Clean Windows = Warmer House?

[pen]

Does radiant heat actually pass through the glass? I would guess no, it doesn't pass through as electromagnetic waves. Visible light passes through but doesn't have very much energy compared to longer wavelengths. My first thought about this subject was that the ash on the glass blocks radiant heat transfer through the glass, but now I am not so sure. I think the way heat transfer through the glass works is the fire radiates heat to the glass, the glass absorbs the heat and gets hot, then the glass re-radiates heat into the room. Maybe the ash layer makes the glass reflect more of the radiant heat rather than absorb it, so the glass stays cooler and thus radiates less heat into the room.

Radiant heat is photonic, radio is electromagnetic. Stoves ain't radios. Not all photons are moving fast enough to be visible.

visible light and IR are electromagnetic as well.

pen

 

[Milton Findley]

Does radiant heat actually pass through the glass? I would guess no, it doesn't pass through as electromagnetic waves. Visible light passes through but doesn't have very much energy compared to longer wavelengths. My first thought about this subject was that the ash on the glass blocks radiant heat transfer through the glass, but now I am not so sure. I think the way heat transfer through the glass works is the fire radiates heat to the glass, the glass absorbs the heat and gets hot, then the glass re-radiates heat into the room. Maybe the ash layer makes the glass reflect more of the radiant heat rather than absorb it, so the glass stays cooler and thus radiates less heat into the room.

Radiant heat is photonic, radio is electromagnetic. Stoves ain't radios. Not all photons are moving fast enough to be visible.

visible light and IR are electromagnetic as well.

pen

True enough, but then pine trees and broccoli would both be vegetables given their properties wouldn't they?

 

[pen]

True enough, but then pine trees and broccoli would both be vegetables given their properties wouldn't they?

Just making the point as it sounded like you were trying to seperate them from radio waves.

Also (nitpicking) There is no scientific classification called vegetable but they are both plants.

pen

 

[DiscoInferno]

Googling 'pyroceram' it does seem clear that it is intended to block IR to some extent. I didn't find anything quantitative but I'm sure it's out there. Nonetheless, it clearly does not block it all; when I start a hot kindling fire in a cold stove and first close the door, I can feel plenty of heat through the glass even though it is still cold to the touch. Now, does a thin coating of ash have a measurable effect on transmittance at IR? I can't say with scientific certainty, it seems so but could easily be "all in my head", as it were.

 

[Milton Findley]

True enough, but then pine trees and broccoli would both be vegetables given their properties wouldn't they?

Just making the point as it sounded like you were trying to seperate them from radio waves.

Also (nitpicking) There is no scientific classification called vegetable but they are both plants.

pen

Thermal radiation is most assuredly different from radio waves. They affect the environment much differently, have different physical properties, and are not in any way whatsoever equivalent enitities. Which would you use as an analog to a wood stove, an incandescent light bulb or a ham radio transmitter? Just because it happens within the realm of the electromagnetic spectrum, which extends from here until the farthest reaches of the universe by definition, does not mean that what happens at one frequency is in any way analogous to what happens at another frequency. Hence, photonic, not electromagnetic.

Nit picking is only required when there are lice present.

 

[CTYank]

Wood stoves have ceramic glass

Radiant heat passes through ceramic glass. You are correct in your observation

http://www.vetrotech.com/us/download/Heat transfer and fire.pdf

A good article that speaks directly about wood stove glass. Copy and past the URL. The forum removes the percent symbols that should be in the gaps in the url

IR transmittivity of stove window glass probably varies, but for my stove the glass seems pretty opaque to IR.

Testing ...
Moderate fire going, cruising at ~650 F stove top.
Hand held ~1' from center of window, door closed. Note IR heating sensation.
Open door, hold hand in same position as above (or try to.) Note IR heating sensation, and that it's enormously more intense than w/door closed.
Conclusion: window glass greatly reduces IR intensity outside window, largely absorbing or reflecting it.
You can see the same phenomenon with IR spot thermometer.

Read what you want, trust what you want, but verify.

 

[CTYank]

True enough, but then pine trees and broccoli would both be vegetables given their properties wouldn't they?

Just making the point as it sounded like you were trying to seperate them from radio waves.

Also (nitpicking) There is no scientific classification called vegetable but they are both plants.

pen

Thermal radiation is most assuredly different from radio waves. They affect the environment much differently, have different physical properties, and are not in any way whatsoever equivalent enitities. Which would you use as an analog to a wood stove, an incandescent light bulb or a ham radio transmitter? Just because it happens within the realm of the electromagnetic spectrum, which extends from here until the farthest reaches of the universe by definition, does not mean that what happens at one frequency is in any way analogous to what happens at another frequency. Hence, photonic, not electromagnetic.

Nit picking is only required when there are lice present.

Ye'd better check with a physicist, or do a little research on the electromagnetic spectrum. Then your statement above would read "most assuredly SAME AS radio waves" in that they're both electromagnetic radiation, of different frequencies. If you have red hair, can I say that you're of a totally different species from me? No, that too would be nonsensical. How do you deal with 5-10 GHz microwaves (and their heating effects)?

Ever hear of the "duality principle"? Wherein certain entities have properties corresponding to matter and properties corresponding to radiated waves? That'll get you over your "hump."

 

[CTYank]

Does radiant heat actually pass through the glass? I would guess no, it doesn't pass through as electromagnetic waves. Visible light passes through but doesn't have very much energy compared to longer wavelengths. My first thought about this subject was that the ash on the glass blocks radiant heat transfer through the glass, but now I am not so sure. I think the way heat transfer through the glass works is the fire radiates heat to the glass, the glass absorbs the heat and gets hot, then the glass re-radiates heat into the room. Maybe the ash layer makes the glass reflect more of the radiant heat rather than absorb it, so the glass stays cooler and thus radiates less heat into the room.

Radiant heat is photonic, radio is electromagnetic. Stoves ain't radios. Not all photons are moving fast enough to be visible.

Huh? Don't be silly. They both originate in phlogiston.

 

[pen]

Huh? Don't be silly. They both originate in phlogiston.

ha! Long time since I've heard that term.

Sorry, but to me photonic is just a discriptor for the visible spectrum and "stuff" near it and it's study. It's like saying German Sheppard. But they are still all dogs.

pen

 

[DonNC]

http://www.youtube.com/watch?v=JSY8BzXYpoY&feature=related

In that video you can watch radiant heat set a room on fire

 

[DiscoInferno]

I agree that stove glass knocks down IR, maybe by a factor of 5 to 10 (or whatever). But whatever percent that makes it through clean glass is the baseline in question; I was interested in the comparison between clean and ash-covered glass. Perhaps the next time mine ashes up I'll only clean half...

 

[henkmeuzelaar]

I agree that stove glass knocks down IR, maybe by a factor of 5 to 10 (or whatever). But whatever percent that makes it through clean glass is the baseline in question; I was interested in the comparison between clean and ash-covered glass. Perhaps the next time mine ashes up I'll only clean half...

Well you sure opened an interesting can of worms here!! Moreover, I think it was indeed a good idea to repeat the question since, as far as I was able to figure out, no one appears to have given you a completely straight answer so far because of the many different technical issues that intermingle here.

I tend to agree with you that the simplest answer is that you felt it clearly yourself. Because of the possible psychological/neurological components involved (as pointed out by several responders) you might perhaps consider making a suitable dummy by filling a plastic or rubber waterbag with room temperature water (most of our bodymass is water, after all), dress it up in some of your clothes, stick a thermometer "anywhere the sun don't shine" (but preferably in the center of the waterbag) and hang the dummy in the same spot you were sitting when you noticed the difference. Make sure to do that at a well-defined point in the combustion cycle (e.g. just at the point where you normally close the bypass flow) and that you are ready to read the thermometer every 30 seconds or so till the dummy's temperature stabilizes.

Now do the experiment you were describing (i.e. just clean half) or better, simply repeat the full cleaning experiment you already conducted earlier (while taking yourself out of the equation, so to say).

Why should such a crude Ghostbusters approach yield any better data than already obtained by the people using fancy radiant thermometers?
The short answer is that these thermometers are not made to measure hot window temperatures since the amount of thermal radiation emitted by any body is a function of temperature AND emissivity. A two-wavelength radiation thermometer is needed to get an accurate estimate of temperature (and emissivity). Manufacturers of home-quality radiant thermometers, either preset the emissivity correction at a so-called "gray body" level or allow one to manually choose an emissivity setting before doing the measurement. The problem with (semi)transparent windows is that their emissivities tend to be extremely low and often a function of the temperature (thus creating a circular problem).

However, the dummy I just described is essentially a so-called calorimeter, capable of integrating the total amount of heat received over a fairly long amount of time.

Henk

PS: In a separate post I plan to discuss some thermal window transparency issues that further plague any attempts at simple temperature or radiation measurements but may help make a functional distinction between fireplaces and wood stoves (one of my pet topics ).

 

[woodgeek]

Dude, give me a break. If you think the radiant energy from the fire goes through the window, then the expt is to compare radiant heat with the doors open and closed! And I think we all know the radiant flux we feel is WAY higher with the doors open than closed. The radiant heat you feel sitting in front of the window is b/c the window itself is hot, so it radiates too. To a first approximation, if you put steel plate in place of your windows, you would feel pretty much the same thing.

Along the same lines, point an IR therm at the doors when closed, and then at the fire with the doors open. Then point the IR through the hot, open window at a cold wall. Does it now read the temp of the wall, or the same hot window temp? The huge diff in readings will suggest that the radiation the IR thermo measures sure doesn't get through. Of course, the stove puts out energy at different wavelengths that the IR therm reads as well, but basically, the window blocks radiant energy, and is engineered to do that.

Absolutely nothing that you have suggested is logical or proper in determining if the glass is allowing radiation to pass.

It is absolutely not possible for any glass to pass 100% radiation. If glass passed 100% radiation then it wouldn't ever heat up from radiation hitting it. If you have a stove with an air-wash to keep hot convective currents off the glass then this means your glass will never get hot.But guess what, even with the air wash the glass gets hot. So we have now established that the glass at least partially absorbs radiant heat. However, with suggestion #1 above you illogical concluded that bc your glass absorbs some radiant heat it must absorb it all.

Did it occur to you that maybe it is only absorbing some radiant heat? When I shoot the temp of my glass it normally shows around 1000 degrees. Do you think my glass is really that hot or do you think maybe there is also some radiation passing through and throwing off the temp? I think the temp and/or radiant feeling you get is a combination of both.

next you illogically concluded that because your IR thermo says the fire is hotter than the doors that they aren't passing radiation is along the same lines. It doesn't at all prove that no radiation is coming through the doors, it just proves that not all of it is coming through.

Lastly we come to the final very, very wrong conclusion that when the IR thermo is pointed through the door at a cold wall the radiation doesn't pass if the temp doesn't read similar to the cold wall. If you think that will tell you anything then you clealry don't understand radiation. The IR thermo is basically just "adding up" all the radiation it is seeing. The hot door is still radiating heat itself, the radiation of the wall is so small compared to the door that you will, in all reality, just be "seeing" the door.

A better comparison to determine if you're seeing any radiation through the glass is to point the IR thermo at the closed, hot door with a good, hot fire going, note the reading. Then open the door and point the IR thermo through the door at a cool wall. If the second reading is any lower than the first then you are, in fact, seeing radiation through the glass.

No need to let me know your results, I already know the answer.

BTW, to the OP, yes, I also notice an increase in radiant heat after cleaning the glass.

I guess our windows are different--I get almost the same temp off my windows as I do of the door frames around the windows, and the same temp looking 'through' the window at hot coals and a cold wall. IOW, no evidence that my IR thermo can see anything on the other side of the window, nor that it is giving in inaccurate temp on the ceramic glass.

I'm glad I phrased my response as an experiment to be done...seeing that folks can get different outcomes.

Of course some radiation gets 'through' the window--it depends on the coating formulation, and yes, many IR thermos use different wavelength bands--the difference in our measurements might be b/c your thermo sees the transmitted band, whereas mine sees a blocked band. My point with the open/closed door is that the radiant intensity on my face (a broad band IR sensor) seems >5x higher with the door open than closed. Given that that 20% could be re-radiation, it is hard to believe much is getting through.

Other fun fact--the IR coating on the glass is conducting, and you can sense it with an ohmmeter. Its supposed to be on the inside surface.

I will go somewhere now and continue to not understand radiation.... :red:

 

[henkmeuzelaar]

Ah, I did it again! double-posted after correcting and don't know how to completely remove the double post, so just removed the text. For the real message, see next post..... Henk

 

[henkmeuzelaar]

I agree that stove glass knocks down IR, maybe by a factor of 5 to 10 (or whatever). But whatever percent that makes it through clean glass is the baseline in question; I was interested in the comparison between clean and ash-covered glass. Perhaps the next time mine ashes up I'll only clean half...

Well you sure opened an interesting can of worms here!! Moreover, I think it was indeed a good idea to repeat the question since, as far as I was able to figure out, no one appears to have given you a completely straight answer so far because of the many different technical issues that intermingle here.

I tend to agree with you that the simplest answer is that you felt it clearly yourself. Because of the possible psychological/neurological components involved (as pointed out by several responders) you might perhaps consider making a suitable dummy by filling a plastic or rubber waterbag with room temperature water (most of our bodymass is water, after all), dress it up in some of your clothes, stick a thermometer "anywhere the sun don't shine" (but preferably in the center of the waterbag) and hang the dummy in the same spot you were sitting when you noticed the difference. Make sure to do that at a well-defined point in the combustion cycle (e.g. just at the point where you normally close the bypass flow) and that you are ready to read the thermometer every 30 seconds or so till the dummy's temperature stabilizes.

Now do the experiment you were describing (i.e. just clean half) or better, simply repeat the full cleaning experiment you already conducted earlier (while taking yourself out of the equation, so to say).

Why should such a crude Mythbusters approach yield any better data than already obtained by the people using fancy radiant thermometers?
The short answer is that these thermometers are not made to measure hot window temperatures since the amount of thermal radiation emitted by any body is a function of temperature AND emissivity. A two-wavelength radiation thermometer is needed to get an objective, fully automatic estimate of temperature (and emissivity). Manufacturers of home-quality radiant thermometers, either preset the emissivity correction at a so-called "gray body" level or allow one to manually choose an emissivity setting before doing the measurement. The BIG problem with (semi)transparent windows is, however, that their emissivities tend to be extremely low and often a function of the temperature itself (thus creating a circular estimation problem).

However, the dummy I just described is essentially a so-called calorimeter, capable of integrating the total amount of heat received over a fairly long amount of time.

Henk

PS: In a separate post I plan to discuss some thermal window transparency issues that further plague any attempts at simple temperature or radiation measurements but may help make a functional distinction between fireplaces and wood stoves (one of my pet topics ).

 

[Battenkiller]

Radiant heat is photonic, radio is electromagnetic. Stoves ain't radios. Not all photons are moving fast enough to be visible.

Milt, you seem to have the technical background I lack, but I have to say that I do know for sure that all photons move as the same speed, and all electromagnetic waves move at the same speed. What varies throughout the EM spectrum is the frequency and wavelength. Higher frequencies = short wavelengths = more energetic photons, but they all travel at the same speed as visible light.

 

[Battenkiller]

Manufacturers of home-quality radiant thermometers, either preset the emissivity correction at a so-called "gray body" level or allow one to manually choose an emissivity setting before doing the measurement. The BIG problem with (semi)transparent windows is, however, that their emissivities tend to be extremely low

The emissivity of Pyroceram is 0.85... not really extremely low at all. My IR gun has adjustable emissivity, so if I ever get a stove that actually has glass in the doors, I'll be able to get a temp reading that will be close enough for country. Others with fixed emissivity guns (almost always fixed at 0.95) can probably use a conversion factor that will allow them to get the proper temp.

 

[henkmeuzelaar]

Manufacturers of home-quality radiant thermometers, either preset the emissivity correction at a so-called "gray body" level or allow one to manually choose an emissivity setting before doing the measurement. The BIG problem with (semi)transparent windows is, however, that their emissivities tend to be extremely low

The emissivity of Pyroceram is 0.85... not really extremely low at all. My IR gun has adjustable emissivity, so if I ever get a stove that actually has glass in the doors, I'll be able to get a temp reading that will be close enough for country. Others with fixed emissivity guns (almost always fixed at 0.95) can probably use a conversion factor that will allow them to get the proper temp.

Nah, it's a bit more complicated than that alas. According to the laws of physics (at least those I'm aware off) the emissivity of a body at a given wavelength is identical to its absorptivity at that wavelength while absorptivity is the inverse of transmittance. When you look at the attached optical transmittance spectrum of Pyroceram you will see that a ~3/16" thick window has 90% transmittance up till 2.3 micron, i.e. well into the infrared region, and then drops off steeply to zero at 2.7 micron. Even if we ignore the second, increased transmittance window that peaks at 3.8 micron, and correct for increased window thickness in most woodstoves, we should not expect an absorptivity, and thus emissivity, value much above .2 (20%) below 2.3 micron.

Now when we look at the Boltzman distribution of black body radiation in the second figure, we can see that already at at a flame (or glowing coal) temperature of only 1000 K (a meager 1346 F) the radiation emission maximum is being reached below 2.3 micron. In other words, at that temperature, a fairly massive amount of direct radiation is already coming through the Pyroceram window. Against that background, the radiation from the window itself (let's say at a surface temp around 500K) will be dwarfed by the radiation from the flames (which can easily reach 2000 F). Even if you do have a double-wavelength optical thermometer with two narrow-band wavelength filters you might be unable to measure the window surface against such a much brighter background. Probably you would need a third or even fourth wavelength to solve for the two different temperatures (average window surface T and average flame/coal T) and emissivities. Been there, done that.

To make matters worse, any surface irregularities, ash particles etc. on the surface of the window, will become powerful emitters in their own right, as anyone who ever had a high-temperature coal stove with mica windows has seen happen as the normally transparent mica window became dirty or damaged and started glowing ever more strongly. Finally, as already mentioned, nearly all the transmittance spectra found in the literature were recorded at room temperature but can undergo significant changes at higher temperature as people doing laser beam experiments know well (as soon as one of your lenses or other optical components start heating up a bit you are likely to get a destructive,
runaway event).

Henk

Edit: I just noticed that the wavelength scale of the Pyroceram transmittance spectrum is not properly labeled; so let me add a (rather similar) Neoceram spectrum from the manufacturer's website.

 

[loon]

hot stove = warm house :cheese:

glass or no glass...


loon

 

[Jerry_NJ]

Moving away from the doctoral thesis dissertation approach, I can say I frequently clean the window on my Quad 4100I, love that bay window. I think I like it even better now that I think there maybe some heat gain from cleaning the glass.

Keep warm.

 

[henkmeuzelaar]

Moving away from the doctoral thesis dissertation approach, I can say I frequently clean the window on my Quad 4100I, love that bay window. I think I like it even better now that I think there maybe some heat gain from cleaning the glass.

Keep warm.

Yes, good idea.

Even as a retired emeritus (rather than a doctoral student) I find it hard to follow a thread so close to my heart without jumping into the fray.

Moreover, in my personal experience, when posters are hitting each other over the head with highly technical arguments over what radiation is and how to measure temperatures, etc. there's nothing like a few nice graphs to get things settled down.

I guess, I must enjoy playing the role of a poorly seasoned log....

Henk

 

[Battenkiller]

Now when we look at the Boltzman distribution of black body radiation in the second figure, we can see that already at at a flame (or glowing coal) temperature of only 1000 K (a meager 1346 F) the radiation emission maximum is being reached below 2.3 micron. In other words, at that temperature, a fairly massive amount of direct radiation is already coming through the Pyroceram window. Against that background, the radiation from the window itself (let's say at a surface temp around 500K) will be dwarfed by the radiation from the flames (which can easily reach 2000 F).

Gosh, I've been looking on and off all day for specs on Pyroceram. Where did you come up with so much useful stuff?

So, I guess that answers the question of whether or not IR is coming through ceramic glass. Now I want to know, do these specs include the IR reflective coating that (supposedly) in put on the inside of stove glass? If not, that changes everything, at least the way I am understanding it here. It would also explain why you aren't feeling all this heat with the doors closed like you do when they are open.



FWIW the emissivity I quoted was just something I grabbed off an engineering site.

http://www.engineering.com/Library/...ype/ArticleView/articleId/151/Emissivity.aspx

You have proven to me that it's a lot more complicated than I thought. I can't wait to see your thread on this subject, should be real interesting. ;-)

 

[DiscoInferno]

That's the plot I wanted to see. So evidently a dimly glowing coal bed would have a higher percentage of its radiation blocked than would a bright flame? I think that matches my experience. But is that by design? I can think of some rationale for it, like preserving a coal bed for the longest time. Certainly seems like if the goal was to pass only visible light the upper passband edge would be at a considerably lower wavelength. I have no idea how hard it is to tune a passband in such materials, but I guess I'd be surprised if such a flat bandpass structure arose naturally.

I like the dissertation approach, myself. Engineers, physicists, and the like rarely get to use their training in everyday discussion. Seem to be quite a few here, but I guess we're natural pyros.

 

[henkmeuzelaar]

Gosh, I've been looking on and off all day for specs on Pyroceram. Where did you come up with so much useful stuff?

So, I guess that answers the question of whether or not IR is coming through ceramic glass. Now I want to know, do these specs include the IR reflective coating that (supposedly) in put on the inside of stove glass? If not, that changes everything, at least the way I am understanding it here. It would also explain why you aren't feeling all this heat with the doors closed like you do when they are open.

FWIW the emissivity I quoted was just something I grabbed off an engineering site.

http://www.engineering.com/Library/...ype/ArticleView/articleId/151/Emissivity.aspx

You have proven to me that it's a lot more complicated than I thought. I can't wait to see your thread on this subject, should be real interesting. ;-)

Battenkiller you're gonna love this Google search tip:

When you type "neoceram OR pyroceram transmittance spectrum" or something like that, don't choose the WEB but the IMAGES!

One warning, though, don't switch "Safe Search" off or you might get sooo distracted that you may forget what you were trying to look for.....


Anyhow, at my age graphic scientific images are about all I can handle!!

Henk

 

[Battenkiller]

I like the dissertation approach, myself. Engineers, physicists, and the like rarely get to use their training in everyday discussion. Seem to be quite a few here, but I guess we're natural pyros.

I'm with you 100%. I'm surprised that with so many engineers, scientists and teachers as members here, there is so little contribution regarding highly technical matters that concern us all. I hope this isn't the last of it we see here, I really love this kind of stuff.