I have a main boiler which on very cold mornings does come on. When i check it, it has low pressure and shows an error code. I top it up with a small amount of pressure and it runs ok for a while but then sometimes cuts out because the pressure goes too high. When the weather is ok the pressure doesnt degrade with time so i guess theres no leaks.
Is it possible the expansion tank is too small i have 10 rads in total 9 are 1400 x 600 doubles and 1 is 2400 the expansion tank says its 24 liters which is a fairly big one.

Any help appreciated.

Cheers pete

My Potterton Myston boiler has a bladder which is meant to inflate/deflate as the system warms up cools down, last time my engineer was out for the same problem you have, he drained some off.


It has an automatic cut off for low system water pressure, mine drops under 1 bar after about 6 weeks and switches off.


Boiler man been here for drips from the boiler, and now Im not sure where the water is going, but I top it up every 6 weeks or so.



Do you know how ?

Is this an existing boiler which has suddenly developed a fault or a newly installed boiler ?

Unless you are trying to use an especially miniscule boiler to run an especially huge heating system then i would be very very surprised if the fault was caused by too small an expansion vessel.

It is far more likely the expansion vessel has either gradually lost it's air pressure or it has gone faulty. As you will be aware an expansion vessel contains two parts, the outer chamber for the water and an inner' bag' of air. Release some of the water pressure if you can then find the Schrader valve on the pressure vessel and check this for air pressure- anything that you'd use to check tyre pressures would be fine. Release a bit of air first to make sure it does actually contain air and not water. If there is indeed some air present but not much of it you should increase the air pressure to what is specified in the boiler's booklet, look on the boiler manufacturer's website if there no book to hand.

If it neededa lot of air pumped in to build it up to the right air pressure check it again after an hour and at regular intervals to make sure there isn't pressure loss. If there is serious pressure loss you will need to replace the expansion vessel, they aren't really that expensive or difficult to replace usually.

When all is sorted in that department you should repressurise the ssystem (water pressure) and use as normal.

Check expansion tank pressure with sytem drained
Also check pump is running if its not boiler could be overheating and pressure in the boiler will shoot up releasing at the prv

[Edited on 24/2/13 by adam1985]

Ok i checked the expansion tank without draining and it was showing at over 3 bar with the boiler showing 2.5 which didnt seem right. So i drain the system down and rechecked and still at the 3+ so it looks like the tank had been left at the precharge. The boiler manual says 0.5bar minimum but no set range so i set it at around .8ish and filled the system. It now only goes upto about 2.5 when at temp which is lower than previously. Ive just got to wait and see if it comes on in the morning now but i guess it should work better now as the expansion will actually be doing something.

Thanks for the help guys.

Cheers pete