My plumber will be installing a Rinnai RU98 tankless water heater. He wants to install a 10 gallon tank to avoid the wait time for hot water given the size of my house. I would prefer not to have a tank because it waste energy and is a big reason why I'm getting rid of my two electric water tanks.

The RU98 has re-circulation built in. Plumber said he could install timers to turn on the unit but that is more complicated than it needs to be so a 10 gallon tank is better (his words).

What is my best option given I am trying to avoid extra energy cost yet not have to wait a LONG time for hot water. I'm fine with waiting a little bit for hot water. My current system has re-circulation built in. The location of the tankless unit and most of the faucets will be far away and we have no option to move the unit to a different location.

Here is what one article said on the internet about this issue (although they did not state which is best option):

1. You could use a re-circulating line that keeps hot water following through a loop in your water system and then through your Rinnai. This is a poor choice for two reasons. First it wastes a lot of energy (probably at least $100 / year). Second, it will shorten the life of your Rinnai and reduce your warranty.

2. You could add a small electric hot water tank (only a few gallons) to the hot water side of your Rinnai and run a re-circulating loop through it. The Rinnai will still heat all the water and you will not shorten the life of your Rinnai or impact its warranty but you will still waste energy. You could lessen the energy waste by running the pump on a timer.

3. You could add a cold water line re-circulating system to your common points of use. This is a system that will circulate water from your Rinnai to a point of use by utilizing your cold water line as a return. This does not shorten the wait period for hot water but it does eliminate water waste and will not increase your energy cost like standard re-circulating systems.

Thanks for your help,

Mike

10 gallons is too much for your needs, a 2 gallon heater would suffice. I let the tank heat the water that's in the loop, that way you have instant hot water at the farthest fixtures. The loop doesn't circulate thru the tankless , that it doesn't fire unless you turn on the hot water somewhere.