My warm water was brown and I flushed the water heater twice already - it stays clean for 3 days then it turns brown again. (It is a natural gas waterheater.) Any ideas on what I should do. Thank you

You have iron oxide, (rust) build up in the bottom of your tank. I know you've flushed it twice but give my way a shot. Attach a hose to the boiler drain at the bottom of the tank. With the pressure on, open the boiler drain and let it run until the water runs clear. You will see a spurt of red,(rust) and some white grains,(lime or calcium carbonate). The whole thing shouldn't take more then a few minutes. Don't forget to flush it out every month. Your heater will thank you for it. If there is so much mineral build up in the bottom that the water is still tinted, shut the water and gas off and drain the tank. Now remove the boiler drain, take a metal rod and go up and break the mineral up as small as you can get it. A second way would be to drain the heater, leave the boiler drain shut and pour a plastic jug of CLR solvent that drug stores sell. It will desolve rust, lime. Or calcium carbonate. Let it set over night. Now hold a large pan under the boiler drain valve opening and turn on the water and try to flush the rust out. Now wrap the threads with teflon tape and reinstall the boiler drain. Now with the boiler drain open flush the tank until thr water runs clear. That should get it but if you water's got that much mineral in it you8 might want to consider a whole house filter or a water softner. Either way best of luck! Tom

I forgot to thank you for this reply - THANK YOU

How old is the heater and where does your water supply come from?