Boilermate A Class HP-DEM

This Boilermate is heated by a Mitsubishi air source heat pump, installed summer 2009. It has started to make a loud humming noise/vibration whenever the heating is on and it wants to start heating the hot water. The noise stops once the DHW is up to temperature. It doesn't sound healthy and takes longer to heat the DHW than when the heating is off. Should the DHW demand override the heating? I suspect it should as the radiators cool off when heating the hot water which normally takes about 20 minutes (with heating switched off) but making the noise it does is not natural. Any ideas?

Hi Terry,

Yes the heating is held OFF until the store is up to 65 degrees normally.

Unfortunately I've no experience of the humming noise you mention.

Cheers, Mike