I need some ideas on diagnosis of an overheating boiler in a conventional vented central heating system.
The system has two valves controlling heat circulation through radiators and a water cylinder respectively. When either of these two valves closes, without the other valve being open, causing water to run through a auto bypass valve, very frequently the boiler overheats. The overheat seems to occur only when the boiler is burning and the valves shut off. As the valve closes the pump continues to run circulating water through the boiler and seemingly through the bypass valve but the water starts to boil in the boiler causing the overheat thermal switch to operate and the boiler to throw overheat status then need resetting. The boiling and overheat can be heard in the boiler.
The boiler didn't overheat until an extension of a circuit was made adding two new radiators. At this point a Honeywell auto bypass valve was added replacing a gate valve as it was found there was insufficient circulation in the two new radiators. Pipework adjacent to the pump was replaced at the same time as bores were reduced due to calcium build-up. A new electric valve was fitted too. This corrective work did improve circulation but the new rads would still not get as hot as the others in the house.
Since then the system has been flushed and back flushed with a specialist flushing/debris extraction system and a new pump fitted as the original was getting a bit noisy. Circulation of heat to all radiators was much improved but the overheats continue.
On average the overheats occur 4-5 times a day. The sequence seems correct, the thermostat clicks, the boiler stops burning, valve operates, the pump continues to run and does not sound particularly strained. You can't tell if the bypass is operating as it's very hot anyway.
Sadly the plumber who has taken us this far has given up suggesting the only way forward is a new system. This I completely disagree with. The system circulation is fine - all the rads heat up and the system is balanced. The boiler works fine - it heats water does not bang and clatter. The pump's good and quiet. It's just when the valves shut, the heat in the bypassed circulation does not get the heat out of the boiler quick enough and water boils.
The auto-bypass valve (Honeywell DU144) has been tried on highest and lowest settings.
The thermal sensor and overheat switch have been replaced on the boiler (Baxi Solo 70PF).
Lower boiler settings generally level 3-4 and no higher.
All laundry and airing items are banned from the airing cupboard in the vain hope that heat dissipates efficiently from the pipework between boiler and bypass valve. To be honest this has reduced the frequency of boiler failures.
Any ideas please?
The system has two valves controlling heat circulation through radiators and a water cylinder respectively. When either of these two valves closes, without the other valve being open, causing water to run through a auto bypass valve, very frequently the boiler overheats. The overheat seems to occur only when the boiler is burning and the valves shut off. As the valve closes the pump continues to run circulating water through the boiler and seemingly through the bypass valve but the water starts to boil in the boiler causing the overheat thermal switch to operate and the boiler to throw overheat status then need resetting. The boiling and overheat can be heard in the boiler.
The boiler didn't overheat until an extension of a circuit was made adding two new radiators. At this point a Honeywell auto bypass valve was added replacing a gate valve as it was found there was insufficient circulation in the two new radiators. Pipework adjacent to the pump was replaced at the same time as bores were reduced due to calcium build-up. A new electric valve was fitted too. This corrective work did improve circulation but the new rads would still not get as hot as the others in the house.
Since then the system has been flushed and back flushed with a specialist flushing/debris extraction system and a new pump fitted as the original was getting a bit noisy. Circulation of heat to all radiators was much improved but the overheats continue.
On average the overheats occur 4-5 times a day. The sequence seems correct, the thermostat clicks, the boiler stops burning, valve operates, the pump continues to run and does not sound particularly strained. You can't tell if the bypass is operating as it's very hot anyway.
Sadly the plumber who has taken us this far has given up suggesting the only way forward is a new system. This I completely disagree with. The system circulation is fine - all the rads heat up and the system is balanced. The boiler works fine - it heats water does not bang and clatter. The pump's good and quiet. It's just when the valves shut, the heat in the bypassed circulation does not get the heat out of the boiler quick enough and water boils.
The auto-bypass valve (Honeywell DU144) has been tried on highest and lowest settings.
The thermal sensor and overheat switch have been replaced on the boiler (Baxi Solo 70PF).
Lower boiler settings generally level 3-4 and no higher.
All laundry and airing items are banned from the airing cupboard in the vain hope that heat dissipates efficiently from the pipework between boiler and bypass valve. To be honest this has reduced the frequency of boiler failures.
Any ideas please?
