I have a worcester bosch boiler, whose pressure dial is all over the place. Sometimes it's at close to zero in the red zone, sometimes it's just in the green zone.
It's been behaving this way for many months.
Is this a problem with worcester bosch boilers in particular, or boilers in general? When we get a British Gas engineer to fix it, he increases the pressure so it's in the green zone. After a few weeks it's fallen again into the red zone.
The boiler is ten years old. Can it be fixed? Should I be looking at a new boiler? Apart from the pressure dial, the boiler is fine.
That would be my guess too, or perhaps it just needs topping up? Ever since I had an old Vaillant getting on for 30 years ago where the vessel was totally inaccessible, I have had an external one on my system. Modern stuff seems to be much better designed for servicing.
If you are half handy with DIY, it may not be difficult to remove the outer casing, then it is just a question of unscrewing the valve cap (if water comes out, the diaphragm has failed), attaching a bicycle pump and pumping it up to 20 psi or so (you will need to look that up). You should be able to do all that without doing anything like disturbing gas or opening the combustion chamber.
You imply that you never need to top it up. Also maybe worth taping a clear polythene bag over the external overflow and checking after a few days whether any water has come out.
And if its into the green when cold, does it go very high as the boiler heats up?
It sounds like this is not a fault with the dial- its probably telling you the truth. Its more likely that the system is losing pressure over time for some reason.
Probably
Not if that is the only faulr
I expect the pressure dial is fine, and you either have a system leak, or a non working expansion vessel.
With a leak, you would expect the pressure dial to act fairly normally - rising a bit as the system warms up, and falling back to where it started as it cools - however you may see a steady drop over a period.
If its an expansion vessel problem then the pressure change will be much larger, swinging very high as the system heats, that causes the emergency pressure reduction valve to dump some water out of the blow off pipe outside. Then as the boiler cools it dropping to a much lower pressure than when it started.
Expansion vessels are basically two sided chambers with a rubber sheet separating the two sides. One side should be filled with air at pressure, the other connected to the wet side of the CH circuit. As the system heats and the water expands, it compresses the air. When the pressure falls the air expands and pushes the water in the expansion chamber back into the system.
The have two common failure modes; one is where the air side has just lost some or all pressure. That can be fixed by pumping it up like a bike or car tyre (same air valve as a car tyre). Alternatively the rubber separator has perished, and the air bubble lost. If you find the vessel and give a brief push on the valve centre you should get a puff of air if working, or water if knackered.
If its knackered then you either need to replace it, or add another expansion vessel elsewhere in the system. As a stop gap you can bleed some water out of a rad and leave a pocket of air at the top - that will act like an expansion vessel for a bit.
The expansion vessel is the silver thing on the RHS in this Vaillant:
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You can see the shrader (car tyre) valve on the front in the middle.
On many boilers these tend to be positioned right at the back behind the main heat exchanger and are a PITA to get at. (they are often painted red as well). So these often get abandoned and another fitted somewhere more accessible. On ones like in the photo, changing the built in one is very easy.
Pressure vessels/system pressure has been exercising my mind lately (never mind the just discovered manifold drip).
While the system was out of use, I checked the expansion vessel pressure. Around 7.5psi but with a slight dribble of water. Pumped up to around 1 bar.
Room temp to full system heat gives a pressure rise of around 0.8 bar so within reasonable limits. However the slight drip has made me aware how sensitive this set up is to system water volume changes.
My inclination is to reduce the expansion vessel pressure to say 0.8bar (static head in a chalet bungalow not an issue) and the room temp. system pressure to say 1.2 bar. This would use up some of the available expansion volume but make the system more stable.
As has been said elsewhere before, I should really fit an additional expansion vessel!
Oh! That's a bit worrying. I suspect mine has been opening due to water hammer when the changeover valve operated. I have put in a relief valve set at around 0.8 bar shunting the boiler output through the DHW coil and then back to the boiler.
Yup can happen - although probably more likely if it does it a few times and can get a chance to scale up. (and if its happing often that also suggests more top up water, less inhibitor and more scale in the first place just to compound things).
Sounds like a sticky needle in the dial gauge. If it were the faulty expansion vessel the pressure would drop below the point at which a pressure switch would prevent the boiler from firing.
We had a British Gas engineer a couple of days ago, and he said the loss of pressure over a period of two or three months is natural, that the boiler needs to be topped up every two or three months, and not to worry about it.
Is he right? I'm not into this stuff, should I worry?
If it has been recently drained and refilled (or its a completely new system) then there will be a fair amount of air dissolved in the water. This will tend to come out of suspension and accumulate in a radiator somewhere. When you bleed that rad, you will drop the pressure a bit and may need to top up.
However once a system has been running a while and the the water is basically stagnant, then it ought not change much or at all.
It is possible to have tiny leaks in a system - so slow the water evaporates before you ever see it. This is generally not a problem if you only need top up once or twice a year. Although you should periodically add some extra corrosion inhibitor since it will get diluted over time, and the fresh oxygen introduced each time you top up needs to be "mopped up" by the oxygen scavenger in the inhibitor which will get used up by the fresh water being introduced.
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