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I'm shopping for a tankless water heater. When the salesman talked to me, he told me they install them outside and this is better because then the tanks do not take up room inside the house. I'm worried that the tank will freeze in the winter and that the installation will require a lot of extra installation costs such as plumbing, wiring, lines, etc.

Wouldn't it be better to just remove the old water heater, which is close to both bathrooms, then put the tankless in the same place, which would keep the installation costs down to a minimum? And, wouldn't keeping the water heater indoors eliminate any future weather concerns of freezing, draining in cold weather (which totally confuses me as to how this would be done and how you would then get hot water flowing again)?

Bottom line, which is best, which has the least maintenance, durability, and is less costly to install?

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  • What type of tankless system is it? If it's gas, then indoors you need a flue, a gas line (assuming existing heater is electric), and there's increased fire risk. Most continuous flow heaters I've seen advertise frost protection, likely a small electric element on a thermostat.
    – SomeoneSomewhereSupportsMonica
    Jul 8, 2017 at 8:20
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    You can mount them on the outside of the house if you live in a southern non-freeze area like southern Florida.
    – d.george
    Jul 8, 2017 at 9:16
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    The salesmen's comments don't make sense for the reasons you stated. Is your old tank gas or electric? Is the new going to use same fuel type? Where on the planet are you located?
    – Tyson
    Jul 8, 2017 at 12:08
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    When water heaters are relocated there can be a very large expense with the plumbing , electric. And or gas lines. Putting a waterheater that generates no heat for many hours outside is a dumb idea because now that outside area will need to be heated to prevent freezing. Where I live it may get down below 20f a few times a year at 27f is low enough to freeze insulated water lines and we now have heat tape on exterior water lines to reduce broken pipes.
    – Ed Beal
    Jul 8, 2017 at 15:47
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    @SomeoneSomewhere That anti-frost only works if the power is on. That would make sense in places like parts of CA or AZ or coastal towns where freezes are rare and short, and almost never involve ice storms, blizzards, wind storms and other winter nastiness that tends to put people out of power for days at a time.
    – Harper - Reinstate Monica
    Jul 8, 2017 at 17:51

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Your salesman is evidently an idiot. Perhaps you want to do more shopping and find a vendor with smarter people working for them.

  1. You already have the space.
  2. The space is well-located for short pipe runs to the bathrooms and has plumbing in place.
  3. Presumably the old water heater has a fuel supply and venting means, or you might have mentioned that as a concern.
  4. Evidently it freezes in your area.
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It will be much cheaper to use an inside model and put it where the current tank is located. But even in this case you might need to install a new and larger gas line to feed the tankless heater. The burner on a 50-gal tank pulls about 50 kBTU/h of gas, but the burner of a tankless heater that most people would think is adequate for a 3 or 4 bedroom house with say 2 full baths will pull 200 kBTU/h. You would probably have to run a new line to a capped off T in the attic to get that much supply. Also you will need a new flue (may be able to use the route of the old one), and will almost certainly need a 120-V receptacle since most new tankless require it for controls.

If I were building a new house, I would use a tankless and put it on an outside wall. If I lived in an exremely cold climate, I would wonder if an outside tankless heater could be prevented from freezing, and would inquire about this.

Twelve years ago I was considering "at some point" on replacing natural gas fired 40-gal tanks in two 1970s 2000 sq ft tract houses. (One our residence and one a rental property.) In each house the tank was centrally located in the house giving short plumbing runs to points of use. On an impulse I bought on sale two identical bottom of the line Bosch tankless water heaters (117 kBTU/h) intending to install them myself in the exact location as the tanks.

Once I had the heaters on hand I began to consider installing the one in my house in the garage, but I ultimately rejected this because it would lengthen the hot water lines and I would have to put water pipes in the attic. I even just used the existing gas supply lines. I suspect that in the winter I am not getting the full 117 kBTU/h output from the water heater when the natural gas furnace comes on (they share a 3/4" gas line), but it works well enough for us. Almost certainly if a 200 kBTU/h heater was installed the gas line would have to be upgraded.

Another possibility is that putting a tankless heater on a shared gas line with the NG fired furnace could make the furnace malfunction. The operation of our furnace was (apparently) not affected (i.e., not starved of gas when the water heater was operating), but our furnace is now 26 years old and due for replacement. When the new one is installed I will have to insure it is unaffected by the water heater. If it is, I will have to install a new separate gas line to the water heater from a capped T in the attic. I think the capped T in the attic was placed there by the builder to give an option to more easily install a gas kitchen range, but we have just stayed with an electric kitchen range.

Edit

Also note that the best installation of a tankless water heater requires special cut-off valves designed for a tankless water heater in both the incoming cold supply and outgoing hot water lines. Each of these is a double valve which allows disconnecting the water heater from the house plumbing and opening the water heater to the outside for flushing of the tankless heater to descale it.

I intended to install these heaters myself but thought better about it and hired a plumber to do both installations. I used a different plumber for each job because I was not satisfied with the job on the first one--costly and quality not what I expected for the price.

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