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In building a new house, I have the choice of one 100 gallon gas water heater vs two 50 gallon gas water heaters. What are the tradeoffs I should consider before deciding?

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    Will the two 50s be side-by-side in the basement? Or one near the kitchen and one near the bathrooms? Dec 8, 2022 at 1:07
  • There is apparently a new(ish) standard for water heaters over 55 gallons that may impact your decision. diy.stackexchange.com/questions/104155/…
    – RetiredATC
    Dec 8, 2022 at 3:35
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    @RetiredATC -- that's only for electric heaters, not gas Dec 8, 2022 at 4:27
  • @ThreePhaseEel Thanks for the info.
    – RetiredATC
    Dec 8, 2022 at 4:32
  • @VtC - this is asking for "tradeoffs to consider" (i.e. pros & cons), not which should I get (i.e. opinion based). This is exactly the type of question most of the opinion based questions should be!
    – FreeMan
    Dec 8, 2022 at 13:20

6 Answers 6

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The distance between the use points and the heaters. I have two heaters because the kitchen and utility room are at one side of the house and the bathrooms are at the other side. Long piping runs take time for hot water to travel. On the other hand, my son has a recirculating system so there is always hot water at each faucet from a single heater.

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If you are building a new house, and heating water with gas, designing in a sufficient gas supply for a tankless water heater beats any size of storage heater while wasting less utility space.

Gas tankless (with sufficient supply) does not have the inherent drawbacks of electric tankless.

Layout of plumbing would still rule the "one or two" decision (recirculation wastes a lot of heat [thus, money] unless the insulation is superb.)

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  • tankless seem over rated... maybe the quality initially was just low... what do you have, navian? Dec 8, 2022 at 17:39
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    I have an electric tanked heater. My parents had a propane tankless heater that worked well, but I don't recall the brand. Given the flue is always open and uninsulated, standby losses on gas tanked heaters are non-trivial, and it's fairly easy to get the huge BTU inputs that make electric tankless the wimpy stepchild, especially if you design the gas system at build time for that.
    – Ecnerwal
    Dec 8, 2022 at 17:51
  • on a new build unless you live somewhere that has temps below 40F for most of the winter, I don't see why you'd bother with gas appliances or a chimney. if you do have below 40F then a ground source heat pump is likely a better investment than the chimney and gas lines for gas appliances. Dec 8, 2022 at 18:02
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One advantage of two is that if they can be positioned to split the hot water demand so water doesn't have to travel the length of the house before getting hot, ie., a water heater at each end of the house. the disadvantage of this arrangement is additional gas lines and additional venting. If the two heaters are in the same room, the advantage would be if one fails, you'll still have hot water. The disadvantage is the cost of two.

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  • You could turn one off in the summer maybe? Or you could run them at different temperatures
    – gbronner
    Dec 8, 2022 at 1:33
  • People still need hot water for showers, cooking, washing dishes and other activities in the summer, do they not @gbronner? What activities are curtailed/eliminated in the summer that would reduce the demand for hot water? (I presume there's no hydronic heating, as a "boiler" is usually used for that, not a "hot water heater", though that could have been what was meant.)
    – FreeMan
    Dec 8, 2022 at 14:26
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The only disadvantage is the cost. Two will cost more than one.

After that there only advantages for having a two heaters.

For example 2 heaters would be better hot water distribution through the house by placing the heaters in proper locations.

Another one would be the energy consumption could be lower if one of the heaters is used more frequently than other or a large heater.

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    @Gil tell me more, I want that, how much new plumbing is required
    – Traveler
    Dec 8, 2022 at 5:11
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    Two 50 gal heaters will cost more than one 50 gal heater, but will they cost more than one 100 gal heater?
    – FreeMan
    Dec 8, 2022 at 14:28
  • How will two heaters used asymmetrically save energy? Dec 8, 2022 at 15:17
  • @manassehkatz-Moving2Codidact if they are used independently and and in parallel set up
    – Traveler
    Dec 8, 2022 at 19:11
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A lot of the answers touched on a few of the benefits. This takes into account gas water heaters only - there would be some differences if you were talking about electric.

Advantages of two tanks:

  1. Redundancy - if a component of one of the tanks fail, you'll still have hot water.
  2. Serviceability - two tanks are smaller than one and you can more easily haul a smaller tank around and get it into position.
  3. Possibility to locate in different areas and serve different fixtures for quicker hot water ( this is difficult with gas as there are venting requirements ).

Disadvantages:

  1. Two tanks take up more square footage.
  2. Two tanks cost more both for the unit and for installation
  3. Two tanks take longer to service than 1.

I am really surprised anyone goes for a gas hot water tank these days especially on a new build. The hybrid (heat pump) electric hot water tanks are so much more efficient and so much easier/cheaper to install.

Advantage of a hybrid hot water tank

  1. Way cheaper to install ( no gas line, no chimney, no make up air ).
  2. Way more efficient 200-300% more efficient than the best gas hot water tank
  3. Doesn't generate products of combustion so overall way safer ( no co devices means no requirement for co detectors ).
  4. PV panels are getting cheaper and cheaper, the payback is now less than 10 years even in countries with cheap electricity. You can use PV to power your hybrid tank.

Disadvantage:

  1. The unit itself is more expensive than a gas hot water tank
  2. The heat pump hot water tank has components ( compressor ) that likely won't last as long as a gas hot water tank. I kept my gas hot water tank for 26 years before replacing it.
  3. It might be more expensive to service it when something does break.
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    heat pump WHs are fine if you are in a hot/warm climate. Not so good up north. They steal heat from where they are installed which has to be re-supplied from somewhere, If in a hot clime, you get free A/C, great. But they are a LOT more expensive and complicated than a simple electrical resistance WH. I have a simple electric WH with re-circ and have hot water within 3 seconds on every sink / bathtub in the house. I know...ppl will probably B***** me out for staying old school, but sometimes simple is best. Dec 8, 2022 at 8:55
  • Sure if the temp around the unit is below 4C/40F it stops working. They do steal heat from where they are installed but typically this is going to be a mechanical room which possibly already has a boiler / furnace and is already too hot. Re-circ is horrible for energy... really a properly designed water supply system should have hot water to you in less than 30s and unless your water is crazy expensive it is cheaper to wait the 30s. I do like the simple electric hwts though. Dec 8, 2022 at 17:37
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    In my neck of the woods it's typical to install the furnace and WH in the garage. Few homes here have mechanical rooms. Even if they did, if there is so much heat escaping from a boiler or furnace, that's troublesome as well. I have recirc bc the bathrooms in my house are pretty spread out. No "single wall" plumbing here. It's on a timer so it turns off at night. I don't think it's "horribly expensive" to run, the pump is a Grundfous circulator...maybe 40 watts. Yeah, there's heat loss in constantly heated piping, but it should be an individual choice. Dec 8, 2022 at 19:00
  • I am in the land of detached garages if you even have one... the heat loss in the walls is the part I don't like about recirc ( your money / your choice ). My hwt is 40 gallon ele in my crawl but I'll think about replacing it with a hybrid when this one dies. Dec 8, 2022 at 20:07
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    If the HW plumbing is well insulated I don't think the heat loss is all that much. Bare pipes....yeah, it could be significant. I'm retired and home most of the time, so it's REALLY nice to have HW within 3 or 4 seconds everywhere, not only that most dishwasher's first cycle doesn't draw enough to get hot water so it's mostly cold. Some may turn on a heating element to get up to temp, but that's just like an electric water heater. Thanks for the resp. polite discussion. To each their own, Take care and DIY on! Dec 8, 2022 at 20:44
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I have two, a 75 and a 50 currently in series. The first is set at about 100 F the second at about 130 F. I have a pump so I have "instant hot water", about 2-3 seconds it is hot at any hot water faucet in the home. That little pump is inexpensive for the convenience. The hot water return from the instant hot is circulated through fin tubing in the basement during the winter, keeps it nice and warm where I spend most of my time. A hot water heaters fail in sub zero weather on Christmas Eve, No problem Just raised the temperature of the first tank and bypassed the second (failed unit).

I use in floor hydronic heating in our 4 seasons room as well. That returns to the bottom of the first heater while the Instant Hot returns to the bottom of the first heater. In the summer I turn off the first heater and change the valves so all the water (city water) comes through that heater before supplying everything else in the home. At that point the pump circulates the water through the 4 seasons room floor and saves me about 24,000 BTU of cooling.This actually preheats it for the hot water. The downfall is the cold is luke warm but since we use refrigerated water that is not a problem. It cools off nicely when the sprinklers run so they are scheduled accordingly.

It took several days to do. The heaters are adjacent to each other to keep the plumbing runs small. They are not insulated as the area they are in is heated so the loss only helps except in the short summer months. It took over a dozen valves and a lot of fittings and maybe 20' 3/4" copper tubing to do not counting the instant hot returns.

There are three pumps involved, one for the Instant Hot, one small one for the outer loop in the 4 seasons room and the third the main for the 4 seasons room. The controls are all custom. When the temperature falls to 40F or colder the small pump runs but when the thermostat requires more heat it is shut off and the larger loop heats the room. When it gets colder maybe 20 or colder I raise the set points on the hot water tanks, both max, first about 120 second about 140. Please note these heaters are rated for heating as well as hot water.

A bit of warning be sure you are allowed to do this in your area, especially when using potable water as I am. Everything was inspected and approved. Start by drawing a schematic and figure out how to set the valves for heating, cooling, and tank failures. This is not a simple task and it will take a lot of soldering so be sure you have the needed skill set and tools.

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