The impact of ceramic glass on your stove's heat transfer

[53flyer]

Given that it feels like so much heat generally comes through the ceramic glass and an insert is almost completely behind a shroud: How important do you think the glass is in relation to the heat obtained from the stove? How big a difference does a dirty window make?

Weather you think it's a positive (or a negative) then how important should comparisons like this be to your overall decision? (broken link removed to http://www.chimneysweeponline.com/viewwoodi.htm)

 

[woodgeek]

The window doesn't transmit heat, it absorbs it and re-radiates it. In other words, if you swapped 'em for steel plates, the heat xfer would not be much different. They are engineered that way, with a special coating on them (that can be detected with an ohmmeter) that is basically opaque to the appropriate IR wavelengths. If you have an IR gun, you can just swing a door open, point the gun through the window at a cool wall, and it will read the hot temp of the 'glass'. Equivalently, close the door, point the IR gun at the coals at >1000F, and you'll still get the lower 'glass' temp.

 

[Hogwildz]

The window doesn't transmit heat, it absorbs it and re-radiates it. In other words, if you swapped 'em for steel plates, the heat xfer would not be much different. They are engineered that way, with a special coating on them (that can be detected with an ohmmeter) that is basically opaque to the appropriate IR wavelengths. If you have an IR gun, you can just swing a door open, point the gun through the window at a cool wall, and it will read the hot temp of the 'glass'. Equivalently, close the door, point the IR gun at the coals at >1000F, and you'll still get the lower 'glass' temp.

Sorry, I just ain't buying that.
I had an older insert that had plate steel doors. No where near the heat could be felt from those doors as I can feel standing that same distance from my insert with a glass door.
If ya put a giant ceramic glass between the earth & the sun and did the same with a giant pc of steel, I'm guessing that the earth would feel more heat through the glass than through the steel.

 

[Beetle-Kill]

I'm with you Hog. Radiant heat does transfer faster, and farther, than convective heat.

 

[lobsta1]

It just so happens that tonight I had used my IR gun on my NC13. The fire was pretty well burned down as I had only put 4 splits in. Aiming at the glass I got 610 F. Opening the door & aiming at the same spot in the coal bed, I got a reading of over temp. In other words, over 900 F. This was at the white coals in front of the dog house. Anywhere I aimed on the glass, I got the same 610 F.
Al

 

[woodgeek]

The window doesn't transmit heat, it absorbs it and re-radiates it. In other words, if you swapped 'em for steel plates, the heat xfer would not be much different. They are engineered that way, with a special coating on them (that can be detected with an ohmmeter) that is basically opaque to the appropriate IR wavelengths. If you have an IR gun, you can just swing a door open, point the gun through the window at a cool wall, and it will read the hot temp of the 'glass'. Equivalently, close the door, point the IR gun at the coals at >1000F, and you'll still get the lower 'glass' temp.

Sorry, I just ain't buying that.
I had an older insert that had plate steel doors. No where near the heat could be felt from those doors as I can feel standing that same distance from my insert with a glass door.
If ya put a giant ceramic glass between the earth & the sun and did the same with a giant pc of steel, I'm guessing that the earth would feel more heat through the glass than through the steel.

I'm not saying you can't feel heat radiating off the window. Of course you can, it's really hot and radiating a ton of heat into the room. While the coating allows visible light to get through, so we can see the fire, it doesn't transmit the peak IR wavelengths emitted by objects at stove temps. I'm gonna guess that your glass doors are hotter than your old steel doors, so you feel more heat radiating off them.

Another experiment--if you open the doors, don't you feel a lot more radiant heat from the fire on your face? If the heat goes through the window, why do you feel so much less when the door is closed? A while back Craig explained that engineering this property of the glass was a breakthrough for putting glass in stoves--it lets the fire run hotter than an IR transparent window would--allowing a clean burn with a large glass area, and keeping the glass cleaner.

 

[EJL923]

I believe some wood stove glass has the IR coating, and some do not. I think most inserts have the coating to keep the firebox hotter, therfore saving more heat for the blowers.

 

[Corey]

Several things going on here. Of course the ceramic glass transmits some heat. If it didn't, there would be no need for the IR coating some have. Or you can just look up the properties and see it does transmit quite a lot through the IR spectrum.

(broken link removed to http://www.ceramicglass.co.uk/downloads/neoceram_charts_1.pdf)

The glass is also heated by the fire, so it is radiating as well, and it's very rare to have one mode of heat transfer without the others so there has to be convection and conduction, too. But what of the heat which is reflected back into the firebox? - it can't just disappear. Part of that is absorbed by the firebox which makes it hotter - which leads to a cleaner, more efficient burn - generating even more heat. The hotter firebox raises the overall temperature of the stove - which means more heat coming out the top, sides, etc. In short, what heat doesn't come through the glass leads to a more efficient burn and will come out of the stove as heat from other surfaces.

As for dirty glass, the IR coating is generally a metal film which is highly reflective in the IR spectrum, though good transmission in the visible. Carbon soot is the opposite...good absorbance in the IR and visible range. Hence, it's most likely defeating the purpose of the IR, coating - letting the glass get a little hotter, radiating a bit more heat to the room, but letting the firebox get a little cooler and less efficient.

 

[karri0n]

Hog, I believe the reason you feel so much more heat. through the glass than through the steel plate is due to the thickness. If you put several layers of the neoceram to make it 1" thick, I bet you would feel a lot less heat being radiated.


As to the giant ceramic class blocking out the sun vs. steel plate blocking out the sun, you wouldn't necessarily feel more IR directly from the sun, but the UV, visible light, and microwave emissions would still cause the surface of the earth to heat.

 

[firefighterjake]

. . .

. . . If ya put a giant ceramic glass between the earth & the sun and did the same with a giant pc of steel, I'm guessing that the earth would feel more heat through the glass than through the steel.

And now we know how the world will end in 2012 . . . it will end when someone sticks a giant piece of plate steel between the earth and sun blocking the heat.

 

[53flyer]

Thanks for all the great replies. If I'm reading most posts correctly it seems like the general though is that you get more heat into the room via the glass front than you would with solid steel door like they used to come with right? Given that to be the case:

What are the thoughts irt the second part of my original post regarding the actual size of the window? To the point, how much do you think the benefit is of having a larger surface area of glass (like they compare at the following site): (broken link removed to http://www.chimneysweeponline.com/viewwoodi.htm)

 

[Slow1]

Thanks for all the great replies. If I'm reading most posts correctly it seems like the general though is that you get more heat into the room via the glass front than you would with solid steel door like they used to come with right? Given that to be the case:

What are the thoughts irt the second part of my original post regarding the actual size of the window? To the point, how much do you think the benefit is of having a larger surface area of glass (like they compare at the following site): (broken link removed to http://www.chimneysweeponline.com/viewwoodi.htm)

Ok, yes you can get more transmitted IR energy directly from the fire through a window than through plate steel or cast iron doors.. so what?

Keep in mind that the glass in only one part of the way any stove is going to get heat to you. I believe that to pick your stove based on the amount of ceramic 'glass' only on the belief that this somehow will determine which stove is going to give you the most heat is rather, well, silly. The value of looking at how much class there is (as in that link) is to know how much viewing area you will have - i.e. how much of the burning wood/coals/fire you will be able to see at any given time. Perhaps it will also reflect the amount of light you can expect in the room from a blazing fire.

Stoves are designed to get the heat out of the wood and into the room. Modern stoves are rather well designed and fairly efficient. Some are better than others, they are not all equal.

A better gauge of how much heat you will get from the stove is to look at the rated output, firebox size, and options such as blowers all as part of the overall stove design. The viewing port is just one part of the design (albeit one of the most attractive parts perhaps).

 

[53flyer]

Ok, yes you can get more transmitted IR energy directly from the fire through a window than through plate steel or cast iron doors.. so what?

Keep in mind that the glass in only one part of the way any stove is going to get heat to you. I believe that to pick your stove based on the amount of ceramic 'glass' only on the belief that this somehow will determine which stove is going to give you the most heat is rather, well, silly. The value of looking at how much class there is (as in that link) is to know how much viewing area you will have - i.e. how much of the burning wood/coals/fire you will be able to see at any given time. Perhaps it will also reflect the amount of light you can expect in the room from a blazing fire.

Stoves are designed to get the heat out of the wood and into the room. Modern stoves are rather well designed and fairly efficient. Some are better than others, they are not all equal.

A better gauge of how much heat you will get from the stove is to look at the rated output, firebox size, and options such as blowers all as part of the overall stove design. The viewing port is just one part of the design (albeit one of the most attractive parts perhaps).

Hi slow,

Thanks for your thoughts. Regarding rated output, firebox size, etc. yes they are all good to look at but as has been discussed numerous times, many of those factors don't directly relate to more or less performance. Replacing an outer material with another that allowed more heat transfer would have a direct relationship

Obviously, the point of that link was viewing area; I'm simply putting another twist on it. Many innovations, when first theorized, seemed rather, well, silly at first glance by most people. However, a major reason innovation occurs in the first place is that a few people look beyond the silliness to find the benefit and make it work.

Yes, we all think modern stoves are rather well designed but the same statement would likely have been made by people in the 50's when speaking of the then current technology. As with anything, we can always make improvements. I don't know what the "exact" difference is but imo every little bit helps and "if" it where ever shown that putting a larger window on (as well as the side panels I described earlier) would provide, oh lets say 5% more heat transfer vs. the exact same model without those changes I'll bet a decent number of people would opt for that extra 5% (as long as they made it look attractive of course...). Actually, since an insert has so much of it's body behind the shroud it could make an even bigger difference in those situations (perhaps 10%). Who knows...it's simple food for thought and some people are more hungry than others.

 

[Slow1]

No offense intended I assure you. My comments related to using the performance numbers is simply a matter of convenience - I believe they are far more indicative of the stoves actual heat output than the size of the viewing window since they take into account the rest of the stove design. My main point is that one cannot select a stove based solely on the window size.

You're points are quite valid and I don't disagree that there are some great innovations that have occurred. I don't, however, believe that stove designers primary motive for the installation of the window is to get the heat out of the stove.

Bottom line, however is that you can perhaps ignore the exact mechanisms of how the heat is getting out of the stove as long as it is indeed leaving the stove. You can simplify the equation quite a bit perhaps - Consider the following; The total amount of heat you can possibly get from a load of wood is a finite quantity "X". The amount of heat you get into the room perhaps could then be expressed as "Y". "Y" will always be less than "X" unless you supply another energy source to provide draft and move the combustion air into and the byproducts of combustion out of the stove. So the game then is to try maximize "Y" as much as possible. There are several areas you can lose energy:

Incomplete combustion "I" - let's lump smoke and 'dirty' ash with burnable coals in here together.
Flue temperature "F" - the heat that goes up your chimney - required to maintain your draft, but in many cases is even more than that.

So, the equation becomes: X - I - F = Y

Now not all stoves are equal - some achieve more complete combustion more easily than others in more varied conditions. Likewise some run with a higher flue temperature when operated than others (burning with optimal combustion to minimize "I"). So to start with I would want to find a stove that has achieved optimal "I" and "F" values if I could so identify them.

The best indicator of a low "I" value that I can find is the EPA emissions numbers - the implication being that if the smoke being emitted is minimal on particles it is being burned. However this number doesn't measure CO output so even so it is incomplete.

I've not seen any manufacturer or EPA numbers on flue temps, however I have seen many threads here reporting on them so this may be the best source for that information.

Now the "Y" heat value is going to go somewhere since it is not leaving the stove up the flue it is going to initially heat the stove and/or radiate directly out the window and heat the room. Eventually it will radiate out the stove as well. This may be where we diverge on our feelings of the importance of the viewing window - I don't feel it matters from a performance standpoint whether the heat comes through the window or radiates second hand from other materials or is pulled off fins via convection by a blower drawing air to be heated.

In any case, the total "Y" value may or may not be useful to you depending on your install. For example, if your stove is stuck in the corner of a concrete basement and the heat radiating is heating those walls which in turn are heating the ground outside then you are wasting that "Y" heat, but that is not the stove's fault. Now, if you have an insert in an old fireplace that is on an external wall and the insert heats up that old masonry fireplace which then heats the outside air that same portion of "Y" heat is again being wasted but in this case I believe one can fairly fault the insert design as they SHOULD consider this factor in the design of the stove given it is supposed to be inserted in on old fireplace.

Just another opinion - thanks for the discussion!

 

[smokingolf]

The window doesn't transmit heat, it absorbs it and re-radiates it. In other words, if you swapped 'em for steel plates, the heat xfer would not be much different. They are engineered that way, with a special coating on them (that can be detected with an ohmmeter) that is basically opaque to the appropriate IR wavelengths. If you have an IR gun, you can just swing a door open, point the gun through the window at a cool wall, and it will read the hot temp of the 'glass'. Equivalently, close the door, point the IR gun at the coals at >1000F, and you'll still get the lower 'glass' temp.

Sorry, I just ain't buying that.
I had an older insert that had plate steel doors. No where near the heat could be felt from those doors as I can feel standing that same distance from my insert with a glass door.
If ya put a giant ceramic glass between the earth & the sun and did the same with a giant pc of steel, I'm guessing that the earth would feel more heat through the glass than through the steel.

Putting a giant ceramic glass between the earth and the sun, wouldn't that give you global cooking. I bet ya Al Gore would love to sink his teeth into that one, he could make a new movie.

 

[smokingolf]

. . .

. . . If ya put a giant ceramic glass between the earth & the sun and did the same with a giant pc of steel, I'm guessing that the earth would feel more heat through the glass than through the steel.

And now we know how the world will end in 2012 . . . it will end when someone sticks a giant piece of plate steel between the earth and sun blocking the heat.

I've got a $90 bottle of Cabernet Sauvignon that the Wine club I belong to said you can age this for a few years. I wrote 12/21/2012 on the bottle and I'm going to start a fire and drink that bottle of wine and be toasty that night. I hope!

 

[woodgeek]

Several things going on here. Of course the ceramic glass transmits some heat. If it didn't, there would be no need for the IR coating some have. Or you can just look up the properties and see it does transmit quite a lot through the IR spectrum.

(broken link removed to http://www.ceramicglass.co.uk/downloads/neoceram_charts_1.pdf)

The glass is also heated by the fire, so it is radiating as well, and it's very rare to have one mode of heat transfer without the others so there has to be convection and conduction, too. But what of the heat which is reflected back into the firebox? - it can't just disappear. Part of that is absorbed by the firebox which makes it hotter - which leads to a cleaner, more efficient burn - generating even more heat. The hotter firebox raises the overall temperature of the stove - which means more heat coming out the top, sides, etc. In short, what heat doesn't come through the glass leads to a more efficient burn and will come out of the stove as heat from other surfaces.

As for dirty glass, the IR coating is generally a metal film which is highly reflective in the IR spectrum, though good transmission in the visible. Carbon soot is the opposite...good absorbance in the IR and visible range. Hence, it's most likely defeating the purpose of the IR, coating - letting the glass get a little hotter, radiating a bit more heat to the room, but letting the firebox get a little cooler and less efficient.

thanks for the info Corey--can you say whether the transmittance curve is for uncoated or coated neoceram? I thought most stove glass in use had the IR coating--readily detectable with an ohmmeter. Even my ol POS stove.

 

[precaud]

Hog, I believe the reason you feel so much more heat. through the glass than through the steel plate is due to the thickness. If you put several layers of the neoceram to make it 1" thick, I bet you would feel a lot less heat being radiated.

I think karri0n was on the right track here, though it has to do more with the mass and specific heat capacity of the materials. The specific heat is the amount of heat per unit mass required to raise the temperature by one degree Celsius. So it is often viewed as a measure of heat storage capacity.

The specific heat of ceramic glass vs steel are not hugely different (.12 to .18). But a steel plate of equal size weighs much more. So steel takes longer to heat up, and it holds more heat. So that is why the glass gives off heat more quickly. Once the stove comes up to operating temperature, then it all normalizes out. After the fire dies, the glass will cool more quickly, as it has less heat stored to give up.

 

[rickw]

I wonder what the spectral response of your IR thermometer is. Most of those things are in the 7+ um range. What I'm getting at is that maybe that instrument is simply not going to pick up the shorter wavelength at the higher temps. Also, it may be possible that the ceramic glass has an IR cutoff such that it doesn't pass much radiant heat at low firebox temps but does at high.

 

[Corey]

thanks for the info Corey--can you say whether the transmittance curve is for uncoated or coated neoceram? I thought most stove glass in use had the IR coating--readily detectable with an ohmmeter. Even my ol POS stove.

Can't say for sure, though there is a suspicious 'notch' in the transmittance of neoceram-0 around 2.75um. If this is coated, uncoated would transmit even more. The neoceram-11 appears to be mainly used in microwave ovens. I suspect a thin metal film in that application would be very bad based on the way most metals react in the microwave.

Though looking strictly at transmittance - either neoceram does transmit well into the IR. Steel, of course, does not 'transmit' IR, just the way it does not transmit visual light. Any heat you feel from steel would be absorbed and re-radiated.

RE: specific heat - while that is a factor, it doesn't really come into play in the transmittance / reflectance subject. Overall, it's pretty minor - a few tenths of a btu per pound per degree F. So even considering 970F down to room temp of 70F, (900 degrees x .2 btu x 10 pounds of glass) barely gets you 1800 btu. For speed of 'giving up heat' we'd have to look at thermal conductivity - I'm guessing steel has ceramic glass beat by a long shot there.

 

[carinya]

How important do you think the glass is in relation to the heat obtained from the stove? How big a difference does a dirty window make?

Whether you think it's a positive (or a negative) then how important should comparisons like this be to your overall decision? (broken link removed to http://www.chimneysweeponline.com/viewwoodi.htm)

I think the placebo effect is the most critical. Our new nc13 has a 16x9 glass and even though it's a small firebox the crystal clear view of the fire is worth a couple hundred degrees in perceived warmth :coolsmile:

 

[precaud]

Cory, the original question was not about IR, but about the perception of heat from the glass. We both agree that, on it's own, the specific heat difference is insignificant. But the mass part of the equation is significant, and for steel it puts a larger time constant into it's response, even in the transmittance / reflectance equation, which is a function of temperature.

It may be that it feels like most of the heat is coming from the glass, but it is not, by far. This just speaks to people being more sensitive and responsive to radiant heat than convective. We've been well-trained by the sun, I suppose.

 

[Corey]

Well, that is an interesting perspective, could very well be true. I guess I was tending to the side that more heat 'does' come through the glass as opposed to steel. Not only does the glass conduct, convect, and radiate - just like the steel, but it can also transmit IR directly through. But the other side of the coin is all the heat has to go somewhere. As I mentioned way back when, more heat coming through the glass means less going back into the firebox and a slightly cooler overall stove. But probably talking a few dozen degrees one way or the other in any case. It's not like stove with big glass can't get above 300F and stove with little/no glass burn 700.

It really gets tricky to tell if a 625 degree stove with X btu/minute coming through the glass is putting out more heat than a 600 degree stove with X+2000 btu/minute coming through the glass. Overall, I would probably buy on looks, features and reputation (roughly in that order - at least for me)