Cause Of Excess Air in CH System?

macman

My mum has a conventional gas boiler CH system. The boiler was changed two years ago (Worcester Bosch Greenstar 18Ri). No other work was done to the system (which is a 1960's single pipe layout) other than to free off some seized TRV's, and put a motorised valve and tank 'stat in to in to control the hot water temp properly, and fit a wireless room stat.
A couple of weeks later she had the pump changed as well, as it has become rattly/noisy (the old boiler masked the noise).
She had some air in the 2 rads at the top of the house at first, and was advised to bleed these as necessary and it 'should settle down in a few weeks.'
2 years on, I now find that she's still bleeding those two rads once or even twice a week. So obviously air is being sucked in somewhere in the system.
Before I get someone in for her, can anyone advise the most likely source of the problem?
As far as I can tell, the problem was not there before the new boiler/pump were installed.
Am I right i thinking that this will cause corrosion of the rads and should be sorted ASAP?

gas4you

Several possibilites;

Pump too powerful
cold feed blockage
wrong pipe work configuration on cold feed/vent
no water in F & E tank

Have you been into the loft to check it out? Take a jam jar with you, fill it with water and place it over the vent with the pump off. Get her to turn on the heating and see if the water in the jam jar is sucked up the vent.

The above are just the first basic checks to carry out.

macman

Many thanks, I'll check those out when I'm next down there. I doubt it's pipe work config, as no work was done on that. Pump is currently set on 2, so might be worth trying it with pump turned down to 1 for a few days, or is that too slow?
Where would I check or cold feed blockage?

gas4you

A cold feed blockage, or partial blockage is usually where the cold feed from the F&E tank tees into the main pipe work.

Sometimes running a magnet over this are of pipes can detect a build up of iron oxide.

If your mother is frequently bleeding the rads and they are filling ok, then it will not be a full blockage.

If by some lucky chance there is a gate valve above this tee, then shut the gate valve, drain a bit of water off and undo the bottom nut and see if you can poke a long screwdriver down it.

Also check there is no crud blocking the feed outlet in the F&E tank.

The pump on 2 would not normally cause this problem. If the pump on 1 is too slow, then it would just mean the rads struggled to get hot quick enough, especially the furthest away ones.