Water in Central Heating filler tank is bubbling

[Nifill]

I recently had to flush the CH system due to a leaking valve which needed to be replaced. I had the boiler serviced at the same time.
Since the system has been refilled and radiators balanced I experience an intermiitted thunderous bubbling sound coming from the loft.
It looks like the water in the CH filling tank is getting hot, bubbling over and in some cases the loft is becoming full of steam.
The CH folk claim it must be air in the system and have fitted an automatic air valve. They also changed the pump just in case.
Guess what.. same problem.
My system has two motorised valves , I notice that when the "thunder occurs", the CH is invaribaly not demanding heat and the hot water is demanding. Turning up the room stat, causes the the CH valve to open and the "thunder" stops shortly after. I also hear air hissing from the automatic valve at the same point.
Question:
Is this likely due to an airlock or is the problem due to the "heating water" actually boiling ?
If it's an airlock, why does it seem to appear only when water is being heated.
Could the problem be due to the Boiler thermostat, ie boiler not shutting off when it reaches temp ?

Any ideas much appreciated

 

[oilman]

I wish professionals would stop changing pumps to fix problems, it's the same thing as changing thermostats in car cooling systems, mostly pointless.

You may have a boiler thermostat problem, but you need to say which boiler you have.
If the CH works reasonably ok, and the HW doesn't, any air has probably been pumped out. This indicates that steam may be generated by the boiler.

We need to know how old the system is, and how you flushed it.

It may be the HW motorised valve has a problem too.

 

[ollski]

If both your motorized valves are closed at the same time but due to a faulty one one of them is still calling for heat at the boiler this will cause your hot boiler water to expand up the feed and/or vent pipe and boil in the header tank. Check your boiler bypass is open or operating correctly. Need more details to confirm this as the problem but it's worth pointing out.

 

[Nifill]

Hi There,
The system is a Potterton Flamingo 50S and it's approx 20 years old. Both Central Heating and HW work OK.

I noted that the "boiling" occurs when HW MV is open and CH MV is closed.

I also note that the system seems to be working when the Boiler temp knob is turned right down. If I set the boiler temp know back to po 4 where I normally have it, the problem will occur.

 

[Nifill]

How the system was flushed:

It was simply drained off from the drain point at the boiler, refilled and inhibitor added.

 

[Nifill]

Bolier By-pass;
Where do I find that please, is that the "balancing tap" near the two MV's ?

The system shuts down OK when both valves are shut.. shuts down the boiler every time.

Thanks.

 

[ChrisR]

Check the new pump is in the right way round. If it's connected to the upper (flow)boiler pipe it should be pumping away from the boiler, if the lower one (return) then towards the boiler.

 

[Nifill]

Found the problem
Boiler Thermostat faulty. Boiler would not shut down at higher thermostat settings! Problem proven by: Turning down CH themostat so CH circuit switched off. Turned up Hot Water tank stat to ensure hot water circuit was demanding. Turned up boiler temp setting to normal (mid ) level.
Within 10 mins loud rumbling from CH filler tank, water in tank was boiling.
Bolier was on high flame and causing heating system water to overheat.
Bolier was not shutting down when heating water reached max temp.
A simple test for Boiler thermostat operation is to turn up CH and Hot water stats. with both systems demanding heat, boiler flame should be controllable by the boiler temp setting.

Thanks for all your help.. Next time you have a problem with CH and you call in a CH company, ask the bloke to draw you a diagram explaining how the system actually operates. Ask him how the Heating, Hot Water and Boiler temp and Pump action are controlled. If he cannot do so clearly, send him away... since he'll probably try changing everything first and may still not resolve the problem.

 

[oilman]

Thanks for all your help.. Next time you have a problem with CH and you call in a CH company, ask the bloke to draw you a diagram explaining how the system actually operates. Ask him how the Heating, Hot Water and Boiler temp and Pump action are controlled. If he cannot do so clearly, send him away... since he'll probably try changing everything first and may still not resolve the problem.

Thanks for the fault description, feedback like this is always helpful.

The idea of getting a drawing of the system is useful, (more work for me ) but it can be a problem working out the tortuous routing of pipework in some installations. I would guess at £100 to work out what goes where, what it does, and provide a drawing. Some installations will be easier and so cost less to do. I would like to explain to people about their heating system, but there is a strange glassy look appears in their eyes on occasions, 1) they possibly can't understand, 2) maybe they are uninterested and don't want to know, then they are possible prey for parts changers.