SylvanLMP
Before you just try to remove the existing valve you should take into consideration how old this tank is.
If it is more then 5 years old you may want to just place a hose bibb cap on the valve threads.
This repair will cost you about $1.00
rather then shutting off the gas and water supply and draining down the system replacing the existing valve and re filling and hopefully not run into any unseen problems
SylvanLMP
