I have recently had the heating system replaced, boiler, cylinder, pump, motorised valve but kept all the old radiators. System was completely flushed at the time and the new system is working very well except for 2 probably linked issues. Two of the upstairs radiators start making a gurgling and tapping noise after a while which then comes and goes and the upstairs radiators need bleeding every few days or stay cold at the top. Any ideas how the air is getting in and how I go about fixing it?
if the gas is air, then the air may be pulled into the system via the header tank if the tank level gets low (when cold). Air can also be pulled into heatng systems thru "non-barrier" plastic pipework. Some plumbers use non-barrier pipe as it's cheap. With non-barrier, the spaces between polymer strands are small enough to stop water leaking out, but big enough to allow air to be sucked in. If this is the case you'll need to rip out non-barrier pipe & replace with barrier or copper.
Alternatively, the gas may be hydrogen gas. Hydrogen is a by-product of electrolytic corrosion. This usually collects in the upstairs rads. next time you bleed a rad, use a cigarette lighter to see if the gas burns (mind the curtians though).
This is easier to deal with. Simply double dose using a good corrosion inhibitor & open up all rads to ensure inhibitor is thoroughly mixed. Electrolytic corrosion will stop & so will hydrogen production.
Best to use is Fernox inhibitor. It's the most expensive but is the best. you can get from B&Q, Plumb Centre, Homebase in a tube which you can inject using a silicone gun thing. The Fernox one is called Protector i think.
I had this hydrogen problem in my house last year and I havent had to bleed a radiator since.