Baxi Bermuda 401 Back Boiler Problem

polybear

I'm having a problem (I think) 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 has just been 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, then shut off briefly (a minute or so?) then light up again for a brief period and so on. At all times when this is happening the room 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.

Any ideas please, and at which setting should a boiler stat normally be left for such an installation please? (Up until now I've been running it at 2.5, in a hopeful attempt at minimising the gas bill......)

Thanks.
polybear I'm having a problem (I think) 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 has just been 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, then shut off briefly (a minute or so?) then light up again for a brief period and so on. At all times when this is happening the room 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.


Any ideas please, and at which setting should a boiler stat normally be left for such an installation please? (Up until now I've been running it at 2.5, in a hopeful attempt at minimising the gas bill......)


Thanks.
polybear

DTDfanBoy

Generally higher is better, when it's not firing it turns into a radiator and sends heat up the flue. If you have an adjustable pump you could try a higher speed.

Crinkmeister

Thermocouple or gas control valve. Get a Gas Safe registered engineer to look at it for you. It's not a DIY job.

SGM1

Doesn't sound like a problem unless it's cycling when rads still cold. The boiler will always run to its own thermostat, once it reaches temperature it shuts down for a little while and will turn on again once cooled down.

polybear

UPDATE:

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.

I'm currently running the boiler stat on position 4, which is one notch off the "HIGH" setting. The boiler burner ran for quite a while from cold this morning, then when boiler got up to temp it started to cycle off/on again - around 30 seconds "OFF" followed by around 50 seconds "ON". I've not noticed this behaviour before, so something's changed/not right.... :cb

polybear