Warm water w Gas tankless hot water heater w built-in recirculation?

last year

Dear all, thanks again for your help.

I'm new to gas powered tankless hot water heater. I heard from my plumber that he would not recommend installing a new gas tankless hot water heater with built-in recirculation system (e.g., Navien's ComfortFlow or Rinnai's RE Series such as the RE199iN), as he said that with such a system, even with a dedicated hot water return line, the cold water will still become warm.

My question is, why? I will have more discussion with him tomorrow to learn more (and will update the forum about what I learned), but my first impression is that does not make sense.

Can folks who have experience about this share your experience please? Also, would you choose a "condensing" (more efficient but more complex system) unit or a "non-condensing" unit (less efficient, but simpler design so might be more reliable because of less moving parts etc) as of now (March 2025)? It seems that the "non-condensing" unit might be going away, so are we stuck with buying only "condensing" unit now? Any thoughts or comments about this? Thanks again!

By the way, here is what I learned about "hot water sandwich effect".

For reference: Navien’s patented ComfortFlow® system is the first to include a buffer tank, recirculation pump and fine-tuned controls into a tankless water heater, resolving the cold-water sandwich effect* and issues of minimal flow rates commonly found in other tankless water heaters.

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Comments (2)

3katz4me
last year
No answers but I did have the warm cold water problem. We bought a house with tankless heater. No problem at first but then we turned recirculating off while we were gone for an extended period. Then we turned it back on 24 hours a day rather than a few hours. We then got the warm cold water. My solution was to just turn recirculating off entirely. We’ve adapted to running the water for a while to get it hot. Waste water or waste enrgy recirculating. Never had this problem with previous tanks.
W C thanked 3katz4me
Jake The Wonderdog
last year
last modified: last year
I've never heard of warm cold water with a dedicated return line. I don't see how that's possible.

If you're talking about condensing versus non-condensing tankless water heater, condensing is the only way I would go. Non-condensing requires stainless steel venting which is very expensive. I've used several of the Rheem condensing tankless units and they work very reliably.

Since you're talking about recirculation, I would prefer a separate recirculation pump rather than one built into the water heater. TACO and Grundfos both make good circulating pumps.

Consider using a hybrid electric heater instead though. Depending on your cost of electricity, the hybrid electric may be cheaper to operate than a condensing gas tankless. You have an ideal location for a hybrid electric and it avoids dealing with any venting - again which is very expensive for non-condensing but cheap (PVC) for a condensing gas water heater.

It also avoids some other issues such as a longer wait for hot water without a circulating pump.
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