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3 Bar = 43.5 PSI When you heat water it always expands increasing pressure.
In a closed vessel volume can increase by 2-3%. Either expansion tanks allow for this and PSI is maintain or without an expansion outlet the PSI will increase in proportion to heat supplied.
Is this a low temp hydronic boiler operating at or <140F, a hi temp hydronic boiler @ >180F or steam boiler?
over heat limit stat on central heating circuit ? if hot water is ok and hot to temp when run for 5 minutes the expansion vessel is doing its job.remember hot water is the maximum demand for any boiler ,,heating is secondry?once you establish hot water at maximum rate then the only real problem is on you demand for heating??
When water is heated it expands. To deal with this all boilers are fitted with a small expansion vessel which is like a reservoir which holds the excess water.
This expansion vessel is like a balloon in a metal pot. When the water goes in the metal pot the baloon squashes a bit to make room for the extra volume of water. When it cools the balloon pushes the water back into the heating system.
What has happened is your expansion vessel no longer works. When the water heats up there is nowhere for it to go so the pressure jumps up to 4 bar. The Worcester boiler lets high pressure water escape (most boiler allow this to go through the safety valve at say 3.5 bar).
When the water cools the pressure drops very low as the volume of water contracts.
The problem is your expansion vessel - normally a little red painted metal cylinder inside the boiler.
Your boiler has an expansion vessel that is like a bladder full of air. Yours has lost it's air pressure. Check your manual for this vessel and where the valve is to re-inflate it and what pressure to inflate to. It should have a simple tyre valve, pump it back up (usually to about 1 bar after releasing the water pressure) and should be all fixed. If you can't re-inflate the bladder is holed and the expansion vessel will need replacing.
Sounds like the expansion vessel in the boiler has lost pressure or is faulty. If you have a tyre pressure gauge, locate the expansion vessl and check the pressure via the schraeder valve. It should be around 1.5 bar. If there is no pressure you can re pressurise it with a bicycle pump. Should it not pressurise then you will need a new one. The pressure relief pipework outside is damp because the system is over pressurising at relieving pressure at 3 bar. This obviously causes a system pressure drop. You can install a pressure vesel into the heating return pipework should accessing the boiler be difficult.
That overflow is the Pressure Relief Valve.usually they are set at 3-3.5 bar but over time they can be gunged up inside and leak/allow water to flow out. Dismantle & clean or replace. Turn off mains water and ch pipes before removing - operate valve remove & allow for some water to leak from it
The first thing you need to do is to put some more water into your heating system. This is usually done by allowing water from your (pressurised) domestic cold water supply to flow into the system: On older boilers, this is done with a connection (often made with a removable hose) on the pipework just outside the boiler. Newer boilers may have a built-in connection with some sort of removable link (sometimes looks like a plastic key).
Once you've got the boiler up to working pressure (typically about 1 bar - 15 psi), you should be able to bleed your upstairs radiator properly.
This may be all it takes, though the pressure excursions you're getting when you run the hot tap suggest there may be another problem. See what happens to the pressure as the boiler heats up and cools down. There really ought to be very little pressure difference between the two conditions. If you see significant changes, this usually means that the "pressure vessel" (or in hydraulic terms, system accumulaor) has lost its air charge.
The pressure vessel is a metal container inside the boiler, usually painted in red enamel. Somewhere near the top, you will find a schrader valve (like the one on a car tyre), and you can use this along with a tyre pump to replenish the air in the top of the vessel. This will reduce your pressure transients for a while, but you need to keep an eye on it. If the problem returns, you probably need to replace the pressure vessel (or clag another one into the plumbing alongside the boiler).
a couple of things , is your filling loop disconected ? , and are all your radiators blead from air, have you got a high point where the air can get stuck in system say pipes going into roof etc you may need to fit a air release valve in line at highest point to remove the air from system , your boiler should be around 1.5 bar running preasure average setting . if you are constanly filling with water you could be exceeding the pressure valve limet which releases pressure from system this could be faulty ?