3 ms·

> If they ever manage to make electric water heating efficient enough to just heat on demand I'd probably use it. It's more a matter of efficiency than through

> If they ever manage to make electric water heating efficient enough to just heat on demand I'd probably use it. It's more a matter of efficiency than throughput. Mark-one googling suggests that a shower head might have a flow rate of 2 gallons per minute (0.125L/s). Suppose that the incoming water supply is at 10C, and the target hot water temperature is 43C (110F) for a shower. The necessary heat flow rate is 33C * 0.125L/s = 4.125 kilogram-degrees per second. Multiply that by the specific heat of water (4184 J/kgK) to give a net energy flux of 17.3kW. On a 240V circuit, that would require 36A of current for electric resistive heating, a hefty circuit indeed. I'm having trouble sourcing a reliable comparison, but the calculated 17.3kW draw appears to be larger than a typical natural gas stovetop with all burners on. > There's just no return there for a big investment. And I can never have solar so free electricity is not a thing (I live in a big city and don't own the roof of the apartment building) You might not personally have free electricity, but utilities are experimenting more with time-of-use billing. Hot water tanks allow heating demand to be time-shifted from use. The finances clearly don't make sense for you right now, but there might come a time when electricity at some part of the day is cheaper (heating equivalent) than natural gas.

5ersi 3y ago
3 phase then? https://www.stiebel-eltron.com.au/3-phase-electric-instant-hot-water-system https://www.stiebel-eltron.com.au/3-phase-electric-instant-h...
- quickthrowman 3y ago
  Unfortunately, residential electrical service in the United States is 120/240v single phase (technically it is split-phase, two 120v legs 180 degrees out of phase with each other plus a center-tapped neutral) in almost 100% of cases. It would be cheaper to just buy a 240v single-phase instant hot water heater than to buy a three-phase water heater and a VFD to convert the single-phase to three-phase. It’s definitely worth doing a three-phase water heater if you have three-phase electrical service though.
wkat4242 3y ago
> You might not personally have free electricity, but utilities are experimenting more with time-of-use billing. Hot water tanks allow heating demand to be time-shifted from use. > The finances clearly don't make sense for you right now, but there might come a time when electricity at some part of the day is cheaper (heating equivalent) than natural gas. Understood. And yes I can get time of use billing here already. Some of my friends are really into it, they schedule their whole day around it with planning and automation. They watch and analyze the prices like greedy traders watching the stock market lol. But personally I don't care and I don't want to. I want to live my life as I see fit and I want my systems to adapt to me, not the other way around. I want to shower when I need and wash when I need fresh clothes, I hate planning days ahead of time. So I'm on fixed billing. Even if time of use billing becomes a mandatory thing I'll just disregard it and do whatever I feel like. My energy use is so low it's never going to be something I want to worry about.
BizarroLand 3y ago
I see a flaw in your logic. Resistive heating is considered to be 100% energy efficient in heat conversion when measured on a watt-per-watt basis. That's to be expected as they are essentially entropy production systems. However, heat pumps are more than 100% efficient on a watt-per-watt basis as they are not directly converting electricity into heat, but rather are moving heat from one area to another. Heat pumps currently are between 150% to 400% efficient on a watt-per-watt basis, which could mean that your ideal hot water heater could run on as little as 240v/10a, or 120v/18a (120v/20a breakers are common in kitchens, to power microwaves and refrigerators, so this wouldn't be a huge stretch for a hot water system, either). Additionally, the state of the art is always improving. Recent heat pump advancements have been made with CO2 for use in electric vehicles that may push past the 400% efficiency rating, which could make it even lower than that.
- Majromax 3y ago
  > However, heat pumps are more than 100% efficient on a watt-per-watt basis as they are not directly converting electricity into heat, but rather are moving heat from one area to another. To reactivate this dead thread: I thought of that, but I don't think it's an argument in favour of on-demand electric heating. A heat pump capable of delivering ~16kW of heat equivalent on demand would be something like 50,000BTU/hr, or larger than many whole-home air conditioning systems. Instead, a heat pump hot water heater would be far more appropriately sized to work with a high duty cycle at lower intensity – a hot water tank.

5ersi 3y ago

3 phase then? https://www.stiebel-eltron.com.au/3-phase-electric-instant-hot-water-system https://www.stiebel-eltron.com.au/3-phase-electric-instant-h...

quickthrowman 3y ago
Unfortunately, residential electrical service in the United States is 120/240v single phase (technically it is split-phase, two 120v legs 180 degrees out of phase with each other plus a center-tapped neutral) in almost 100% of cases. It would be cheaper to just buy a 240v single-phase instant hot water heater than to buy a three-phase water heater and a VFD to convert the single-phase to three-phase. It’s definitely worth doing a three-phase water heater if you have three-phase electrical service though.

wkat4242 3y ago

> You might not personally have free electricity, but utilities are experimenting more with time-of-use billing. Hot water tanks allow heating demand to be time-shifted from use. > The finances clearly don't make sense for you right now, but there might come a time when electricity at some part of the day is cheaper (heating equivalent) than natural gas. Understood. And yes I can get time of use billing here already. Some of my friends are really into it, they schedule their whole day around it with planning and automation. They watch and analyze the prices like greedy traders watching the stock market lol. But personally I don't care and I don't want to. I want to live my life as I see fit and I want my systems to adapt to me, not the other way around. I want to shower when I need and wash when I need fresh clothes, I hate planning days ahead of time. So I'm on fixed billing. Even if time of use billing becomes a mandatory thing I'll just disregard it and do whatever I feel like. My energy use is so low it's never going to be something I want to worry about.

BizarroLand 3y ago

I see a flaw in your logic. Resistive heating is considered to be 100% energy efficient in heat conversion when measured on a watt-per-watt basis. That's to be expected as they are essentially entropy production systems. However, heat pumps are more than 100% efficient on a watt-per-watt basis as they are not directly converting electricity into heat, but rather are moving heat from one area to another. Heat pumps currently are between 150% to 400% efficient on a watt-per-watt basis, which could mean that your ideal hot water heater could run on as little as 240v/10a, or 120v/18a (120v/20a breakers are common in kitchens, to power microwaves and refrigerators, so this wouldn't be a huge stretch for a hot water system, either). Additionally, the state of the art is always improving. Recent heat pump advancements have been made with CO2 for use in electric vehicles that may push past the 400% efficiency rating, which could make it even lower than that.

Majromax 3y ago
> However, heat pumps are more than 100% efficient on a watt-per-watt basis as they are not directly converting electricity into heat, but rather are moving heat from one area to another. To reactivate this dead thread: I thought of that, but I don't think it's an argument in favour of on-demand electric heating. A heat pump capable of delivering ~16kW of heat equivalent on demand would be something like 50,000BTU/hr, or larger than many whole-home air conditioning systems. Instead, a heat pump hot water heater would be far more appropriately sized to work with a high duty cycle at lower intensity – a hot water tank.