I just installed a 30K Btu/h natural gas vent-free radiant heater in
a house near Allentown, PA, where kerosine now costs $3.50/gallon
and natural gas costs about $1.50/therm (roughly equivalent).

Empire Comfort Systems (Enerco) makes this "Mr. Heater." Tractor Supply
sells it for $229.99 as sku #2151954. It comes with a thermostat and
a blower and a digital temperature display. It has an oxygen depletion
sensor, and it can work without grid power, but there have been some
problems since installation.

The thermostat only has 5 temp settings, as well as "pilot only." It is
supposed to make the room about 55 F min, with 5 F steps above that, but
the temp sensing bulb is on the back near a cold floor under a cold single-
pane window. When the first setting heats most of the room to more than
70 F, the owner turns the thermostat back to pilot before the heater
turns itself off, while the temperature display still only reads 58 F, ie
the thermostat isn't doing much. The owner says with the knob between
the lowest and pilot settings, the heater emits interesting flaming blue
gas footballs instead of the usual red glow. And it makes condensation
on the indoor window surfaces.

We might fix the first 2 problems by putting a 25 watt light bulb near
the temp sensor with a $15 line-voltage thermostat on the wall that
turns the bulb off when the room is warm enough.

This old house seemed drafty enough to avoid window condensation, but
it also has a damp basement, with puddles of water after rain. Keeping
water out of the basement might help a lot. Indoor storm window shrink
films could also help. Allentown is 31.8 F on an average December day.
An R1 window with a 1.5 Btu/h-F-ft^2 still airfilm indoor conductance to
70 F room air and (70-31.8)1ft^2/R1 = 38.2 Btu/h-ft^2 of heatflow would
have a glazing temp (dew point) of 70-38.2x1ft^2/1.5 = 44.5 F (504.5 R),
with indoor RH = 100e^-(9621(530-504.5)/(504.5x530) = 40% at 70 F (530 R).
Basement puddles at 55 F could condense on window surfaces.

If indoor film makes the windows R2 and raises the film temp to 70-19.1/1.5
59.4 F (519.4 R), the a max indoor RH = 100e^-(9621(530-519.4)/(519.4x530)
= 69%, with no condensation from basement puddles.

NREL says Allentown has an average humidity ratio wo = 0.0028 pounds of water
per pound of dry air in December, with Pa = 29.921/(1+0.62198/0.0028)
= 0.1341 "Hg. Air at 70 F and 100% RH has Psat = e^(17.863-9621/(460+70))
= 0.748 "Hg, approximately, so merely heating the outdoor air to 70 F
with no basement puddles would make the indoor RH = 100Pa/Psat = 18%.

The ASHRAE HOF says pure methane (vs a different natural gas mixture) has
a high heating value (HHV) of 23,875 Btu/lb, when we condense the water vapor
from combustion and a low heating value (LHV) of 21,495 (11% less) when we
don't, and 1000 Btu can evaporate a pound of water, so a vent-free heater
that makes 20K Btu/h also makes 0.11x20K/1000 = 2.2 lb/h of water vapor.
With window films and no basement puddles, we could keep a 50% indoor RH
(wi= 0.016) by moving in C cfm of fresh air (at 0.075 lb/ft^3), where
2.2=60C0.075(wi-wo), so C = 37 cfm, with a heat loss of about 37(70-31.8)
1420 Btu/h, which lowers the heater system efficiency to 93%, compared to
an HHV- 100%.

We might move outdoor air into the room with a $30 humidistat and a muffin
fan, or (more efficiently) use a homebrew HRV with condensation outside
and fresh air inside Coroplast (plastic corrugated sign material) plates,
or run a dehumidifier or an $80 low-airspeed window AC inside the room.

If the $1275 DV-20E 81.5%-efficient direct vent and $3268 93%-efficient
Mantis condensing gas heaters are measured with LHV-based efficiencies
and we subtract 11% to compare apples to apples, the $229 vent-free
heater is more efficient, as well as a lot cheaper.

Kiddie's 900-0113 plug-in CO and explosive gas detector with battery backup
($48 from Amazon, with free shipping) would go well with this.

Nick