My dad had a new Rheem hot water tank installed for floor heating. The weather now is cool and rainy. He lit the tank and a short time later it went out. He kept relighting it until it stayed lit after 20 mins. A plumber saw a drip when examining it when it was cool. He thinks the humidity in the flue is dripping down and putting out the burner until it is heated enough to displace the humidity in the flue. Has anyone had this problem?
If it's brand new and just installed, call back installer/manufacturer and let them know of the situation. Should be some kind of installer or manufacturer warranty.
If you installed it, recheck your work and make sure everything is installed to manufactures specs as stated in owners manual, especially venting. If it's not venting properly, getting the appropriate amount of air, or bad thermocouple the pilot will not stay lit. Just a suggestion.