Electrical requirements for tankless water heater

Mikepier · 2011-08-04T00:05:44+00:00

I currently have a standard 40 gallon electric water heater on a 30amp breaker. The panel is 100A service which also services an electricdryer. I was thinking about getting a tankless water heater to savespace, but after reading some electrical specs on these units, itseems I would have some issues with the electric.I have 1 bathroom and 1 kitchen and a washer. From the calculations Iwould need at least a 5 gpm unit.If you look at the specs for the Rheem units, the RT18 needs 2seperate 40A circuits, with maximum power being 75 amps. my panel cannot support that. I could maybe drop to the RT9unit that requires 1- 40A breaker, but I'm concerned it might beundersized at 3 GPM, and also concerned that it uses 38 amps. If mydryer is on, thats another 26 amps, and my 220V water pump using 5amps, totaling about 70 amps.That leaves me with about 30 amps to spare.So I could either upgrade my panel, or just stick with what I have.This is for a seasonal home in the summer. Just wondering whateveryone else thinks I should do.

Aug 04, 2011 22 Replies

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sam E 11 years ago

Now I'm wondering how long it would take that 15 watt water heater to heat enough water for a bath.

Probably forever, since it cools off faster than it heats. 0% efficiency.

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makolber 11 years ago

assuming your delt T is in F degrees I get 19.8 kW

using 3.785 kg per gallon and a specific heat of 4.187 J/g K

it is interesting at $0.15 per kW hour it costs about 5 cents a minute to take a shower.

also a 45 deg F rise is a pretty cold shower, need more like 70 F rise, 50 to 120...no?

Mark

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Tony Hwang 11 years ago

Hi,

1 BTU/hr is ~0.3W

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