Rocket mass heater as waterbed pedestal/heater?

I'm still in the planning/searching for the right piece of property stage of building my Ultimate Off-grid Homestead and I've run in to a problem with how I'm going to heat my waterbed. Since I refuse to sleep on anything else, it's a big problem.

I can't run a regular AC powered heater because it will burn up my power inverter. If anybody makes a DC powered one, I haven't been able to find it. Then I hit upon the idea of using a rocket mass heater after learning about them here.

Since I have zero experience with one of these contraptions I have a couple questions. The first is just how hot does the cob surface get? I don't want to roast myself out of bed. The second is will the cob structure support the weight? Close to a ton spread out over about 40 sq/ft.

Anyway, there's my problem and my idea for fixing it. Any help or insight would be appreciated.

And one more question that's a little off-topic: Why does everyone use cheap galvanized duct work on these things? There's a lot of moisture flowing through there and galvanizing only slows down the corrosion process. It's seems to me that cast iron drain pipe would be a better choice. It may cost more but, I guarantee it will last a lifetime...or two and the pipe itself could serve as part of the thermal mass.

If you were to build a mass to put your waterbed mattress upon, it should not be cob. Simply not strong enough. Water weighs 8 pounds per gallon, and I'd say your waterbed is something near 100 gallons. Also, the surface of the mass can easily get over 150 degrees for an extended period of time, which might compromise the tensile capacity of the plastic liner of your mattress. However, if you were to build a strong enough pedastal mass with a layer of concrete construction grade (closed cell) foam board between the mass and your mattress, the latent heat in the mass would surround your bed without directly heating it.

Steve M. : There are two Different types of Cob, Insulating Cob, and Structural Cob, we can work around the first, and Structural Cob can be used to
make multi-story buildings. A Structural Cob thermal bench, even using light weight cheap ductwork, can easily take the weight of your waterbed
though you will certainly need to plan on a slab floor under that weight combination. For insulation between the Cob and your slab floor you can use
several inches of recycled,rough cut wood not even worrying if its level, or even nailed together, as the cob bench will allow you to sculpt your bench
to meet your needs .

The water can take away the heat faster then the thermal mass can give it up, you will need to regulate the temperature by the amount of covers on
the bed, pretty much the way you do it now. Failing to keep a fire going (more later) and no covers on the bed will give you a warm room and a cold
bed.

Generally we build Rocket Stoves on the cheap. Light weight ductwork is in the Thermal Cob just to act as a form, as soon as the Cob dries its only
other job is to allow for sealed inspection/clean outs though many go a couple of years w/out vacuuming out the Thermal Mass. Your Cast Iron Drain
Pipe will need to be at least 6'' I.D. if not 8'' I.D. depending on how much of your total house you plan on heating.

Did you see that there is also a section here at Permies dedicated to all Cob subjects ? There is lots of good information there !

A mild warning/disclaimer, The Dragon that lives in your Rocket Stove likes frequent feedings of small, very dry wood, as often as 2-3 times an hr
steadily over an 8-10 hr period to give you 20-25 hrs of stored heat to keep your house warm during cold weather. Any one who deviates from this
routine will come home to a house cold as would be in proportion to the lapse in attending to his House Dragon ! Be forewarned !

Please go to rocketstoves.com to download your PDF copy $15.oo U.S. of Ianto Evans' great book 'Rocket Mass Heaters', you could also go to
ernieanderica.info for a quick view of whats out there for D.I.Y. plans for building Rocket Stove Mass Heaters !

There are two new companies out there selling Kits to help you build your Rocket Stove parts - though I would not recommend buying the first one
of any new product.

You might want to check with your local sustainable living or permaculture groups to see if there are going to be any Rocket Stove Building, or Cob
building workshops in your area. both Kansas and Nebraska have a common history of Sod homes so generally speaking cob construction will work
in your area.

Be Safe, keep Warm, PYROmagically - Big Al