radiators warm even though the heating is turned
Whichever reason is causing radiators to get hot, a drain-down is required to
allow replacement of the faulty component, but there is no reason not to cope
with the problem by turning off the radiators that are getting hot. No harm will
Radiators NOT getting hot when the heating is turned ON:
Sometimes pump failure, sometimes failure of the motor in the motorised
valve. With pump failure, sometimes the upstairs radiators will continue
to heat up by natural convection leaving just the downstairs rads
System pressure repeatedly falling to zero:
This pressure gauge on the boiler repeatedly falling and the user
needing to 'top up' the radiator system by opening a valve means water is being lost from the system. Adding water using the
filling device restores pressure but it slowly falls back to zero
Sometimes this is a plain ordinary leak somewhere on the radiator
system but often the cause is the pressure relief
valve (PRV) 'letting by'. This, in turn, is usually caused by failure of
the expansion vessel to operate correctly.
The function of the expansion vessel is to smooth out the wide swings
in pressure that would otherwise occur in a seal system as it heats up
when ON, then cools down when OFF. It does this using an air cushion
inside the vessel. The air in the expansion vessel escape or dissipates as
time passes, leading to the system pressure rising too
high when turned ON so the PRV opens to discharge a little water to limit
the pressure. Then when the system cools, the pressure displayed on the
gauge will be lower than the user set it to, or even zero.
The expansion vessel needs to have it's air charge replenished. This is
done with an air pump connected to the Schrader valve on the vessel, and
it needs to be pumped up to 1.0 Bar. Do this with the pressure relief
valve one or a drain tap open to achieve an air pressure of 1.0 bar with
the system pressure gauge still reading zero. Then close the PRV or drain
tap and add water to pressurise the system as before, but now the problem
should be fixed.
A subsequent problem can be that the PRV noe fails to close fully,
having been operated, and it needs dismantling and cleaning or better,
replacing. This can be seen by checking the end of the discharge pipe
attached to it. There should be no signs of water. A good way of detecting
water discharging intermittently form the PRV is to tie a polythene bag
over the end of the discharge pipe and inspect it again a day later.
First published 16th January 2004
Last updated 26th September 2014