If you ever get one make sure it's a clean burner. Either multi-stage or catalytic. I hate it when people just chuck in wood, walk away and smoke crawls out for hours. That's the reason why politicians are banning wood burning in a lot of places and put us at the mercy of some racket or monopoly. The majority of wood stove owners in our neck of the woods simply does not know how to operate them properly.
If those work like the resonant transformer they used in former East Germany (to make their TVs work on a lousy power grid) then they would be energy hogs.
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I have an outdoor wood furnace (it came with the house) and would not advise anyone to buy one. Their one advantage is that the ashes are outside. Disadvantages; You are heating the outside air and ground where the water lines are buried, You need to keep this thing running all winter, (or the water will freeze), When something breaks you are stuck doing plumbing outside in the middle of a blizzard....
George Herold
Thanks for the information, George.
I thought they used anti-freeze to avoid this problem, at least until it gets really, REALLY cold?
---Joel
Usually not. Given the typical volume of water (hundreds to thousands of gallons), that would first be very expensive, and after a few years, (as it breaks down with age and needs to be dumped) a large disposal problem. On the third hand, 99% of "outside wood burners" are designed exactly wrong, and rather than producing less pollution than a woodstove, produce much more - to the point where Vermont instituted fairly harsh restrictions on them because of air quality impacts (filling valleys with smoke).
If you prefer to have a wood boiler out in a shed, fine. Buy a wood boiler that could be installed in the house (it will burn clean, particularly if it's a "gasifier"), and build a nice insulated shed for it. Insulate the lines to it, and avoid self-defeat by insulating the lines from each other, too. Several otherwise clever folk have built unintentional heat exchangers and keep getting cool water out the far end, even though they pump out hot water and get warm water back ;-)
If all is insulated properly, you should be able to set a circulator control so your alternate heat source will keep the wood boiler and its shed from freezing (say, when you go on vacation) without wasting too much oil, gas, or whatever else you heat with.
Proper setup of a wood boiler should also have a large heat storage tank, but that should be inside the heated space, ideally - the idea being that the storage will absorb all the heat of a full load of wood burning full blast (thus cleanly), and then the stored heat can be used to heat the house, domestic hot water, etc.
That's because most people do not know how to burn wood properly. They just chuck in a few logs, close the door and walk back to watch Jay Leno or whatever. The worst are the boilers that burn (or rather, smolder) a
4' long round like a cigar, belching smoke all day long.
Full blast isn't always the most clean and efficient method, as evidenced by the amount of soot in pellet stove vents. We achieve the cleanest burn when there is a blueish-white gas cloud above the wood. However, this requires closing primary air and proper adjustment of the secondary air so it matches the properties of the wood type. For example, while you can burn almond with the throttle pushed far in (like on really cold days) this gets plain ugly with some kinds of pine.
An added benefit: While I have to clean the pellet vent every year I take a peek into the wood stove vent. Clean like a whistle, year after year. Heck, I don't even take the brush extender for that up the roof anymore because I now know it'll be clean (but of course I take a look anyhow).
Naw. All the better ones use PWM to induce sinusoidal currents. With all the added circuitry that entails. Especially VFDs.
The added circuitry actually isn't all that much. You need a half- or full-bridge converter before the communtation bridge anyhow. All you have to do is modulate the pre-converter with a sine. Either from a table or, if the last penny counts, from an oscillator. Of course, to do a thorough job you wouldn't be able to play the usual trick to add the output onto the incoming DC-voltage in order to save a few pennies in copper.
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Most significant to serious VFDs are 3-phase input and output. Some of the single phase assumptions don't work for them.
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Find yourself a vfd on ebay of a suitable rating. Remove the control board, that will leave you with all the power electronics you need. All you would need to build is a board to supply a pwm signal to 4 of the driver opto couplers.
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Yeah, it'll be a little more effort. But the OP's case does not sound like more than one phase.
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