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

[ninah]

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 1 bar
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 bar 1.
Then I put the hot water on and the pressure gauge rose to way beyond 3.5 bar *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 bar 1.
Next time I put the hot water on it rose to about bar 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.

[gucciman]

i really dont know what is happening here as the pressure guage is for the central heating system and shouldn't be affected by the hot water at all.
the only thing i can think of is that there is a leak within the secondary heat exchanger and water from the hot water circuit is leaking into the heating circuit

[matt1e]

Hi ninah
I agree with gucciman - well sort of !! I dont think he noticed that you have a system boiler rather than a combi (you do have a hot water tank right?) well I think its more than likely the indirect coil (which is a heat exchanger) inside the tank thats leaking
.you are looking at a new hot water tank if it is

[matt1e]

and of course I mean "hot water cylinder" when i say hot water tank