What you are referring to is a buffer tank after a tankless, and defeats the purpose of a tankless unless there is a circulator to maintain temp in the tank, otherwise is will go cold unless heated with electric elements, which again defeats the purpose.

With regards to twin tanks, although the initial draw of tanks piped in series is higher, the 10% reduction would be better given up to pipe two tanks in parallel, so that both had equal usage and flow rates would be higher.

Are you looking at a gas or electric tankless?

I was looking at an electric tankless. I already have an electric tank heater that is working properly. After doing some research it seems like the most sensible thing to do is just to replace the broken tank heater with another tank heater.

Thanks for the advice.

Yep, I would rarely recommend electric whole house tankless. And there would be no reason to do it in your situation.

There could be a good argument for one gas tankless to replace both of the 50 gal electric though, if you have gas available.

The water heater pros in my area is Clarke & Rush. They know everything about water heaters. I was dealing with a leaky water heater and looking at maybe getting a tankless system. Talked to the reps there and they gave me all kinds of information. Ended up with a new 50gal that works great and was super affordable. Tankless seems legit, but not really for me, I guess.

The disadvantage of tankless is that you may not have enough hot water to supply multiple simultaneous uses (running 3 showers at a time, for example). So what I'm thinking is a tankless and a tank heater feeding into a mixing valve such as a Watts LFMMV. Set the tankless to deliver 110F and the tank to deliver 120F and set the mixing valve to supply 110F. Then under normal conditions the the mixing valve calls for "cold water" of 110F from the tankless but if the tankless can't keep up and falls below 110F then the mixing valve mixes in hot water from the tank to maintain the 110F. Would this work?

Yo, this is an 11 year old thread... move on already!

nwsvndr, I'd love to know what you ended up with and how it turned out.