Baxi Banama 100 Combi Boiler internal water noise problem

[grollema]

Hi

(Sorry, posted this in the appliances section as well)

I own a Baxi Bahama 100 (came with the house) and have had al the usual problems in the past 3 years (flow switches, leaking tubes to pressure switch, faulty PCB, faulty automatic bleed valve etc.)

However there is a new problem to which I have no answer. 3 weeks ago, the expansion vessel blew (prior to which the boiler had worked perfectly for 6 months). I have replaced it with a new one but unfortunately during repressurisation, some water leaked from the attachment onto the control panel blowing the control PCB as I had not tightened the nut enough. So I replaced the control PCB as well.

All seems to be working now apart from the following. There is a very funny noise coming from the boiler during the initial stage of the cycle (both on CH and DHW), when the boiler is initially warming up. It sounds almost like a groaning sound, low volume to start with, getting quite loud after about 5 seconds or so, then deminishing completely after about 10 seconds or so. Once boiler is up to temperature, problem dissappears. This problem also occurs if you increase the required temperature during the cycle. Say boiler has run at min temp for about 1 hr, you then increase temp to say halfway, problem occurs until boiler settles at new temperature. It is so bad, that my wife refuses to use it any longer.

My initial thought was air in the system, but I have bled all radiators completely and am sure now that it can not be air in the system.
My gut feel is that the water is boiling in the pipes within the boiler. During start-up, the boiler goes to max gas supply (max output), it then measures internal water temperature and reduces gas supply as to attain required water tempurature. I assume that this does not work properly: The water gets too hot and boils inside the pipes before the boiler decides to reduce gas flow.

I don't know what to do about this (as it was fine before I replaced the control PCB and pressure vessel). Could it be that my new control PCB has a slightly different response time/sensitivity to the thermistor? Should I replace the thermistor maybe? Something wrong with the amount of gas supplied (can I reduce the max gas flow to the burners?). Should I get another control PCB?

Anybody has experienced this before, any solutions?

Any help is very much appreciated
Best Regards
Gareth

 

[tws750]

sounds too simple but have you checked the metalwork surrounding it?

Mine used to vibrate on the side of the internal casing around the heat exchanger and sound something like a droan from a helicopter overhead. Finger pressure on the panel, something wedged against it is enough to shut it up.

 

[grollema]

Thanks for your suggestion. I've checked all casings and yes they vibrate a bit, but are not the cause of the noise.
Regards
Gareth

 

[Mikeyboy]

Very common problem on your boiler is that the auto air valve can fail , normally due to iron oxide sludge build up . This will result in noise on start up , due to control driving gas valve up to full rate . The AAV is located on the top right of the boiler , with a small black plastic dust cover . This dust cover should be loose , to allow air to escape . You may already have checked it out ,but there is no mention in your posting .Easy way to check , when boiler is cold , very carefully loosen the AAV nut connection to enable trapped air to escape , and re-fix nut when water comes out .If water only comes out , then the AAV is working .If so , then theonly other case would be that the boiler heat exchanger requires a chemical cleanse .Hope this is of use to you .

 

[grollema]

Many Thanks.
Sorry for not mentioning that. Due to all the problems in the past I clean the flow switches and the AAV every time I drain the boiler.
I thought of flussing the system next, but it makes no sense to me that prior to my expansion vessel failing, the boiler workt perfectly without noises.
Regards
Gareth

 

[Mikeyboy]

Try increasing internal water pressure to one and a half bar .

 

[grollema]

Hi
Thanks for the reply. Pressure is running at 1.4 bars cold, and approx 1.7 bars when hot. I'll try increase it so see if it makes any difference.
however, I have noticed that if you turn on the HW, the pressure drops slightly to say 1.2 bars when cold. I assume this is because the boiler is downstairs and it does not get the water pressure from the upstairs radiators anymore.

 

[grollema]

After all the above, I still have no solution.
I am thinking of reducing the maximum gas pressure to the burner by turing the outer nut on the gas regulator a little (anti clockwise to reduce pressure according to the manual). This should then reduce the amount of gas to the burners at time of startup, reducing the initial maximum water temperature. I have no equipment to measure the maximum/minimum burner gas pressure.
How many turns between max and min on this valve? Would this be a way forward?
Any advice welcome.

 

[ChrisR]

I am thinking of reducing the maximum gas pressure ....... I have no equipment to measure the maximum/minimum burner gas pressure.
How many turns between max and min on this valve? Would this be a way forward?
Any advice welcome.

No idea how many turns! You don't know what the pressure is to start with and they can jump as you turn , if you change the max you may change the min ..... advice - DON'T TRY IT! Corgi time .