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small condensing heaters
30 oct 1996
jim ramnes   wrote:
>nick pine wrote:
>> surely someone makes a small externally vented, condensing gas heater...
>would this be as opposed to a small *internally* vented, condensing gas 

sure. a gas or propane or kerosene space or water heater which condenses the
combustion water vapor out of the combustion products, at 1000 btu/lb, like a
swedish woodstove with the chimney joints lapped downwards, and keeps that
heat inside the house, with a small container or drain for the water, like a 
dehumidifier, and puts the cooler fumes outside the house, at a temp less
than 212 f. perhaps it doesn't make much difference where the input air for
combustion comes from, for a small heater. using house air seems easy and
perhaps safer, since there is less danger of input/output air mixing and
possible subsequent leakage of fumes into the house. that might also help
dehumidify an airtight house in the winter.

suppose we had a 10k btu/hr radiant kerosene heater with a cylindrical
parabolic reflector, with a thin layer of ir-transparent teflon or tedlar
film enclosing the face of the parabola, and 80% (?) of the heat emerged as
ir through the film, leaving 2k btu/hr of heat in combustion products. how
much chimney area inside the house would it take to cool those products to
212 f, with room air moving at 10 mph past the outside of the chimney? if
the chimney surface were rough on the outside, it might have a thermal
conductivity of 2 + 10/2 = 7 btu/hr-f-ft^2, so (212-68)ax7 = 2k btu/hr, so
a = 2 ft^2? a 6'long, 1.5" diameter flue pipe inside a 3" diameter pipe,
with a 50 cfm blower and thermostat near the top?

that might save about 20% of the heat, vs putting the water vapor and hotter
combustion products out the chimney. i'd like to have the version built into
a trash can, with a swinging lid and propane auto-ignition, to burn junk mail.


william shurcliff, phd, author of _new inventions in low-cost solar heating:
100 daring schemes tried and untried_ lives at 19 appleton street in
cambridge, ma 02138. mail addressed to 19b appleton street ends up in
his woodstove.

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