[HayZee518]

in order for any hot water heater LPG or Nat Gas to operate properly, adequate make up air is a must! as heat rises so does the exhaust gases from a hot water heater. you have to have makeup air coming in to function properly. even more so with propane because the gas is heavier than natural gas and sinks to ground level. that is why you're getting the whooomp upon start up. open a window in your "machine room" to get this makeup air, put in a louver in the door where your HW heater is. make sure the flue is of proper size and put a weather cap on the outside flue.