Air Problem in Open Vented System after Boiler Change

[fixitflav]

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,

If the HW zone valve is normally closed, sounds like the air is getting into the HW circuit via the return pipework. That might be connected with the return pipework layout you described earlier, hard to say without knowing the details. But it doesn't alter the fact that the air shouldn't be there anyway.

 

[fixitflav]

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?

That tends to support my theory - heat exchanger. Theory also means there'd be more condensate than expected, which sounds like you're getting. Can you catch it, and measure volume? Measure gas volume over same time and compare actual condensate with estimated from gas volume.

 

[fixitflav]

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?

Agree with kop. Not right.

 

[fixitflav]

Water level in expansion tank had risen approx 6mm this morning, but system was slightly warm. Will keep an eye on it and check again at the same time and temperature.

Mmm! I'd expect it to drop if water is being drawn into the h/ex.

Personally, I wouldn't use pressurised on a new system, and I definitely wouldn't take out an existing F/E tank and convert. Each has its pros and cons.
And I doubt it would cure your problem. It's possible higher pressure at the boiler would stop air ingress (if that's the cause), but you might get outward leaks instead! If the pressure is kept low the boiler doesn't know it's not on gravity.

 

[fixitflav]

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.

It might, but the leak only seems to happen when the h/ex is hot, otherwise you would expect water leaking out when the system is idle. BTW what's a Monument gauge?

 

[ThePook]

What makes you think your pipework (which has survived for decades) won't be able to take the relatively small increase in pressure from a sealed system?

I take your point. Honestly, its probably just a somewhat irrational fear of undiscovered leaks under floors. Genuinely, I believe it originates from discovering dry rot under the floor of our first house on day 3 of ownership, having skrimped and saved for years for a deposit, and being pennyless to sort it out quickly!

Oh and you have a manual vent on the return, I'd probably replace that with a bottle vent which would at least prevent any trapped air from being recirculated.

Good shout and an easy job for a quick win. Leave open all the time or close after initial fill and bleed? Worth putting one on the top flow into the DHW coil, too? Both places are manual at present.

But it doesn't alter the fact that the air shouldn't be there anyway.

My thoughts entirely, and why I want to remedy at source the air problem.

Mmm! I'd expect it to drop if water is being drawn into the h/ex.

What if the HEx was also somehow letting the air in? Long shot, I guess.

BTW what's a Monument gauge?

Sorry, Monument-branded water pressure gauge - on a hose with a 1/2 bsp female connector. Bought when our first house kept popping joints, and eventually proved that our incoming water mains was over 10bar at times - see above, sad times!

Thanks all for the constructive comments - appreciate you taking the time.

 

[gmartine]

A bottle vent does so automatically, yes I'd replace both manual vents. Intergas condenses all the time due to nature of hex, doesn't explain colour but may explain volume.

 

[fixitflav]

Thanks all for the constructive comments - appreciate you taking the time.

Keep us posted on F/E tank level and condensate production. I still think it's most likely the h/ex.

 

[ThePook]

Let the system cool tonight, much to the disgust of Mrs Pook, and the cold level in the tank is exactly the same as 24 hours earlier.

Going to struggle to catch and check the condensate, also cook with gas, so volumes will be misleading. Will have a think on that one...

 

[fixitflav]

Let the system cool tonight, much to the disgust of Mrs Pook, and the cold level in the tank is exactly the same as 24 hours earlier.

Going to struggle to catch and check the condensate, also cook with gas, so volumes will be misleading. Will have a think on that one...

As you do not appear to be losing water (out of the h/ex) checking condensate vs gas consumption won't tell you much.
The h/ex could still be drawing air in but not leaking water. I can't think of anything else, and you've tried nearly everything. I can see the installer will be loth to change the h/ex (or the complete boiler). Maybe if it's at your cost if it doesn't cure the problem.