Sealed central heating system pressure loss?

[AndyR]

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 are:

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

- 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?

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

Your advice/thoughts would be appreciated.

Regards,

Andrew

[MarkyMark]

I was having a 1 bar pressure drop over a 8 hour period due to a leak from the heat exchanger and two 500ml tubs of fernox f4 cured that leak or they have as of yet (fingers crossed)

Regards Mark

[John Laird]

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.

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

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.

[Robert Laws]

I suggest you charge the system up to 2 Bar and then draw water off into a vessel to see how much loss is needed to cause the pressure to drop by 0.3 Bar, or whatever. This will tell you how big a leak yo uare looking for. I am thinking that iuf the expansion vessel is failing there could actually be very little loss at all.

R

AndyR wrote: