We got a call the other day from a homeowner that had a contractor install a solar water heating system. The system had been installed for just over a year and had been installed with a new water heater. The water heater had rusted out and failed in that period of time. The homeowner was concerned that somehow the solar system caused the problem. After some discussion back and forth we discovered that the installing contractor had removed the anode rod from the tank in order to provide an extra port to the tank. That was a major mistake. You should never remove the anode rod from a water heater. The anode rod is made from a metal that is more reactive than the steel of the tank so it serves to react (or sacrifice) with the ions in the water to prevent the ions from attacking the steel. Without the anode rod, the ions in the water will react with the steel of the tank and prematurely rust out the tank.
If you need to add an additional port to a two port tank you have several options for doing it. First, purchase a tank with 1 or 2 extra ports. The American water heater tanks (sold as Whirlpool at Lowes) generally come with a third port in the top of the tank. Rheem, Bradford White, and AO Smith all offer special solar tanks with extra fittings in the top. Generally the “solar” labeled tanks are sold at a premium so I avoid them if possible. Finally, even if a tank has only two fittings (one for hot and one for cold) you can create an extra fitting in the tank by using the P&T port. Remove the P&T, install a nipple and tee, install an extended probe P&T (Watts 100XL-8) in the end of the tee and then the side port to the tee becomes an extra hot out from the tank. A final option that is available is to use an outlet anode rod that suspends the anode rod below the anode connection while providing you a port to remove water from the tank.
You can now have the ports that you need in a standard tank without sacrificing the longevity of the tank or your solar heating system.