On Thu, 19 Jan 2012 20:30:29 +0000, AmateurRob
wrote:


Thanks for all the replies - I'll try to address most of them with this
reply:

- System has not been emptied recently (other than a isolating a couple
of radiators when I've re-PTFE'd the rad tails) and I've been getting
rid of air for a few months now - can't believe there's that much air in
there!

- No leaks from pressure relief valve near expansion vessel - there's a
pipe that goes to a drain that's been dry the whole time.

- I've assumed expansion vessel is fine since (a) no water from air
inlet (b) only a small rise in cold-hot pressure change (0.1bar).

- Pressure holds all the time - it only loses pressure when I bleed the
air out.

- Got some inhibitor to add, but I wanted to make sure I'd sorted the
leaks before I added it (and then diluted it with refills). Might add
that to eliminate the 'hydrogen' gas possibility.

- I suppose boiler leak is a possibility - I'm going to get it serviced
and will find out then.

- I'll re-bleed all the rads again to eliminate as much as possible.

- Can someone tell me what gauge pressure (in the cellar) I should set
so that my ground floor, 1st floor and 2nd floor rads are at the right
pressure (plus it's a Victorian house so the ceiling/height of each
floor is 9 or 10feet)



You may need a local expert's help if thorough bleeding doesn't work.
A pressurized system can't let air in unless it comes from a make-up
water supply. Never saw that in a water supply.
I've had expansion tanks on top floor and basement, and they were
never an issue.
They had no "air inlet."
Their purpose is to absorb system hydraulic shock.
They can get water-bound, but I think that's from years of oxygen
dissolving into the water.
The one I had in the basement could be isolated and drained.
I never got significant water from it when I drained it as a check.

As far as pressure to maintain, that's something that's specced for
the system.
One of mine had a straight city water connection.
The other had a reducing regulator between city water and system.
But no gauges at all, so I never knew if it worked.
I drained that system a couple times, and had no problem refilling.
City water pressure was about 40 psi in both cases.
I expect any hot water system can easily handle that.
I always shut off the water supply after completely bleeding.
No need for make-up water in these systems.
And if you did spring a leak, you'd have a continuous one.
Once full, the only pressure of meaning is head pressure.
And pump capacity as measured by pressure if not naturally
circulating.
Mine had no gauges, but the circ pump working was evident by a slight
flow noise.
That circ pump system is still running fine after maybe 50 years.

The other system I had was even older, and entirely natural
convection.
Both were 2-story houses.
These are really simple systems, and I suspect your problem is
incomplete bleeding.
Could be wrong though.
Please let us know how it gets resolved.

--Vic