Do stove-mounted blowers inhibit combustion efficiency?

[Martin Strand III]

Never said I don't like or don't use electricity.
Actually, I'm very grateful for it.
Just don't like to waste it or use it
when I don't have to or want to.

And, I am not prejudiced against electricity.
Electric power in my area fails several times
a year including each winter. If I were dependent
on electricity for heat, I would be SOL, sometimes
for days. For me, one of the main reasons
to have wood heat is that it lets me, in a small way,
function more independently and lessens the amount
I pay to the power company. I grow the wood, cut and
split the wood, dry the wood and I burn the wood.
So, when when the power failure occurs, it's no BFD,
and I feel good.

I just asked why run a fan blade (when you
don't have to).

As for 'indoor weather', it comes from hot metal
stove surfaces creating superheated air (room air
on hot stove surface), currents and drafts (convection)
accentuated with the inherent metal stove
heating/cooling thermal cycling.

Add a fan to compound the natural convection from
any wood burning device and push air around the
room increasing air movement, you have 'wind chill'
not to mention needing more BTUs.

Aye,
Marty

 

[jharkin]

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.

"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)

There are a couple different efficiencies at play here - the efficiency of the burn (i.e. how much wood mass is actually consumed to produce heat vs going out the chimney as unburnt particles - we can measure this by grams/hr of soot)) and the efficiency of heat transfer from the fire to the house (how much heat goes into the room vs up the stack as hot gas - we can measure this by the difference in temperature between the flame and the flue gas temp at the chimney cap).

If you are transferring more heat to the room you are increasing heat transfer efficiency, but in our case if the temperature of the fire is reduced to the point below which you get optimum secondaries you could reduce the burn efficiency... again I say this is theoretical and may not be noticeable in practical terms as we might just be talking 1200F secondary's vs. 1600F secondaries, both of which are hot enough to burn off everything. So yes the overall system efficiency could be increased.

You are right that the difference in heat transfer is exponential... I guess it depends on which dictionary definition of the word proportional you want to use - it can mean simply "to vary with something" or it could be more strictly defined as "having a constant ratio variation with something" Im no expert but I believe the dictionary allows both interpretations.

But still, the bottom line is that just like there is no such thing as perpetual motion there is no such thing as energy from nothing. heat is energy - its coming from somewhere.... that's the only opinion in this discussion I was humbly disagreeing with, but as I stated I agree for practical purposes these difference are probably not noticeable.


Just MHO

 

[laynes69]

We heat with a EPA certified woodfurnace. Without a blower, we would have very little heat in our home. Our blower is a 1300 cfm 4 speed blower. While the blower probably cools firebox temps, we maintain secondary combustion during the burn. A stove or insert blower is much smaller than our furnace. I think design plays a big part in maintaining temps within the firebox. Our firebox is lined with 1/4" insulation and firebrick up to the baffle. So heat is maintained inside the firebox, the top of the firebox and the large secondary heat exchanger is where most of our heat is extracted. During the burn as the blower cycles off and on, I see no difference with the burn. Just like any stove or insert, it's important for us to get the firebox temps up before closing the primary damper, otherwise things won't burn correctly. Unfortuantely I have no way to monitor temperatures, so I rely on the look of the fire and chimney. Flue temps run lower than a stove due to the heat exchanger.

 

[raybonz]

Never said I don't like or don't use electricity.
Actually, I'm very grateful for it.
Just don't like to waste it or use it
when I don't have to or want to.

And, I am not prejudiced against electricity.
Electric power in my area fails several times
a year including each winter. If I were dependent
on electricity for heat, I would be SOL, sometimes
for days. For me, one of the main reasons
to have wood heat is that it lets me, in a small way,
function more independently and lessens the amount
I pay to the power company. I grow the wood, cut and
split the wood, dry the wood and I burn the wood.
So, when when the power failure occurs, it's no BFD,
and I feel good.

I just asked why run a fan blade (when you
don't have to).

As for 'indoor weather', it comes from hot metal
stove surfaces creating superheated air (room air
on hot stove surface), currents and drafts (convection)
accentuated with the inherent metal stove
heating/cooling thermal cycling.

Add a fan to compound the natural convection from
any wood burning device and push air around the
room increasing air movement, you have 'wind chill'
not to mention needing more BTUs.

Aye,
Marty

tu déconnes, tu racontes n'importe quoi, t'es bidon Marty ;-)

Ray

 

[HotCoals]

Never said I don't like or don't use electricity.
Actually, I'm very grateful for it.
Just don't like to waste it or use it
when I don't have to or want to.

And, I am not prejudiced against electricity.
Electric power in my area fails several times
a year including each winter. If I were dependent
on electricity for heat, I would be SOL, sometimes
for days. For me, one of the main reasons
to have wood heat is that it lets me, in a small way,
function more independently and lessens the amount
I pay to the power company. I grow the wood, cut and
split the wood, dry the wood and I burn the wood.
So, when when the power failure occurs, it's no BFD,
and I feel good.

I just asked why run a fan blade (when you
don't have to).

As for 'indoor weather', it comes from hot metal
stove surfaces creating superheated air (room air
on hot stove surface), currents and drafts (convection)
accentuated with the inherent metal stove
heating/cooling thermal cycling.

Add a fan to compound the natural convection from
any wood burning device and push air around the
room increasing air movement, you have 'wind chill'
not to mention needing more BTUs.

Aye,
Marty

Wind chill only effects living things,by blowing the heat off of said living things.
But I'm like you..by practice I hardly ever run them.
But..
I'm still heating off a 1/2 load from about 10 last night.
Had some decent wood at 7 this morning.
I opened the BKK all the way up with fans on high for a hour and half.
Man it went from 71 to 76 in the next room in that time.
At that point i was down to coals..turned the fans off and left full air.
It is now 77 in that room next to the stove with decent coals yet..but burning down.
I doubt I add any wood till after dark.
I'm burning ash and and the coals seem to last a long time.
Doing this I also am keeping my ash way down.
I'm going to try burning this way for a week.
So when I load tonight I'm going to do another 1/2 load and fire her hard(with fans) for awhile then put the t-sat on 2 to see how it goes.
I want just some wood left in the am to crank her again with fans then when I hit just coals turn off the fans and leave the air on high.
I dunno if I will use less or moor wood then just filling her up for the long slow burn...24 hours or more.
I guess I should weigh out the wood I use both ways.

I just read what I posted..it seems if I do the short hot fires I'll only burn a 1/2 laod in 24 hours compared to full load..hmmm.
But that's on;ly if I make it till tonight for the reload at 10ish.
I bet I end up added some before that and burn that off fast with fans on...we shall see!

 

[Martin Strand III]

On voix la mer d'ici.

Aye,
Marty

 

[bokehman]

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).

Of course the car uses more fuel, the energy has to come from somewhere. A good example is air conditioning. On average about 10% more fuel is used; in town maybe 20-25% more.

Really, efficiency is about keeping the fire in the zone. If you take too much heat out (with the blower) you will need more air to keep the fire alight. More air means more heat wasted up the chimney. Efficiency means the maximum energy released per unit of fuel put in the stove. It is nothing to do with how warm or comfortable you feel.

 

[raybonz]

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).

Of course the car uses more fuel, the energy has to come from somewhere. A good example is air conditioning. On average about 10% more fuel is used; in town maybe 20-25% more.

Really, efficiency is about keeping the fire in the zone. If you take too much heat out (with the blower) you will need more air to keep the fire alight. More air means more heat wasted up the chimney. Efficiency means the maximum energy released per unit of fuel put in the stove. It is nothing to do with how warm or comfortable you feel.

It takes less fuel to drive down the highway with the A/C on than have with the windows open due to wind drag.. I get good milage with the A/C running in the car. Better than winter in fact with no A/C running.. Sorry I don't buy any of the B/S being slung here lately..

Ray

 

[begreen]

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

Many modern stoves are convective - by design. There's less radiant heat coming from their sides and back and sometimes top because of a second shielding skin around the firebox. The firebox is insulated so that turning on the blower does not dramatically affect the stove's combustion efficiency. But they are designed so that running the blower will improve its heating efficiency. This is true of PE, Quadrafire, Napoleon, and many other brands of modern, close clearance stoves. And true for most modern inserts.

 

[begreen]

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).

Of course the car uses more fuel, the energy has to come from somewhere. A good example is air conditioning. On average about 10% more fuel is used; in town maybe 20-25% more.

Really, efficiency is about keeping the fire in the zone. If you take too much heat out (with the blower) you will need more air to keep the fire alight. More air means more heat wasted up the chimney. Efficiency means the maximum energy released per unit of fuel put in the stove. It is nothing to do with how warm or comfortable you feel.

It takes less fuel to drive down the highway with the A/C on than have with the windows open due to wind drag.. I get good milage with the A/C running in the car. Better than winter in fact with no A/C running.. Sorry I don't buy any of the B/S being slung here lately..

Ray

Many new hybrid cars are more efficient with the windows closed and AC on, than with the windows open and the AC off. The AC is electric and does not scavenge energy off of the engine.

 

[raybonz]

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).

Of course the car uses more fuel, the energy has to come from somewhere. A good example is air conditioning. On average about 10% more fuel is used; in town maybe 20-25% more.

Really, efficiency is about keeping the fire in the zone. If you take too much heat out (with the blower) you will need more air to keep the fire alight. More air means more heat wasted up the chimney. Efficiency means the maximum energy released per unit of fuel put in the stove. It is nothing to do with how warm or comfortable you feel.

It takes less fuel to drive down the highway with the A/C on than have with the windows open due to wind drag.. I get good milage with the A/C running in the car. Better than winter in fact with no A/C running.. Sorry I don't buy any of the B/S being slung here lately..

Ray

In many new hybrid cars, they are more efficient with the windows closed and AC on, than with the windows open and the AC off. The AC is electric and does not scavenge energy off of the engine.

BG the engine charges the batteries which run the A/C so yes the engine runs the A/C .. The energy has to come from somewhere and the alternator can be quite a mechanical load when delivering amperage to the batteries.. Windows open create a wind drag which makes the engine work harder to move the vehicle forward so the A/C with the windows closed makes the car more aerodynamic etc..

Ray

 

[LLigetfa]

In many new hybrid cars, they are more efficient with the windows closed and AC on, than with the windows open and the AC off. The AC is electric and does not scavenge energy off of the engine.

BG the engine charges the batteries which run the A/C so yes the engine runs the A/C .. The energy has to come from somewhere and the alternator can be quite a mechanical load when delivering amperage to the batteries.. Windows open create a wind drag which makes the engine work harder to move the vehicle forward so the A/C with the windows closed makes the car more aerodynamic etc..

Ray

Ja, the savings come from aerodynamics. The MythBusters tested it.

 

[LLigetfa]

The second skin theory, not counting open air radiation shielding... many stoves draw secondary combustion air through chambers between the skins to preheat it so in theory, there could be heat stolen away reducing the temp of the secondaries. The question then is whether too much heat is stolen away.

 

[raybonz]

In many new hybrid cars, they are more efficient with the windows closed and AC on, than with the windows open and the AC off. The AC is electric and does not scavenge energy off of the engine.

BG the engine charges the batteries which run the A/C so yes the engine runs the A/C .. The energy has to come from somewhere and the alternator can be quite a mechanical load when delivering amperage to the batteries.. Windows open create a wind drag which makes the engine work harder to move the vehicle forward so the A/C with the windows closed makes the car more aerodynamic etc..

Ray

Ja, the savings come from aerodynamics. The MythBusters tested it.

They also found that a dirty car got better milage than a clean car which I found strange.. Did you see that episode?

Ray

 

[begreen]

Some yes, some no. The question posed is whether the blower negatively affects combustion efficiency. For a modern, insulated firebox my thought it no.

Does anyone know if the blower has to be on during EPA testing of a stove? I searched but so far have not found info on this.

 

[raybonz]

Some yes, some no. The question posed is whether the blower negatively affects combustion efficiency. For a modern, insulated firebox my thought it no.

Does anyone know if the blower has to be on during EPA testing of a stove? I searched but so far have not found info on this.

I have never heard of a test with the blower on. I think they are looking at emissions, efficiency, and clearances.. I agree with the insulation that retains the heat to enhance complete combustion and feel the impact of a blower is negligible to stove performance with the small amount of heat extracted.. If this were a major concern to the EPA I am sure they would factor it in or make mention of it in their testing.. In my opinion a blower is a must with a convection stove and without it would make more sense to run a radiant only stove.. Think about running an A/C, would it make sense to run one without a blower? I think not and it's really a convection cooling unit and worthless without an airflow.. A convection wood stove however will still function OK without a blower but with less efficiency and reduced thermal transfer as there is still natural convection that will take place in the even of a power outage..

Ray

 

[Martin Strand III]

Guyz,
if blowers are so cool
why are they usually
an OPTIONAL accessory?

My thought is because
they are NOT NEEDED.**

They are sold because
they make the mfg and
dealer more MONEY
taken fromyou, the buyer.

Please put that into your
combustion equations.

Aye,
Marty
**Grandpa used to say,
"A sucker is born every minute."

 

[begreen]

That's a bit too general Marty. It depends on the house too. We run our T6 most of the time with the blower off. It convects well naturally and the ecofan mixes the air a bit. This coupled with stove placement in an open floorplan keeps temps comfortable throughout the house until it gets below about 25F. Below that I will run the blower more during the day.

Besides the blower there are lots of things missing in the EPA tests if concerned about combustion efficiency. The tests are not done with cordwood, not tested E/W burning vs N/S in a square firebox, and they don't test for how long it takes for a stove to go from no smoke to smokefree on startup. These are all options in the hands of the user. To its credit, they have tested the difference between well maintained and poorly maintained stoves and saw quite a difference. Dry wood in a well maintained, modern stove provided the best consistent results.

 

[Armoured]

I think JHarkin has this right - it's the overall system efficiency that matters, and that has to include the measure of getting what you want out of the system. Which is (usually) heating the living space to the desired level. Granted, that is complicated by whether we mean 'average' temperature or some other measure that accounts for the fact that most people don't want 300 degrees within five feet of the heat source and 55 degrees everywhere else. It's like the pedantic argument that lightbulbs that convert only ten percent of the energy into light and the rest to heat are not 'inefficient' because the heat is not destroyed (basic law of physics, energy can't be destroyed) - which is true, but most people don't need heat at or near their lightbulbs or light fixtures and it won't appreciably help them feel warm. Users of woodstoves/inserts want useable heat, however they may define that. If they need a blower to get the heat around and make the space liveable, that will improve system efficiency. Of course, not everyone will need that, but if you need one to get the heat useable, it won't (all else being equal) decrease efficiency.

The 'all else being equal' condition is important, but probably marginal in most cases - as far as I can figure out, whether removing heat using a fan/blower would reduce the stove's efficiency (the combustion efficiency, not the 'system' efficiency) due to falling out of the optimal operating temperature (range).

The better parallel is probably to automobile heat, not the A/C. Most of the time, a car will be using waste heat (by which I mean heat that cannot otherwise be used/converted into movement or secondary system power) to heat the passenger cabin. Using it won't impact mileage appreciably (assuming fan power is negligible) most of the time, and having the passenger car compartment be 'useably' warm is part of the car's usage constraints.

The one exception is really only where the engine is too cold and below optimal temperature range for petrol combustion. During warm-up, stealing heat could slow down the process of getting to that temperature and reduce combustion efficiency dramatically (and I understand cold start-up is precisely when a lot of pollution occurs). (I say 'could' because it's possible the car could be taking heat downstream enough - off the exhaust - so as to have no impact). In practice, you can run your heating while the engine is cold ... and get cold air, but you would be delaying the point of getting to optimal temperature range.

In both cases (car and stove), "best" practice would probably be to wait until the system is in good operating temp range before turning on the fan (removing heat, delaying the point when optimal temperature range is reached), and not running the fan enough to drop it down to below the right temperature combustion range for efficiency.

For cars, my experience has been that modern cars are designed very well to get to the right temperature range when used as instructed and to maintain their temperature (apart from start-up, my car NEVER varies from it's midpoint range, pretty much no matter how it is operated). Long warm-up should not be needed even in cold weather - wait a very short period until the engine stops running rough and then drive, they get up to heat best when used under load. Except in very extreme weather/older cars, you don't need to put cardboard over the radiator like in the old days, nor do they overheat - engine compartments are often insulated so they get to the right temperature quickly. And yes, cranking heat probably slows the process, no idea whether it is significant enough to matter - probably not under load.

How does this relate to stoves/inserts? Well, stoves/inserts and installation and operating conditions/fuel vary a lot more. But can't hurt to use (what seems to me to be) common sense: use the fan/blower if it helps spread heat in a useable way; 'too much' fan is too much. Don't use the fan to spread heat when there isn't any (i.e. when the stove is cold), and cautiously during 'edge periods' (warming up and cooling down at points below good operating temps). Probably most important not to turn up the fan full blast when warming up (when cooling down, hopefully it's because the fuel has mostly already combusted). In practice will probably depend somewhat on specific stove/home operating conditions, but mainly at margin.

Oh, and of course it would be great if every installation/home were designed perfectly so as to spread heat optimally for comfort through simple convection/radiation and no fans/blowers needed. And everyone should get a pony, too.

(Just my somewhat ill-informed opinion)

 

[savageactor7]

Do stove-mounted blowers inhibit combustion efficiency

No, I don't think so but the stove top temp will foll you because of the lower temp reading.

rich I have that same stove but experience with has dictated I use the blower...with a fan.

We have a huge old farmhouse and heat with wood 96% of the time. imo the stove is too small for the house but the wife wanted this one NOW so that was that.

We burn the stove WOT all the time only adding a log or 2 at a time. The blower stays on low 24/7 once fully into the season and most of the time there's a floor fan 20' away on the porch blowing cold air toward the stove...

...which does warm up the kitchen at the far end of the a few degrees.

As an aside I was reluctant to use the blower at first because I thought it was too loud. I guess you get use to it. The blower not only helps fan out heat but imo keeps the inner firebox cooler so I'm less likely to over-fire the unit.

Don't get wrapped up in stove top temps because these jacketed stoves are deceiving.

 

[Treacherous]

My fan, on high, can overcome my snap disk though if I am running in the 400-500 range.

I have moved it around everywhere as well.

I put in an optional bypass switch to address this though.

In the 500-600 range it isn't an issue.

Do stove-mounted blowers inhibit combustion efficiency

No, I don't think so but the stove top temp will foll you because of the lower temp reading.

rich I have that same stove but experience with has dictated I use the blower...with a fan.

We have a huge old farmhouse and heat with wood 96% of the time. imo the stove is too small for the house but the wife wanted this one NOW so that was that.

We burn the stove WOT all the time only adding a log or 2 at a time. The blower stays on low 24/7 once fully into the season and most of the time there's a floor fan 20' away on the porch blowing cold air toward the stove...

...which does warm up the kitchen at the far end of the a few degrees.

As an aside I was reluctant to use the blower at first because I thought it was too loud. I guess you get use to it. The blower not only helps fan out heat but imo keeps the inner firebox cooler so I'm less likely to over-fire the unit.

Don't get wrapped up in stove top temps because these jacketed stoves are deceiving.

 

[Corey]

Guyz,
if blowers are so cool
why are they usually
an OPTIONAL accessory?

My thought is because
they are NOT NEEDED.**
...

I would say that is exactly how you know they ARE needed! Manufacturers always leave out the important stuff because it allows them to lower the price and they know once you get the new gadget, the options to make it actually work are an easy sell. Ever buy a computer, printer, home electronics, anything which runs on batteries? The one A/V cable, network cord, bit of software, battery, USB cable or anything else which makes your new device work to it's fullest potential is usually MISSING! and you have to run back to the store and get it.

But you don't have to take my word for it, the forum is littered with 'couldn't get heat until I added a fan' and 'added a fan... WOW much more heat!' threads.

 

[oldspark]

Let's see this is hard, house is warmer with the fan running do I really need it, hell no I can just put on a sweater.

 

[ddddddden]

I dunno how the numbers work out for bottom-line efficiency, but the BK Princess insert does not do marathon burns like its freestanding sister stove does, reportedly because the insert has to keep the blower running. So it looks like a blower does increase heat transfer, but you have to burn fuel at a faster rate to support that.

 

[raybonz]

I don't buy it.. The blower is extracting heat from the metal not blowing into the fire.. Other than a slight cooling of the stove surface I see no negative effects at all..

Ray