It seems to me that the most fuel/cost efficient way to heat water is obviously with microwaves. So why are there no microwave home water heaters on the market? Why would we continue to use old fashioned heating elements?
These things are highly insulated anyway, so it can’t be the cost of the housing, can it?
Old fashioned heating elements are 100% efficient, and the circuits in a microwave heating system that create the radiation are not 100% efficient.
Can you supply more info? From my rule of thumb based on a 1200W MW oven, about 4000Wh are necessary to heat 200l to 60 °C, in what way would that be more efficient than solar or aerotherm heaters?
I am not sure microwaves are more efficient than resistance heating. Much of the efficiency is transferring the energy to what you are heating. In a hot water heater, all the electricity is used to heat the element and nearly all that heat is all transferred to the water. Some of the heat then leaks out.
In the microwave, I am sure there there is some some heat lost in the circuitry that generates the microwaves. There are massive heat sinks and a blower. Since the heat is produced in the food, none of it is lost being transferred to it, although some is lost heating the container and the insides of the oven.
On the stove top, the electricity all goes to heating the element. The heat must then transfer to the container and then to the food. Much of it is also transferred to the air around the element, container, and food. So the microwave is more efficient at heating food than the stove top.
Microwave tankless hot water systems are already in use.
This is one company doing it, since 2005 I believe…
Huh. I went to their web site, but I don’t see any advantage of microwave heating over conventional heating. Just marketing is my guess. They do claim over 99% energy efficiency, although only 90% thermal efficiency. Not quite sure what those definitions are.
They’re “seeking investments”. Current stock price is $0.0041. I can’t find any evidence that these water heaters are actually available for sale.
I assume they’re foreign, so I’ll give them a pass on the bad grammar and atrocious web design, but what I don’t like is when I see things like “produce up to about 6 to 7 gallons per minute” and then later “4 GPM” also I’m curious as to how a water heater can claim it “Conserves Clean Drinking Water”
Several years ago, I studied conventional tankless water heaters as a possibility to solve the problem of my water tank in the garage remote from the point of use. I was hoping their small size would allow relocation closer to where we use water. The ‘‘instant hot water’’ they hype is at the outlet of the water heater, leaving the same 20’ of pipe with cooled off water if located where the old tank was. Due to the larger clearances they require, there was no locating one where I needed it. But the deeper I dug, the more hype I turned up. I have no use for liars.
So if the conventional tankless heaters rely on hype, why not go for investors money with a story that sounds good?
Well maybe if you can solve the clearance problem and locate the thing close to the point of use, it would eliminate running all the cold water down the drain waiting for it to get hot.
Well, I suppose they could be referring to the fact that conventional electric water heaters have their heating element immersed in the water, and over several years of use a small portion of the metal (iron?) dissolves into the water, whereas with their microwave heating that doesn’t happen. But that’s really, really stretching it a great deal, though. And most people drink cold water, rather than the hot water.
100% efficiency is not the same as being more fuel efficient. If it takes* 4watts to send the microwave, and you lose 1 watt that’s a 25% loss. But if those remaining 3 watts heat the water more than 15 watts of resistance heating, then I’ll take the inefficient one every time.
I’ve never heard of Aerotherm, please explain? To you mean wind-produced electric? I’m not really trying to get into the production method for the electricity, that’s another thread, and there are several out there already. I’m talking about the delivery of the energy to the water and what will produce hot water with the least amount fo energy overall.
I’ve always understood that resistance heat lost an enormous amount of energy in heating the element itself. Granted, if it’s an immersion unit the heat would almost all transfer into the water, but that doesn’t mean all the energy turned into heat.
I’m perplexed. I think I needed a thread to define the question before the thread asking it. 
I guess that means the answer is 42?
*These numbers pulled from thin air to illustrate my query and nothing else. If I could find any real numbers I wouldn’t be here annoying you kind folks. LOL!
In a light bulb, the efficiency is important. The amount of energy that goes to heat instead of light Is energy you’re paying for, but not getting use from. For resistive heating, all the energy goes to heating; there’s no where else for it to go. So resistive heating is essentially 100% efficient. All those watts go directly into heating the water. For the microwave heater, some of those Watts go into generating microwaves, which then heat the water. There’s some waste heat, and they absorb that with the water before the microwave heating section. So again, they essentially have 100% efficiency. But there’s no “3 watts heat the water more than 15 watts of resistance heating” possible with the microwave.
They’re converting energy into microwaves, then into heat, instead of directly into heat, but they end up with the same amount of heat at the end.
I think you’re thinking of electrical inefficiencies in the generation end of things. Overall, including the generators, electrical heating is relatively inefficient compared to, say, gas heat. But all those inefficiencies are in the generation and electrical transmission. The part that’s in your house is essentially 100% efficient. Using a microwave instead of resistance heating won’t do anything about those inefficiencies.
If that were the situation, then the 75% efficient microwave system would have a higher efficiency than the other one- its efficiency couldn’t be higher than 20%, since its dumping under 3 watts into the water, and consuming 15.
A resistive heating element is about as efficient as you can get (99% or more). Almost all the heat transfers to the water, and it does mean almost all the energy is turned into heat. Some of the energy goes into heating the element itself, but that’s not very much compared to the total.
It would be pretty much impossible for a microwave heater to beat that.
The only way you are going to beat a resistive heater is with a heat pump, which can be nearly 300% efficient at heating.
But, nobody is going to pay for a heat pump water heater…
Cheating! You’re just moving heat around with that 
Even the heat to heat the element beyond the water temperature will transfer to the water after it shuts off. A little heat will be wasted from the end of the element outside the tank.
Well, now, wait a minute. You can get the same amount of hot water with less energy, if you use a tankless heater. “Efficiency” in the context of power generally means power out divided by power in, and that’s not this, exactly, but less energy to get your hot water is still a good thing.
There are two reasons this is possible. One, the same thickness of insulation will let more energy leak out of a large tank than out of the small heating zone in a tankless heater, as the energy flowing through insulation is proportional to its area. And, two, in a tank type hot water heater, you can’t put the heater right under the tap, so to get hot water you must run the tap for long enough to clear the recooled water out of that run of pipe. And you spent energy to heat that water, even though it cooled off again since you did it.
I don’t see why microwaves would be a better way to make tankless heaters. Maybe they are, for some reason. Though, resistance heating works awfully well - I do it all the time with air. Of course, if you build a tankless heater in such a way that it doesn’t fit right under the tap, it loses one of its advantages. And any one particular manufacturer could do all sorts of slimy things.
I don’t know if you get more hot water for the same energy with the cited heater, if it’s even been manufactured. But in principle it should be possible to do so generally with tankless heaters.
Well, I did. It’s part of my heat pump HVAC system.
I’ve never heard of that, please tell us more. Who makes it?
Mine’s a WaterFurnace brand. The household hot water part of the HVAC system is provided by a desuperheater built into the compressor cabinet. A small, electric pump constantly circulates the water through the water tank (a commercial, electric hot water heater not connected to the mains). Any time the HVAC is operating, the water temperature is increased by a small amount every time it passes through the desuperheater. In the winter, it takes a minuscule percentage of efficiency away from the HVAC heater; in the summer, it uses 100% waste heat that would otherwise be pumped into the ground.
That tank is in series with the original, natural gas water heater. As hot water is drawn from the gas heater, it’s replenished with already-hot water from the storage tank. Ideally, the gas tank will never fire. Of course there are a few weeks a year where the HVAC system isn’t operating, and so I don’t get “free” hot water. Also, if you never draw hot water (causing the tank to fill with new hot water), then the gas tank eventually gets cold, and so the gas burner will fire in order to maintain its temperature.