Do stove fan's make a stove more efficient, or does it just help spread the heat around.

[SuperJ]

Does having a fan on a stove make it any more efficient? Or does it just help even out the temperature in the room. Or worded another way, will I burn less wood with a fan installed, than without (assuming the same room temperature)? I'm assuming EPA stove ratings do not include a fan.

My wood stove will likely be a bit over sized anyways, so it shouldn't have a problem keeping the room warm with or without a fan.

I'm considering buying a stove without a fan, and then adding one if I have trouble moving the heat around. The BK Ashford 30 is already more than I wanted to spend and adding a fan is another $300.

 

[Michael Sean]

My personal opinion would be to run the stove with out the fan. As you progress through the burning season and get into some cold weather, you will be able to see how your stove heats and if it’s suitable for your needs. If it’s not, then go for the fan. If it heats just fine with out the fan then you save your $300.

 

[WoodyIsGoody]

Does having a fan on a stove make it any more efficient? Or does it just help even out the temperature in the room. Or worded another way, will I burn less wood with a fan installed, than without (assuming the same room temperature)? I'm assuming EPA stove ratings do not include a fan.

Well, that's two different questions.

Yes, a fan does increase the burn efficiency but you will likely end up with a warmer house rather than burning less wood (since you said the stove was over-sized for the application). However, if you will be warming the house up from cold (like a 20 degree temperature rise) the fans will not only save you wood, they will heat the place up considerably faster. If you plan to burn 24/7 on a low setting, you don't need them.

 

[lsucet]

Another thing to take into consideration is if the stove is convective or radiate the heat. We can say BK is a convective stove, what can make the fan a must to have. But everything depend if you can get away without them and keep/stay warm. I don't use them much on my princess but it makes a different those time i use them and just in low..

 

[Highbeam]

On the thermostatic bk fans will drop efficiency. These cat stoves are most efficient at the lowest burn rates. Fans strip heat from the stove which makes the thermostat open and increases the burn rate. Any increase in burn rate results in a decrease in efficiency.

On a noncat the fans help to capture some of the waste heat and can increase efficiency when used properly.

 

[lsucet]

I don't understand how the fans are related to efficiency. I really will like to know more about it to understand better. Then a furnace has no efficiency cause depends on fans? I understand that the fans can make the thermostat open more often but at the same time you can dial it lower if what you are looking is distribution of that heat vs more heat. I am not trying to argue or start anything. I just want to know and learn.

 

[jetsam]

Imagine that you are burning your BK very low. The thermostat almost completely closes the air as the firebox warms up, and opens it a bit when the firebox cools down.

Now you turn the fan on. The firebox cools down, so the thermostat opens, so you burn wood faster and get more heat in the room.

I don't agree that this necessarily implies less efficiency.

 

[yooper08]

There's two efficiencies when it comes to wood stove operation. There's the burn efficiency, which is simply how completely does the stove burn the wood, which is related to the burn rate. The second is the heat transfer efficiency, which is how well the stove transfers heat to the living space.

There's ultimately an equilibrium. If you're pulling more BTUs off the firebox than the burn rate can provide, burn efficiency will end up declining until you increase the burn rate to bring it back to equilibrium or in favor of the burn rate.

So, depending on how you'd run the fan, it could decrease your burn efficiency if you needed to maintain higher temps because of it.

Another option is an eco fan that runs about $100 and you place it on the stove and it uses no electricity. I can't imagine that would impact burn efficiency but it would help move warm air around. A manufacturer-funded study at a university showed the eco fan improves heat distribution while using less wood.

 

[lsucet]

I think that is my point of view. It is true that wood burn faster but I know that different burn rates will give you different efficiency. It is confusing.

 

[WoodyIsGoody]

Wow! I can see how the answers in this thread sound contradictory and confusing even though they are all somewhere between 70%-100% accurate when read correctly.

Let's focus on the claim that a fan could cause the thermostat to open from low-burn to a higher (and less efficient) burn rate. First off, this isn't necessarily true even on a thermostatically controlled stove (using a BK as an example). The BK's thermostat has a huge range that it can be turned past the point at which the air intake is closed as far as it's going to close. So if you turn the thermostat all the way low, turning the fans on is not going to cause the air intake to budge past fully closed as long as your fire isn't in it's last stages.

In a different scenario, say with the thermostat set such that the air intake is just barely fully closed with out the fan running, yes, turning the fans on could cause the air intake to open more and this will cause the wood to be consumed faster. However, it's not a given that the actual efficiency will be less because much more heat will be delivered to the room with the fans running. Even if the thermostat self-adjusts to a higher (and slightly less efficient) burn rate, the amount of heat reclaimed by the fans on even a medium setting would more than make up for the faster burn rate.

The bottom line is that the effect the fan has on efficiency is generally positive but to achieve the benefit of extra efficiency either:

1) You need the additional heat, or

2) you turn the stove to a lower burn rate using the thermostat while extracting more heat due to the fans.

Near the end of a burn the fans could stall the cat sooner which would be the least efficient scenario of all.

So, as your cat temp drops and is heading towards the "inactive" zone, it's a good idea to either re-load (if you need the heat) or to turn the fans off (if you don't need additional heat). Turning the fan off with a dwindling fire will increase efficiency by extending the amount of time the cat is active. But, if you have a good fire going and you need additional heat, the fans will increase efficiency but it may be necessary to turn the thermostat down to maintain the same burn rate (while extracting more heat).

 

[lsucet]

The BK's thermostat has a huge range that it can be turned past the point at which the air intake is closed as far as it's going to close. So if you turn the thermostat all the way low, turning the fans on is not going to cause the air intake to budge past fully closed as long as your fire isn't in it's last stages

This is my understanding. We have members here reporting they can dial the stove to low and the fans have no effect. I know that depends on setup, location, draft, etc. Now, saying that, i think, including burning to faster/higher burn rates to those that are affected by the fans, the stove is supposed to burn into the efficiency rating that manufacture claims. The heat still going into the house, just at faster rates.
In my experience I can dial it lower if using the fans. As we all know, makes better distribution of the heat to us with weird floor plan and the wood consumption is not that much different, but is true that burns faster. At least that is my experience. I know it looks like we are off of the OP question but i think these statements are providing better information to him.

 

[Highbeam]

This is all theoretical and really we're talking about minute differences in efficiency. The fans on all woodstoves move only a small amount of air and hardly have a significant effect on efficiency. Efficiency in this context is always total efficiency not combustion efficiency. Total as in heat delivered to the space vs. heat dumped up the chimney as a portion of fuel energy provided.

 

[WoodyIsGoody]

This is all theoretical and really we're talking about minute differences in efficiency. The fans on all woodstoves move only a small amount of air and hardly have a significant effect on efficiency. Efficiency in this context is always total efficiency not combustion efficiency. Total as in heat delivered to the space vs. heat dumped up the chimney as a portion of fuel energy provided.

At high burn rates the fans can improve efficiency substantially. They do this by reducing flue gas temperatures and the effect is not small. Last week I measured the air coming out of the blower outlet at around 550 degrees. The blowers blow across the flue collar and extract heat that would otherwise be wasted right up the chimney. The temperature of the flue collar and top of the upper chamber is reduced by the blowers and that lowered temperature causes those components to absorb more heat from the exhaust stream (due to the temperature differential being larger) before it exits out the stack.

I'm pretty sure the EPA efficiency figures are calculated using test data from stoves without blowers (because the blowers are optional equipment). Maybe if BKVP sees this he can correct me if I'm wrong about that.

 

[cd36]

I believe you are correct about the rating being based on having no blower. I remember when I got our zc fireplace the rsf manual stated that a fan would improve heat output slightly.


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[SuperJ]

One thing to keep in mind is that to calculate BTUs it involves a volumetric flow rate (CFM in this case) and a delta T. You can get big BTUs moving around off a low flow rate if the delta is high enough. So even though the stove fans don't move many CFMs they are still moving a lot of BTUs off the metal and into the room air.
I guess to simplify my original question it would be...

Do you think you burn less wood to keep your house warm if you run a stove fan?

I would think the answer would likely be yes; a fan should theoretically save a bit of wood (but the amount would be debatable), since you wouldn't have to unnecessarily run the stove as hot to get heat further from the stove. If the immediate vicinity around the stove is overheated you will be losing more heat thru the walls, ceilings, and windows, the flue, than if you had a cooler but more even temperature in the room.

 

[lsucet]

Do you think you burn less wood to keep your house warm if you run a stove fan?

It will be hard to tell you if it will consume less wood. everyone setup is different. like i mentioned before some members claim no changes in consumption with fan on or off. i think they have enough draft or better location, etc, allowing this. that is not my situation. I know that my burn times are shorter with the fan on but the heat is there and better distribution thru the house. I also can dial it lower to the point of not make the cat goes inactive. that helps to not see much differences in wood consumption between fans on and off. Another point to take into account in my situation, when I run the fans, I run then on low 90% of the time. I run the stove without fans 80% of the time.

 

[WoodyIsGoody]

Do you think you burn less wood to keep your house warm if you run a stove fan?

I'm going to have to agree with Isucet and say that will largely depend upon the particular install (and how the fans are used). In general and with typical usage, I would say the fans are likely to make the house warmer (and warm it up more quickly) with not a big change in fuel consumption. Probably the most utility offered by the fans in most situations is not that you will burn drastically less wood but in being able to increase the stoves output to warm the house sooner or deal with extra cold, extra windy winter conditions, especially in installs where the stove is not oversized for the situation (without burning more wood). The fans increase the maximum effective heating power of the stove.

A scenario in which the fans could reduce wood consumption significantly is if the stove (without fans) is required to run wide open to maintain comfort but could be run at half-throttle with the fans going and maintain the same temperature.

In reality, seasons change, weather varies and the fans are useful at least a small percentage of the time in any given install. But if they are only beneficial 10% of the time, and make the stove 15% more efficient, that is only an annual wood savings of 1.5% (less wood).

Honestly, I prefer silence to the sound of blowing air so I only run the fans on low and turn them to high only if the structure needs maximum heat fast, to bring it to room temperature from dead cold. Even on low they increase the efficiency of the stove (more heating power for less wood). If the room becomes too warm, I can turn the fans off and reduce heat output while maintaining about the same level of wood consumption. So, yeah, I'm burning less efficiently but the room is more comfortable (not as warm). And turning the fans off doesn't make the emissions dirtier (if anything, slightly cleaner or no change). Emissions do not necessarily rise and fall in direct proportion to how efficiently your stove is burning.

The fans are worth $300 but the stove will perform very well without them too, just less versatility. You can always add them at a later date.

 

[yooper08]

One thing to keep in mind is that to calculate BTUs it involves a volumetric flow rate (CFM in this case) and a delta T. You can get big BTUs moving around off a low flow rate if the delta is high enough. So even though the stove fans don't move many CFMs they are still moving a lot of BTUs off the metal and into the room air.
I guess to simplify my original question it would be...

Do you think you burn less wood to keep your house warm if you run a stove fan?

I would think the answer would likely be yes; a fan should theoretically save a bit of wood (but the amount would be debatable), since you wouldn't have to unnecessarily run the stove as hot to get heat further from the stove. If the immediate vicinity around the stove is overheated you will be losing more heat thru the walls, ceilings, and windows, the flue, than if you had a cooler but more even temperature in the room.

Bingo, you got there before I did. It comes down to how many BTUs are needed to make the space comfy and where they end up vs where you wanted them. Piling up near the stove and relying on natural convection can be inefficient use of BTUs, which is where fans come in and can provide the benefit of flattening the temperature gradient of the space by spreading them out. In certain setups, there may not be a significant difference in wood used. In others, it can be rather significant.

Here's what I'd recommend to experiment with this before you make a fan purchase. If you have a small floor fan, set it up so that it's blowing cold air towards the stove, just run it on low. That alone will help you figure this out given your setup.

If all you need is a little push to move some air around, the eco fans I mentioned earlier can do that, are silent, don't consume electricity (think power outage), and cost about $100. It may be all you need.

 

[yooper08]

Unless someone can correct me, fans don't increase BTU output per se, they just redistribute what's already been put out. Meaning, if my stove's heat transfer efficiency is 75%, 75% of the BTUs will get transferred regardless of fan status. If I run the fan, I'm just distributing that 75% differently. It's not like the flue is sucking a portion of that 75% back in and to the outside (except for combustion air but that happens anyway). The only way to increase BTU output is a hotter firebox.

 

[jetsam]

Unless someone can correct me, fans don't increase BTU output per se, they just redistribute what's already been put out. Meaning, if my stove's heat transfer efficiency is 75%, 75% of the BTUs will get transferred regardless of fan status. If I run the fan, I'm just distributing that 75% differently. It's not like the flue is sucking a portion of that 75% back in and to the outside (except for combustion air but that happens anyway). The only way to increase BTU output is a hotter firebox.

Depends on the stove. On a BK, a fan blowing on the stove increases BTU output and wood consumption. This is because the stove's thermostat increases input air to make more fire as the stove cools off.

 

[cd36]

Unless someone can correct me, fans don't increase BTU output per se, they just redistribute what's already been put out. Meaning, if my stove's heat transfer efficiency is 75%, 75% of the BTUs will get transferred regardless of fan status. If I run the fan, I'm just distributing that 75% differently. It's not like the flue is sucking a portion of that 75% back in and to the outside (except for combustion air but that happens anyway). The only way to increase BTU output is a hotter firebox.

It went increase how much heat your wood is capable of produce, that is set with the wood you're using.

It will increase how much of that heat your wood produces actually ends up in your house. Without a fan your stove will get to a certain temperature, heat the air around it and make itself a little insulating blanket that reduces how much heat can be conducted from the stove into the air. The heat that can't be conducted goes out the chimney.

The fan blowing cool air onto the stove will disturb that insulating blanket of hot air around the stove, allowing the stove to give up more heat. The stove, now surrounded by cool air can conduct more heat out of the stove itself, instead of just sending it up the chimney.

So yes fans will increase how much heat ends up in your house from the wood you are burning. It's just simple thermodynamics. Think of windchill, do you feel colder on a still air day or on a windy day? The windy day disturbs your warm insulating blanket of air around your skin, allowing more heat to conduct out of your body into the air, making you feel colder. Really stoves are no different.

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[Randy Acton]

Here is my 2 cents based on my heating situation.

A woodstove can/will exhibit all three types of heat transfer: conduction, convection and radiation. Adding a fan, whether ceiling fans, a fan blowing across the stove or a fan on the stove, will interrupt what the stove does best all by itself (convect and radiate). As soon as I turn on any kind of fan, any part of the house more than 20' away from the stove, starts to cool down.

I don't personally think a fan all by itself can improve efficiency, all it does is change heat transfer. We all know that a fire needs 3 elements to burn and those elements need to have the proper ratio in order to get the most efficient burn. Sure a fan can extract more heat but not without changing that ratio so somehow you have to get the heat back, so you increase air which in turn burns more wood and increases the firebox temp. If all you do is extract more heat, efficiency goes down.

 

[lsucet]

Here is my 2 cents based on my heating situation.

A woodstove can/will exhibit all three types of heat transfer: conduction, convection and radiation. Adding a fan, whether ceiling fans, a fan blowing across the stove or a fan on the stove, will interrupt what the stove does best all by itself (convect and radiate). As soon as I turn on any kind of fan, any part of the house more than 20' away from the stove, starts to cool down.

I don't personally think a fan all by itself can improve efficiency, all it does is change heat transfer. We all know that a fire needs 3 elements to burn and those elements need to have the proper ratio in order to get the most efficient burn. Sure a fan can extract more heat but not without changing that ratio so somehow you have to get the heat back, so you increase air which in turn burns more wood and increases the firebox temp. If all you do is extract more heat, efficiency goes down.

I understand that everyone's situation is different and i did mention that before. I don't know what type of stove you have, but I can tell you this. The way the fans on BK, blows over the top,( between top of stove and convection deck ) it really make a difference compare to my previous stoves. It is more effective and move the heat very good without the stove room feel overheated compared to the rest of the house. That was not my experience with previous stove fans, the was more noise than heat distribution. Like mentioned before, BK has a wide range of burn rate allowing you including sometimes depending outside temp, house insulation, etc, to use lower thermostat settings when using the fans. Now , I know depends on many factors. floor plans, home dimensions, heat demand needed, etc.

Like also mentioned before here, depending on draft, setup, etc. Some don't see much different on wood consumption between fans on or off. Not my case, but still, not enough differences to take into consideration.

 

[Highbeam]

Probably the most utility offered by the fans in most situations is not that you will burn drastically less wood but in being able to increase the stoves output to warm the house sooner or deal with extra cold, extra windy winter conditions, especially in installs where the stove is not oversized for the situation (without burning more wood). The fans increase the maximum effective heating power of the stove.

It's magic. Drastically less wood consumption with increased output! Not true in my experience.

Those of us with experience know that fans will increase wood consumption on a thermostatic controlled stove. Give it a try this year! You'll see.

The fans do increase output, but they also increase wood consumed. No free lunch!

 

[LondonNeil]

first think about inefficiency in a stove, then think if a fan can affect that.
A stove can be inefficient/wasting energy in 2 ways, #1 failing to burn clean and wasting energy as unburnt fuel(smoke), #2 wasting excess heat up the flue. unclean burn has various causes but that smoke going up the flue is wasted energy. one cause could be a cool stove, perhaps just lit or one dieing down, or just running very low and smoldering. Here a fan will make things worse by sucking more heat from the stove / firebox. So don't run a fan on a smoldering stove.

scenario 2, heat wasted up flue, occurs at the other end of the spectrum when the stove is running really hard. At this point a fan will help efficiency by sucking a bit more heat off the stove and into the room.

So yes a fan can help efficiency if you a running your stove hard, or it could hinder efficiency if your stove is just lit or running very low.