Air in Central heating system


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Can you help please troops , I cant even bang a nail in straight.
My condensing boiler central heating system keeps getting air in it . I have to bleed the thing every 5/ 6 weeks. It's always the upstairs rads that get the air in .

My boiler is only 3 years old and has just been fully serviced and passed muster.

I have a header tank up in the loft cavity which seems to be full of water so I should say it is ok ?

I have followed professional advice when bleeding the rads .

Some one told me to up the pressure in my boiler through two taps and a silver metal webbed tubing but my condensing boiler does not have such an attachment .

Any one out there can assist me please ??
I would help if you could post your boiler make and model. Is the tank in the loft for heating or cold water? Having a header tank for your heating system would suggest it's not a pressurized system so the line about the braided hose would be incorrect particularly if there's a cold water feed into the tank in your loft.

When you bleed the Rads is the water black? This would indicate magnetite in the system which gives off gas.

If indeed you have an old style expansion tank for your heating, sometimes over-pumping results in water constantly entering the tank by the expansion pipe stirring it up and getting aerated in the process.

You should also check that the end of the expansion pipe is above the highest possible level in the expansion tank as a closed loop in a non-pressurized system can cause these problems too.

A modern condensing boiler with a pressure gauge would indicate you have a pressurized system. To top mine up does not require a hose but there's a valve connected to the cold supply inlet to do this. The chap who serviced your boiler should be able to sort your issue though, get the fella back in.
Sounds like a new boiler grafted on to an old system and they didn't want to risk pressurising the system in case they had to chase leaks...... not really good practice.

The system could be pumping over, you can tell by checking the open- ended pipe which sticks up over the tank.

It must be sorted, otherwise your rads will pinhole, and the magnetite will collect inside the boiler and cause failure of parts.

Joshua Slocum

have you got an immersion tank in the system and a diversion valve ?
if you have an immersion tank it will have a coil running through it to heat the water by thermal transfer, might be letting air in through a leak there
also check around every thermostatic valve for leaks underneath
any radiators rusty ?
also any damp smells near where the pipes run that could indicate a leak under pressure and it drwaing back air as it cools
you may not have a link pipe arrangement for pressure, some boilers have a large key underneath and a black knob next to it to top up
google the make of boiler and download a PDF of the operating instructions
the header tank in the loft may just be for domestic cold water ?
however on an open system it tops it up and allows any high pressure to release into the cold water
try for a local GAS SAFE registered plumber, check their website
get a few quotes as soem of them have expensive lifestyles ( round here anyway)


Book Reviewer
thanks troops , I shall work my way through your suggestions .
I certain ly did have a new boiler grafted onto an older system .
The water comes out water coloured not black .
the water tank is for the heating only cold water feed .
the tap and hot water feeds are in two tanks under the eves below the header and not connected , i think.
At first I thought you might have a tank- fed combi, but now it appears you have a system boiler, my mistake.

You have to be careful installing this type of system, as a pump only pumps halfway around a circuit, then it sucks, to simplify the theory.

This means that if the vent, the pipe which comes over the top of the tank(the one for the heating circuit,) is too close to the positive side of the circuit, hot water is "pumped over" the vent, which is easily seen.

If the vent is too close to the suction side of the pump, air can be dragged down into the system, but this is less common.

As J S has said, another cause can be slack glands on rad valves, or the pump valves, which will allow air in when the pump is running, it's a good idea to walk around a stone- cold system, checking under the rad valve caps or the valves either side of the pump for white scale deposits, or green staining.
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