Heating sucking air from expansion pipe.

[tubaman]

I have a bit of a bizarre problem with my central heating.

Problem is that when it's on hot water only its gets itself into a weird cycle of pulling air down the expansion pipe and pushing hot water up the fill into the header tank.
It starts ok when cold, but once hot you can hear a whooshing noise as it pulls air down the expansion and pushes water up the fill. It does this for about 10 seconds every minute or so. You can hear the aerated water then going through the hot water cylinder and into the boiler.
It does this on pump speeds 2 or 3 - it's ok on 1 (but then not very good for fast heat-up of radiators!).

Configuration is:- boiler - expansion pipe(22mm with straight of 'T' going to expansion tank) - feed pipe (15mm pipe about 150mm from expansion) - pump - 3 way valve.

I suspect that it has been doing this for a long time, as I have noticed the expansion tank hot before. However, we have just had a new boiler fitted which is now in the kitchen (was in utility) and the noise of the air going through the boiler does not please me!

Hope this makes some sense and that someone can help.

It has been suggested that I convert to a sealed system, but as the current system is about 25 years old, with pipes buried in concrete floors, the thought of putting it under any increased pressure fills me with dread!

 

[giblets]

What is the vertical distance from the water level in the feed tank when the system is hot and the bend where the vent pipe turns over the top of the tank? Should be a minimum of 450mm but may need to be higher.

Did the installer who fitted the new boiler thoroughly clean out the existing system?

 

[twgas]

my suspicion is you now have

boiler - expansion pipe(22mm with straight of 'T' going to expansion tank) - PARTIAL BLOCKAGE - feed pipe (15mm pipe about 150mm from expansion) - pump - 3 way valve.

 

[Terrywookfit]

It would be much better with an Aerjec fitted.
Use comp fittings as even these are susceptible to blockages.

 

[tubaman]

Wow - such fast replies!

Distance from top of tank water to vent pipe bend is about 1m - this has just been increased from about 400mm, as water was pumping over with pump on speed 2 or 3 (and making strange whooshing noises as described before) - only did this with old boiler on speed 3. Position of new boiler has shortened the pipe runs quite a bit, so this may have some bearing on it.

System has been drained/refilled a couple of times during boiler fit, but has not been power-cleaned. Installer still has some work to do adding extra rads in an extension. His only suggestion to any problems is 'convert it to a sealed system', but I really don't want to go there.

Haven't heard about an Aerjec before - I'll investigate.

What is odd about this is that it is not sucking air constantly - it seems to cycle.