Water Heaters - Double anodes; Mg vs. Al anodes; venting tankless WH

Two natural gas 40 gallon A.O. Smith tank water heaters originally installed in our 7 year old home are (were) hooked in series, with the first one set a a slightly lower temperature than the second. The second one sprung a leak a few days ago, so I have now bypassed it.

(Perhaps my situation is evidence that two tanks in series, rather than one big tank, is a good idea. I'm now free to determine the replacement method/model/style of the new tanks without being in a panic! Besides, I can lug the 40 gallon tanks up and down the basement stairs with slightly greater ease than the effort needed for a single larger tank.)

Anyway, here are my questions:

1) I've read that some manufacturers have two anodes in their water tanks. One rod is accessed via the typical hex head bolt on top of the tank. The other anode is apparently part of the water inlet (or outlet?) tube. Which manufacturers have such setups...or is this always part of the "Best" system every manufacturer seems to promote?

2) Which manufacturers use Magnesium instead of Aluminum anodes? I'm having a difficult time finding this specific information in advertising literature. I understand that either magnesium or aluminum anodes may combine with chemicals in the water to form hydrogen sulfide. We have never had a problem with such smelly water previously, and I'd prefer no to install a water heater with anode rod(s) which initiate such a problem.

3) Our home has a gravity feed hot water circulation system in place. That system circulated the cooled hot water to the second tank in the series. I'm guessing that this constant circulation, as well as the higher temperature of the second tank, contributed to its earlier (premature?) failure. Any thoughts? Would I be advised to connect the cooled water return point to the first tank?

4) Do all tankless systems require a stainless steel flue liner? We have a two story masonry chimney with a single steel flue liner that combines the flue gases from two furnaces and two water heaters. Unless I can use the old flue for a new water heater, I am somewhat disinclined to go with a tankless system. It will be difficult to install a separate new vent for the tankless system because our home has so many windows...I cannot identify a vent exit location that is the required distance from all windows and doors. Also, we have a rather open floor plan home, and routing new stainless or PVC pipes upwards through the two story home will be nearly impossible.

BTW...we have very hard well water that is not soften. Our hot water pressure had been dropping slowly over the years. I had assumed that my foolishness in not having a water softener was the primarily cause for the loss of hot water pressure (due to scale build up in the copper pipes). To my pleasant surprise, the problem was simply that the outlet pipe on the failed water heater was the culprit...that outlet was nearly blocked with rust. There was no significant accumulation of rust on the outlet of the other water heater...we have hot water pressure again!

Thanks in advance,

Mr. Timewise

Tankless require stainless steel due to the high btu and the high moisture content of the exhaust. You cannot put this into to a galvanized or masonry flue. There are power vent models which do not require stainless, but you could also NOT put this into a shared flue or a masonry chimney