Posting a message to the forum will remove the above advertisement
  1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.
  2. Checkout our Plumbers Insurance area - heavily discounted Public Liability Insurance and Van Insurance specifically designed for plumbers.

Discuss Air Problem in Open Vented System after Boiler Change in the Central Heating Forum area at

  1. ThePook

    ThePook New Member

    Hi All,

    First post here, so please be kind!

    I am currently having a lot of trouble with air and related noise in my open vented heating system. Long story, with lots of things tried already, so please bear with me…

    Firstly, a photo of the heart of the system in the garage as it is today (lots of spare cable length to allow me to move bits!):[​IMG]

    The system originates from 1970, fully pumped, 9 radiators, 1 towel rail and a vented indirect DHW cylinder. Two-storey house, gas boiler, pump, zone valves in the garage, header tank in the loft. I changed all the radiators about 3 years ago and installed a Honeywell Evohome system with TRVs on every rad plus hot water control kit and an automatic bypass valve I added when I added the TRVs. The system was Y-plan at this time. Worked fine, no air problems ever, drained and filled fine for the radiator replacements, but the 35 year-old boiler always kettled badly. Always dosed with inhibitor. All plumbed in soldered copper, suspended floors throughout, so pipes not buried in screed.

    Recently, the boiler was changed – system flushed, new Intergas HRE OV condensing boiler, Magneclean Pro2 on return, new Grundfos UPS2 15-50/60 pump on flow, taller towel rail, new header tank in loft (only ’cos the old one didn’t have a lid!), converted to s-plan (but heating zone valve locked open as Evohome does the zoning – recommended ‘figure 4’ solution from Honeywell). The other thing that was changed at this time was that the GSR installer converted to ‘combined feed and vent’ (single 22mm to bottom of header tank, tee’d in just before the pump on the boiler primary flow, flow/return done correctly at boiler). System is dosed with Sentinel X100 and tested to be the correct strength. The filter is always very clean.

    The header tank is (and always was) approximately 4.5m above the pump, and 1.7m above the top of the new towel rail, which is the highest part of the system.

    Ever since this, I have not been able to get all air out of the system (or is it getting in?). The radiators make a terrible noise (sounds like ball bearings or gravel being fired through the TRV’s) which I am sure is air or gas bubbles. This is worst first thing in the morning, or when only 1 or 2 radiators are calling for heat. I have bled it literally tens of times bottom-up (all rads, pump, DHW cylinder, filter), the gas doesn’t burn or smell, so I am confident not a chemical/corrosion problem. Level in header tank is fine (always above outlet, always below overflow).

    Things that have been tried (in chronological order):

    1. Pump has been tried on all 3 speeds (it actually circulates fine even down at 1). No improvement.

    2. I noted that the primary flow actually went slightly downhill before joining the vent pipe to the loft, so the installers (who are very helpful) returned, installed a Tower air separator (type without separate cold feed) and re-routed the pipe to run uphill. No improvement.

    3. I installed a Spirovent RV2 air eliminator on the inlet to the pump. No apparent improvement, even though it was definitely ejecting air!
    4. I isolated the flow at the pump valves and return at the filter, then connected a hose pipe to the drain point at the bottom of the boiler. I watched in the tank in the loft while somebody else turned the hose on – there was a huge belch of air into the header tank, suggesting that it was trapping in the feed/vent at some point. No improvement.

    5. Based on point 4 above, I traced the feed/vent and found that there were significant unavoidable horizontal runs (crossing floor joists) and dips in it, so possible air locks/traps. I therefore moved the header tank in the loft, and made a new route for a new 22mm feed/vent, with significant uphill slope all the way to the loft. Guess what? No improvement.

    6. I ran a separate 15mm cold feed from the loft, converted the 22mm to a vent over the tank (bend 450mm above water level), tee’d in just after the Tower separator in the boiler flow, before pump inlet (to replicate the layout of the version with the cold feed). In other words, back to a properly piped separate cold feed/vent. I.e. Boiler Flow->Tower Separator (with Vent Pipe)->Cold feed->Pump. Both pipes have nice constant fall from loft to boiler, system fills easily, no pumping over. No improvement.

    7. I changed the pump valves, in case of air leak around the spindle on inlet side. No improvement.

    So this is where I am today, with no improvement! As I understand, the neutral point is at the cold feed connection (approx. 0.42bar or 4.2m water), so the only section that could be under negative pressure and could conceivably admit air at any point is the short run of pipe (all soldered connections) down to the pump inlet. Is that correct?

    The installers want to convert to a pressurised system, but I am very reluctant due to the nearly 50 year old pipes under floors and I sense that it would mask the problem, not cure it. I would really like to understand the problem.

    Is there anything wrong with the current vent/feed/pump layout?
    Could the pump itself be sucking in air?

    The only very minor point in the piping design I know about is that all heating returns tee into the return line after the DHW return, but I don’t think this relates to my air troubles at all, and I get no reverse circulation in the summer.

    Sorry for the very long post, but I wanted to put as many details of the steps taken to date as possible.

    I would be extremely grateful for any thoughts you have.

    The Pook
  2. SimonG

    SimonG Trusted Plumber

    Seal it.
    • Like Like x 1
    • Agree Agree x 1
  3. Riley

    Riley S. Mod Trusted Plumber GSR Top Contributor!!

    Seal it it will solve your probs
    • Like Like x 1
  4. king of pipes

    king of pipes Trusted Plumber GSR

    Take a snap of the boiler connections right hand should be the flow are they crossed ? , personally don't like the Airjek more trouble than they are worth I would take that out . kop
    • Agree Agree x 3
  5. fixitflav

    fixitflav Active Member

    Can't offer much, but can reassure you there isn't negative pressure between cold fill and pump suction. The friction loss in that stretch will be well below the 4.2m static.
    That might be significant. Do you still have to bleed the rads though air is being ejected? It seems a long shot, but could it be the boiler heat exchanger? If that is letting air in and water out the water will evaporate and go out the flue, so no sign of a leak. It might be worth closing the inlet valve to the F/E tank, and see if the level falls.
  6. ThePook

    ThePook New Member

    Hi all,

    Thanks so much for taking the time to read and reply.

    Unfortunately, I am rather afraid of going sealed in the short term, as the old pipes look tired...I do intend to seal the system long term, but will repipe the whole house first and that is definitely a summer job!

    As an engineer with an enquiring mind, I just have to try to understand and sort this!

    Don't know how to post a picture from my phone, but the flow and return are definitely the correct way at the boiler. Flow on right through a red grommet, return on left through a blue one. Intergas colour coding seems sensible there! Checked the manual, too.

    Agreed on the suction head, I think the piping is basically OK from that point of view.

    With respect to the boiler heat exchanger, I will try what you suggested with the tank, but would it also be possible to pressure test? I have a Monument gauge that will fit on the boiler drain terminal.

    One further observation I should have mentioned above - the air seems to hide in the DHW circuit. If I manually open that zone valve, I hear the air rush into the return and filter, before hearing it again coming out of the flow, through the air separator and into the pump. If I manually close it again, the radiators, make a horrible rushing air bubble sound for a couple of minutes. Any clues there? I have bled the DHW coil a million times!

    Is there any possibility that the new pump could somehow stir up or pull in air in itself? Other than the boiler heat exchanger, it's about the only thing I haven't tried!

    Finally, I notice that if I pull a small sample of water out of the boiler, it seems very foamy. Is that right for water treated with x100?

    Thanks again all.
  7. ThePook

    ThePook New Member

    Also forgot, the condensate trap on the boiler seems to run quite often and has a black/dark dusty sediment in it. Is that normal?
  8. dancinplumba

    dancinplumba Trusted Plumber GSR

    who installed boiler ?
  9. king of pipes

    king of pipes Trusted Plumber GSR

    No that's not normal the condense should be like water out the tap it's quite possible you may have a faulty heat exchanger does the feed and expansion take ball valve trickle water into the tank may only be dripping ???
  10. ThePook

    ThePook New Member

    Independent GSR installer, they have been very helpful, and would seal it for me, but at least short term would like to avoid that and fix the issue open vented.

    I have spoken with them before attempting mods myself, as a common courtesy!
    • Dislike Dislike x 1
  11. dancinplumba

    dancinplumba Trusted Plumber GSR

    It's an Intergas
  12. dancinplumba

    dancinplumba Trusted Plumber GSR

    I'm out
  13. king of pipes

    king of pipes Trusted Plumber GSR

    Can still leak bud fitted a few
  14. dancinplumba

    dancinplumba Trusted Plumber GSR

    rare as hens teeth, guv job this, i'm out
  15. ThePook

    ThePook New Member

    Sorry, don't understand that, is it something I said?
    • Like Like x 1
Similar Threads - Problem Open Vented Forum Date
Vaillant 937 Ecotec Plus problem New Member Introductions 13 minutes ago
Urinal Auto siphon problem Plumbing Forum Wednesday at 8:11 PM

Users found this page by searching for:

  1. condensing boiler pumped flow or return

  2. vented heating system is taking in air

Users found this page by searching for:

  1. condensing boiler pumped flow or return

  2. vented heating system is taking in air