Air in gravity primary feed and return

[Hargreaves 79]

Hi, wondering if anyone experienced with gravity hot water and pumped central htg systems can help!!
I'm a general builder and have a lot of experience with plumbing systems but need some help on this one, i undertook some building work for a customer and part of the work was to tidy some pipe work and reduce the size of the boxing in, amongst the pipe work was the primary feed and return and the basic layout of the pipe when work when replaced was the same just a bit more stream lined.
it has now started accumulating air and wont circulate round the coil in the cylinder properly it has a towel rad tee'ed off just before the cylinder which circulates fine, its a floor standing ideal mexico boiler.
It was fine for about 3 months after i did the work and if i drain the system and re-fill it works fine for a couple of days then won't circulate properly.
someone has said to me about putting a bypass between the fill pipe and the vent would this help?

 

[Best]

From experience, I always tell people not to let builders have anything to do with plumbing. :grin:
Too many questions need asked, but here goes, -
What else did you do? Is the cylinder still the original? Did you alter the pipes at the cylinder? How do you know "accumulating air"? Where is this air? There should be no air ever accumulating in gravity pipes. Hope you haven't fitted bendy plastic pipes onto the gravity pipes?
A bypass won't help the cylinder to heat, but just sounds like an easy fix. Really mad idea.
If the towel rad is heating fine, then the original cylinder should heat (if original pipes unaltered at the cylinder).
Usual mistake is treating gravity pipes as if they are pumped. They need fitted well. Perhaps you have been unlucky & pulled dirt into pipes.
Begs the question - Why did you not get a proper heating guy in to do the work?

 

[jtsplumbing]

Have you reduced the size of the Flow & Return pipes ? (Streamlined as you put it) have you crossed them over ? have you added extra bends (elbows) Possible air trapped in coil,

 

[SimonG]

Maybe a picture or two would be a good idea, then probably an experienced heating guy.

Sometimes you just need to bite the bullet.

 

[Ermintrude]

Errr 22mm push fit on solid fuel bbu with unvented.... Who needs experience

 

[Hargreaves 79]

The feed and return are in 28mm all the way from the boiler to the cylinder and the towel rad is tee'ed off it 22mm just before cylinder, the towel rad is circulating fine.
i can't have them crossed over because when you fire the boiler from stone cold the top pipe from the heat exchanger becomes hot and upstairs you can feel the hot water flowing into the top of the cyli