Hot water flow and pressure drop off when any tap is turned on.Cold is fine.Happens after the system has not been used for a few hours. If you leave the tap running long enough, can hear a very loud bang noise and pressure is fine. Almost like a valve is sticking. After this happens, theflow/pressure will be good for several days. Water heater is a Superflue powervent. Would it be a flow valve in the heater.