System is a WB boiler suppling heat to the thermal store (Boilermate 2000). DHW is supplied on demand by a pump pushing heat-store water through a plate heat exchanger (and back to the store) with mains-pressure cold water fed into the other half and then on to the hot-water taps.
This morning…no DHW.
Ah! This has happened once before, caused by a dicky speed selector on the DHW pump.
Set a hot-water tap running, switch speed selector to 3 (fastest), to result in a surprising gurgle from the PHE. Sure enough, HW now flowing nicely.
Only problem is…why was there air in the PHE? We have had some suggestions of air in the incoming cold water in the form of occasional slight spitting at higher flow rates, so presume that there has been a build-up in the PHE.
Hmm…solve one thing, discover a second problem…