I've checked all over this forum as there's a load of info here on issues with a combined feed and vent but I can't find an answer to my specific problem. I'm helping someone out with their new house where we've had to replace a couple of the radiator valves. Partly drained the system, replaced the valves and refilled. And then we found that the system has a combined feed and vent pipe to the header tank. Tried for days now to get all the air out the system but its still gurgling like a crazy thing - especially when it's just trying to heat the hot water. All rads are working fine apart from one that we drained plus another one. When we try to bleed them nothing comes out - even taken the whole bleed fitting out of the rad and still nothing comes out. There's just no pressure in the water to push the air out. And now we've found that the hot water is no longer heating up. I'm sure it's just air locks but we just can't get them out the system. Any advice would be great. cheers