gravity feed central heating nightmare... help needed pls

i have a gravity feed system with the boiler downstairs, pump, 3-way value and hot water tank upstairs and overflow tank in the loft.
the system has not been very well, i have swapped out the pump and the 3 way value in the last 6 months.

but then it died again, the pump seems to be pumping but i get a massive kettling effect, it seems as if the overflow tank is not feeding the system.
there is air down the pipe to the boiler then as the boiler heats up, you can hear water pouring down the pipe and getting vapourised and exiting the system via the overflow pipe (22mm)which runs up into the loft with an open end over the overflow tank.
i have all the symptoms of having a sludge filled system. eg failed pump, noisy kettling boiler, and micro leakage from the odd rad. And now this failure, so i though there was a blockage in the pipe 15mm feeding the system from the tank in the loft... (but not sure)
so i hired a power flow pump and washed out the whole system today.
the results seemed good, during treatment i had the boiler on for 3 hours.
all the rads got hot and i got hot water from the taps.
i thought that that ment i had no more blockages and the system was nice and clear.

but no

i started the system and am getting this strange kettling effect, as described earlier.

i don't think the overflow tank is feeding the system.
do i have a blockage from the overflow tank?
has my pump failed a second time?
(pump sounds like its running and is spinning when i remove centre cap)
anyone got any ideas?????

many thanks

could be a blocked cold feed,if it is the system wont fill, try running a magnet around the pipework if it sticks to the copper you can have a build up of sludge in that area,

Your System cannot be Gravity, if it contains a pump & a 3 port Valve,
I think you mean "Open Vented"?

Sounds like your Cold feed pipe may be blocked

Your boiler stat could be faulty.

Most probably the boiler stat but have you checked your ball valve is not stuck in your tank.

Just noted the 3 port
and would defo go for a blockage where the Flow, Vent and Cold feed connect

Thanks for the help. I guess it is a vented system I have. I have tried a 15ft pipe snake going down the cold feed pipe but get stopped by a sharp bend lower down. Looks like I will have to cut the pipe near the funnel shaped junction. And try and clear the mystery blockage from there.

wadespade said:

Thanks for the help. I guess it is a vented system I have. I have tried a 15ft pipe snake going down the cold feed pipe but get stopped by a sharp bend lower down. Looks like I will have to cut the pipe near the funnel shaped junction. And try and clear the mystery blockage from there.

Click to expand...

Thats the most likely place for it to be

yep it was a blockage in the cold water feed pipe, i cut the pipe just before it entered near the funnel shaped piece.
there was about 6ins of crap in there.

thanks for your help ppl

until next time \o