Glow-worm boiler intermittently trips off on water only

[JohnED]

I have had a 5 month saga with an intermittent problem that my local plumber has given up on. It's a system about 18yrs old with Glow-worm Fuelsaver 80F boiler which works fine when the central heating is running but if the hot water is running by itself there is occasionally a banging noise in the boiler and the thermal trip operates. The pipework from the boiler to the cylinder has been relaid (because it was thought to be blocked around a compression fitting - not true in the event). The whole hot water path has been flushed - perhaps a minor improvement. The system will run for a couple of weeks OK then crash off once every day for a few days then carry on OK. I notice that the Honeywell mid position divertor valve is allowing some heat to transfer to the CH pipework when on HW only - too hot to touch 2ft from the valve. Is there any experience of these valves sticking and perhaps reducing the flow? With the warmer weather approaching the CH will be off and the problem can only get worse.
Any suggestions would be gratefully received.
Thanks

 

[gas4you]

Was the pipe work (flow & return) removed from the h/w cylinder and the coil inspected for blockages? I have had joint on coil flow totally blocked before even though it is 28mm

If this is the model I think it is has the thermostatic heat controlled pump over run/thermostat been checked. This should let the pump keep running after a call for heat has finished until water temp has dropped, although this would work the same for ch side.

Slight possibility of a valve blocking, if it is an old one. Have you a bypass fitted in the pipe work, if so is it adjusted correctly?

Banging in the boiler is usually lack of flow especially if overheat is tripping out!

 

[JohnED]

Thanks Dave, I believe the coil was flushed and found OK. The thermostat in the boiler was replaced as part of a service prior to this saga. The bypass was opened fully by the plumber to allow us to get some hot water without crashing off the boiler while he got around to replacing the pipework. I specifically asked and he reassured me it had been closed again afterwards. I assumed the bypass was either fully open or fully closed, how do I determine the optimum position ?

John

 

[JohnED]

PS The pump does run on after the boiler shuts down.

John

 

[gas4you]

As an experiment I would open the by-pass fully and see if fault still occurs. If it doesn't then we know that circuit is good as far as this. As a general rule you would normally close the by-pass fully then open approx 1/2 turn, assuming it is a gate valve. If it is an old gate valve then there is always the possibility that it appears to open/shut by the head turning but the 'guts' of it has stuck shut!

 

[gas4you]

PS The pump does run on after the boiler shuts down.

John

The pump obviously has nowhere to 'pump' then

 

[JohnED]

Thanks, will try the gate valve. It seems in reasonable condition.

John

 

[JohnED]

The system seems to run OK with the bypass gate valve fully open. The pump and divertor valve are next to the cylinder in the airing cupboard. With the bypass gate valve fully closed and only the HW circuit on, the system is noisier but the overheat on the boiler has not tripped out. There is a pronounced noise of trickling water from within the cylinder which I have not noticed before. I guess the result is inconclusive so far - but as the system could go for a couple of weeks before tripping out before it was unlikely to perform straight away. There is certainly no major permanent blockage in the water circuit but I'm still suspicious about the divertor valve as the temperature on the CH side seems to vary widely - hot for a couple of inches to hot for a couple of feet on the pipe with the CH set to off.

Thanks for your suggestion Dave, I have a better idea of what's going on and I will monitor it for a while.

John

 

[andypenney]

Morning Did you sort out your 80F Boiler ??
I am just replacing mine with a Worcester,, This one works fine ..

 

[JohnED]

I have to admit that I have just been living with the problem as it is intermittent and occasional. However during the Summer months when the radiators have not been on it has become more than irritating and I plan to call my local plumber back in a couple of weeks as he did a good job on an unrelated CH problem. The only extra information that I have got over the months is that when the fault is about to happen the system gets very noisy and the cold feed from the header tank in the loft gets very hot. The thermal trip in the boiler goes when the boiler turns off and shortly afterwards the pump runs, there is a "rushing" sound through the cold feed which goes cold very quickly. After resetting the thermal trip the boiler can then continue for a week or so before it happens again. It still does not happen if the central heating is working. I do not think it is a boiler problem - although it will need to be changed before long I want it sorting before then.

 

[mickyg]

obviously the circuit is restricted somewhere. Have you checked your honeywell MV? the rubber ball inside has been known to swell causing a blockage.

 

[andypenney]

You dont need any spare parts before I bin this fine 80F ????

 

[JohnED]

Thanks Mickyg - I have had my suspicions about the diverter valve but a plumber on a previous visit discounted it. I will be getting another plumber in shortly and I will get him to check it.

Thank for the offer andypenny but the boiler is so old that when it next breaks I think I will replace it.