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Our central heating system stops working occassionally.

The whole house gets cold, because the radiators are cold. This persists for several hours. I've tried turning the room thermostat up which results in the relay clicking over and the water pump running up but the gas does not light up to heat the water. So the system is just pumping cold water around. The timer is set to ON by the way.

This intermittant fault continues for several weeks with the heating working properly and then not working for 5 hours or more and sometimes even a day.
The other symptom is that after a few weeks of the above carry on the system water content has disapeared and the central heating is permanently cold (as it won't fire up cus there is no water). I have to top it up again and we end up with the initial problem again.

As mentioned It's not a timer problem BTW as this is set to permanently ON.

We've had four visits from British Gas Homecare. Each time the focus is on the expansion tank and the water loss. They end pumping stuff - not quite sure what they are doing - but it hasn't made any difference.

I have a theory though and wondered if it makes sense. I am no central heating engineer so apologies for talking rubbish if that is the case.

For me the problem could be one of two things:

(1) It's a sticky gas valve. Since the system is clicking on ok, the pump is running up, but there's no release and burning of gas to heat up the water.
Not sure that this one explains where the water is going though over a period of a few weeks. Still, its a thought.

(2) It's the water temperature sensor (thermocouple or whatever it is).
Since if this is faulty the system may think the temperature is too high when it isn't and therefore prevent the gas firing up as a safety measure.
This may explain where the water is going over the few week period since conversely the temperature sensor may be indicating too low a temperature causing the water to be overheated and forcing it out of the external expansion vent.
Funnily enough the central heating water temperature gauge on the front of the boiler (Vaillant) consistently reads low (<30 deg C) and the needle never moves. Now this can't be right as the radiators themselves are almost too hot to touch (50 deg C) so the water inside them will be even hotter especially where it leaves the boiler. I've pointed this out to the Homecare people and they've just shrugged their shoulders.

Any thoughts, advice, ideas?

Driving us mad this one and getting worried given we have a baby and the house gets damned cold when the system decides it aint gunna work for the night.

Cheers

N

How old is your boiler? Who made it?

What are the radiators like? (In good shape or looking a bit old?)

Do you have trv’s on your radiators?

What’s the tank made from?

Hi Peter,

It's about 8 years old. Radiators all good. There are control valves on each radiator - all set on high.

Made by Vaillant. Its an open-flued VCW GB 240H combi-boiler.

Not sure what the tank is made from.

Heating of hot water is fine. Always fine.

It's just that it decides not to open the gas and light up. The pump starts up fine - you can here a click and pump running but you don't hear or see it lighting up that you normally do. So it just pumps cold water around. This will last for hours or longer meanwhile the house is getting cold.

Thermostat in the hall (the only one) clicks the system into operation when you turn it down then back up again. Just not lighting up the gas. Pilot light is on.

Then, after a while it will just decide to start working again. I think it must be an intermittant fault with either the gas valve / relay or the the central heating water temperature sensor. I keep suggesting this but they don't listen. Not that I know what I'm talking about! But, their solutions have all been ineffective - replacing expansion tank valves, pumping the expansion tank and so on. I really dont think its anything to do with this.

It's got to be something to do with the control associated with heating the central heating water - gas valve, relays, or thermocouple.


What do you think?

I would guess at - and it is only a guess as I haven't seen the boiler and don't know the make / model.

Lazy gas valve, fan or air pressure switch is to blame - probably the air pressure switch - normally the case with intermittent faults with the burner not igniting.

I don't rate the BG homecare and I wouldn't pay for it - or advise anyone else too. Heard too many people complain that they have to keep calling them back and the boilers never get fixed - have they tried to sell you a new boiler yet ? (wont be long before they do).

When i go to a job I fix it, if i'm not 100% confident it's working correctly - I'll call back later on - or the next day to check it. BG seem to turn up - do things - without telling the customer what they're doing - then leave - and that's it until the next time you call them back.

Think its been sorted. I went through the following possibilities from first principles with the chap.

Yeh, we had the.. we can sell you a new boiler thing. But told them we were not interested.

Here is the summary of what I discussed...

The central heating stops working for several hours, sometimes as long as a day. Then it will start working again!
When we have the problem the following should be noted…
[list]The pilot light is on ok,
The timer is set to Always ON,
There is also enough water in the system[/list]
Symptoms…
When you adjust the room thermostat you can hear a relay click over and the pump starts running BUT the gas does not fire up to heat the water. It’s just pumping cold water around. The house is cold and the radiators are cold so it should be firing up but it isn’t.

[b]Why is the gas not firing up to heat the water?[/b]

[b]What controls this?[/b]

[b]Water temperature sensor[/b]
If the water is too hot already (e.g. sensor reads a high temperature) then the system should NOT allow the gas to fire up
CAUSE: Faulty temperature sensor or controller board u/s

[b]Flow rate of water[/b]
If the water flow rate (via the pump) is too low or weak then the system should NOT allow the gas to fire up as the water would heat up far too quickly and cause an over-temperature scenario
CAUSE: Faulty pump (not enough torque) or Faulty pressure sensor / switch arrangement (sensor probably mechanical / hydraulic which in turn operates the relay controlling the gas valve) or controller board u/s

[b]Water content[/b]
If there is insufficient water content in the system then the system should NOT allow the gas to fire up for obvious reasons.
CAUSE: No water in the system or system believes there is no water in the system thus could be a pressure sensor problem

[b]On-boiler central heating thermostat (sets the temperature of the central heating water content). Various settings for Winter, Summer etc.[/b]
If this setting is too low (perhaps artificially so due to a fault) then the system will NOT fire up the gas as it believes it does not need to since water content temperature is greater than the thermostat setting (which is incorrectly low)
CAUSE: Faulty thermostat or controller board


[b][i]The problem by reductive analysis was associated with water flow rate. There is a hydraulically actuated rod that pushes in and out dependent upon water presure being high (pump running) or low (pump not running). The water pressure was fine but the rod wasn't always making good contact with the microswitch that operates the relay to open the gas valve. A simple screw adjustment seems to have fixed the problem by bringing the microswitch closer to the rod.[/i][/b]

For now!

Well if its pumping cold water around the system and working well for a while, I think it could be the PCB board.

I think something on the PCB board is over heating and the fuses are tripping.

5 hours does seem about right for a fuse to reset it’s self.

But it could be anything.

Get hold of the manufacturers instructions, if you have not already got a set. There is a telephone number to ring on their web site. Also one for technical help.
At the back of the manufacturers instructions, should be a detailed fault finding guide. Intermittent faults are particularly difficult because they never appear when the service engineer calls. This will enable you to make a start at possible fault finding or at least heading the service engineer in the right direction. You need to find one who is competent, Corgi registration is not sufficient as you have found. Strongly recommend you do not try hands on fault finding as gas and live electrics are involved, but use the guide as an intellectual exercise.