[andrewvc]

One other challenge is for existing homes a water heater may only have a gas line running to it. Want a heat pump hot water heater? Hiring the electrician alone, not to mention potentially ripping up walls will ruin any economic advantage.

[underbluewaters]

This was a major barrier for me. I had to replace an existing natural, tanked gas water heater. Ultimately I just bought a $750 replacement because I could easily swap it out myself. Installing a heat pump would have involved an electrician to install a new circuit, and possibly other changes. While there were some 120v models available locally, they all had pretty bad reviews. So I would have paid a couple thousand dollars more. Maybe I could break even over 10 years paying less for gas but that seemed like a poor use of funds.

[atrus]

A possible challenge, yes, but there exist 1500w, standard outlet hpwh, and it's a lot less common to not have any power near a water heater.

[quickthrowman]

A 1500W electric water heater would be painful to use in a home. A typical 30-60 gallon EWH is 6kW.

And for the record, every single natural gas water heater is connected to 120V power for the ignition circuit.

[adrianmonk]

> 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.

[somehnguy]

> And for the record, every single natural gas water heater is connected to 120V power for the ignition circuit.

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.

[SigmundA]

a 1500W heat pump water heater with a COP around 3 will put 5500 watts of heat into the water.

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.

[tonyarkles]

Keep in mind that there's going to be a CoP associated with a heat-pump water heater. Depending on (a bunch of factors) that 1500W HPWH could approach the performance of a 6kW standard EWH.

[quickthrowman]

Yeah, I neglected to compare my hypothetical 6kW EWH against a 1500W HPWH, and did it against a 1500W EWH.

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)

[SigmundA]

AFAIK there are actually no 1500w HPWHs currently, the normal hybrid 220v models have a small 500 watt compressor thats very efficient and keeps the airflow requirements low which helps with installation placement and ducting if needed, then still have the electric elements if needed to boost.

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.

[maxerickson]

It's not uncommon for a gas heater to have an always on pilot.

[quickthrowman]

I completely forgot about thermopiles. And also that heat pump water heaters exist :facepalm:

Thanks for catching that :)

[harshreality]

I think this is right...

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.