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enter image description hereI am designing a house for myself and want to use a tankless hot water heater. The issue is I am going to be on LP gas and I want to have a recirculating loop. Id rather not consistently use gas to keep the loop hot all the time. and to put a wrench into things I also plan to use the Return loop to warm my hydronic towel loop in the master bathroom shower.

So this is my idea, I use a gas tankless water heater to feed the home during water consumption and use a electric tanked water heater to feed the recirculating loop using a pump when water is not being consumed. The pump will be set on a timer and use check valves to keep the flow of the water in the correct direction. I also plan to have all of my hot water lines insulated to keep the water hot as long as possible.

Im looking for input on this design and if it will work properly. any input is greatly appreciated.

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    A) You don't need a recirc loop with tankless if the heater is located properly. It should be close enough to points of use that the delay shouldn't be an issue. B) Plumb a small heating loop through the towel warmer - you only need the towel warmed when you're actually using the shower, right? It doesn't need to be warm when you're in the office or sleeping...
    – FreeMan
    Nov 28, 2023 at 13:52
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    @FreeMan you wouldn't need a recirc loop with a tanked heater if it is located properly either. tankless vs. tanks is a meaningless difference for that. Nov 28, 2023 at 13:54
  • @FreeMan =I don't disagree with either of those statements but the only place to properly center the heater isn't possible and the reasonable place to set them is upwards of 75 feet from the furthest fixture. Also I would like the towel warmer to stay hot longer than the shower to help dry the towel.
    – Daniel
    Nov 28, 2023 at 13:59
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    Energy to keep the recirc loop hot has to come from somewhere - are you saying you'd prefer the "heat maintenance" energy be electric, while the "bulk heating" of fresh, cold water be gas? Or do you intend instead that the electric tank would not be powered at all and is just a heat reservoir to be expended over time in heating the towel rack? (In the latter case, there's a problem around how the tank will be re-heated.)
    – Greg Hill
    Nov 28, 2023 at 14:06
  • @GregHill that's exactly what I am saying. I want the loop to be heated electric and the bulk heating to be gas. I could use a tankless only for the loop (I've looked at a tankless with a built in recirculating pump) and it wouldn't be a big deal but I don't want the consistent gas consumption from maintaining the loop temp.
    – Daniel
    Nov 28, 2023 at 14:12

2 Answers 2

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You have tankless injecting into the wrong water pipe. It should be adding to the input water of the tanked heater, not the output water.

The reason is, in a tankless, the water isn't already hot. So the water in the tankless and on the supply leg from tankless to loop will be icy cold, and will show up in your pipe as a slug of ice cold water. If you feed tankless output into the output line of the tanked heater, it'll show up at the showerhead as a slug of 5 seconds of ice cold water in the middle of your shower. If you inject the tankless water into the input of your tanked heater, it will diffuse into the 30 gallons or whatever of water, and will be easily reheated by the thermostat in the heater by the time it rises to the top.

Also, this will solve a stagnation problem you'll have in the tanked heater. Imagine all the water in the tank and circulation loop is green. When you take a shower in your current diagram, you get this high flow of fresh blue water from tankless via that little bit of pipe to the shower head, displacing the green water in that pipe section... and that's it. A little bit gets pushed through by the recirc pump, but you're not flowing gallons per minute there. Since the green water isn't moving, the chlorination will decay and bacteria will grow. This, plus keeping the water temperature below 140F, is what breeds dangerous bacteria, such as legionella.

Whereas if you feed the tankless into the tanked inlet, you finish with the bottom half of the tank being full of blue water, ensuring near total water change-out every day or two.

And here's a Technology Connections on how water tanks work.

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  • The slug of cold water will heat while it travels the pipe which is at tank temp but you are right it would be noticeably colder. As far as the stagnation goes it shouldn't be a problem if the tank is set to 140F? Nov 29, 2023 at 5:07
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    @Fresh the problem is the stagnation would bleed the chlorine out of it, the 140F assumes normal chlorination. Nov 30, 2023 at 6:45
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Energy in = energy out

It really doesn't matter whether you use tankless (large amounts of energy at one time) or tanked (small amounts of energy for a longer period of time). It really doesn't matter whether you use electric (except heat pump - those are far more efficient, but I don't think that's really an option being considered here) vs. gas (LP or natural gas) - in the end it is all BTUs (or another unit of your choice).

This seems to be a really complicated plan based on a few premises that really don't hold up in practical real terms:

  • Tankless is somehow better than tanked - because that's what you want to use for most of your water heating
  • Tanked is somehow better than tankless - because that's what you want to use for your towel bar
  • Recirculation pumps and long lines when you are building a house and can therefore put your hot water source closer to point of use

I am not a big fan of tankless. Absolutely not (except in very limited circumstances) for electric. Potentially OK for gas - but they have minimum flow rates/temperatures which make them not so great for a minimal use application such as a towel warmer.

Altogether, this seems to me to be an exercise in making things more complicated than they need to be.

My recommendations:

  • If you can, design your house so that your primary water heater is near the bathrooms. Typically (but not always) there is one or two locations with most of the plumbing. I have the water heater in the laundry room and the kitchen is directly above the laundry room. On the other side of the house are the bathrooms (two on one floor back-to-back above a third bathroom right below one of them). So I have effectively instant hot water in the kitchen but not in the bathrooms. Prioritize design for planned use by putting the water heater close to the bathrooms.
  • If you can't put a gas water heater (tanked or tankless - the issues of location are basically the same - where can you safely put it for air in and exhaust on) near the bathrooms then install a large tanked electric water heater as close as possible to the bathroom. You are designing a house, so you can design the space needed to make it fit. With electric it is super-easy - you just need a small closet - no fresh air or exhaust issues. And then there is no need for recirculation pumps. Two electric water heaters (or one electric, one gas depending on what can go where) is possible as well. The big cost is running all the plumbing, but that is easy when designed in.
  • Install an electric towel bar. But that isn't as efficient as the water heater, you say? Well, actually it is the exact same 100% efficient as an electric water heater. And in fact more efficient because it runs exactly and only when you need it and only heats the towels.

If this were a retrofit situation then recirculation, second water heater, etc. might make sense. But at the design phase you can do much better.

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    This is what I was getting at in my <500 character comment. +100%
    – FreeMan
    Nov 28, 2023 at 15:46
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    Two gas-fired tankless water heaters, one at each end of the house, would be a great choice. Save on pipe, save on recirc, still get endless hot water. Some tankless heaters have very minimal clearance requirements and might be designed into a closet or cabinet to make them "disappear" very near the point of use.
    – Greg Hill
    Nov 28, 2023 at 16:21
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    As for undersizing tankless: the result depends on the model and its use. If I set my Rinnai for 106 °F and turn on only the hot water tap in the shower then I get a comfortable shower. If somebody else turns on another shower or sink, we both still get 106 °F water and I stay comfortable, although the shower head spray is much weaker -- this particular model, operated this way, restricts the flow as necessary to maintain its set temperature.
    – Greg Hill
    Nov 28, 2023 at 16:25
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    well everyone has helped me a lot on bouncing ideas. I actually did some digging and found a wet rated kit for a electric towel rack. so that solves that problem. also I do think going to two water heaters separate from each other is the smart move but I will probably go with the navien tankless and still do the instant hot water loop with a return line. these tankless have the recirc pump built into them. ill also probably still insulate all my hot water lines under the home for efficiency reasons. thanks so much for the input and help.
    – Daniel
    Nov 28, 2023 at 16:37
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    not a huge fan of recirc as it wastes a lot of heat. i like to homerun 1/2 hot lines to showers so you only have to dump the water in the 1/2" pipe back to source before you get hot water ( hot in sink and hot in shower can be off same pipe since you only use one at a time typically ). crazy how much longer truck/branch/twig plumbing takes to get hot water to show up but does save on pipe. Nov 29, 2023 at 5:10

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