CH pressure keeps drpping

[Matthew.Ridges]

Hi,

I was just wondering what might cause a CH to drop from 1.5bar to 1 bar over a week ?

Sometimes when its on 1 bar it increases pressure to one and a bit.

None of the rads are leaking.

I'm stumped.

[Roger Mills]

Are you comparing like with like - in other words only observing the pressure when the system is in the same state? The pressure is meant to rise when the system gets hot, and to reduce again when it cools.

It should be about 1 bar when cold, and upwards of 2 bar when hot.

If you are genuinely losing water, suspect a leaking pressure relief valve. [Hang a container over the external discharge pipe and see whether it collects any water.]

If water *is* coming out of the PRV, it might be a faulty valve - or its blow-off pressure might actually be being reached, so that it is simply doing its job. This shouldn't normally happen,and frequently indicates a problem with the pressure vessel.

[BartC]

I had similar problems a year or two ago; it eventually got bad enough that I had to top up the water pressure every couple of hours. I can't remember the exact details but:

Initially it might have been a pressure relief valve leaking. When these operate once (perhaps due to overpressurising), apparently they get dirt trapped in the valve which then continues to leak even when the pressure drops.

I think in the end it might have been to do with the expansion vessel (either faulty or with the wrong pressure), which means that when the water heats up, the pressure rises rapidly and gets discharged through the pressure relief valve, so water is lost again.

In fact I had low water pressure just last week, but I'm hoping it's just a one-off due to the recent cold weather: a system pressure which was already borderline, dipping below the minimum operating pressure overnight as the pipes cooled down more than normal, and the boiler refusing to come on in the morning.

[TMC]

Just curious but on my system the cold pressure is 0.8 bar and the running pressure is 1.0 bar (the expansion vessel pressure being set at 0.9 bar before the system was pressurised)

A pressure of upwards of 2 bar when hot would suggest that either the air pressure in the expansion vessel is set at 2 bar or that the expansion vessel is mostly full of water.

Is it that different boilers work to different pressures or that expansion vessels are too small to fully absorb the expansion?

Regards

Apologies Roger did not intend a personal reply clicked wrong button

[Phil L]

in oe, right click on the toolbar at the top>customize

move the 'reply' button as far to the right as possible by using 'move up and 'move down' buttons - having it next to the 'reply group' button is fraught with danger :-p

[Roger Mills]

That's a strange arrangement! I would always set the air pressure in the expansion vessel to *less* than the cold system pressure - (say) 0.7 bar air pressure, 1 bar cold system pressure. Otherwise, you've got no resilience in the cold system - you've only got to lose a thimbleful of water, and you've got no pressure left. By the same token, you won't be able to bleed the radiators.

Not necessarily. The amount by which the pressure rises will depend on the system volume and the capacity of the expansion vessel. Clearly, the vessel needs sufficient capacity to allow the water to expand when fully hot without generating excessive pressure - but 2 bar isn't excessive. 3 would be!

Dunno. Not quite sure what you mean by "absorb the expansion". As the water expands, it goes into the vessel and further compresses the air - so the pressure *will* rise (Boyle's Law and all that)- but that's what it's designed to do.

No worries - like it says in my SIG, I don't check that address - so hadn't seen your personal reply!

[The Natural Philosopher]

- gas boiling out of the water.- - de gas and re pressurise.

- faulty pressure regulator allowing high temps to cause high pressure and safety valves to pop a little . Replace or repressurize the regulator.

- a leak you haven't spotted because its to small and the water evaporates off the hot pipe anyway. Wait and hope it seals itself..

[TMC]

Checked the boiler manual and indeed you are correct

they specify expansion tank pressure as 0.7 to 0.9 bar

they say to set the water pressure when cold to 1.2 bar which I have now done

(Actually the say to pressure to 1.5 bar and then use the pressure relief valve to reduce it to 1.2. Did not do this as may introduce debris into the valve seat which is currently fine) I wa misled by some stickers on the front of the boiler which showed pictures of the pressure guage one showing 0.8 bar as ok and should light and another showing 0.5 bar not ok will not light

I will check the running pressure when up to temperature but on the basis of the earlier figures do not expect much rise in system pressure

Cheers