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We are installing a tankless heater system that has the specs of 122,000 btu, 36 kWh, 150 Amp.

The current systems looks to have only a single dual 60 Amp circuit to both the well pump and oil furnace.

I take it we need to upgrade the electrical. But for what exactly? 3x 60 Amps just for the tankless system?

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    You may want to tell your installer to hold off, until you get more clear on what electrical service you have. Can you take photos of the various electrical panel(s) around your house along with any labeling that says what loads are on which breakers? – Harper - Reinstate Monica Feb 21 '19 at 23:47
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    Can you post photos of your various electrical panels? Also, what sort of climate do you live in, why are you interested in a tankless heater, and how do you heat hot water right now? – ThreePhaseEel Feb 22 '19 at 1:32
  • @Harper it's a DIY system, I'm the installer but didn't plan on electrical not being enough. But yes, I'll take picks but the 200 Amp panel is pretty full. – eduncan911 Feb 22 '19 at 2:27
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    @eduncan911 the number of breakers in the panel has nothing to do with the load your house can bear. The extreme is a cell phone store with a panel full of circuits to the cell phone displays. They don't take anything. I am astonished that the people who sold you this thing didn't grill you with questions about your electric service, you oughta return it just to spite em :) – Harper - Reinstate Monica Feb 22 '19 at 3:11
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The tankless water heater will have instructions detailing how many breakers of what size. What is the make and model?

The Rheem 36 kW model requires four 40-A double-pole breakers. Probably it has four separate heating elements each 9 kW. I would think you would need to upgrade your electric service to at least 300 A to supply this heater. How large is your house and how many people in the household?

  • I believe that's exactly the same tankless heater the kit comes with. Though, the dimensions don't match the size is the same: 36 kWh for 122,000 btu. Himm, this is going to be a problem since the 200 amp panel is pretty full already. It's for radiant floor + domestic hot water, 4 bed/3 bath. – eduncan911 Feb 22 '19 at 2:29
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    This is almost certainly not a large enough heater for domestic hot water for this house in New York if the incoming cold water is as cold as I would suppose. I think you would need a natural gas fired tankless heater of 180,000 BTU/h to 199,000 BTU/h. How are you heating the water now? An electric tank water heater? You will probably have to stick with that and forget about a tankless. – Jim Stewart Feb 22 '19 at 3:21
  • 36kW is about 170A@220V so 200A may be enough. I think, if you face issues, you could fit an 'automatic load reducer' to turn off 'unessentiashowerl' heat when hot water is in use. Surely you can't cook while cooking (if you're on electric range). Usually a stove element is about 3kW or 16A@220V. I agree with those suggesting to get a gas tankless water heater if it's impossible to increase the service – DDS Feb 22 '19 at 13:24
  • 200-A service is standard for a 4 bdrm 3 bath home without an electric tankless water heater. Since the OP says he has an oil-fired furnace this suggests a rural setting and he may not even have natural gas service. If he wanted to go tankless, he could have a propane (LPG) fired tankless heater. – Jim Stewart Feb 22 '19 at 13:31

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