Bungalow built in the 70s with a vented system. Impossible to raise the storage tank level which is already in the roof space. Everything has worked fine for over 20 years since we moved in , with gravity fed hot water taps through the property and power showers. Recently, and this has been examined by a number of plumbers without success, air has been getting into the gravity hot water flow and causing serious water flow failures. The height difference between the header and the hot tank is minimal. If I fit a pump, will that simply draw more air - presumably from the vent pipe (nowhere else for it to come from) ?? We are struggling here. Oh - and mains water pressure is excellent. So no problem whatsoever with storage tank capacity.