air bubbles coming up cold watre feed

[stevegreen]

Having some problems with my dads central heating which has run well for nearly 30 years! It started with the furthest radiator not coming on. flushed out, checked combined inlet/outlet valve for blockages - clear, also checked feed and return pipes to rad all clear. The system has a pumped feed to a mid position valve. i have noticed that when the pump is only delivering to the heating system the pump sounds good (speed setting 1) but when the domestic hot water loop is switched in the pump starts to make a whoosing noise in sort of waves combined with air bubbles entering the feed tank. The vent pipe is connected to the suction side of the pump within 4". The boiler is a conventional boiler sited on the ground floor (bungalow) the pump is in the attic just below the mid position valve . Two circuits of flow and return pipes run down opposites side of the bungalow and then drop feed in 10mm pipe to the rads. I am guessing the air going into the header tank is also getting into rest of circuit causing our radiator problems. Can anyboy shed any light onto this problem Thanks Steve.

 

[justlead1]

Hi. When you refilled the system, is possiable that the coil in the cylinder has air trapped in it. With pump off try cracking the joint on the primary flow as it enters the cylinder to release any air. (it can be ongoing if there is air present in the rad circuits.) Good Luck

 

[Paul Barker]

Pump sucking on vent is modern method of piping. You should have a T with vent off it then cold feed then pump. Between vent and cold feed get's restricted and eventually blocks, water then gets sucked down from f/e tank and to keep things balanced the other side of pump presurises system until it cascades over open vent into tank. BG fix for this is to combine them (cf/ov) just beneath tank so it short circuits the water fall. It would do you no harm to either fit an airjec in place of the two pipes aforementioned, or to exp[and the open vent T with next size up, so if your system is 22mm or 3/4" go 28mm with the T. The expanshion at this point slows flow and allows air to escape properly where it should, with same size T as used standard air isn't released, hence you are finding it has found a way up the CF.

It is a problem area. While improving the design clean out the pipe between and the bottom few inches of the cold feed.

 

[stevetheplumber]

id go for cold feed blocked usually in the last foot of pipe drain down and cut out

 

[Bernie2]

Whilst I agree with Paul, I would also ask myself "If it was working okay, what has changed to make it not work?" Often its air in the system. The idea of putting the vent near the cold feed is that the cold feed then acts as the neutral point of the system.

They arrive at a neutral point by imagining a pump fitted on a circle of pipe, but it both pushes and pulls (not strictly true of course, it makes a low pressure area) around the circle.

So at some point there should be a point where the pull and push of the pump balance themselves out and the pressure never changes or as it is known, the neutral point. In an open vent system its where the cold feed joins the system, in a sealed system its where the expansion vessel connects.

So if you put the vent and cold feed close together, the vent is largely unaffected by any pressure changes in the system under normal system working.

Basically it lessens the chance of either sucking down or blowing over in the vent.

If the BG fix, is a combined feed and vent, it would seem a step back in time to solid fuel days, and not as intrinsically safe as a close coupled arrangement.

 

[Paul Barker]

Excellent advise.

 

[stevegreen]

Paul thanks for reply - I have checked pipework - The cold water feed from expansion tank is actually tee' d into the return leg! just before it goes down from the attic. The return leg then connects to bottom stabbing on boiler. I don't think there can be any blockage in the cold water feed leg as and when we drain the system down the tank drains down very quickly through this leg. do you think the cold water feed is piped up incorrectly? What does BG mean?

Bernie2 - thanks for reply - do you think it is possible for there to be a restriction somewhere within the boiler so the pump can't pull the water up at the speed it wants to and therefore pulls on the vent pipe instead. The pump sounds fine when it only has the central heating loop to deal with and no air bubbles are seen coming up the cold water feed pipe into tank, but when its got to push water around the DHW loop aswell there's not enough water supplied on the inlet to the pump (because of a possible restriction), the pump then starts to make the whooshing noise at the same frequency as air bubbles are seen to rise in the cold water feed pipe. or am i totally on the wrong lines. any thoughts are much appreciated.

 

[bonesy60]

BG means British Gas

 

[RoryD]

On flow there should be Vent and then cold feed within 150 mm, then the pump (within a metre). Excess air / hydrogen SHOULD come up the feed pipe, however your's is in the wrong place. I think your system is "sucking in" thru the vent. hold a saucer of water below the vent and immerse the end into the water ... watch it drink!! BG don't do the above ... they reconfigure the pipework.. correctly to overcome the problem