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Posted by on December 18, 2007, 8:46 am
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
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Posted by George on December 18, 2007, 8:56 am
nicksanspam@ece.villanova.edu wrote:
> 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.
>
>
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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Posted by sylvan butler on December 18, 2007, 6:51 pm
On 18 Dec 2007 08:46:13 -0500, nicksanspam@ece.villanova.edu
> the temp sensing bulb is on the back near a cold floor under a cold single-
> pane window.
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.
> 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.
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.
> 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.
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. :)
> 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.
Absolutely.
sdb
--
What's seen on your screen? http://PcScreenWatch.com
sdbuse1 on mailhost bigfoot.com
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Posted by on December 18, 2007, 7:20 pm
>> 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.
>
>The 93% and 81.5% already account for the 11% loss in non-condensing.
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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Posted by Jim Elbrecht on December 18, 2007, 7:29 pm
[reading this on alt.home.repair]
On 18 Dec 2007 08:46:13 -0500, nicksanspam@ece.villanova.edu wrote:
>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).
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-
>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,
I'd ask him/her to leave it alone until it can even itself out.
-snip-
> And it makes condensation
>on the indoor window surfaces.
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-
>
>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.
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-
>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.
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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