Boiler Pressure Problems

I have a Glow worm Compact Electronic (combi-boiler).

Recently the pressure has been going up to between 2.5 & 3 and the "Low water pressure" light flashing red. Also hot water has been coming out of the overflow pipe on the side of the building.

Apparantly the pressure should normally be about 1.5. When the boiler is turned off the pressure gradually decreases.

So.. the questions, what does the flashing red light mean? How can I stop it? Could it be that the radiators need draining?

Thanks for reading & even more thanks for replying

Many hot water boilers run at around 15 pounds pressure. Check your gauge to see if it is marked x10, so 1.5 would be 15 pounds. The discharge may be the temperature/pressure safety valve. If the pressure really is getting to 25 pounds, then it is doing its job and discharging the water to relieve the pressure. Because the water expands and contracts with temperature, if you shut things off, the pressure would normally drop.

Somewhere in the system there is an expansion tank. Often, when the pressure goes up during operation, it is because the pressure tank's internal bladder has failed. This tank has a flexible membrane (bladder) that separates the wet side from trapped air. If the bladder has lost all or most of its charge, or has sprung a leak, then there is no place for the water to expand when it heats, the pressure rises, and the discharge system dumps it.

Many systems, but not all, have an automatic water makeup valve which refills the system if there is a leak or a loss of water. If yours does not have one of these, after the system dumps water because the expansion tank isn't doing its job, then you will be missing some water.

Low pressure (if the system is actually working properly), is a problem that needs to be fixed. A hot water boiler, just like your car, needs a little pressure to keep the water from boiling and creating steam. The system is not designed for steam. Many boilers won't operate if they detect low pressure, yours at least is warning you of the situation.

Note, I'm not a pro, but have dealt with my own boiler for the last 15 years...a blown expansion tank is probably the cause of your problem. It should sound hollow on the end towards the air valve, and my guess is that yours sounds full (dull thud when knocked) on both ends. If so, replace, and you'll probably be okay for awhile.