Hi, new poster, and a DIYer that just doesn't understand water. I've a 20 year old house with Y-plan system, bit of a Trigger's broom (I've previously separately replaced the pump, 3-port valve, cylinder, plumber replaced the boiler... but the pipe-work is all original!) Oh, 12mm microbore plastic to the rads. Pump is < 6 months old, and a new magnaflow installed at the same time. I power-flushed the system whilst replacing these, and cleaned/inhibited the system whilst doing this. Until recently - all has worked well. Today, no heating. Well, a little from the upstairs bathroom rad (this is a bypass), but nothing significant from the others. I've bleed the ground floor rads (no air expelled) and tried opening the two lower bleed valves (lowest parts of the system) - some discoloured water but no sludge. Previously when I power-flushed the system (6 months ago, when replacing the pump) there was lots of magnatite (or whatever it's called) but after a couple of hours the system ran clear. I know that I can feel the heat hit the CH side of the 3-port if I turn the CH off, leave to cool, then turn CH on (pipe heats up in seconds to hot). Thermostat is generating demand. 3-port can be heard motoring when alternating between hot water/CH. But no heating!? I'm thinking it's time to power-flush again (at least that justifies me buying the second-hand unit last year) - as all I can imagine now is a blocked manifold, but would happily try anything easier if anyone has any clever idea?