The water pressure in my Potterton combi-boiler seems to fluctuate wildly. In the morning when it is cold, the pressure is usually around 0.2 bar. I open the valve to manually increase the pressure above 0.5 bar (recommended minimum). When the boiler fires up and the water temperature increases, the pressure slowly rises to around 3 bar (recommended maximum 2.5 bar). I initially assumed it was down to the expansion of trapped air, but I have bled every radiator and they all seem to be fine.I'm guessing the safety valve is opening when the pressure gets too high, which is why it is low again the next morning, but I have no idea why there would be such a big difference in pressure between when it is hot and when it is cold.Any ideas gratefully received.
Peter W is spot on,the expansion vessel on the boiler will either be burst or more likely be needing recharged with air.
have you bled the actual boiler there is usually a automatic bleed valve on top that can leak but needs manual bleeding occasionally, if the top of your boiler is full of air that would explain it
the water in your system expands by 4% when heated this increases the pressure in your system, the expansion vessel (a big metal cylinder usually red) which is in the back of the potterton takes up this expansion when the system is hot and returns to normal when the system cools. if this expansion vessel is not working (no pressure or split inside) then when the water in your system heats it will discharge out of the pressue relief valve. it will continue to work until the system cools. as the system cools it will loose pressure hence why you find it with just .2 bar in the morning. remedy, you need to try to recharge the pressure in the expansion vessel this can be done with a foot pump with a pressure gague on, if this will not charge you will need a new expansion vessel.
Normally, you will find a little tap valve under the boiler, this comes from the mains cold water. There may be some kind of braided flexi hose attached to this, for filling the heating system. Turn this tap valve gently till the pressure rises back to 1 1/2 bar. If the pressure has dropped, there may be a problem(leak) with the heating system, or the boiler itself. Keep a close eye on it.
Do you have an expansion tank? If not that is your problem. Water expands when it heats and if it has nowhere to go it will blow off through the PRV valve. Then as it cools down it shrinks - low pressure. Or, your expansion tank could be saturated. In either case, this would explain the problem. If you have a bladder-type tank, make sure it is at the proper pressure (typically about 15 pounds (~1 bar)). If you have an old-fashioned air-tank, close the feed valve and open the drain valve, draining it entirely. Close the drain valve, open the feed then bring your boiler to the correct pressure. If no tank at all, get one installed properly sized for the system.
