Boiler low pressure problems

I have a Saunier Duval Thelia Twin 28E combination boiler which was installed when the house was built seven years ago. Over the last 3/4 years it has caused us a lot of problems usually due to lack of pressure but for the last 8 months (since it was last repaired) it has been okay.
Now it is loosing pressure so quickly that it needs to be topped up every 3/4 hours. I have placed a bucket underneath and this is catching water leaking from an overflow pipe under the boiler. There is also an overflow pipe outside but this is dry. The boiler only seems to leak when I initially top up the pressure, it then settles down, but after 3/4 hours has lost so much pressure that it cuts out.
I have found that if I turn down the CH thermostat so that the boiler is off and ensure no hot water is being used, then I allow water in to top the pressure up, the pressure still falls slightly over a short period e.g. 15 minutes even though the boiler isn't being used. During this period no water is seen to be coming out of the overflow pipe under the boiler. Initially, this suggested to me that I had a leak in the CH system and so checked all the radiators and pipes in the house but couldn't see any evidence of this. The only pipes I can't check are those under the downstairs floorboards. I would expect to see damp patches in the ceiling if pipes between the floors were leaking. Besides, if there was a leak in the CH system then surely this would result in the highest radiators (in the loft) having "cold patches" where air had replaced the lost water. This is not the case, in fact, I have bled all radiators and none had any significant amount of air in them. Does this suggest that the CH system is NOT leaking and that the problem is definately with the boiler?
We have already had an engineer replace a part last week (after waiting for 2 weeks for the replacement part to arrive) but this hasn't made the slightest bit of difference. Sorry, but I don't know what part he changed!
Are there any more tests I can perform to try to confirm that this is definately a boiler problem and not caused by leaking radiators/pipes? The boiler is covered by an insurance policy, but the radiators and pipes aren't so I want to make sure that the engineer doesn't try to blame a leaking system for the fault rather than a faulty boiler on his next visit.
If it is the boiler, any idea what might be causing it?
Thanks.
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The first thing you need to do is to stop it leaking from the " overflow pipe " which I guess is the pressure relief pipe. As it is leaking underneath the boiler that could be the reason for no water from it outside. The pressure relief valve may need renewing. Also I would look at the expansion vessel on the boiler. To see if it is faulty you would need to fill the boiler to about 1.5bar, turn on the heating and see if the pressure rises to 3bar in about 15 mins or less. That would indicate that the expansion vessel is knackered taking the pressure too high, opening the pressure relief valve, losing your water in the system, hence " a leak "

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Ian,
I tried what you suggested and that's exactly what happened. Having set the pressure to 1.5 bar while it was "quiet" I then left it and returned 15 minutes later to find the pressure over 3 bar and water pouring out of the pipe at the bottom.
We have boiler insurance through Homeserve GB Ltd (because we have so many problems with the boiler) and I'm not sure if the engineer who visited worked directly for them or was on contract. I'm guessing that they have a policy of attempting to replace the cheaper parts, such as the pressure relief valve, before changing the biggies like the expansion vessel.
I get the impression that the expansion vessel is quite an expensive part and is difficult to change. Is this true do you know?
I'm guessing that the engineer doesn't really know what the problem is otherwise he would have fixed it during his last visit. He is due again next Tuesday at which time it will have been 3 weeks since we first reported this problem to Homeserve. I'm afraid their service leaves a lot to be desired and I'm tempted to have a chat with him to try and persuade him to change the expansion vessel.
Thanks.
IAN CAPEL wrote:

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In an earlier contribution to this discussion,

I would say that the expansion vessel was pretty certainly the prime suspect.
However, it may not actually need replacing - it may just need re-charging with air. A fairly simple DIY job as long as you've got slightly more than half a clue.
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Roger
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They are not that expensive, but first I would try pumping it back up/ repressurising it. The problem with some boilers is that to renew the expansion vessel you need to remove the boiler Now you know what the problem is you can amaze the engineer with your knowledge when he appears and charge him 80 call out and 80 per hour plus vat ( isn't that the norm when it comes to heating engineers )( not me)
Good Luck

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On Thu, 30 Mar 2006 04:28:29 -0800, Milleniumaire wrote:

Try the SealedCH FAQ. It's the expansion vessel for sure.
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Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
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I checked the pressure on the expansion vessel with the boiler "quiet" and this showed 2.25 bar. According to the technical data in the manual:
Expansion vessel charge pressure: 3 bar Expansion valve setting: 6 bar
On the expansion vessel itself (this is a large silver container at the front of the boiler) it shows that the vessel should be set at 1 bar.
I'm therefore a little confused at these different figures, so I tried a couple of different settings!
When set at 3 bar the pressure gauge on the boiler went up quickly within about 5 minutes and it started venting water. I then released some pressure, down to 1.5 bar and this reduced the pressure shown in the gauge to just below 2 where it hovered for a short time before jumping up to over 3 bar again and started leaking.
So, setting the expansion vessel pressure to 1.5 or 3 bar doesn't seem to solve the problem and appears to just affect how quickly the boiler starts loosing water.
I did determine that no water is leaking out of the expansion vessel valve so I guess this means that it isn't punctured.
Any comments on my findings?
Paul. Ed Sirett wrote:

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In an earlier contribution to this discussion,

I'm not sure where most of those figures came from - they sound like garbage to me!
Please explain what you mean by "I checked the pressure on the expansion vessel".
I suspect that you are talking about water pressure which is *not* what is required.
The expansion vessel has a synthetic rubber diaphragm inside with air on one side and water on the other. The air pressure has to be set with zero pressure on the water side. It is checked using a car-type tyre pressure guage on the built-in schrader valve. The air pressure typically needs to be between 0.7 and 1.0 bar - which is probably where the 1 bar message on the vessel itself comes from. It it's too low, you pump air in with a car tyre pump.
Once you've got the air charge pressure correct, you can *then* pressurise the water system - introducing water via the filling loop. The water pressure when the system is cold should be very slightly higher than the air charge pressure - so that it just starts to compress the air in the expansion vessel. When the system gets hot, the water expands - compressing the air in the expansion vessel further. The vessel provides resilience to allow the water volume to increase without the pressure rising to too high a level. A water pressure of 2 bar when the system is hot is about typical. If the pressure rises above 3 bar - which it can easily do if the expansion vessel is faulty or not correctly charged with air - the safety valve operates and lets water out. Then when the system cools, the pressure falls to zero - which *ain't* what you want!
I've no idea where your information came from stating a charge pressure of 3 bar and blow-off pressure of 6 bar - unless you've got a highly atypical system. I don't think I'd want those pressures in a domestic situation!
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Roger
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Hi roger,

I got the figures from the boiler manual, but I have no idea what they mean!

I checked the pressure using a standard car pump with a pressure gauge attached to the valve on the expansion vessel. This showed 2.25 bar which seems high so I reduce it to 1 bar but this doesn't resolve the problem.
I have an engineer coming tomorrow and will find out what he thinks the problem is and how he plans to fix it.
Paul.
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In an earlier contribution to this discussion,

Is the manual available on-line? If so, please supply a URL where we can look at it.

But did you check it without any pressure in the water system? If not, you must! If there's pressure in the water system, it will compress the air in the expansion vessel - so that will read pretty much the same as the water pressure gauge.
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Roger
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On Sat, 01 Apr 2006 08:51:32 -0800, Milleniumaire wrote:

This sounds like it might be a boiler with a built in unvented store. The principles outline in the FAQ will be the same but the numbers will be bigger. I'm not familiar with this model and would have to look through the manual.
Not every tradesman who comes to your is automatically dishonest. In fact IMHO most are honest but the dishonest ones have a disproportionate presence.
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Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
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The engineer appeared yesterday and I got to ask him a few questions. It was a different guy to the last one and so he had to start from scratch in determining the problem.
He was able to tell me that the last guy had replaced the Pressure Relief Value but obviously this didn't resolve the problem.
The new guy thought the problem was with the Expansion Vessel (no surprise there) and set about re-pressurising it. He obviously did a better job than me at the weekend as this seems to have fixed it although I will monitor it over the next couple of days.
Your right Roger, I didn't ensure that there wasn't any pressure in the system before I attempted to pressurise it (not sure how to do this, but I guess switching it off for a while and letting the water cool down would do the trick). The other problem I had was each time I pumped some air in and then attempted to remove the pump, some air would leak out. Not a huge problem, but when trying to get it to 1 bar, I wanted to be accurate.
Thanks for your help and, for now at least, it looks okay - I haven't had to fill it for over a day!
I'm seriously considering replacing this boiler in the Summer. Any recommendations of brands? I have 3 showers on 2 floors (upstairs and loft), 2 baths and 16 radiators in a 5 bedroomed house.
Paul. Ed Sirett wrote:

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In an earlier contribution to this discussion,

No - that would only take it down to the static cold setting, which would still be about 1 bar if it were operating correctly. To take it down to zero, you'd have to physically let some water out - either by operating the release valve manually (not recommended) or by bleeding a litre or two out of a radiator. Or you could let some out at the filling point, without the filling hose connected.
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Roger
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On Mon, 03 Apr 2006 23:56:10 -0700, Milleniumaire wrote:

I strongly suggest something other than a combi-boiler but the house really does warrant something with a reasonable store of HW.
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Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
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Ed,
I know what you're saying, when we first bought the house I was sceptical about having only a combi boiler as all our previous houses had traditional gravity fed systems with hot water cylinders. In fact, in one of the three similar houses that were being built the elderly couple that were buying it paid extra to have a standard heating system fitted (hot water cylinder + header tanks in loft).
I guess the advantage of the combi / sealed system is that we don't need to worry about header tanks (which would be difficult now that the loft has been converted into a bedroom & bathroom) and we don't loose space for housing a large cylinder.
The down side, as I have found too ofter, it that when the boiler packs in that means no central heating and no hot water until it is repaired. Unfortunately, I think the builder (or the plumber) cut costs by installing a Saunier Duval boiler and I certainly wouldn't choose one again given the problems we've had with it.
Having said that, when it is working we have never had a cold house or run out of hot water except when filling a large bath - we have to wait 5 minutes half way through. I guess this is due to the small hot water cylinder in the boiler being replenished. Most of the time we have showers and the pressure from all three is good, even the shower in the loft is acceptable. I'm not sure we would get the same pressure from a gravity fed system.
So, I'm quite happy to stick with a good quality combi boiler and would prefer something more powerful (and more efficient) than the Saunier Duval boiler as I always get the impression it is being over stretched. It's also a little noisy which is unfortunate as it is located in the garage below our bedroom so it tends to wake us, especially when it's not well!
Paul.
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