[alancalverd]

The rate of heating is directly proportional to volts x amps, so to get the same power from a 12 volt unit, you need 10 times the current that you would use in a 120 volt unit. Higher voltages are preferred because you lose less power in the transmission lines.

If you have solar power, consider using a heat pump and a big hot water storage tank, and also underfloor low temperature (30 - 35 degree) heating rather than traditional 50 degree "radiators" (actually convectors) - "low and slow" is best if your property is adequately insulated.

[Moleculiar]

If microwaves are so energy ineficient, why not just cook with electric devices?

It's a matter of perceived convenience, not science-based common sense, which dictates whether or not to purchase & or operate a microwave oven in a household. I doubt there are any typical consumers who would consider the energy efficiency of a microwave oven vs something like a toaster oven

What about the matter of keeping the inside of the oven clean from food splatters? Microwave ovens are very good at collecting junk, and then combined with how well they condense moisture onto the inner surfaces of the oven, it's really a wonder that very few cases of food poisoning result! I would like to see a microwave have a removable liner that can go right into a sink or dishwasher and also a blow-down mode for the fan so it runs after the oven turns off

ps - microwave oven is also an 'electric device'

[Moleculiar]

I have been thinking about this exact thing recently. I have worked on furnaces for over 20 years and am wondering why microwaves can't be used instead of say natural gas? A standard furnace has a heat exchanger that is made up of 2 to 4 chambers. Picture cucumber shapes (standing about 1 1/2 ft. tall and approx. same depth), upright with about 2 inch gaps between them. The inside air in these chambers is heated which heats the metal itself which in turn heats the air surrounding each chamber. The fan blows this air throughout the house via ductwork.

Can these chambers not be heated using microwaves? If needed one could make sealed chambers with water or whatever inside if one needs something else to hold the heat. The microwaves themselves would always be enclosed in each chamber. I have no idea offhand the cost of electricity to power microwaves or if it would be safe using it this way. I also thought that since microwaves will run on 12 volt could one not put up a couple solar panels to power it? If so it could be a 100% free standing, self contained unit. Environmentally friendly as well, no?

There are electric forced air furnaces. But they are like using a conventional oven to melt a slice of cheese compared to doing it in a microwave cost wise.

I would love some opinions.

Yes, you can heat the inside of your chambers, using microwave energy. But is it a worthwhile idea? Possibly not! The common magnetron used has air-cooled fins for a reason, as the magnetron itself is converting a sizeable portion of mains energy into heat. You could do what the microwave oven manufacturers do - route the warmed air from these cooling fins into your heating chambers. But there will still be lost heat due to radiation off of your ducting. You might try insulating all the ducting with aerogel paint. Even better if you place the magnetron itself, inside your chambers, so that all the heat is in the right location. But this will create some overheating issues for the magnetron

So, you will be drastically increasing the cost of heating your chambers without making any improvement on the energy-exchange from mains power to room air. If you can find an application where this increased cost can be offset by a as-yet unknown benefit, such as industrial or laboratory processes, maybe it's worth considering further

Perhaps, if you were to make use of excess microwave energy that is being produced for some other purpose, in order to provide an 'added value' of heated air?

[Halc]

As for microwaves they just seemed to heat things up so much faster which is why I thought it might be feasible. As the heat exchangers could be made of or contain the best heat transferring products.

Any microwave device consuming X power can heat something no faster than a standard radiant electric heater at the same power.  Only a heat-pump can be more efficient than this, since it uses heat from a sink in addition to the heat generated by the power source.

[Moleculiar]

As for microwaves they just seemed to heat things up so much faster which is why I thought it might be feasible. As the heat exchangers could be made of or contain the best heat transferring products.

Any microwave device consuming X power can heat something no faster than a standard radiant electric heater at the same power.  Only a heat-pump can be more efficient than this, since it uses heat from a sink in addition to the heat generated by the power source.

Being that the region of energy exchange between radiated energy & the material substance of a target food item is many times greater for a microwave oven when compared to resistive heating elements, and that for a microwave oven, such a region could allow a reduced efficiency of exchange and yet a greater total source of conducted heat. And also, that the greater exchange region of a microwave oven also presents a much reduced distance of food material to traverse via secondary heating due to conduction - it may very well be possible that a microwave oven is 'faster'?

[Halc]

Any microwave device consuming X power can heat something no faster than a standard radiant electric heater at the same power.  Only a heat-pump can be more efficient than this, since it uses heat from a sink in addition to the heat generated by the power source.

Being that the region of energy exchange between radiated energy & the material substance of a target food item is many times greater for a microwave oven when compared to resistive heating elements, and that for a microwave oven, such a region could allow a reduced efficiency of exchange and yet a greater total source of conducted heat.

For food, yes, but for house heating, there is 100% efficiency in either case.  No energy goes to waste heating something else, as opposed to gas or oil which sends heat up a chimney.
All these fancy technology heaters they push in the stores are still just plain electric heaters which are all 100% efficient.  A heat pump might be 130% efficient.  All depends on the sort of heat sink available to it, and the temperature differential between the output of the heating unit and that sink.

And also, that the greater exchange region of a microwave oven also presents a much reduced distance of food material to traverse via secondary heating due to conduction - it may very well be possible that a microwave oven is 'faster'?

Yes, a microwave heats food faster since it heats all of it, not just the outside surface.  The middle of the food is heated directly in part (although still not as fast as the edges), not requiring all heat to conduct from the surface into the interior.  This all seems irrelevant to heating water for a house-heating unit.  A resistive heater can be used directly, or indirectly with some sort of forced-air setup.  A microwave system would need water and then a heat exchanger to move that heat from the water to the house air.  All adds up to more steps and more costs.
I got free electricity one winter and managed to mostly heat my house with several $10 electric appliances put here and there, kind of maxing out my breaker box.  Was sad to see that end, I tell ya.

[Moleculiar]

Any microwave device consuming X power can heat something no faster than a standard radiant electric heater at the same power.  Only a heat-pump can be more efficient than this, since it uses heat from a sink in addition to the heat generated by the power source.

Being that the region of energy exchange between radiated energy & the material substance of a target food item is many times greater for a microwave oven when compared to resistive heating elements, and that for a microwave oven, such a region could allow a reduced efficiency of exchange and yet a greater total source of conducted heat.

For food, yes, but for house heating, there is 100% efficiency in either case.  No energy goes to waste heating something else, as opposed to gas or oil which sends heat up a chimney.
All these fancy technology heaters they push in the stores are still just plain electric heaters which are all 100% efficient.  A heat pump might be 130% efficient.  All depends on the sort of heat sink available to it, and the temperature differential between the output of the heating unit and that sink.

And also, that the greater exchange region of a microwave oven also presents a much reduced distance of food material to traverse via secondary heating due to conduction - it may very well be possible that a microwave oven is 'faster'?

Yes, a microwave heats food faster since it heats all of it, not just the outside surface.  The middle of the food is heated directly in part (although still not as fast as the edges), not requiring all heat to conduct from the surface into the interior.  This all seems irrelevant to heating water for a house-heating unit.  A resistive heater can be used directly, or indirectly with some sort of forced-air setup.  A microwave system would need water and then a heat exchanger to move that heat from the water to the house air.  All adds up to more steps and more costs.
I got free electricity one winter and managed to mostly heat my house with several $10 electric appliances put here and there, kind of maxing out my breaker box.  Was sad to see that end, I tell ya.

Irrelevant to the original question, yes  But I was addressing the speed of heating being questioned, and not the OP. I think that the rate of energy exchange is in itself worth looking into. The dynamics affecting that could be interesting

Even if the originally proposed system was designed to be safe for home use, it still can not avoid the facts that it involves the addition of microwave radiation leakage concerns, high-voltage required for operating the magnetron, additional points of mains power being brought into close vicinity of a water system, and all of that without removing the existing hazards of traditional hydronic heating systems

But, I wasn't interested in considering a practical application of the OP's idea, only in evaluating the idea by itself. Could it be made to work? I think it can. Should it be done? I don't care! I only want to see that it works, and that it works well. If any individual wants to try such a thing, such as the OP, then why not see what they can accomplish? It could be at the very least, very entertaining!

So what I would consider, is to use an existing microwave oven for a base design. It already has a correctly sized tuned cavity. Then I would add testing instrumentation to allow the realtime evaluation of just how well this cavity remains in good working order. The very first item to purchase for any instrumentation should be a well-made microwave leakage detector. But temperature & flow sensors, along with waveguide tuning equipment should also be looked into. I would drill holes in the walls of the cavity and braze into place steel tubing. The diameter of such tubing is a variable to consider from the perspective of waveguide design, so as to avoid generating secondary radiation on the inside of such proposed tubes. But it should be worth noting that the diameter of the holes already present in the walls of the cavity, such as the ones in the door, are the best size already chosen for safety. These holes are engineered to be as large as possible to allow viewing the inside of the cavity while remaining just small enough to block the radiation. So it might be a good idea to choose an ID of steel tubing which is that size or slightly smaller. These tubes then should be coated on their outer surface with a microwave absorbent material that will act as the energy conversion stage. I already know of some very practical research into choice of materials, for melting precious metals inside standard microwave ovens. This prototype design will be much like a water-tube boiler. And as such, the rest of the design should be already established. A set of manifolds will be attached at either side of the cavity, perhaps as a simple box enclosure brazed directly to the outside of the existing cavity. The flow rate of water traveling through this boiler will need to be controlled, maybe using a pump with higher flow rate in combination with  restricting orifices & a throttling bypass valve?