CH Expansion Vessel

[Bart]

I had the expansion vessel on my CH boiler repressurised recently.

Now the system pressure is starting to rise imperceptibly again (0.3bar cold pressure increase over 2 weeks).

Can someone please explain why, if the expansion vessel is losing air and therefore the CH water has more space, why the CH pressure should go up and not down? (The pressure guage I assume is on the 'wet' side of the vessel.)

Thanks.

..Also, if I need to repressurise it myself, what's the simplest way to do this without completely draining the system?

Bart

[Roger Mills]

Odd! I suppose that a possible explanation is that the weather is getting warmer - so that the system water doesn't cool as much when the heating is off - but I'm not sure that that would explain it.

Thanks.

You need to set the air pressure with the wet side de-pressurised. You don't need to drain the system - just let some water out at the fill point - or from a radiator bleed screw - until the gauge reads zero. And yes - the pressure gauge *is* on the wet side.

Use a car foot pump (operated by hand) to pressurise the air side to about

0.7 bar (10 psi). Then use the filling loop to re-pressurise the wet system to your usual cold pressure - probably about 1 bar.

[Ed Sirett]

Could be filling loop letting by or corrosion.

[Roger Mills]

That's a very valid point. Is the filling loop permanently connected? [It shouldn't be!]. If it *is* and if there's a slight leak in the valves, additional water will flow in from the mains - slowly increasing the pressure.

[Guy King]

The message from "Roger Mills" contains these words:

They almost always are though.

[Bart]

The loop is disconnected, and there is no leak. There *was* a leak a couple months back, but that wasn't the cause, and I fixed it anyway. And the DHW heat exchanger was just replaced.

My fix had been to drain off water every so often, but with no new water getting in, it's a mystery how the water seems to replenish itself.

If the expansion vessel had no air at all, I'd imagine the system pressure would go up (a lot) when hot then down again. But the behaviour appears to be this regular rise over a period of weeks. Very strange.

Bart

[Andy Hall]

On Sun, 4 Jun 2006 22:01:11 +0100, Bart wrote (in article ):

logic would suggest that if the filling loop is not connected that the water is entering via the DHW heat exchanger. There aren't any other places where mains water and the primary circuit come close together.

Could be a dud?

[Bart]

The problem was there with the old heat exchanger (which was full of sludge). The probability of there being a leak in both the old and new heat exchangers seems small.

[Andy Hall]

On Sun, 4 Jun 2006 22:59:32 +0100, Bart wrote (in article ):

I know, but to quote Sherlock Holmes:

"Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth."

I can't think of anywhere else that it can be coming from.

[<me9]

Either that or corrosion producing hydrogen. Bleeding teh radiators should determine which.

[Bart]

Well, there are certainly corrosion problems.

What should I get in this case when bleeding the radiators, water or gas, or some combination? I remember getting slightly foamy liquid which the engineer who worked on it recently thought was inhibitor.

Anyway the CH has been off the last few weeks; would the gas produced in the rads still have an effect?

Bart

[Roger Mills]

If the system is working properly, you should get just clearish water coming out when you open a radiator bleed valve.

If a gas comes out, you need to investigate further. It is normal to get some air coming out for a while after the system has been re-filled, but all the dissolved air should come out in the first few weeks. If corrosion is occurring because there is insufficient inhibitor in the system, hydrogen will be produced. If you hold a lighted taper by the valve when you open it, hydrogen will burn with a blue flame, whereas air will probably blow out the flame.

[Ed Sirett]

It could be pressure generated by acid corrosion? This would be due to hydrogen being formed so if any radiator needs bleeding the it should be tested for hydrogen.

[Ed Sirett]

Take care, there are urban myths around about scorched wall paper/ curtains/hands.

[Andy Hall]

On Mon, 5 Jun 2006 18:49:03 +0100, Ed Sirett wrote (in article ):

I suppose possibly, but it would take a lot of corrosion in such a short time I would have thought.

[Ed Sirett]

A litres or two of gas would bring the pressure up measurably.