moorfield

Long story short: this manifold has been working fine for 10+ years until the combi boiler feeding it was swapped out last summer. It feeds UFH only 4 zones, no rads, heat is circulating but nowhere near as well as it used to. So I think that boiler job could have introduced airlocks which firstly I want to try and bleed out myself. I know how to open the actuators but can't figure out how to do the bleed or if there is any valve for that. I've put some photos up if anyone can help me with, also I'd like to understand what the grey thing at the bottom, and the outlet at the top are for. I've seen stuff online involving hoses etc. but want to avoid going down that route if I can. The one plumber I've spoken to about it has suggested I power flush it, which probably involves taking the whole thing apart. Again want to avoid that unless absolutely necessary and I'm not convinced UFHs ever need power flushes really. Thanks from a newbie here for any help.