Replacing 2 conventional water tanks with Navien NPE-240a

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Mr.Miller

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So here goes. I previously had two 35 gallon tanks in series, feeding household domestic water needs a well as 2 radiant heat zones. . The hot side of the first fed my 2 zones of radiant heat as well as the input for the second tank. The return line from the heat zones, along with the main clod water line make up the first tank input. The output of the second tank was then the feed for all domestic needs. This system was already in place when I purchased the home and seemed to work fine for our family of 4. Until this weekend. Second tank was leaking. Easy fix was to by pass it with a quick PEX line. Now I have replaced with a tankless unit. All seems good on the domestic side, but if I open the valves to the heating zones, I get no hot water at any faucets in the house. Lots of water, but no hot water. Unit indicates it is running (flame on front) but is not. Are larger pumps needed for the zones now? Do the pumps need to be moved to the cold side of the zones, and pull water through instead of the current "push" water through? Is the increased pressure differential across the water heater to blame, as it previously worked fine with the conventional tanks in place?

Any insight would be great.

Thanks
 
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Mr.Miller

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Here is quick drawing of the previous set up. The new set up has the tankless in place of the two conventional units. Nothing else has changed besides some piping adjustments to move the unit over to the wall.

previous water tank set up.jpg
 

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