I am building an addition with a basement underneath. The existing home is on a slab foundation, and the 20 year old tank water heater is in a closet. I was planning to replace this water heater with a tankless heater in my new basement. However, I now realize that this will significantly increase the distance from the water heater to the kitchen (and a couple bathrooms).

The new plan that I am forming in my head is to put my large tankless water heater in the basement to service the new bathroom/laundry room in the addition directly, and in series (where the original tank heater was) put a much smaller tankless heater that is near the other two bathrooms and kitchen. The thought being that the smaller one will shut-off when the water from the larger heater reaches it, but it will provide hot water quicker to the faucets/shower much farther from the large heater. Is this a bad idea? Are there any other ideas that I should consider? (I've considered a recirc pump, but it seems that sort of defeats the purpose of tankless since it is always keeping hot water in the lines - your tank just becomes the lines).

Note: my tankless water heaters would be propane fueled

  • Why not just size each tankless for the load it serves?
    – Ecnerwal
    Commented Dec 24, 2020 at 3:39
  • I should have mentioned in the original post. I am also trying to avoid running new gas lines in the existing home. A new gas line will be run from my propane tank to the addition that will be sufficient for the new larger tankless heater. But I was looking at only a 70,000 BTU heater for the smaller unit which my current gas lines could support.
    – Bill
    Commented Dec 24, 2020 at 14:25
  • At a place I worked we used 2 tankless heaters in series, and after much problems we learned that they must be both designed to work in series, and properly installed. Until we threw enough money at them to solve the problems, the second header would regularly dump cold water, cooling off the hot water produced by the first heater.
    – izzy
    Commented Dec 24, 2020 at 17:41
  • @izzy can you clarify that story? Wouldn't it be cold water pipe -> tankless1 -> tankless2 -> hot water pipe where the outlet on tankless 1 connects to the inlet on tankless 2. It seems logical to me that tankless 2 might refuse to turn on if its inlet water is too hot. But how could it "dump cold water, cooling off the hot water produced by the first heater"? Where did tankless 2 even get access to a source of cold water?
    – bobpaul
    Commented Jan 11, 2023 at 0:42
  • 2
    @bobpaul I don't fully understand it but the plumber said that without the electronic link cable that we were missing, the 2nd heater in series was cooling the hot water produced by the first. I think it was because it measured the input temperature and thought the incoming water didn't need heating.
    – izzy
    Commented Jan 11, 2023 at 2:14

1 Answer 1


Won't work. Either the point-of-use heater is big enough to heat the water, or it isn't.

  • If it is big enough, you don't need the main heater at all.
  • If it isn't big enough, the water will come out tepid for the first minute.

Both of these scenarios are losers. You're spending a lot of gas to heat water that'll just go down the drain, because nobody's going to step into that shower if the water is coming out 80F.

You are better off rearranging the furniture so that the 70,000 BTU water heater is adequate for that end of the house, and having it be the primary water heater.

Last time I did a calc like this, it was with a 13KW heater that would barely make a shower work with a 1.2-1.4 GPM flow on the shower head. Let's convert your 70,000 BTU/hr into KW.

1000 watts provides 3400 BTU/hr. So it looks like your 70,000 BTU/hr heater is equivalent to a 20.5 kilowatt (85 amps) electric heater. That's doable; put a low-flow showerhead on there and you should have some operating margin. Simply have that heater supply that end of the house (the closer to the shower, the better - for response time) and have the new heater serve the new wing.

Keep in mind that if you draw water at a faster rate than the heater can keep up with, the heater heats the water less. That's when the water goes tepid and users go "WHAT THE HECK on-demand heaters!" and they wrongly conclude that on-demand's suck and get rid of em. That's the wrong move. When the water goes tepid, reduce flow and it will heat up again. Of course good luck teaching that to users; so I advise putting a gate valve on the inlet of the water heater, and adjust it to impede flow, to make out-drawing the heater impossible :)

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