YACH Problem

[Alan]

My sealed CH system appears to be playing up, although is still heating the water (tank) and the rads.

I noticed that the pressure guage on the boiler dropped to zero yesterday when the main room themostat reached the temp limit. Prior to that the pressure had been at around 2.5-3 bar. Last night I shut it all off and the pressure settled at 1 bar. I turned everything back on and this time when the thermostat clicked off the pressure settled back to 1 again.

But this morning it clicked off and dropped back to 0.5bar.

Anyone got any ideas what I should check? I don't know much about the setup, but it has got 2 zone valves under the HW tank (one of which the BGas engineer changed recently)

[John Laird]

I'd check the expansion vessel first. An operating pressure of 2.5-3 bar is too high for most systems. You'll need to locate the vessel (often in the boiler casing), and then release water from the system so that the boiler gauge drops to 0. If you put a tyre pressure gauge on the expansion vessel valve, it should be around 1 bar (iirc). If not, pump it up. If it holds pressure, then it has just gone down slowly over the years (using Schrader valves pretty much guarantees this in time). Then refill the system to the correct pressure on the boiler gauge (1 bar plus natural head on mine). If your vessel won't hold pressure, it will need to be replaced as the internal diaphragm is shot.

Without an effective expansion vessel, your system is "full" of water and any temperature rise produces large pressure rises. The expansion vessel is there to provide a "cushion", when it's working properly, that is.

I'm at a loss to explain the variation in non-running pressure, though.

[Alan]

Thanks John, The tank/piping in my airing cupboard looks like this picture:

(sealed system)

Would this expansion vessel be the one I need to check (or is there another in the boiler casing, boiler is an Ideal Classic). I did point out the high pressure to the BGas man, but he said it was ok. It wasn't dropping to zero at this point in time though.

Could the gauge be faulty?

[Alan]

And I checked the tundish yesterday, there was no sign of it having been wet recently (and in fact in was coated in dust on the inside.)

[Set Square]

system: (sealed system)

NO! Not if your airing cupboard looks like the one in the diagram. This shows an unvented (mains pressure) hot water system - whereas your problem is in the *primary* circulation system - i.e. the water which circulates from the boiler, through the internal coil in the hot cylinder, and through the radiators.

(or is there

If it's a sealed primary system, there'll be another one somewhere. It may be spherical or - if it's inside the boiler casing - it could look like a large fat pancake.

It's possible. Usually, if the pressure genuinely fluctuates by too much, water is lost through the safety valve - and when the system cools, the pressure then goes to zero.

With the primary circuit de-pressurised, the gas in the expansion vessel should be at about 0.7bar (10psi) as measured at the Schrader valve. If it's less than this, pump it up with a car tyre pump. Then use the filling loop to pressurise the wet primary system to about 1bar when cold. When the system gets hot, the pressure should rise to about 2bar. If it goes significantly higher than this, you have a problem. The safety valve is designed to operate at 3bar - but you *don't* want that to happen!

[John Laird]

See Set Square's response. I would note that the expansion vessel in my boiler is actually hanging on the back of it, nearest the wall, and can be seen without removing any panels. It's a large, red, pancake thing :-)

They can be completely external, however.

[Alan]

The weirdest things happen. I found the red flat pancake thing under the top cover on the boiler, along with a valve with a wire going to the pressure gauge - I had a poke around, but I didn't touch anything switch/button like. I checked upstairs in the airing cupboard, no sign of leaks, and the expansion vessel upstairs didn't look like it had been touched in ages (covered in dust), didn't see a valve (although there must have been one somewhere).

The boiler was burning away, pressure was approaching 3bar again - I went outside to check the pressure relief pipe, which appeared to have been dripping at some point (slightly wet). Went back in and the boiler pressure had dropped to 1 bar and the heating was off (boiler themostat). I adjusted the boiler themostat, which kicked the boiler back in, turned it down, it cut out as expected. At this point I noticed an 'orange' glowing light from the control board, which I don't remember seeing before but I might have just missed it.

Anyway, the boiler now appears to be ok - cold pressure settles at 1 bar, running pressure is just above 1.5 bar. I have no idea what happened, maybe poking around (and moving the cover) fixed something, or there was a blockage somewhere that's now gone. The pump now appears to run once the boiler themostat cuts out, which I definitely couldn't hear before (and neither could the BGas man the other week). Perhaps I've got a faulty control board?

I wish I had a nice simple heating system!

Thanks for the help.

Alan.

[<me9]

Do you keep your filling loop connected? You could have a slight leak into the system, causing intermittent overpressure and the releif valve is lifting, and not always re-seating properly.

[Set Square]

That sounds about right.

Unlikely! Someone else has asked a very valid question about the filling loop being left connected - which may explain things if it is.

Does the pump run for ever, or only for a short time after the boiler stops? If the latter, this is intentional - to carry the residual heat away and stop the boiler from overheating.

[Alan]

The filling loop, at least the one in the airing cupboard, which is the only one afaik and does affect the pressure gauge, is permanently connected by the look of it. I've never tried to remove it, and the 2 BGas engineers that looked at the system didn't comment on the fact it was. I'll check if it has got a leak.

[Alan]

The pump only runs until the room themostat cuts out, which also cuts off the boiler if it was burning. I can definitely hear it running now, which I couldn't before when the boiler stopped burning.

Hopefully it's ok, I'll check the filling loop and pressure relief pipe again tonight.

[Set Square]

That's ok - that's how it should be.

The filling loop *must* be disconnected after use. Otherwise, any leakge through its valves can over-pressurise the system from the mains.

[John Laird]

Care to put a figure on the percentage that actually are, though ?

The system over-pressure valve would presumably have to fail for damage to occur, though. So that's three valve failures before real problems arise.

[Besides, I think I might rather that a faulty filling-loop valve leaked into my CH system as opposed to leaking all over my house, having seen the damage done by an uncontrolled burst on a mains cold-water system.]

[<me9]

I wonder what proportion are disconnected, even though it is a legal requirement. I haven't seen a sealed system (other than my own - I fill it with a hand pump) with it disconnected.