Central heating - losing pressure

Hi,
We have a Worcester Greenstar 27 HE system boiler which was installed in combination with a Megaflow unvented cylinder about 2 years ago when our extension was built.
The system has worked well but I noticed last week that the pressure gauge was reading zero bar. I repressurised the system and checked for leaks (none found). Since then I have been in a cycle of having to repressurise from zero every couple of days.
The system is on contract with British Gas and they have been out a total of three times in the last week. Most recently (today) a new siphon was installed. Unfortunately this hasn't solved the pressure loss.
It seems most likely that a leak exists, but I have inspected all radiators, plus ceilings and under floors.
One observation that was made -though not necessarily related - is that the boiler seems to be plumbed in with two zone valves and no bypass circuit. I had noticed that the temperature in the boiler climbs to around 90 degrees whenever the heating shuts off. However, there's nothing in the installation instructions for the boiler about requiring a bypass circuit. Just wondering if there is a built-in automatic bypass on this model?
Any help or suggestions extremely welcome. Please go easy, this is not my field of expertise!
Gareth
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In an earlier contribution to this discussion,

The chances are that your pressure vessel has either lost its air, or is shot - thus not providing any expansion resilience - and causing water to be lost through the pressure relief valve when the system heats up.
The outlet pipe from the pressure relief valve should go out through the wall into fresh air. Find it and tie a plastic bag round it. Then re-pressurise the system, and see whether the bag collects any water when the system gets hot.
Come back when you've done that.
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Bagged the discharge pipe, re-pressurised to 1.5 bar (hot). After a couple of hours pressure down to 1.0 bar, bag empty.
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In an earlier contribution to this discussion,

Sounds like you may have a leak somewhere else, then!
You really ought to pressurise it to 1 bar *cold* though, and see what happens when it gets hot and then cools down again.
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Okay - we left the heating on a low setting overnight (to keep junior asleep!) but when I got up this morning the boiler had gone onto an F7 error code (flame detected but appliance not running). This happened last week, prompting replacement of the siphon.
Anyway, I took advantage of the system being cold to check the expansion vessel pressure (i.e. system pressure was virtually zero, plus I took some air out of a couple of radiators). The expansion vessel was showing about 0.5 bar so I pumped in some air until it was at around 0.75 bar which is what the manual recommends. I then repressurised the system to 1.0 bar and reset the boiler. Once warm the pressure gauge had only risen by around 0.5 bar, which is better than before.
However, still nothing from the discharge pipe.
By the way, the original installers have responded to my query about the lack of a bypass circuit by agreeing to send someone round to check it out. Discussion was along the lines of "if it hasn't got a bypass we'll have to fit one".
Thanks for the advice so far. Gareth
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In an earlier contribution to this discussion,

Is it still losing pressure, though?
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Apparently it held for a while but has now dropped back to zero :-(
Guess I should recheck the expansion vessel pressure tonight to see if it has fallen again.
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In an earlier contribution to this discussion,

Have you still got the bag on the pressure release pipe? Did it collect any water prior to the pressure drop?
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Just took the bag off - it has collected maybe 20ml over 24 hours.
This evening I found a significant leak on one of the radiators - I walked into my son's room and there was a very noticeable whistling coming from the bleed valve. Even after tightening it up it didn't stop - only covering the hole with my finger killed it. The carpet under that end is pretty wet. Now, this is not the sort of thing you could easily miss - it's possibly significant that I had just shut down the system so I could recheck the expansion vessel pressure. Anyway, I closed off that radiator at both ends and was briefly optimistic - repressurised to 1.0 bar but it has already fallen a little :-(.
Regarding the bypass circuit, the installer came round today and pointed out a 1.5 metre loop of pipework in the ceiling above the boiler, incorporating a balance valve. However, to me this seems all wrong as it branches off the main flow pipe *** after *** the zone valve. As soon as the room thermostat switches off, the zone valve closes and there is no flow into the circuit at all. Am I making any sense?
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In an earlier contribution to this discussion,

Well, there shouldn't be any at all coming out of there. Presence of water in the bag indicates either that the pressure relief valve is faulty and is opening at too low a pressure or that the pressure is becoming too high (>3bar) at some point.
[Are you sure that it *is* the pressure relief pipe, and not the condensate drain for a condensing boiler?]

Was it air or water coming out? Do any or all of your radiators often need bleeding? Is there any corrosion inhibitor in the system?
The carpet

The by-pass circuit is designed to provide a path for water circulation during pump over-run conditions when all zone valves are closed. There *must not* be any zone valves between the boiler and the by-pass valve!
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On Tue, 07 Dec 2004 15:01:04 -0800, no_heat wrote:

See FAQ
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Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
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