P
polybear
Sorry for the long post , but here goes....
I'm having a problem with a BB 401 Back Boiler; the system is fully pumped with cylinder/tank & room stats and a 3-way valve. A faulty boiler pilot light thermocouple was recently replaced and all appeared well; however I'm recently finding that, when the room (or tank) stat is calling for heat the boiler will light up for a while (e.g. 20 mins), then shut off briefly (30 secs or so) then light up again for a brief period (50 secs) and so on. At all times when this is happening the room stat (or tank stat) is always calling for heat - it's not switching off/on/off etc.
It seems that, even when the room (or tank) stat is calling for heat, the boiler stat beneath the gas fire is controlling the burner - you can hear it clicking off/on etc. in synchrony with the burner flame. Winding the boiler stat up to a higher setting initially causes the burner to immediately light up again and burn for a longer period before commencing the off/on/off cycle, but then the same process kicks in again - albeit at a higher boiler/heat exchanger temperature. The boiler stat is currently at it's highest setting as I type this, and the burner is cycling off/on/off at regular intervals.
The pump is quiet with no strange noises etc; the pump shaft is nice and free to turn and self-starts straight away when demanded; the pump speed setting switch (I never knew it had one) was on position 2 - it's now on position 3. The 3-way valve plays nicely and the hot water and rads warm up quickly; room & tank stats control the system correctly.
There's plenty of hot water, and all rads get nice n' hot with no cold spots etc. There's no sign of the system venting into the header tank in the loft, and the small amount of water collected when accessing the pump shaft is clear - it's always run with Sentinel/Fernox inhibitor in the water.
I've checked that, when the boiler shuts down, the pump (a Grundfos UPS 15-50) is still running (visible check that the shaft is turning). The boiler is nice and quiet also - no knocking, kettling sounds etc. The 3-port valve is playing happily also (visible check that the actuator mechanism is moving thru' the zero/45/90 degree positions) and the valve shaft itself is free to turn.
I've now had the boiler thermostat changed by a GSE, with no improvement.
Apart from draining & flushing the system and then refilling & running the system with X400 included for several weeks is there anything I've missed or not tried please? I'm running out of ideas now - something has definitely changed and I'm a bit concerned that the Gas Valve ultimately won't like all this switching; the heat exchanger is being repeatedly heated and cooled also. It's not too much of a problem for hot water (usage is low) but at the moment I'm laying off using the rads. too much - I'm tending to run the heating until it starts cycling (20mins) and then giving it a break of half an hour or so. Not practical thru' the night though!
Thanks!
polybear
I'm having a problem with a BB 401 Back Boiler; the system is fully pumped with cylinder/tank & room stats and a 3-way valve. A faulty boiler pilot light thermocouple was recently replaced and all appeared well; however I'm recently finding that, when the room (or tank) stat is calling for heat the boiler will light up for a while (e.g. 20 mins), then shut off briefly (30 secs or so) then light up again for a brief period (50 secs) and so on. At all times when this is happening the room stat (or tank stat) is always calling for heat - it's not switching off/on/off etc.
It seems that, even when the room (or tank) stat is calling for heat, the boiler stat beneath the gas fire is controlling the burner - you can hear it clicking off/on etc. in synchrony with the burner flame. Winding the boiler stat up to a higher setting initially causes the burner to immediately light up again and burn for a longer period before commencing the off/on/off cycle, but then the same process kicks in again - albeit at a higher boiler/heat exchanger temperature. The boiler stat is currently at it's highest setting as I type this, and the burner is cycling off/on/off at regular intervals.
The pump is quiet with no strange noises etc; the pump shaft is nice and free to turn and self-starts straight away when demanded; the pump speed setting switch (I never knew it had one) was on position 2 - it's now on position 3. The 3-way valve plays nicely and the hot water and rads warm up quickly; room & tank stats control the system correctly.
There's plenty of hot water, and all rads get nice n' hot with no cold spots etc. There's no sign of the system venting into the header tank in the loft, and the small amount of water collected when accessing the pump shaft is clear - it's always run with Sentinel/Fernox inhibitor in the water.
I've checked that, when the boiler shuts down, the pump (a Grundfos UPS 15-50) is still running (visible check that the shaft is turning). The boiler is nice and quiet also - no knocking, kettling sounds etc. The 3-port valve is playing happily also (visible check that the actuator mechanism is moving thru' the zero/45/90 degree positions) and the valve shaft itself is free to turn.
I've now had the boiler thermostat changed by a GSE, with no improvement.
Apart from draining & flushing the system and then refilling & running the system with X400 included for several weeks is there anything I've missed or not tried please? I'm running out of ideas now - something has definitely changed and I'm a bit concerned that the Gas Valve ultimately won't like all this switching; the heat exchanger is being repeatedly heated and cooled also. It's not too much of a problem for hot water (usage is low) but at the moment I'm laying off using the rads. too much - I'm tending to run the heating until it starts cycling (20mins) and then giving it a break of half an hour or so. Not practical thru' the night though!
Thanks!
polybear