I recently took off an end stop on the vent/expansion pipe over the heating header tank. I now know why it was capped off. When the 2 way valve opens to heat the hot water, water pours out of the vent pipe. I had a heating engineer out, he said get rid, open vented systems are now obsolete, failing that, cap off the pipe again. That great 2minute advise cost me £40. So, I've checked a lot of advice on here, and firstly the whole system is 22mm pipework, except for the vent pipe which is 15mm, should this be 22mm as well. The cold feed goes to the top of the heating coil in the HW tank comes out at the bottom 2metres of pipework, then a tee to the vent pipe, then the 2 way valve. Then I guess, to the boiler in a detached garage with hidden pipework, a nightmare to chase. I can't find anywhere with this layout of feed and vent and wonder if anyone has any advice as I can't afford a new boiler and sealed system. My boiler does have an overheat thermostat, so the blocked vent is probably not dangerous.