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Natural gas space heaters

Discussion in 'Home Power and Microgeneration' started by [email protected], Dec 18, 2007.

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  1. Guest

    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
     
  2. Not an ideal location... I wonder if the bulb could be relocated a
    bit, and perhaps some insulation could be added on two or three sides of
    the bulb to prevent cold drafts from the window above and perhaps allow
    more effect from the radiant transfer of the warm room to the bulb.
    Might could even put insulation on up to 5 sides of the sensor.
    Hmm, I prefer a passive solution if possible. And if that second
    thermostat is plugged into an outlet on the exterior wall, it is likely
    to have the same problems as the thermostat built into the heater.
    The 93% and 81.5% already account for the 11% loss in non-condensing.
    It is erroneous to subtract it again. Never the less, the non-vented
    heater is nearly 100% efficient, and the price looks very good in
    comparison. :)
    Absolutely.

    sdb
     
  3. Guest

    I finally heard from the manufacturer, who tested all of them with gas
    with a heating value of 1035 vs 930 Btu/ft^3, ie the HHV.

    Nick
     
  4. Jim Elbrecht

    Jim Elbrecht Guest

    [reading this on alt.home.repair]

    In how big an open area? I like my unvented gas stove & have never
    had a bit of odor, nor has it ever registered more than the slightest
    amount of CO on my detector----- But I remember when I bought it
    10[?] years ago the main concern was whether or not it was too big.

    My house is pretty open downstairs & the stove is in a 12x20 room with
    an outside door and 5 windows. We [my gas supplier & myself]
    decided that 30K would be doable.

    Mine is just used as supplemental [and on the odd occasion that my
    furnace goes out or we lose power. But it has never fogged windows.
    -snip-
    I'd ask him/her to leave it alone until it can even itself out.

    -snip-
    That could be poorly insulated windows, high humidity in the house- or
    excess humidity from the gas. [LP adds water to the air- I think
    natural gas does, too]

    -snip a whole lot of ASHRAE gobbledygook-gook-
    Call your local fire department and ask them to invite you clean out
    the next house they get called to remove a body that succumbed to CO
    poisoning because some gadget failed.

    -snip-
    It [or a similar alarm] 'goes well' with any combustion device in your
    house. With a ventless stove you're a damn fool for not plugging
    it in before you cranked up the stove. Especially one as big as
    you're running there. spend a little of the time you've invested
    poring over ASHRAE charts and read what your local building department
    suggests in regards to those stoves.

    Jim
     
  5. Vent free heaters produce COPIOUS amounts of water vapour - a much
    more serious issue in most cases thanoxygen depletement or CO
    concentration.
     
  6. You

    You Guest

    That would depend on the Relative Humidity of the area you live in....
    if your RH is very low, you may just want a bit of humidity enhancement
    in your house....
     
  7. Guest

    Exactly 2.2 pounds per hour, with 20K Btu/h of heat.
    No, but removing it does... 37 cfm of fresh air will keep the indoor RH at
    50% at 70F on an average 31.8 F December day in Allentown, PA. In a perfectly
    airtight house (vs an average US house that naturally leaks 224 cfm), we
    might do that with a humidistat and a muffin fan, or open a 2'wide x 3'tall
    window with an A ft^2 gap at the top and bottom with A = cfm/(16.6sqrt(HdT))
    = 37/(16.6sqrt(3'(70-31.8)) = 0.21 ft^2, ie a 2'wide x 1.25" gap at the top
    and bottom. In a non-airtight house with a window 24' below a leaky upstairs,
    the gap might be 37/(16.6sqrt(24'(70-31.8)) = 0.074 ft^2, ie a single 0.44"
    max gap at the bottom of a first-floor window.

    Nick
     
  8. Except my humidifier (on a humidistat) runs nearly all winter long
    anyway. So added humidity from unvented gas heater would not likely be
    an issue. I'm thinking in climates where heating is needed (not just to
    take the chill off, e.g. southern C.A.) lack of humidity is frequently
    more of a problem than too much humidity.

    sdb
     
  9. Guest

    It would a help...
    An average 2400 ft^2 US house leaks naturally leaks 0.7 ACH, ie 224 cfm,
    much more than ASHRAE's standard of 15 cfm per full-time occupant (which
    used to be 5 cfm.) This makes house air very dry in wintertime and makes
    the system efficiency of a vent-free heater higher than any vented heater.

    A good (IDEAS) Canadian house only leaks 2.5 cfm. With people and green
    plants evaporating water inside, it needs DEhumidification (eg mechanical
    ventilation) in wintertime.

    Nick
     
  10. Guest

    That was sort of an accident. It might possibly change.
    Ah yes. I was just thinking about the sensible loss. Then again,
    we might build that Coroplast condensing air-air heat exchanger.
    Higher, I'd say, in this drafty house.
    The owner seems reluctant to add window films or fix the basement puddles...

    Nick
     
  11. Guest

    The original (19th C?) standard was based on productivity. Somebody noticed
    that coal mine workers with less than 5 cfm fell asleep :)

    The newer version is based on odors and other pollutants.

    Nick
     
  12. Guest

    Maybe we need 2 fans, with a Thermocube and a relay to turn off the outdoor
    fan if the outgoing airflow is less than 35 F. With equal capacity fans and
    outgoing condensation, freezing seems unlikely. One $9 sheet of Coroplast
    and a tube of 3M 4693 H adhesive could make a 2' wide x 6" high x 8" deep
    box that sits in a window with about 64 ft^2 of heat transfer surface and
    Cmin = 50 cfm and NTU = AU/Cmin = 64x1.5/50 = 1.9 and E = 1-e^-1.9 = 0.85.

    Nick
     
  13. Guest

    This one's pretty much already destroyed :)

    Lately I'm thinking a natural convection air-air heat exchanger might be nice,
    with more airflow than a sensible heat exchanger, since humidity adds bouyancy
    and the outgoing air would be warmer, with condensation.

    Nick
     
  14. Guest

    Lately I'm thinking a natural convection air-air heat exchanger might be nice,
    Something like this sensible calc, with poly film plates...

    20 H=4'hx height (feet)
    30 W=2'hx width (feet)
    35 NP=8'number of hx plates
    40 A=2*NP*H*W'hx area (ft^2)
    50 U=1.5'hx conductance (Btu/h-F-ft^2)
    60 CFM=37'hx airflow
    70 NTU=A*U/CFM'Number of heat Transfer Units
    80 E=NTU/(NTU+1)'hx effectiveness
    90 TCI=31.8'cold inlet temp (F)
    100 THI=65'hot inlet temp (F)
    110 TCO=TCI+E*(THI-TCI)'cold outlet temp (F)
    120 TCA=(TCI+TCO)/2'average cold temp (F)
    130 THO=THI-(TCO-TCI)'hot outlet temp (F)
    140 THA=(THI+THO)/2'average hot temp (F)
    150 DT=THA-TCA'average temp diff (F)
    160 AV=CFM/(16.6*SQR(H*DT))'vent area (ft^2)
    170 S=12*AV/((NP-1)*W)'min plate spacing (inches)
    180 DS=4/25.4'Coroplast spacer depth (inches)
    190 NS=INT(S/DS+.5)'# Coroplast spacers
    200 D=(2+(NP+1)*NS)*DS'hx depth (inches)
    220 PRINT E,D,S,NS
    230 PRINT THI,THO,TCI,TCO

    effectiveness depth (in) spacing (in) # spacers

    0.838428 4.566929 0.4124437 3

    Thi Tho Tci Tco

    65 37.16419 31.8 59.63581

    To avoid freezing, a $13 TC-3 Thermocube might turn on a small
    room air exhaust fan when the room air outlet temp drops to 35 F.

    Nick
     
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