Any Physics majors around?

[Backhoe]

There was a thread a while back about the considerable additional heat output of a 30-NC when the fan was running as opposed to the fan not running. That got me to thinking about where the additional heat comes from. Is the stove less clean burning at the chimney with the fan going because of lower fire box temperatures? Do you use more wood with the fan running? Where does all this additional heat come from? These types of questions are the result of early winter idleness syndrome I believe….

 

[begreen]

Not a physics major, but I think the heat exchange would be seen in lower stove top temps with the fan on. We'll see. I'm about to try a fan on the T6. However, we will need to wait for winter to return. This is the first day without a fire in a few weeks. 48 outside right now.

 

[woodgeek]

Oooh Ooooh! Me Me! Certified physics major.

There was a thread a while back about the considerable additional heat output of a 30-NC when the fan was running as opposed to the fan not running. That got me to thinking about where the additional heat comes from. Is the stove less clean burning at the chimney with the fan going because of lower fire box temperatures? Do you use more wood with the fan running? Where does all this additional heat come from? These types of questions are the result of early winter idleness syndrome I believe….

Yes, grasshopper, energy is conserved. With the blower off, the heat is either going up the chimney (as a hotter flue, or unburned fuel), or combustion is slowing down. OR perceptions are deceiving--for example the stove is heating up, emitting more heat as radiation, which is diffusely heating the whole room/space/upstairs. Need to set up one of those IR cams and data loggers the ENGINEERS around here love so much.

Any ENGINEERS around?

 

[dave11]

There was a thread a while back about the considerable additional heat output of a 30-NC when the fan was running as opposed to the fan not running. That got me to thinking about where the additional heat comes from. Is the stove less clean burning at the chimney with the fan going because of lower fire box temperatures? Do you use more wood with the fan running? Where does all this additional heat come from? These types of questions are the result of early winter idleness syndrome I believe….

Running a fan will either lower the radiant heat output of the stove, or lower it's operating temperature, or more likely, a combination of both. The trade-off is that you get a gain in hot air (convection) from the stove which is free to circulate around the room/house.

So there's no additional heat coming from anywhere, you're just converting radiant heat to convection. But you'll be removing heat faster than it was dissipating before, so the firebox temp will go down a bit. In my experience so far with my stove, I can't tell that the fan, on max, is making any big dent in the operating temp. There's just so much heat, I don't think the little bit you can turn into convection via the fan bothers the stove at all. Maybe just a bit at start-up and at the end of the fire.

 

[fossil]

The heat output of the stove is a function of the energy released by combustion of the fuel in the firebox. A blower doesn't change that. I don't think a blower affects combustion in any way or to any significant degree whatever. Without a blower, the heat produced in the firebox is transferred to the living space through radiation and natural convection. The radiation is pretty much omnidirectional, the natural convection is pretty much vertically upward from the stove. What the blower does is add forced convection to the equation. It takes in room air from beneath the stove (in most cases) and directs it around the firebox through the annuli formed by the heat shields and out into the room through the outlets (specific location is stove-dependent) with perceivable velocity. Stand in front of the stove with the blower off, and you feel the radiation. Stand in front of the stove with the blower running, and you feel the radiation plus a bunch of forced convection. More of the stove's heat is washing over you, but the stove isn't producing any more heat because it has a blower running...the heat's just being distributed differently. A portion of the heat produced in the firebox that would otherwise just rise straight up to the ceiling through natural convection is being collected up by the air the blower's moving around the firebox, and then being forced out horizontally into the room. How far is comes out into the room depends on blower speed. It's a perception thing...but it's real, and can make a big difference in the comfort level of the living space. Combine a stove with a blower and a properly sized ceiling fan, and you've got the makings for one very comfortable room. The effectiveness of a blower on a free-standing stove is worth the money, IMO, in most installations, and is especially important in the case of a hearth stove installation, and is absolutely essential to the performance of an insert. Rick

 

[cycloxer]

The fan facilitates heat transfer by moving more air around your hot stove. It's as simple as that. There is a point of diminishing returns. If you use too large of a fan you will just cool down the surface temp of your stove. That is why most manufacturers settle around a 100-200 cfm fan. A fan is a very efficient way to extract more heat from your heater.

We get heat out of our stoves via radiation and convection. Once you reach your max stove operating temp of say 600 degrees you can't increase the radiation. It is what it is. The only way to increase the radiation would be to buy a larger stove with more radiating surfaces (metal or stone). You can increase convection via the fan by circulating more air around the stove, roughly another 30%.

 

[Battenkiller]

It's a perception thing...but it's real, and can make a big difference in the comfort level of the living space.

I've tried the fan thing, and I agree it's a matter of perception. But too much air movement can increase the amount of evaporative cooling on your skin surface, making you feel cooler even though the air is warmer. As well, the lower relative humidity of inside air in heated winter spaces increases the rate of evaporation, adding further to the cooling effect.

I have come to prefer the full radiant energy striking room surfaces to keep me comfortable. I just got my new IR thermometer, and it confirms what I already knew. Just about everything exposed to the direct radiation of the stove is warm. Even outside walls are around 70ºF. When everything is warm, I feel warm - regardless of the actual air temperature. The hotter my stove gets, the warmer those surfaces will get. I would not want to do anything that lowers stove temps.

I agree that the fan doesn't increase the heat output, but since it lowers stove temps, can't you just add more wood more often and open the draft more to bring them back up? This would result in greater heat output in total because you are actually burning more wood.

 

[Beetle-Kill]

Back to the basics, X amount of wood only contains Y amount of BTU's, correct? So the delivery of the heat seems to be the variable, convection vs. radiant heating. As stated by much larger brains than mine( eg. field mouse), Radiant heat feels hotter, faster, but convection finishes the race- so to speak. JMHO

 

[pen]

I've never run a blower on my stove before but here's my question.....

Shouldn't you be able to run the stove harder and keep stove top temps lower when a blower is running since the blower will remove the heat from the stove top ?

For example, w/ a full load of wood, and my primary air just barely open, my stove top is tickling 700 degrees. W/ the blower running w/ the same load of wood and the same air setting, I'd imagine that my stove top would be cooler as the moving air removes the heat from it? Correct? Sooo...... if that is true..... Then I should be able to open the primary air a little more and run the stove a little harder since the blower will help keep things on the top cooler?

Just thinking here. Don't mind me. Any thoughts from those who run a blower? Am I off base?

If I am correct, then the blower would allow you to run the stove harder and therefore you would get more BTU's out of it by burning the load of wood faster.

pen

 

[Slow1]

I expect you might be able to run a stove a bit hotter but that would depend on how well your stove is able to conduct the heat out through the walls and away from any heat sensitive areas. My point being that if you run the stove hotter and cool the surface the inside of the stove may be run at a higher temperature for a longer period of time and thus some parts inside may suffer damage. Naturally if the stove design took this into account then it shouldn't be a problem.

So for example, I would be less inclined to take a stone stove (slower to conduct the heat through the stone) and stick my own fan solution on the outside and then run the surface temps up to the the max with fans cooling the surface expecting that the stove would have the same service life as if it were run as designed. However, running a stove with a fan kit from the manufacturer that was designed for it and then staying within specs for temps (whatever they may be) could in theory take more wood to maintain the same surface temperatures as it would take without the fan kit (more BTU/hr from the stove with fans) and be perfectly fine.

 

[cycloxer]

Shouldn't you be able to run the stove harder and keep stove top temps lower when a blower is running since the blower will remove the heat from the stove top ?

Yes, absoulutely. That is how you can extract more heat out of your stove with a fan. The earlier poster was right that a 50k Btu heater is 50k, no matter what you do. However, with a fan blowing over your stove, you are effectively cooling it down. This allows you to run more wood (fuel) through the stove and then transfer more Btu's per hour to your room without over-heating the stove. You are speeding up the heat transfer process from your hot stove to the room. This concept is no different than how the fans on the radiator of your car work. They are to speed up the cooling down process of your engine coolant.

A further benefit, as pointed out, is that the blower is directional, and it will blow the hot air straight out into the room horizontally where we want it. A free standing stove w/o this option relies on natural convection currents and we all know that heat rises, right up to your ceiling. We want our rooms to be hot in the lower 6' where the people live, not up by the ceiling. Go over to your stove and place your face over the top and you can literally feel the hot air rising. That is a convection current.

None of this affects radiation whatsoever. Radiation is ony dependent on line of sight and the reflectivity of the material it hits. Stoves generally radiate equally on all sides, top, & bottom. This is the same glowing heat you feel from the sun on a bright day. You can even feel it in the winter. Radiation is the same on a calm day or a windy day. It's the same with your stove. The only thing that can stop radiation is a shield placed between the stove and the surface you are trying to protect. Go up to your stove and place your hand about 12" away. You can feel the radiating heat warm up your hand very quicky. Now place an object like a book between your hand and the stove. That heat goes away almost immediately because the book is blocking the radiation from hitting your hand. This is how metal stove shielding works. It is also why a stove placed smack in the middle of your room is ideal - you have the most exposed stove surfaces to radiate into the room. This is also why inserts rely on blowers to make a lot of their heat. Since they are tucked inside a hearth they have much less radiating surface than a free standing stove.

 

[SolarAndWood]

I use the blower when we need more heat. Seems to decrease my burn time by about a third. The thing I am a bit wary of is a power failure when we are not around and are pushing the stove with the blower on. Probably wouldn't matter later in the burn cycle but could overheat the stove in the earlier stages.

 

[Jags]

There is truth to a blower "washing" away heat from the surface of the stove - thus allowing the stove to be run harder without exceeding the recommended stove top temps, but a word of caution: I speculate that the MFGs specs on stove top temps will also have an association to the internal temps of the stove. By washing away surface heat and firing the stove harder you may still be under the stove top temp, but quite literally be over firing the inside of the stove.

I know the arguments that will follow: then why do they have blowers on them if it can be dangerous? Its to move heat. I doubt seriously that the intent of a blower was to reduce stove temp, although that is the trade off of using one. Just a non-physics major pontification. %-P

 

[North of 60]

I use the blower when we need more heat. Seems to decrease my burn time by about a third. The thing I am a bit wary of is a power failure when we are not around and are pushing the stove with the blower on. Probably wouldn't matter later in the burn cycle but could overheat the stove in the earlier stages.

Solar, your stove will gain about 100F or so and then the combustion air will start to throttle back quite fast. The flame pattern will start looking allot lazier as the stat becomes more satisfied with that heat not being removed from the back of the stove.
Try it, pull the plug. As for wood consumption with the fan on, I also loose about a third of burn time which is quite significant.

 

[cycloxer]

By washing away surface heat and firing the stove harder you may still be under the stove top temp, but quite literally be over firing the inside of the stove.

Good point and this is true as the fan can only cool the outside surface of the stove. The good news is that the internal firebox of the stove can withstand higher temperatures w/o damage than the outermost stones, castings, sheet steel, etc. If you run your stove to 600 degrees (common mfg's rec max operating temp), I think there is enough headroom before you start to damage your stove due to over-firing. I would also recommend using the mfg's blower specific to the stove. They designed it and I am sure they have tested it and know that it works.

Now, if you add your own custom blower, cram the firebox full, and run the stove hot, you could have problems and/or reduce the longevity of your stove before a rebuild is required. You can do it, for sure, you just have to use your discretion and watch the temps.

 

[Jags]

Good point and this is true as the fan can only cool the outside surface of the stove. The good news is that the internal firebox of the stove can withstand higher temperatures w/o damage than the outermost stones, castings, sheet steel, etc.

Well...I'm not a stove mfg, but outside of the combuster (cat) or burn tubes, and maybe the firebrick, I will bet that the rest of the internal steel or cast is the same stuff that is used for building the other parts of the stove, but I'm not looking to split hairs either. Burn'em if ya got 'em.

 

[madison]

Observations of a t6 with added OEM blower this season.
1. Stove top temp is lower with the blower on, the higher the blower speed, the greater the reduction in stove top temp.
2. NO difference in burn times, with the fan on. BUT, I have generally not changed (opened) the primary air setting due to a lower stove top temperature with the fan on, but set the primary air on time and visual clues and not stove top temperature.

Comments/opinions:
1. Blower increases heat production/efficiency of a convective heater. If you get into your cold car in the morning, and after the car warms up, the interior of the car will get warmer, faster, with the fan on the car heater on, vs fan off. The cast iron shell around the Alderlea's allows the fan to more effectively remove heat from the stove and disperse it into the house. I would hazard a guess that certain designs of stoves would NOT benefit as much with the addition of a blower. (ie"the radiant stove types")
2. The blower on our T6 significantly increased the effectiveness of the stove in our 3300 sq ft vaulted ceilinged (?sp) "glass house" with ceiling fans galore, no doubt about this, the furnace was turned on just a week or so ago, where the previous season was on the entire month of November. (Ceiling fans are still being used as before last season.) And I believe am using less wood. As I can cycle the stove fully, rather than reloading too soon, the blower on a 200 degree stove puts out more heat than a 200 degree stove without blower.
3. Downsides: increase in ash dispersal throughout house... I now turn the fan off during major raking, ash clean outs (bonehead move and quickly learned that lesson) and noise, though the blower on the T6 is much less than I thought it would be, and less noisy then our nat gas inserts in various homes we have owned in the past.

 

[cycloxer]

I'll tell you what I would not do: run the stove to 800 degrees stove temp with the blower running full speed. Yeah, you will get a ton of heat, but you probably have a raging inferno inside your stove and if that blower were to crap out for any reason, lol, you would be in a world of sh#$%$%.

 

[SolarAndWood]

The thing I am a bit wary of is a power failure when we are not around and are pushing the stove with the blower on. Probably wouldn't matter later in the burn cycle but could overheat the stove in the earlier stages.

Solar, your stove will gain about 100F or so and then the combustion air will start to throttle back quite fast. The flame pattern will start looking allot lazier as the stat becomes more satisfied with that heat not being removed from the back of the stove.
Try it, pull the plug.

That was pretty cool. 30 minutes into a new burn with a full load on top of a large bed of hot coals, fan on high and tstat set well above normal. Thermometer towards the upper end of the active range. Pulled the plug, temp went up but never came close to pegged before it came down a bit and leveled out. Pretty much a non-event. Gotta love that tstat.