Natural gas space heaters

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`C0.075(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
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snipped-for-privacy@ece.villanova.edu wrote:

I have never seen one of those style heaters that didn't have the same slightly acrid odor that is really annoying in a confined space such as a house. Is this one any different?
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On 18 Dec 2007 08:46:13 -0500, snipped-for-privacy@ece.villanova.edu

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
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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
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[reading this on alt.home.repair]
On 18 Dec 2007 08:46:13 -0500, snipped-for-privacy@ece.villanova.edu wrote:

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
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On 18 Dec 2007 08:46:13 -0500, snipped-for-privacy@ece.villanova.edu wrote:

Vent free heaters produce COPIOUS amounts of water vapour - a much more serious issue in most cases thanoxygen depletement or CO concentration.
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clare at snyder.on.ca wrote:

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....
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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
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Winter time in northern states of USA, humidity is rather low. We northern folks often use a humidifier to add water to the air. so, a vent free heater adding humidity is good.
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On Mar 18, 8:29pm, "Stormin Mormon"

Yea adding more pollutants you dont need, smart, unless your house leaks like a sieve vent free are not a healthy choise. I believe many areas in Canada they are banned.
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On Tue, 18 Dec 2007 22:06:57 -0500, clare at snyder.on.ca <clare> wrote:

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
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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
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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
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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
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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 CFM7'hx airflow 70 NTU=A*U/CFM'Number of heat Transfer Units 80 E=NTU/(NTU+1)'hx effectiveness 90 TCI1.8'cold inlet temp (F) 100 THIe'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 AVM/(16.6*SQR(H*DT))'vent area (ft^2) 170 S*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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You could use a small "computer" fan to circulate room air to the thermostat better. Or, add insulation behind the heater.
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is an RH of 74.5%), then losses are more. At 74.5% RH and 31.8F, the energy in air is 10.6 Btu/lbm for a loss of 37*60*.075*(25.5-10.6)$81 Btu/h and efficiency of 87.6%. Of course with lower outside RH, you won't need quite so much ventilation, but it looks like your 37 cfm is based on NREL's 74.5% number, so maybe you will still need 37 cfm.
So with ventilation to control the humidity, you'll get efficiencies comparable to a conventional gas furnace. It'll be nice and humid in the house, but with single pane windows you'll have a lot of condensate and mold growth on the frames.
daestrom P.S. 2.2 lbm/hr of water vapor at 70F is about 31.8 cfm of H2O being added to the space.
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Dunno.

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
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wrote:

As long as you stay above freezing to avoid frost I suppose...

Oh well, his loss(es)... (pun intended)
daestrom
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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
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