Baxi Bermuda 401 Back Boiler - excessive cycling

[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

 

[kris]

Hi there polybear, although your room stat and cylinder stat call for heat and they will break power when the desired temp is achieved but the boiler is governed by the boiler stat , the boiler will come on till all the water in the pipe work reaches roughly 60-70deg and even when the water inside the pipes is up to temp it doesn't mean that the hot water in tank or the temperature in the room is up to temp yet, the boiler has to cycle on and off as you describe to slowly build up the desired temp, however the cycling will get shorter if symptoms like sludge or air in system or pump is slowly dying, by the sound of it your boilers doing what it should be but best to have an engineer measure pipe temps to ascertain if there is a problem, like you mentioned about flushing with chemicals and also check pump for air and air in system, also make sure system is balanced properly ,,
hope any of this helps and welcome to the forum,

 

[kris]

Forgot to say your gas valve is designed to be opened n closed so don't worry bout that, the fact that it's an old system, I'd get the system flushed or drained and while drained I'd clean out the f n e tank in attic of any sludge, poss new pump depending on condition,

 

[polybear]

Forgot to say your gas valve is designed to be opened n closed so don't worry bout that, the fact that it's an old system, I'd get the system flushed or drained and while drained I'd clean out the f n e tank in attic of any sludge, poss new pump depending on condition,

Many thanks; whilst the CH and HW is plentiful it does seem that something has changed recently (i.e. the boiler cycling). I'm probably at the stage where I had planned to drain & flush the system a couple of times, incl. washing out the header tank, followed by a refill with 1L of X400 and run the system for 4 weeks and drain etc. This seems a more sensible step than swapping the pump at this stage, bearing in mind there's no obvious pump problems and the rads warm up quickly etc. I'm a bit averse to the idea of a pressure flush etc. on an old system - all the upstairs rooms have just been completely renovated and I don't want the floors up for a leak.....

I realise it's an old system but (a) I like it, and (b) I'd replace like-for-like if the same was still on the market. Not a fan of these new fanged combi boilers; at least when my boiler goes wrong I can turn on the Immersion, plus I've got an airing cupboard

Many thanks.
polybear

 

[IDCHAPPY]

Your efficiency will be about 65% tops, you can get a condensing system boiler that can reuse most of your original system.
Don't get me wrong, Baxi BBU's aren't bad, but it's 19 oatcake technology.

 

[Atouchofgas]

You could replace with a condensing baxi bbu and electric fire front

 

[Chalked]

Your system is working correctly.
Not the most efficient boiler ( one of the worst) . But if serviced regularly, one of the most reliable bits of kit out there.

 

[polybear]

Thanks for the replies - I realise that my existing boiler isn't the most efficient in the world, however my gas usage is very low so it would take many years to recoup the cost of a new boiler.
I'm not so sure my boiler is working correctly - whilst it's doing everything it should do (i.e. CH & HW, plus the boiler cutting in/out iaw the boiler stat) it's never, ever cycled on/off in such a way before, so something has surely changed?

I've seen mention of the condensing Baxi BBU and I quite like the idea when the time comes (which is hopefully not yet though) - I realise they are more expensive that a Combi, however I would imagine that the reduced installation time offsets this a little. Not sure what the current reliability is though - I understand they've had some issues in the past (are they fixed/sorted now?). A longer Baxi warranty would be nice though. Keep the airing cupboard and tank/immersion would be a big bonus, as would not having the floorboards up....

Thanks.
polybear

 

[ed209]

You may well have a redundant anti gravity valve that has blocked up with sludge restricting circulation

 

[polybear]

You may well have a redundant anti gravity valve that has blocked up with sludge restricting circulation

Many thanks; as far as I know there isn't such a valve - the system installation was a total stripout and re-install, including all underfloor pipework, tanks etc. etc.

I've now measured the Boiler Feed & Return Temperatures once the Boiler had been on for some time (45-60 mins) and was "cycling" again; the temperatures were measured with a good quality/accurate handheld Digital Thermometer & Probe at the boiler feed & return 22mm pipes in the airing cupboard (the closest convenient location to the boiler - the pipe runs to the boiler would be about 16ft at a guess).

1. Boiler Stat at Position 4: 63 & 75 degrees C.
2. Boiler Stat at Position 2.5: 55 & 57 degrees C.

I also measured the rad feed & return pipes, directly below the rad valves on each of the five rads; all valves (not the lockshield valves) were turned fully open:
1. 67/64 degrees C
2. 67/57 degrees C
3. 74/67 degrees C
4. 71/64 degrees C
5. 74/68 degrees C

At the top of the Hot Water tank the temp was 48 degrees C; the tank stat is set to 50 degrees.

Does this offer any clues please? Thanks.

polybear