Gas boiler overheats on valve closure - vented system
- Thread starter penguinventures
- Start date
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That and set correctly
Yesterday I opened the old bypass close to fully open. Boiler has overheated twice.
Any ideas what I do now?
I have two thermocouple meters so can monitor other points in system.
Any ideas what I do now?
I have two thermocouple meters so can monitor other points in system.
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Currently the auto bypass is wound fully out and the old bypass is open too. Flow arrow is correct.
If there is anyone still following this continuing saga who's not lost the will to live...
With the manual bypass valve fully open and the auto-bypass control fully wound out. I monitored the temperature either side of the auto bypass valve seeing a 10 degree drop. As the rad electric valve closed the temperature drop continued and the boiler overheated. This indicates there was no flow through the bypass both while the system was heating the rads and on the overun.
Lots of bubbling and rushing of air occured all going through the pump and into the rads.
I reset the boiler. This time the temperature dif across the valve was negligible. As the rad valve shut this time, if anything, the temp on the outlet side of the valve was rising above the inlet by 3 to 5 degC? The boiler continued to run, valves closed, and then shutdown without overheating.
What is going on?
Could airlocks be causing this? Unequal temps either side of an open bypass? Rising temps on the return to the pump from the bypass? Why? Almost as if water is flowing in reverse with heat coming from the boiler?
If airlocks are the cause would converting system to a pressurised system cure it. Would a pressurised system eradicate airlocks? Is it feasible to convert a f&e system to a pressurised system?
With the manual bypass valve fully open and the auto-bypass control fully wound out. I monitored the temperature either side of the auto bypass valve seeing a 10 degree drop. As the rad electric valve closed the temperature drop continued and the boiler overheated. This indicates there was no flow through the bypass both while the system was heating the rads and on the overun.
Lots of bubbling and rushing of air occured all going through the pump and into the rads.
I reset the boiler. This time the temperature dif across the valve was negligible. As the rad valve shut this time, if anything, the temp on the outlet side of the valve was rising above the inlet by 3 to 5 degC? The boiler continued to run, valves closed, and then shutdown without overheating.
What is going on?
Could airlocks be causing this? Unequal temps either side of an open bypass? Rising temps on the return to the pump from the bypass? Why? Almost as if water is flowing in reverse with heat coming from the boiler?
If airlocks are the cause would converting system to a pressurised system cure it. Would a pressurised system eradicate airlocks? Is it feasible to convert a f&e system to a pressurised system?
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Have you checked the impeller of the pump? You may find it blocked and unable to produce enough flow to lift the auto-bypass. Set the speed of the pump to 3 also.
I think think the pump is ok. It sounds alright? It's less than 5 months old fitted straight after the powerflush. It's already on setting 3 which is what the plumber told me to leave it on.
Currently the measurements either side of the auto-bypass are equal so there's currently a flow. Boiler is working fine cutting in and out as the room stat demands.
- Joined
- 18 Mar 2008
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Unless you remove the pump head and check the impeller you'll never know what to 'think'... Power flush and blocked impeller go hand in hand I'm afraid.
I thought about the hasty response after I wrote the reply and thought it was an unfair reply - sorry. At the weekend, if I can close the isolator valves, I'll remove the motor and have a look.
I've just watched the auto-bypass not flow. The rad valve closed. Temp above the bypass went to 63 deg C, below was 51 deg C.
Since the bypass valve will dissipate virtually no heat then its obviously not opening as expected.
Either wrong way round, blocked or wrongly set !
Tony
The automatic bypass valve was completely blocked with iron oxide. A different valve has been fitted which has a bigger bore. All is fine now.
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