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The whole tank thing is really holding me back from moving away from natural gas. With gas you have unlimited hot water when you need it, it never runs out. I hate planning things in advance. And you're not wasting energy keeping it hot when you don't need to. And no wasted space for a big tank.

If they ever manage to make electric water heating efficient enough to just heat on demand I'd probably use it.

Of course it is more efficient with electric but I hardly use any gas anyway, my bill is like 15€ a month including fixed supply charges. 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)

I have always had tank-type water heaters and I don’t recall ever running out of hot water (family of 4). Is this actually a problem people have or is it just a tankless water heater industry talking point?

Various places will have different-sized water heaters installed. As a child, I lived in one place that had a measly 20 gallon electric heater. It sucked, badly. Dad showered first, then I got woken up to shower - barely any hot water.

Nope it’s a real problem, an undersized tank, high usage devices (eg a big bath tub) or just low temperature set on the unit for safety’s sake can cause the hot to run out.

Modern tanks have a label on them that will let you know how many gallons of hot water it can deliver in its first hour of usage, some of those numbers are much lower than others.

The good thing about the heat pump I've got is that it's also surprisingly quick to heat up. If you do run out (I never have, but I experienced the speed the first time it was turned on when it was installed) you can go and press the 'Boost' button on the controller and you will have 50L in 20 minutes and a full tank (mine is 315L) in 3 hours.

The system also heats the tank to at least 60 degrees C at the sensor point (about 2/3 of the way down) at least once every 24 hours and then has a tempering valve on the output for safety. That way you don't have scalding water at the point of use, but do have lots of hot water available and no issues with Legionella bacteria.

I have a large 2-person tub, even though I have a 80 gallon water heater (double normal size) it cannot fill the tub. That is a rare use case though, and I don't use the tub very often so I'm not sure I'd recommend it to someone thinking about that. (when my wife and I want to relax it is nice).

As a kid my family had off-peak water heating, 200 gallons of water heaters, that were only turned on for a couple hours at midnight. We ran out of hot water about every 5 years - when mom did a lot of laundry and we all took long showers/baths on the same day. One time the breaker blew and we suspect it was 2 days before we noticed.

you can run the tank hotter and use a mixing valve to deliver hot water at a safe temperature. Let's you have a smaller, more efficient tank and also ensures your water heater is hot enough to never get infected with the bacteria that causes legionnaire's disease. Double win!

> 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.

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

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.

> 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.

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.

> 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.

This is from Sanden, the manufacturer of CO2 heat pump water heaters. They are known for the SANCO2. https://www.smallplanetsupply.com/sanc02

Ah, interesting, apparently it is manufactured by Sanden, yes. I thought they were different because you can also buy Sanden branded heat pumps here in Australia too, and I assumed that since Reclaim also sell commercial ones made by Mitsubishi Heavy Industries I though the domestic one might be Mitsubishi too.

You’d think that if you highlighted 150°F iPad Safari could translate it to Celsius, or allow lookup to do the conversion.

65 degrees Celsius.

For context, 54 degrees for 30 seconds will cause full-thickness skin burn.

It does translate it for me (65.56°C).

If it doesn’t for you, that likely is because of your settings

Strange.

It does now.

It’s not as if someone from Apple did a quick fix is it?

Thanks for this!

I'm in Launceston, Tasmania and have access to no-interest credit up to $10k over three years for energy efficiency improvements including heat pump hot water.

Our electricity use is dominated by heating and hot water, and the next step I've been wanting to take is heat pump hot water.

I'll have a read of the link you provided when I get home.

Thanks again.

Yeah, it’s been good. I think you should be able to get it installed for something like $5K after rebates.

My situation was a bit weird since I bought it in late 2021 directly from Reclaim (for about $4600 with the 315L glass lined tank), it sat in storage for a bit while I was doing other work on the house and then I had it installed more recently (cost $950) and then am claiming the STCs myself (which are worth about the same as the installation). It would be much easier to just have it done by an installer who orders it, installs it and claims the credits as part of the deal…

That's really cool. Hope someone does one with integrated aircon soon.

I have often dreamed about solutions for this, but at the end of the day I'm not sure that your average household would really have enough coincident demand for heating and cooling to make it worth it... Definitely for larger buildings it makes a lot of sense though (and there are chillers that have heat recovery now for this kind of thing)

I was not thinking of running heating and cooling at the same time. Simply that if you run a heat pump in reverse, you can use it for cooling. So you should be able to use the same device for heating in winder and cooling in summer. Even portable aircon's do this.

The tricky part here is that you want heating to transfer heat to water, because radiators are much nicer than heated air. These Co2 based heat pumps are good because they can heat to a higher temperature which is necessary if you want to use your existing radiators or your house is difficult to insulate. But I'm not sure it would work well to use 'radiators' for cooling. Apart from any other reason, you won't want to risk freezing them.