Air getting into heating system

[Alfie_Huntingdon]

Hi All

I've been struggling with a problem with my heating system ever since moving into my house over four years ago. It is a townhouse with four floors and has a Baxi system 60/100 boiler that is getting on a bit but seems to be functioning fine on the second floor.

Air is getting into the system, which collects in the heat exchanger within the boiler (the highest part of the system except for a single radiator in the loft).. If left for more than a day or two, the air blocks the flow of water through the heat exchanger causing kettling and it overheats cutting off the boiler. The water pressure rises slightly whilst this happens. There seems to be no evidence of any water leaking from the system.

I have been managing it by running the heating in the top radiator in the loft daily. You can hear all the air rushing through and I can then bleed it though the radiator vent.

Over time I have replaced the heat exchanger, pump, auto air vent and flushed the system a few times. I have also got a plumber out to look at it who recommended replacing the boiler. I'm happy to do this, but am reluctant as I'm not convinced it will fix the problem. The issue doesn't appear to happen in the summer when just heating the hot water tank. This suggests to me that it is a problem with the heating system/associated pipework.

In terms of next steps I'm thinking of:

1. Adding some leak sealer to the system (not convinced this would work).
2. Shutting the radiators off and running the heating through one at a time per day to try and narrow down where the issue might be occurring (waiting for it to warm up before I can do this properly).
3. Adding a loop in the pipework that is slightly higher than the boiler and installing an auto air vent.
4. Adding an auto vent to the radiator in the loft and schedule the heating to run up there for 10 mins a day. This seems like a bit of a bodge to me.

Any alternative suggestions or recommendations would be much appreciated. It has been getting on my nerves for a while!

 

[fixitflav]

Could be gas produced by corrosion, not air. Try lighting it, if it burns, it's hydrogen.
In any case, worth doing a bright wire nail test to check if (more) inhibitor needed. Sample the water, 1" in a jam jar, and add 2-3 nails. If a problem, they will rust in a few days. Can do same with tap water as a control, but not essential.

 

[Alfie_Huntingdon]

Could be gas produced by corrosion, not air. Try lighting it, if it burns, it's hydrogen.
In any case, worth doing a bright wire nail test to check if (more) inhibitor needed. Sample the water, 1" in a jam jar, and add 2-3 nails. If a problem, they will rust in a few days. Can do same with tap water as a control, but not essential.

Thanks. I did try lighting but it was not combustible. I did put 5L of inhibitor in the system after the last flush to be sure! I forgot to mention this on my post.

 

[fixitflav]

Thanks. I did try lighting but it was not combustible. I did put 5L of inhibitor in the system after the last flush to be sure! I forgot to mention this on my post.

Usual inhibitor dose is 1 litre for a typical system, so 5 litre is a bit OTT. They say overdosing isn't a problem, but I wouldn't go that far!
According to the manual the pump has an auto air vent. Worth checking it isn't blocked.

 

[Alfie_Huntingdon]

Yep. The air vent is good. It has recently been replaced as well but it has made no difference.

 

[fixitflav]

Yep. The air vent is good. It has recently been replaced as well but it has made no difference.

Interesting that air/gas builds up in the h/ex. Most boilers including other Baxis have flow and return out of the top, so that wouldn't happen. Have you asked Baxi if they can help? Like is it a problem with that model?

 

[Johntheo5]

When the system is running normally, does water come out of the vent on that attic rad when opened or is there a slight vacuum?

 

[Alfie_Huntingdon]

Interesting that air/gas builds up in the h/ex. Most boilers including other Baxis have flow and return out of the top, so that wouldn't happen. Have you asked Baxi if they can help? Like is it a problem with that model?

All pipework connects at the bottom and the heat exchanger is at the top. Its probably 20+ years old so I doubt they will offer too much support!

 

[Alfie_Huntingdon]

When the system is running normally, does water come out of the vent on that attic rad when opened or is there a slight vacuum?

Water will come out of the vent once the air is bleed out. The system is still under pressure at c. 1.5 bar.

 

[fixitflav]

I doubt they will offer too much support!

If you don't ask you won't get! You might get Baxi to admit this is a problem with that model.

 

[polesapart]

The issue doesn't appear to happen in the summer when just heating the hot water tank.

Check expansion vessel pre-charge and check for water stains below the discharge end of the pressure relief pipe.