my glow worm 30ci boiler keeps losing pressure. any idea why?

I have a similar problem. When the central heating is on my Ultimate C35 is losing pressure at the rate of approx 0.1 bar per day. This means I'm having to top up the pressure every 5 or 6 days. I am wondering where the water is going. There is no loss of pressure when the heating is off so that makes it unlikely that the cause is a leak in the system. My plumber has checked the expansion vessel and has not found any fault. He is suggesting fitting a second expansion vessel. I'm not convinced that this is the solution as the boiler was installed nearly 5 years ago and the loss of pressure only started about 6 months ago. Any ideas?

Andrew M627
March 2023

I had a similar problem today with my 30ci boiler ,due to a mains pipe leakage pressure was 0.4 bar when i got home to the well known [there's no hot water]winging from the wife,suggesting that we call out an engineer, and at about a £100 call out that i would much rather spend down the pub i set about fixing the damn thing.After reading the instructions after one can of fosters i threw them in the bin deciding that a real man does not need instructions on how to fix a man made machine.I tried to refill the reduced pressure system[as per instructions] and had no joy so turned off the complete water system to take the damn machine apart to find that the sudden drop in pressure due to the mains being turned off had caused the pressure sensing valve to become stuck down.sollution is to release any residual water pressure by undoing the water inlet pipe a small amount, resecuring it then winding the main water supply tap to your house to full whilst having the blue system fill tap on your boiler wide open, you will find the pressure on the boiler indicator will rise as high as 3.2 bar after which close the blue system fill valve then reduce system pressure to 1.2 bar using the black boiler drain valve[use a large bowl at this point because it sprays out like billeo soaking all the electrics] who said being a heating engineer wasn't dangerous!!! light up the boiler,run at max heating,bleed the radiators and hey presto, fixed. Dont worry if pressure goes to 1.8 bar when hot because as long as an idiot didn't plumb your house it will be fine unfortunately an idiot did do my plumbing but there you go , i do live in the midlands what more could i possibly expect ! ps:max pressure on the system is 2.5 bar !

nick ball ! IF YOU CAN P*SS YOU CAN FIX ANYTHING
October 2008

Check the expansion vessel if the internal diaphram has gone then theres no where for the water to expand and it gets pushed out of the safety valve resulting in a loss of pressure.

Dennis Scales
November 2007

or just fix the leak

a.n.other
November 2007

I had this problem with a glow worm 35CI. The water was being pumped out through the overflow because the flexible braided pipe leading up to the expansion vessel had blocked with silt. Try manipulating the pipe by hand so that you eliminate some tight bends.

Barry Abel
November 2007

you have a leak somewhere

hi-spec plumbing & heating
November 2007