Please help, at end of my tether System is traditional Y plan, Gloworm 40FF boiler, Grundfos pumped primary circuit for rads/immersion coil, gravity fed HW by loft tank. New boiler fitted Sep 04, great for 5 weeks then HW not heated, only CH. Company called back, said Honeywell mid position valve faulty - problems ever since they replaced it: Pipes howled/rads bubbled - came back month later, fixed by opening last rad valve. Since then one very loud metallic bang comes from valve and pipes between it/boiler, only when HW program starts, not CH. No joy getting them back to check this - guy had problems wiring it up. Any idea what problem is? Even worse, only since valve replaced we keep getting airlocks in all HW system every few weeks - all HW taps gurgle, slow to trickle then stop. They did come back to drain/refill system 5/1/05 & Ok till last night, blocked again no HW! Have checked both loft tanks filling OK/pipes lagged/not frozen. Any suggestion what problem is would be much appreciated! Could it be scaled pipes? Sorry if duplicate post, last one didn't seem to send.