Critique: Tankless with recirc loop and storage tank

[Makemenuconfig]

Hi,
I have a Rinnai gas tankless which I am relocating due to an addition I'm building. The water heater will move to the garage, where it will be centrally located between the house and addition. It is a big enough house that a recirc line will be necessary. Since there is house in either direction, it has been plumbed as two separate hot and recirc lines (4 connections total, two separate "loops").

I am designing the water heating system, and want feedback on this approach. My goals were to:
- Provide recirculation on a timer only when hot water is needed, AND on an aquastat so the pump won't run continuously.
- Use a storage tank for recirculation so that the tankless doesn't need to fire every time the pump runs.
- Use flow limiters to ensure equal flow through each loop.

My main concern would be whether or not the periodic cycling of the pump would be enough to prevent the water in the storage tank from becoming stale. Because it is not in-line with the primary flow, fresh water can only enter when it is pulled from the DHW lines through the recirc return while the pump runs. Do you think that is sufficient?

Would you worry about legionnaires with a setup like this if the tank was set to 120?

Diagram: (Note T&P, gas, elec, flush valves, bypass valves, etc. not shown)

 

[Breplum]

I recommend Navien NPE A series with built in pump and buffer tank. Then, keep it simple with just circuit setters and mini expansion tank.
Pre Navien era, we used to use mini water heaters as you are showing. Wasted effort in my opinion.
If you must keep your overly complex layout:
I would say to consider the D'mand system controller with pump https://gothotwater.com/shop_builder/

 

[Sponsor]

 

[Makemenuconfig]

I recommend Navien NPE A series with built in pump and buffer tank. Then, keep it simple with just circuit setters and mini expansion tank.
Pre Navien era, we used to use mini water heaters as you are showing. Wasted effort in my opinion.
If you must keep your overly complex layout:
I would say to consider the D'mand system controller with pump https://gothotwater.com/shop_builder/

It's just that I already have the Rinnai water heater, it's been in service for about 2 years now. So I'd like to work with what I've got. I'll look into the D'mand system.

 

[Jeff H Young]

I dont see how the return line re enters the the water heater and is reheated ? Id want to use what you have too but breplum only uses Navien as do many others , I have done only a few tankless jobs . Navien I think is a big favorite

 

[John Gayewski]

The reality is legionella grows in 120 degree water and your system could theoretically keep that same water molecule in it for life. I think if you are gonna go with this setup you need to be at 140 degrees with temperature limiting devices on the outlets.

 

[Makemenuconfig]

I dont see how the return line re enters the the water heater and is reheated ? Id want to use what you have too but breplum only uses Navien as do many others , I have done only a few tankless jobs . Navien I think is a big favorite

Return line loops through the small tank, which would do the heating just for the recirc. There would always be hot water in the tank to be circulated as needed.

I guess I got scared away from Navien from that YouTube guy with the heat exchanger issues on the NCB type units.

 

[Jeff H Young]

oh yea my bad I thought it was just a storage tank I looked right at it but the word element didnt sink in . I think of it as a water heater