Strange Goings On with Boiler: Pressure Surging when Hot Water Goes On


I bled my radiators a few days ago as the top of the radiators weren’t getting warm. I probably collected about 1-2 litres of water in the process.
That afternoon I noticed that the pressure on my boiler had dropped below the acceptable green range.
So I let some water into the system until the pressure gauge moved up to the desired level (1).
I noticed a few hours later that the pressure had dropped a little, so I put in a bit more water to take the pressure gauge back up to 1.
Then I put the hot water on and the pressure gauge rose to way beyond 3 *PANIC* (no water was flowing out the pressure relief value was newly fitted about 6 months ago?)
So I drained about 5-6 litres out of the radiators to get the pressure gauge down to 1.
Next time I put the hot water on it rose to about 2.5-3. (hot water not on for as long previously as I was keeping an eye on it)
So I emptied another 3-4 litres out of the system by bleeding a radiator.
Yep you've guessed correctly, next time I put the hot water on it rose rapidly again.
I bled another 3.4 litres from the system.
Pressure not sat at 3 when cold.
I don't want to bleed the radiators dry, so what do I do next? Ay idea as to what has happened?
All was fine before I tinkered with the radiators :(
Any help and advice will be very much appreciated indeed :)
Note the pressure doesn’t rise as much when I put the central heating (only when I fire up the hot water) on if that’s any help. It’s a Alpha SY24 Boiler.

nina, November 2008

Hi, pressure settled down after a couple of days. Can only imagine i had put too much water back into the system, hence the pressure surging, many thanks for your help

Nina, November 2008

sorry Ricky correct me if i am wrong ,but an alpha sy24 is a system boiler not a combi so it will not have a plate hw heat exchanger !!

matt, November 2008

THESE BOILERS HAVE A PLATE HOT WATER HEAT EXCHANGER THE REASON THE PRESSURE RISES MAY BE THAT WHEN YOU ARE RUNNING THE WATER IT IS OPENING UP THE SMALL PLATES IN THE HEAT EXCHANGER ALLOWING THE WATER TO GO INTO THE HEATING SIDE THIS IS QUITE A COMMON FAULT

RICKY, November 2008

you keep draining water out but not putting it back in ?? have you shut off the filling loop fully ? it is also poss the expansion vessel has lost its charge of air , but its unusual for this problem in hw mode most common with ch ! the air you have let out of the rads could of been acting as a mini expansion vessel [ thus not showing a prob with the main expansion vessel ] now you have removed that air it has highlighted the problem

matt, November 2008