Do stove-mounted blowers inhibit combustion efficiency?

I really don't think so, but I don't have a scientific formula as to why. I haven't noticed a difference when we run ours.

I leave my blower switched off till things are warmed up a bit... about 750 degrees stack temperature, in a set-up where 500 is low, 750 is medium, and 1000 is full output. Typically 5-10 minutes after light-up, which is also about the time when I close down the extra startup air. I doubt that I *have* keep the blower off, I just like the idea of "every little thing" to help the draft get established and the fire burning clean more quickly.

In principle, sure... the blower removes a few percent more heat from the surface of the stove, and in turn the firebox runs a few degrees cooler. But the blower mostly removes heat from surfaces that are *downstream* from the actual combustion zone(s), at least on most stoves I've seen... (I've seen Quads in the store but never run one). I really doubt you are having much effect on combustion efficiency with the blower.

Now, if you are having creosote problems, or not-enough-draft problems, then I could see cause for concern. Those could be caused by overcooling the exhaust before it gets to your chimney. But combustion efficiency? Relax, and enjoy the extra heat your stove puts out with the blower on.

EddyKilowatt said:

I leave my blower switched off till things are warmed up a bit... about 750 degrees stack temperature, in a set-up where 500 is low, 750 is medium, and 1000 is full output. Typically 5-10 minutes after light-up, which is also about the time when I close down the extra startup air. I doubt that I *have* keep the blower off, I just like the idea of "every little thing" to help the draft get established and the fire burning clean more quickly.

In principle, sure... the blower removes a few percent more heat from the surface of the stove, and in turn the firebox runs a few degrees cooler. But the blower mostly removes heat from surfaces that are *downstream* from the actual combustion zone(s), at least on most stoves I've seen... (I've seen Quads in the store but never run one). I really doubt you are having much effect on combustion efficiency with the blower.

Now, if you are having creosote problems, or not-enough-draft problems, then I could see cause for concern. Those could be caused by overcooling the exhaust before it gets to your chimney. But combustion efficiency? Relax, and enjoy the extra heat your stove puts out with the blower on.

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x2

Given that combustion in modern stoves takes place inside a chamber which is essentially insulated from the outer skin, blowing air on the stove shouldn't notably decrease combustion efficiency. If it does, I suspect it would be a maximum of a few percent as that inner chamber is then surrounded by a shell of steel which may be 400-500 degrees as opposed to 600-700 degrees.

The flip side is blowing air on the stove SUBSTANTIALLY increases the thermal transfer efficiency from the stove to the air. There is no doubt blowing air on the stove cools it off - hence you are taking much more heat out than natural convection is doing. Given the chance of -2% combustion efficiency vs +30% thermal transfer efficiency, the fan wins hands down.

cozy heat said:

Given that combustion in modern stoves takes place inside a chamber which is essentially insulated from the outer skin, blowing air on the stove shouldn't notably decrease combustion efficiency. If it does, I suspect it would be a maximum of a few percent as that inner chamber is then surrounded by a shell of steel which may be 400-500 degrees as opposed to 600-700 degrees.

The flip side is blowing air on the stove SUBSTANTIALLY increases the thermal transfer efficiency from the stove to the air. There is no doubt blowing air on the stove cools it off - hence you are taking much more heat out than natural convection is doing. Given the chance of -2% combustion efficiency vs +30% thermal transfer efficiency, the fan wins hands down.

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+1 I would not be without the blower I find it makes a big difference in heat distribution and has no effect on the fire..

Ray

I agree with cozy heat and might add that if the fans cool the stove enough the heat in the stove might transfer faster to the shell..thermodynamics..it's magic..but the fire won't change unless more air is introduced into the box.

The bearings starting to go on my blower and it started getting noisier than I could put up with, so I was looking into replacement bearings, but in the meantime I set up a wall mount fan directly behind my stove to blow air past the flue and over the top of the stove. Man does that ever work well! Much better than the stove blower for extracting heat from the stove and warming the house up. I don't think I'll be bothering to fix the blower, I'll just keep the fan arrangement since it works so much better, and is quieter and cheaper.
Anyway, even with this fan extracting more heat than the blower did, I can't say it has effected the burn in any negative way, although with the fan running I find I can run my stove harder (if I want) and still keep the stove top within the acceptable heat range.
I think if there is a negative side to extracting more heat from the stove like this, it would be cooling the exhaust gas might lead to a little more creosote in the flue. But for me, I don't mind giving the flue an extra sweep if I had to, if it means utilizing my heat and firewood more effectively.

I don't think a little rinky-dink stove mounted blower could push it out of the sweet spot unless it has a very small fire going. I have a large 650 CFM central heat option blower and does not appear to have any effect on the combustion. Too much cold outside air via the OAK though, is another matter. Opening up the air too much in very cold weather reduces the heat output.

If the blower on yours is like mine, it's mounted at the rear bottom, which blows the air up the backside and across a relatively small fin that points it over the surface?

The backside of the stove if baffled to death to keep clearance distances minimized (I'm always amazed at how cool it is behind the stove). Running air over this surface shouldn't affect much internally. On my stove (Quadrafire 3100 Millennium), the air pushes more on the local radiant heat over the stove than directly on the stove top, so I'm not sure how that would affect it. If yours is actually pointing down onto the surface, that would presumably cool the 'quaternary' combustion zone, possibly robbing that of some heat production.

That said, I do notice a very slight difference in the fire that I can't put my finger on when I'm using the fan (maybe slight cooling, maybe interference with the air intake?)

raybonz said:

has no effect on the fire..

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This isn't true. Heat doesn't come out of thin air, it comes from the fire. If you take more heat out you have to put more fuel in. And also if you run the blower it may heat the air more by convection but lowers the radiation due to the lower skin temperature.

bokehman said:

raybonz said:

has no effect on the fire..

Click to expand...

This isn't true. Heat doesn't come out of thin air, it comes from the fire. If you take more heat out you have to put more fuel in. And also if you run the blower it may heat the air more by convection but lowers the radiation due to the lower skin temperature.

Click to expand...

You will burn the same amount of wood as the blower is not feeding the fire, that doesn't change.. Yes it reduces the radiance on the blown surfaces but not the front.. I do not like too much radiant heat and find it very unconfortable but can sit in a 75 degree room near the stove in comfort.. This is my preference and I have heated this way with wood for around 24 years..

Ray

bokehman said:

raybonz said:

has no effect on the fire..

Click to expand...

This isn't true. Heat doesn't come out of thin air, it comes from the fire. If you take more heat out you have to put more fuel in. And also if you run the blower it may heat the air more by convection but lowers the radiation due to the lower skin temperature.

Click to expand...

Bokehman: When you get in your automobile, and turn the blower up or down to regulated the heat or defrost your windows, does the gas mileage (efficiency) change in your car? I think not.

IMHO, actually the blower is increasing the efficiency of heat transfer off of the stove/insert, and secondly , possibly as a result less heat is escaping up you chimney because of this.... So in effect the blower increases efficiency in two different ways.

I would also in jest, argue that the statement "heat doesn't come out of thin air" is incorrect (think heat pumps, though I know that this does not apply to the wood stove arguement).

bokehman said:

raybonz said:

has no effect on the fire..

Click to expand...

This isn't true. Heat doesn't come out of thin air, it comes from the fire. If you take more heat out you have to put more fuel in. And also if you run the blower it may heat the air more by convection but lowers the radiation due to the lower skin temperature.

Click to expand...

It is true imo.
With the blowers going the shell cools some..then I think the heat in the box transfers better to the shell in effect saving some heat that would have went up the flue..that is your net gain .

I have to agree with bokhemon on this... laws of physics tell us there is nothing for free. If the blower puts more heat in the room, its taking more heat from someplace, ie the fire. Heat transfer between two materials is proportional to the temperature difference, so if you cool the steel, heat will move from the firebrick to the steel faster.

In a stove with a thermostat like a bk, what should happen is that the fire cools,tsat opens to let in more air and the fire burns faster... so wood consumption goes up. In a stove with fixed air, the cooler fire might mean that it burns a bit less efficient.


Anyway that s what the science says , whether the effect is strong enough that you would notice is debatable..


Btw this one has been beaten to death in at least a dozen prior threads

Yes..the t-stat will adjust the fire somewhat.
I was trying to leave that out of the mix for those that don't have one and to stay on topic with what the op wanted to know.
I'm still going with you gain with blowers on..but I don't like to hear them.

jharkin said:

I have to agree with bokhemon on this... laws of physics tell us there is nothing for free. If the blower puts more heat in the room, its taking more heat from someplace, ie the fire. Heat transfer between two materials is proportional to the temperature difference, so if you cool the steel, heat will move from the firebrick to the steel faster.

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"taking more heat from someplace ie the fire" --> which I think, defines increase efficiency

"Heat transfer between two materials is proportional to the temperature difference" -- not a linear proportion but exponential (i think)

HotCoals said:

Yes..the t-stat will adjust the fire somewhat.
I was trying to leave that out of the mix for those that don't have one and to stay on topic with what the op wanted to know.
I'm still going with you gain with blowers on..but I don't like to hear them.

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My CDW blower was loud and quite annoying but the PE blower is very quiet and effective.. Based on that blower noise needs to be assessed on each stove as each model will be different. Owners of the T-5 and T-6 report quiet and effective blowers. Englander and DW owners report noisy blowers sounds like we need a poll..

Ray

I do have a bias.
I am a big fan (pun not intended) of radiant heat
for many natural reasons.

Why would you want to use a fan?
(realizing some convective heat is
created from 'radiant' stoves)

Have you considered spinning a fan blade
* uses electricity made from burning mostly
bituminous (aka dirty) coal
* attaches you to 'the grid'
* cannot be relied on in a power outage
* violates 'KISS'
* sends your $$ to a big power conglomerant
* creates indoor weather
* spreads indoor fried dust
* is less healthy than radiant heat?

Aye,
Marty

Marty S said:

I do have a bias.
I am a big fan (pun not intended) of radiant heat
for many natural reasons.

Why would you want to use a fan?
(realizing some convective heat is
created from 'radiant' stoves)

Have you considered spinning a fan blade
* uses electricity made from burning mostly
bituminous (aka dirty) coal
* attaches you to 'the grid'
* cannot be relied on in a power outage
* violates 'KISS'
* sends your $$ to a big power conglomerant
* creates indoor weather
* spreads indoor fried dust
* is less healthy than radiant heat?

Aye,
Marty

Click to expand...

Are you serious?
Up here they do not use coal for power but if they did I would still use the power..
I like being part of the grid, my livelihood relies on electricity and I like it that way..
You call a fan complicated, I could fix a blower in my sleep?!?! That's borders on hilarious! It costs very little to operate a blower and keeps me comfy and unbaked.. If you didn't have power we couldn't have this discussion!
What the heck is indoor weather?? Ooops never mind I just saw a rain cloud pass through my livingroom, BRB gotta open my windows lol..
I fail to see what is unhealthy here please enlighten me..

No offense Marty but I will probably always stay on the grid as it's too costly to produce my own power and I for one love electricity.. My favorite inventors are Tesla, Thomas Edison, Ben Franklin and a host others and the vast majority involve electricity!

Ray

My stove's thermodisc doesn't actuate soon enough during cold start fires or long enough during the coaling stages so I added a bypass toggle switch so I can operate it during these times. I've tried moving thermodisc around but the switch option when needed has worked best. I do make sure I have at least a 400 degree flue temp before manually turning on fan. I can always turn switch off and return to stock as well. I have observed on my stove that if I have solid 500-600 stovetop temps then there doesn't seem to be any affect on combustion but if I am running fan hard at temps lower then this it seems to affect secondaries. Maybe there is some bleed over effect from the primary intake air? I will continue to observe and maybe I am drawing a incorrect conclusion at this point. I am now using some of the Idaho energy logs at times so that may change some of my observations as well.

Just a joke , no flame intended -- Marty, I would like to know how you are posting to this forum without using electricity or the Internet ( a power hog in itself).

Coal appears to be ~ 50% http://www.p360.org/dsg.aspx?Data_Set_Group_Id=603

Most of the power in the county my cabin/woodstove is located in comes from wind power at this point. I'm sure hydroelectric contributes a lot as well. I think BPA in Washington state still sells a lot of hydro power to California.

The Wild Horse Windmill Farm in Kittitas County
http://www.youtube.com/watch?v=Xn1OpRJkFWk

If I was going for an off the grid cabin option I could easily power with solar and wind.

Marty S said:

...
Have you considered spinning a fan blade
* uses electricity made from burning mostly
bituminous (aka dirty) coal
* attaches you to 'the grid'
* cannot be relied on in a power outage
* violates 'KISS'
* sends your $$ to a big power conglomerant
* creates indoor weather
* spreads indoor fried dust
* is less healthy than radiant heat?

Aye,
Marty

Click to expand...

Some good points, Marty - it's certainly a personal preference to use a fan or not, but not using one means lower stove efficiency, hence more wood, so have you considered not using one...

- causes you or someone in the wood processing business to burn more (dirty) gasoline for cutting, splitting, hauling, etc
- makes you more reliant on 'big oil' and manufacturers of all the little gadgets we use for processing wood (saws, mauls, vehicles, etc)
- may not be needed in a power outage, but certainly helps all other times
- in 'KISS' the fan is a pretty simple/cheap bit. Burning more wood without one puts more wear/tear on expensive/complex bits like saws, and vehicles.
- more wood processing sends more $$ to big / foreign oil conglomerates to keep all gas powered equipment running.
- reduces stratification of indoor weather - without a fan, you tend to have a hot ceiling and cold floor. With one, temps tend to be more even.
- dust - ya, ok, you got me here...the stronger air current of the fan could cause dust to spread further.
- health...dunno, I'm not a doctor, but an evenly warm body with my fan makes me feel better than a hot head and cold feet which happens when I turn the fan off.

I don't think a fan will change then burn much on a air tube system. Since the secondary burn is below the heat baffle below the top of the stove. But with my King Ultra where the top of the cat chamber is the top of the stove it has a heat probe in the cat chamber I have to play with fan speed vs air settings. Without the fan I can easily over heat the cat chamber on low to mid heat. But I can keep it at 1500 deg at 3/4 to full with full fan speed. On the other hand if I have it on low and leave the fan on I can easily cool it below 600 deg which is too cool. In the manuel the heat range for the cat chamber is from 500 to 1700 deg.

Billy