Heating causes boiler to trip and blow the fuse

Fatman

I have an annoying problem that started yesterday out of the blue

If I put my boiler on, to heat hot water, it works fine

But as soon as I start the heating by turning up the thermostat, it trips the boiler in the fuse box and also blows the 3a fuse where the boiler is wired into the wall?

Anything I can check before calling someone in?

Thanks
Andrew

CrazyRed

The only thing different about a boiler heating hot water and a boiler heating the heating circuit is the pump.

First thing I'd look at is the pump and see if it's seized - this would cause an overload and the fuse to blow.

shaun_from_Africa

CrazyRed wrote: »

The only thing different about a boiler heating hot water and a boiler heating the heating circuit is the pump.

It is highly likely that the heating system in question uses the pump for both heating and hot water as this is still the most common setup there is.

It is far more probable that the fault is with the motorised valve seized up in the "hot water only" position and it is this causing the fuse to blow when it tries to motor to the heating position.

Fatman

Th boiler is about 12 years old if it makes a difference.

How would I see the pump or this motorised valve?

shaun_from_Africa

The age of the boiler won't have any relevance on the problem as it is simply a heater and doen't know where the hot water is going.
If the boiler was at fault and blowing the fuse for heating then it would do the same for hot water.

The pump and valve are normally located close to each other and may well be in your loft.
This webpage gives a few examples of what it may look like.
http://images.google.com/search?tbm=isch&hl=en&source=hp&q=central+heating+motorised+valve&gbv=2
They are often in two parts, the mechanical valve and the electrical actuator and the two parts can sometimes be pulled apart.

If you can do this (don't try it if you are not confident with what you are doing), you will be able to detemine whether it is an electrical problem with the actuator of if the mechanical part of the valve is jammed in one position.
Simply take the electrical actuator off the valve assy and switch the heating on again.

If the boiler starts up and the fuse doesn't blow then it will be the mechanical part of the valve jammed and this will need to be replaced.
If the fuse blows then it will be the electrical actuator or the wiring between it and the central heating controller.

One other possibility is the thermostat or the wiring to it, but my gut instinct would be the valve.

Fatman

Turns out I have 2 separate pumps. One for CH and one for HW.

The CH pump appears to be broken, ill be replacing it on Wednesday!

Thanks for all your help.

barose

My system today started blowing the 5 amp boiler circuit breaker. I tried several things but noted that the fuse blew even with the thermostat turned down (off). So with thermostat off the boiler isolation switch on and pulling the heating timer control on - fuse blows. With the heating timer control 'off'' and the hot water timer control 'on' the boiler runs fine. I tried disconnecting the pump live wire, no difference and unplugging the boiler, no difference. Seems as soon as the heating timer control lever is moved - pop.

Could really use some advice on where to look.