I'm new to the forum but I've been a long time lurker and learned a great deal from this forum. I'm a keen and competant DIYer but on this one I've had to admit defeat so here I am after a little advice
Apologies it's a long post but I've tried to include as much information as I can to assist.
We have an open vented central heating system comprising a Baxi Solo PF2 60, 16 radiators and an unvented cylinder. The system has been reasonable since we moved in last year but not had much care in the last 15 years it's been installed it seems. Over summer I removed all radiators and flushed out with a hose and re-dosed with inhibitor. I've fitted TRV's to the bedroom and living areas and changed the bypass gate valve to an auto bypass. The boiler water temp sensor was changed when the fan failed just before Christmas.
Still here? The problems started a couple of weeks ago. I added some Sentinel X400 to do a proper desludge, knowing it would need about 4 weeks to work. I re-balanced the radiators as they were miles out and the house had never heated so fast. Then the boiler started kettling a lot - like a steam train. Pump was changed, the old one was well past it and totally clogged but the new one made no difference. So I flushed the system using mains pressure (connected the cold feed to the fresh water in the loft), did one radiator at a time. Disconnected the flow and return and flushed out the heat exchanger as best I could with a hose pipe. Re-dosed with X100 and X200 descaler.
The house gets hotter faster than it ever has, the boiler still kettles a fair bit and now the system seems to draw air in ever time it runs and is also heating the water in the loft tank - hot water is going back up the cold feed rather than it pumping over. I've held a cup of water over the end of the vent pipe while the heating is switched on and it sucks the water in then spits it back out. Cold feed doesn't seem to be blocked as the tank emptied quickly and the system filled no problem. Pump is set to speed 2. 1 isn't enough and 3 reduces kettling a little but still get lots of air drawn in.
So, does anyone have any ideas what I should do next? I've read loads of posts on this forum and others but not sure how to tackle the problem.
Below is a picture of the pipework.
A - Cold feed 28mm from pump to 15mm to the F&E tank
B - Vent 28mm to 22mm to the loft, F&E again
C - Mains water to F&E tank
D - Discharge for cylinder
E - Grundfos 15-60 pump
F - Honeywell 22mm auto bypass
G - CH 2port valve
I believe the pipework for the pump/F&E is not optimal but what should I do to sort it out?
If converting to pressurised would help (I'd love to get rid of the loft tank anyway) what do I need, and what pipework changes would be necessary? Can I use the existing discharge pipework for the cylinder or does the safety valve and out let need to be by the boiler?