Problem open vented system

[ironside]

Hi,

I'm new to the forum, I'm a keen and competent DIYer. Lots of good info on here but I've had to admit defeat so my first post is asking for help

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.

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.
Sorry for the long post but I wanted to give as much info as possible. Below is a picture of the pipework.

A - Cold feed 28mm from pump to 15mm to the tank
B - Vent 28mm to 22mm to the loft
C - Mains water to tank
D - Discharge
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?

Many thanks

Ironside

 

[secret squirrel]

If you are having/considering an unvented cylinder you MUST be qualified to install these. These are dangerous bits of kit if not installed correctly. Do a search on youtube and watch one explode......

 

[ironside]

Eh? We have an unvented cylinder already, you can see the big green thing in the picture. This isn't a problem and works perfectly.

 

[secret squirrel]

My mistake long day, confused myself between unvented and pressurised.....

 

[dontknowitall]

Bit tricky to work out without seeing everything, but an unvented cylinder shouldn't be vented to an upstairs cistern or if it isn't, what's "B" for?

It also sounds as if the unvented cylinder is not emptying correctly and the kettling sounds as if there's a potential explosion waiting to happen.

If I were you I'd get a carded unvented cylinder chappy in to take a look this week if possible.

 

[ironside]

The cylinder is nothing to do with this and is not vented. The water in the cylinder is heated by the boiler. You'll see 2x 2port valves, the one bottom left is for the hot water the one tagged G is for the CH.

The pipe B is the vent pipe for the central heating only. The problems occur when only the central heating is running. I've only included the cylinder details as I've tried to give as much information as possible.

Please don't be destracted by the cylinder, yes I know it's an ugly green but honestly it's fine.

 

[Connor Williams]

Hello mate, it does sound like the boiler is overheating, if you have trv s on every radiator this can cause overheating, I know you have a byepass but there really should also be a manual radvalve on at least one radiator to help the circulation while the water cools.
As you say the heating is rapidly getting to temperature, if the pump has been installed in the correct position so it is not drawing in air through the vent pipe, try turning down the temperature of the heating, make sure all the rads,pump and any air vents are bled, and make sure you have at least one rad with a manual valve, this also acts as a bypass. Failing that, well, call in a heating engineer! Goodluck mate