Combined feed and vent with unvented cylinder

[MethodPH]

Got a job today to take out vented cylinder and replace with unvented indirect cylinder (i’m G3 qualified). Only problem is the system has a strange set up. It’s combined feed and vent S plan system. Heating zone valve is on the flow and pump and hot water zone valve on return. My issue is that just after the feed T’s into the vent, it T’s for the cylinder flow. Seems like a strange set up to me. I plan on putting a zone valve on flow into the cylinder which will make it a customised S plan with two zone valves on the hot water. But I’m thinking this system is going to be a nightmare to refil, any thoughts? Never worked with combined feed and vent. Boiler is an old easicom oil boiler. Heating and hot water works fine now, just rubbish hot pressure as gravity. Have attached some pictures

 

[ShaunCorbs]

Never seen anything like that before very strange

 

[MethodPH]

Never seen anything like that before very strange

Thats what I thought, obviously changing the cylinder for unvented isn't going to make any difference to the heating system, boiler has an overheat stat so we're all good there, just think its going to be a pain to try and refill after draining

 

[snowhead]

I'd say that was originally Gravity flow / return to the cylinder, it looks to be 28mm and probably has a 28mm coil.
There's a 22mm vent off the cylinder flow and a 15mm , would have been, feed into the cylinder return at the bottom, is that capped now?

When it was converted to pumped flow / return to the cylinder, the zone valve couldn't be in the flow as that's also the vent.
So it was put in the return, but that was the cold feed, so that was moved onto the vent.

Don't try and fill it too quickly, give the air chance to get up the vent.
Better still, make the ( capped off ? ) old cold feed on the cylinder a fill point.

 

[MethodPH]

I'd say that was originally Gravity flow / return to the cylinder, it looks to be 28mm and probably has a 28mm coil.
There's a 22mm vent off the cylinder flow and a 15mm , would have been, feed into the cylinder return at the bottom, is that capped now?

When it was converted to pumped flow / return to the cylinder, the zone valve couldn't be in the flow as that's also the vent.
So it was put in the return, but that was the cold feed, so that was moved onto the vent.

Don't try and fill it too quickly, give the air chance to get up the vent.
Better still, make the ( capped off ? ) old cold feed on the cylinder a fill point.

They are 22mm pipes going in and out of cylinder. The top tapping on cylinder is obviously the flow and the vent from that T is the heating open vent, f and e tank T's into that in 15mm. Return out of cylinder has a T on as well, the 15mm isn't capped off it has a capillary vent on it. Good idea with the fill point, could T in under the capillary vent, but would also have to keep combined feed and vent T on the flow otherwise zone valve will cut the vent off when hot water circuit is closed

 

[SJB060685]

I've not experienced it but have seen posts about this before on other pages. I believe the idea is to prevent venting over, however the system takes a while to fill from start

 

[Ric2013]

I have worked on a combined feed and vent system (which has other issues with poor system design but is now happily hobbling along).

Surprisingly, I filled and drained it a few times when adding some radiators and had no problems filling it. The one I worked on didn't have a vent pipe at all. I would imagine that with your arrangement, once there is enough water in the system to prime the pump, the pump will circulate enough water to shift the air to high points, hopefully to the radiators which can then be bled. I can't really imagine that the pump on return will suffer from airlocking.

The venting system you show with the vent splitting off the feed is not dissimilar to one shown in the IOM for the Thorn Apollo boiler.

Hope you are as lucky as I was!

 

[John.g]

Got a job today to take out vented cylinder and replace with unvented indirect cylinder (i’m G3 qualified). Only problem is the system has a strange set up. It’s combined feed and vent S plan system. Heating zone valve is on the flow and pump and hot water zone valve on return. My issue is that just after the feed T’s into the vent, it T’s for the cylinder flow. Seems like a strange set up to me. I plan on putting a zone valve on flow into the cylinder which will make it a customised S plan with two zone valves on the hot water. But I’m thinking this system is going to be a nightmare to refil, any thoughts? Never worked with combined feed and vent. Boiler is an old easicom oil boiler. Heating and hot water works fine now, just rubbish hot pressure as gravity. Have attached some pictures

A bit late in posting. I have the combined feed and vent from new (40+ years) and have no problems with re filling. My system is almost identical to your sketch, the only difference is that the HW zone valve is just before the cylinder coil flow entry and I manually latch this open when refilling the system.