I've an old (2010ish) Vaillant ecoTEC Pro 28 combi boiler much the same as the one in this video:
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When the heating's on the system pressure rises and it cuts out after about 5 minutes. Also, there's a steady slow drip from the overflow. I gather from google searches that a depressurised or faulty expansion vessel is a prime suspect - sound about right?
If so, I intend to:
Depressurise the system by bleeding a radiator until the pressure gauge goes to zero (I can't see a drain point).
Check the vessel pressure and repressurise to 0.75 - 0.9
The typical sequence with lack of expansion space is that the pressure rises dramatically as the system warms up. At some point the emergency PRV operates. That does not necessarily cause a cutout immediately since it will reduce the system pressure to an acceptable level. The problem normally occurs when the system turns off and cools, and then the pressure falls too low due to the loss of primary water, and it then locks out with a low pressure warning.
Doe the drip from the overflow happen all the time, or just after the system gets hot?
If you watch the pressure gauge, what do you see it do?
If there are bits of the system that are higher than the boiler, it might never go completely to zero since it will have some static head of water above it.
Check that you don't get water coming out of the shrader valve on the vessel when you test it. If you do, then its diaphragm / bladder has ruptured. If that were the case you would need to replace it, or fit another elsewhere.
If the vessel is knackered, you can get a temporary fix by draining some water from a rad so that you have a pocket of air at the top of it. That will give you some expansion space until you can fix/replace the expansion vessel.
If the PRV has been operating frequently it is also possible that it might not reseal reliably and let through a trickle of water even when the pressure is within limits. The normal fix then is to replace it - but sometimes descaling it can help.
You helped me somew time ago with an expansion tank problem, which was fixed by fitting an external expansion tank.
The same system had another problem in warm weather. All the TRVs would close meaning the pump couldn't turn much. (It was an old Ideal Isar and I believe more modern pumps can work at variable speeds.)
The boiler would stop showing a code for a "pump fault". The solution was simply to open a TRV.
Not certain, but seems to be all the time. Very slow drip - maybe once every
10s.
It rises to near maximum, the system cuts out (pressure too high error), and the pressure then returns to (and stays at) normal. I've not needed to top up the system pressure at all.
Ah OK - would that matter for the purposes of repressurising the expansion vessel?
Some boilers have an internal spring bypass to cope with such cases. However it is common to have one rad without a TRV (usually in the room with the main stat), and that also fixes that.
Yup, that would do it. You can always unscrew the TRV head to make a rad "uncontrolled"
Mostly closed TRVs would in effect reduce the amount of primary water that is available for circulation. So the system would come up to temperature faster. So it may not change the overall problem, but could well make it happen more quickly after the system fires up.
All done - I replaced the air valve, PRV and expansion vessel (about £70, using a non-Vaillant vessel). I found it easier to isolate and drain down the boiler.
I'm not sure how long the boiler is for this world - it must be at least 10 years' old, with some nasty looking corrosion around the hot water heater. But it's very quiet and seems to do the job, so while spares are cheap and easy to fit I'll stick with it.
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