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I'm trying to evaluate if an electric tankless water-heater would be suitable for my home. There are just too many variables to consider, and I wonder if there is a mathematical approach to it.

I'm thinking these are the factors to consider:

  • Occupants: 2 (no child yet!)
  • Sinks: 2
  • Baths/showers: 1
  • Washing machine: 1 (but little-to-no warm washing)
  • Dishwasher: 1
  • Floors: 1

Still, having these data points tells me nothing. Is there a formula of some kind?

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The numbers you need are flow rates and desired temperatures. For example, Home Depot's guide gives the following flow rates:

Bathroom Faucet: 0.5 – 1.5 gpm

Low Flow Kitchen Faucet: 3.0 – 7.0 gpm

Shower: 1.0 – 2.0 gpm

Dishwasher: 1.0 – 2.5 gpm

Clothes Washer: 1.5 – 3.0 gpm

Tankless heaters are rated for how much heat they provide at a given flow. If your incoming water is 50°F, and you want 120°F, you need a 70°F rise in temperature. Now say you want to both run your dishwasher and take a shower. On the high end, you may be looking at 4.5 gpm. You need to find a way to raise 4.5 gpm 70°F. Going back to Home Depot, they sell a Rheem gas heater that will do 77 degrees at 4.9 gpm.

Alternatively, you could combine a whole-home unit with point-of-use units. This is particularly useful if you want to only heat your shower to 120, but your dishwasher doesn't include a heating element and you'd like to heat that water to 140, or if you want your kitchen faucet to supply hotter water than the bathroom faucet or shower. Also, point-of-use units are useful for providing instant heat.

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  • The gas heater you link to is discontinued and I don't have gas (hence my looking for an electric solution). But the rest of the info is super. Thanks! – MPelletier Jul 25 '12 at 14:38
  • Ah, yeah, I must have glanced over that part. The math still applies, but you may actually get out cheaper with electric, and they're also generally more efficient. I just clicked gas first when pulling up units to demonstrate. – ND Geek Jul 25 '12 at 15:42
  • No worries. You say electric is more efficient? I thought it was the opposite... Your gas unit does 77F @ 4.9 gpm, and the best (and admittedly cheaper than gas) electric units does 77F @ 3.19 gpm. – MPelletier Jul 25 '12 at 15:56
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    Sorry, by more efficient, I mean that gas can convert somewhere around 85% of its energy to heat, whereas electric tankless is somewhere around 98% for the most efficient units. Gas tank heaters are more efficient than electric. – ND Geek Jul 25 '12 at 16:06
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    Apologies for being pedantic and a bit OT, but electric heat is only efficient as far as heat provided by input energy the consumer pays for. If you look at transmission and generation losses, electricity is not so efficient when comparing source energy consumption. More coal or whatever is used to generate electricity to provide a unit of heat than the equivalent amount of gas, including delivery energy, to provide the same unit of heat. The carbon footprint of electricity might be greater as well. This is just a general statement, there are many variables to swing things one way or another. – bcworkz Jul 26 '12 at 2:17

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