Tankless Water Heaters

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Are you doing a standard or optional calc?
new house or addition to existing?
 
Tom Drake said:
When applying tankless water heaters to electrical load calculations. Do you apply the demand factor or is it figured at 100%
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What demand factor? if you have 4 or more appliances then yes you can use 75% but if not the water heater is calculated at 100% as it is tankless so it is not continuous
 
Isn't that 75% factor with the standard calculation only? All Non-HVAC loads in the optional calculation get a 40% factor once you're past the 10KW threshold.
 
Dennis Alwon said:
What demand factor? if you have 4 or more appliances then yes you can use 75% but if not the water heater is calculated at 100% as it is tankless so it is not continuous
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I get that the code is the code, but how is a tankless water heater not continuous, or potentially so? Someone leaves the tap open for 4 hours, it would run continuously. I found a table for electric water heater recovery times, and with 4500 watts of heating, assuming the water coming in is at 40F and you're heating it to 120F, the recovery is 23 gal/hr if all the water in the tank starts at 40F. As long as the tank is under 69 gallons, the storage water tank is not a continuous load in practice, unlike what the code says. 😖
 
gadfly56 said:
I get that the code is the code, but how is a tankless water heater not continuous, or potentially so? Someone leaves the tap open for 4 hours, it would run continuously. I found a table for electric water heater recovery times, and with 4500 watts of heating, assuming the water coming in is at 40F and you're heating it to 120F, the recovery is 23 gal/hr if all the water in the tank starts at 40F. As long as the tank is under 69 gallons, the storage water tank is not a continuous load in practice, unlike what the code says. 😖
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4500 Watts is 15,354 btu/hour.

69 gallons of water is about 576 pounds of water.

You would need 576*80 BTUs to warm up 69 gallons of water from 40 Deg F to 120 Deg F, or about 46,000 BTU.

So three hours is about right.

The tankless is unlikely to run at 100% max current which is what a continuous load has to be, even if the tap is left on for a day. It only uses as much heat as it takes to heat up the water being used. The only way it could ever be a continuous load is if someone drew more water through it at a starting temperature less than what it could heat up at full load. I suppose it could happen but it would be really unusual.
 
gadfly56 said:
I get that the code is the code, but how is a tankless water heater not continuous, or potentially so?
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I just went thru this last year with a huge tankless on new smaller house, it took four 2-pole 40's if I remember.
I had to dig thru some mfr documentation to prove it would internally not run 3 hours or more even if tap left on.
I used the standard calc not the optional and took the 75% i believe, id have to look up my calcs.
Defiantly a good idea to run it by the utility and AHJ first, i was surprised the numbers did work out.
Next make sure they install that as close to the service as you can, I was able to feed it with one conduit.

EDIT I just remembered another one I did, and I upgraded the 200A service to a 320A.
 
suemarkp said:
Isn't that 75% factor with the standard calculation only? All Non-HVAC loads in the optional calculation get a 40% factor once you're past the 10KW threshold.
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Yes, I am not sure what demand the op is talking about so I just thru out one that perhaps he was talking about.
 
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