thermocouple constantly replaced

[endoftether]

Long standing problem requiring long explanation

I have an open system with an Appolo boiler and a thermocouple which includes the interuptor wires. The heating system has always been prone to the occasional loud bang and every now and then, say twice a year the pilot light would go out and not relight until we swapped the thermocouple.

At the moment I am replacing the thermocouple almost monthly and nobody can give me a satisfactory answer as to why. In the last twelve months I have had a new fan fitted, a new injector bray and a brand new pump. In the past the overheat cut off device has been replaced.

The pilot light ALWAYS goes out after a series of bangs. I bleed the radiators weekly but rarely get any air out although you often hear a trickling sound in the pipes as if they are not full. I have drained the system on a number of occasions and added sludge removers and limescale treatments (I live in Manchester which does not have much limescale in the system).

This weeks it's behaving oddly again. Now the boiler seems to be literally boiling. It switches itself off after about two minutes then fires up again moments later. It's as if the overheat cut off device is switching it off but I had always assumed that the cut off device sent a signal to the thermocouple and broke it.

Out of interest the hot water only cycle seems to be well behaved.

Can anyone help me solve this?

 

[HenryRoot]

I would suspect that you had either a blocked cold feed or a duff valve in your header tank.

Check the header tank to see if it's full, if not replace the valve. If it is and if your pump is on the flow side, together with the cold feed and open vent, try this:- Turn all power off, isolate the pump using the pump valves, remove the pump and then carefully open the topmost valve. You should now have filthy water pouring all over the place or better still into a bucket. If you do, you may have a problem with your pump or motorised valve. If not, then you have a blocked cold feed. If this is the case do sa search on this forum as there are about a million posts concerning this.

 

[endoftether]

Thanks for the response.

The header tank appears to be working fine to me and as I said the pump is brand new. When I bleed the rads the water is very clear (probably as a result of all my earlier attempts at flushing the system). The feed tank empties quite readily and refills OK when I let the ballcock go

I'm not sure your reply actually tackles the root cause anyway. is the cold water feed in constant use? I thought it was only required to top up the system if it expanded. But why is it continually expanding?

I've had 33 reviews of the problem to date and only one offer of help.

 

[weargas]

the gas valve, olive green square button or round grey button?

 

[endoftether]

The gas valve is a green square button (the pilot light ignition is a round grey button)

Waiting in anticipation...

 

[endoftether]

I was feeling around in the cylinder cupboard when I came across the instructions for the pump. It syas that it should be installed with the propellor shaft horizontal.

the original pump was installed vertically and the new (identical) one is also installed vertically.

Could any of this be caused by an air lock in the pump which will just keep recurring?

Cheers

 

[weargas]

sorry, didn't see your reply to my gas valve button question.
i know your other thread tackles the overheat issue but my direction was gonna be more the fact that you need a thermocouple monthly. It could be that the heat exchanger needs a good clean between the fins. if there is a restriction then too much heat will be held down inside the boiler and could burn thermocouples out earlier than normal. Are you buying universal thermocouples?

 

[endoftether]

Thanks again.

Like you, I'm not entirely sure that the thermocouple problem is directly related to the overheating.

When the last plumber replaced the fan he left us for a couple of week with no boiler and everything was removed fom the front. I took the opportunity to give it as best a clean as I could with a small brush but I'm no expert on whetehr I'm doing it right. Perhaps the recent application of limescale remover will also help reduce any restriction.

If too much heat is held in the exchanger why does the overheat cut off device not just switch it off I wonder? Isn't that what it's for?

The thermocouples include the interuptor and I'm buying exact replacements at about £10 a time.

 

[weargas]

Perhaps the recent application of limescale remover will also help reduce any restriction.

limescale remover in the water system won't clean throught the fins in the flue passage part of the heat exchanger. which is where the restriction i'm talking about would be. cleaning it with a small brush wouldn't be that effective. I use cleaning blades when servicing apollos, made by Regin and available in any heating suppliers for a couple of quid. you can push them through the fins between the waterways to be sure the flue passage is clear.

If too much heat is held in the exchanger why does the overheat cut off device not just switch it off I wonder? Isn't that what it's for?

the overheat cut off is fixed to a water pipe so it cuts off based on water temperature. it wouldn't necessarily cut out for a restricted flue passage.