AndyR wrote:
Hi all,

I've recently sealed my fairly old central heading system and I'm
experiencing a pressure loss and would appreciate your help and advice.

In brief, the system is around twenty years old with a capacity of
approximately 100 litres. Over the last twelve months I've replace the
boiler, all nine radiators and valves, the hot water cylinder and have
carried out some minor pipe work alterations. Most recently I've sealed
the system with an 18 litre kit preset at 1.5 bar and an automatic air
release valve installed at the highest point.

All appears to be fine with the working pressure up to 2.0 bar, except
I'm experience a system pressure loss of approximately 0.1 bar over a
twelve hour period with the system set at 1.0 bar and remaining cold
through out. This loss seems to increase to around 0.3 bar over the
same period but with the boiler in normal use. Having looked in the
easily accessible places I've only found two small weeps but nothing
more significant. I've not added any inhibitor yet as I didn't want to
run the risk of having to flush it away should I experience problems.

So, my questions a

- Obviously I have a fairly significant pressure loss, but how much
water would this result in?

The maths is not too hard. Your expansion vessel pre-charge pressure
seems a little high - I'm sure it is normally set a little lower than
the typical system pressure when cold. If it really is 1.5 bar, then
any loss of water pressure below that level will result from the
tiniest loss of water, because the air in the vessel is not being
compressed by the water at all. If the water pressure starts at 2.0
bar, then the air has been compressed to 3/4 of its original volume
(pressure x volume is constant). The other 1/4 is now water, which is
4.5l according to your figures. A pressure drop from 2.0 bar to 1.5
bar (when cold) is thus a loss of that 4.5l. Call it a gallon.

If your vessel is at 1.5 bar and the system is at 1.0 bar, then you
have no water in the expansion vessel - it is all air until the water
pressure exceeds 1.5 bar. A loss of 0.1 bar could be the dissolved air
in the water being released.

I may be misunderstanding your expansion vessel settings, but possibly
you should reduce the air pressure in it to about 0.7 bar, and then see
how the system behaves with a cold pressure of 1.0 bar. If it
continually drops down, then a leak is confirmed.

- Could the lack of an inhibitor and the subsequent build up of
nitrogen/any trapped air in the system, and its subsequent loss through
the air release valve, result in a variation in pressure and would
topping the system up slightly continue with this cycle?

That's been my experience when re-filling a system. A few pressure
top-ups were required before it settled down.

- Assuming I can't find any further water loss how effective are liquid
system leak sealers in resolving problems like mine?

I have no experience of leak sealers, but if you assume the leak is of
the order of a small weep, I would imagine they could be effective.
Better to find the leak if you can, though, in case it's something
working loose. I think installers use compressed air to leak-test a
new system.

--
"Life would be easier if I had the source code."