And for the record, every single natural gas water heater is connected to 120V power for the ignition circuit.
Mine isn't. During a long power outage, I still had hot water.
I was a bit surprised the water heater was working since I was pretty sure it had an electronic control system. So I went and looked, and sure enough, it was electronic, and somehow the LED was flashing blue like normal!
It turns out the electronics are powered by a thermopile which is heated by the pilot light.
This is incorrect. Multiple homes I've lived in had no electric to the water heater, including my current.
With a standing pilot a thermopile is used to generate the tiny bit of electric required for the control.
My Rheem hybrid 220v heat pump water heater only has a 500w compressor but puts 1500-2000 watts of heat into the water pulling it from the hot garage.
I have the choice to run it in high demand mode which will run both the heat pump and electric 4500w element for around 6kw of heat into the water if I need fast recovery.
Heat pumps have no problem operating at a COP of 3-4, so the 120V 12A (1440kW) HPWH would be equivalent to a 240V 25A EWH (6kW)
The 120v model HPWH's I have seen do not have electric resistive elements and instead have around a 1000W compressor, so they recover faster purely on heat pump and can run off a standard 15 amp circuit while staying well under the NEC 80% rule which would be 12 amps, they are closer to 10 amps.
They do require more airflow and are generally noisier due to larger fans and compressor.
Then you have dedicated split system HPWH's like SANCO that use an outdoor unit like a minisplit and pull around 1800 watts putting well over 6kw into the water, these are probably the future or whole house heat pump systems that heat both water and air(and cool) as combined unit.
Thanks for catching that :)
kwatts_effective [kJ/s] * heating_time_minutes [min] * 60 [s/min] * COP = 4.184 [kJ/kg/K] * (T₁-T₀) [K] * gallon_capacity [gal] * 3.785 [L/gal] * 1 [kg/L]
6.6 kW, for... COP 4, T₁-T₀ = 30 [K] (lower value for warm climate), allowable 30 minute heating time, 50 gallon capacity. A cold climate could double that power requirement, or alternatively double the heating time.
