A plumber and contractor installed a Laing Autochirc hot water pump under the sink in our main bathroom. We have a ranch and it's on a slab. The hot water only comes after I hear a "knock" in the pipes, usually in the night. If I turn on faucet then, I get hot water. Otherwise, no hot water when we turn on faucet. Contractor, plumber and mfgr tried to solve problem, but no one seems to be able to do so. We want to save water. Anyone with experience with this? Any suggestions?