Air in hot water side of CH system after drain down

[cjb121]

I have a bungalow with conventional CH system, with wall mounted gas boiler ( 3 yrs old ), but all other original plumbing from build.. approx. 1985.
BGas fitted new boiler some 3 yrs ago, with no circulation problems after. The hot water cylinder is the original 36x18 insulated copper, fed by adjacent pump and diverter valve.

Recently,I had to replace the bathroom radiator which was rusting on the outside, which meant draining the system. I have done this before and bled the system after as usual.

Since then, I daily get air bubbling from the hot water side, more noticeable if only hot water running. If the valve opens, then the bubbling gradually decreases as the air goes round the radiators .. until next time. I have to bleed the vent from the cylinder pipework each day, with no smelly gases... or water leaks.

I appreciate air could be getting in from the new radiator, but then why would the problem only occur on the hot water circuit. My suspicion is for some reason the tank coil has now decided to leak/fracture since the draining.. but I don't notice any sediment in the hot water to bath or sinks.

Any ideas please on how I prove the cylinder coil is faulty .. or not.

 

[TCCHeating]

If the coil was leaking you would have water coming from the f and e tank overflow. (The smaller of the 2 tanks you will have, prob in loft)

It might just be some stubborn trapped air. Keep perservering with bleading the cylinder coil. Or you could fit an auto air vent. I find that if you keep switching between heating and hot water for 10 mins or so you will desperse the air quicker and it should all end up at the top of the rads

Also, have you changed the pump speed by any chance? In poorly designed systems this could cause air ingess.

 

[cjb121]

thanks for prompt reply.
The pump was changed by BGas when they did the boiler, but valves are original.
The only problem I had after was the rattling of something going round system .. mentioned to engineer who wasn't interested, so I cancelled the annual contract seeing as boiler was under manufacturer warranty anyway.

He did suggest putting pump speed down to 1 and so I just put up with it but since I drained system, that problem has gone LOL ..
but I have tried blasting the air out by putting speed up to 3.. tried opening the pump air bleed screw, which leaked after, so it is now back on setting 1.

The pump is a little bit rumbly now so had considered changing it, but my thinking is .. if it was the pump leaking, why does problem only seem to occur on the hot water circuit,... or is the air just collecting there. I don't notice any tank overflow.

I now only run the hot water during the evening to prevent constant gurgling, but when it comes on there is a massive bubbling around the pump/cylinder, almost as if the pipes were empty or an awful lot of air there.

I did consider draining down again and re- bleeding system which is probably easiest thing to try.. but I can't get the notion out my head that the cylinder is duff. I cant look inside since the immersion plug is jammed tight.