Question about firebox temps, fans, and extracting the most heat

[ADDvanced]

Maybe this is simple, maybe I'm over thinking it, but here goes:

I've read many places that you want to get your insert's firebox pretty dang hot, the hotter the better really, to put out more heat, and to prevent creosote. Makes sense.

My insert has a blower, with a thermo-actuated switch, that you can manually bypass and turn on. If I leave it in 'auto', the fan only kicks on when the thing is pretty dang hot, but most of the burn cycle is already done at that point, so the majority of the heat has gone up the chimney.

If I put the fan on at the beginning of the burn cycle, the moving air keeps the firebox cooler, and it doesn't get as hot.

HOWEVER.... doesn't the law of thermodynamics say that energy transfer happens faster when two things are farther apart in temperature? By that rational, shouldn't I run my blower fan on full speed, even at the beginning of the burn cycle? This would prevent the firebox from ever getting really hot, but it should also keep the entire insert the coolest.... which is further away from the temps of the fire... which means more heat transfer.

So which is it? Leave it on auto, and it barely ever kicks on? Or leave it on 100% the entire burn cycle and keep the whole insert fairly cool, knowing that by keeping it cool I've extracted the most amount of heat?

 

[Ctwoodtick]

My opinion after using my insert for about 8 years- I turn on the blower manually after roughly 20 minutes after starting from kindling. By that time, the insert’s stove top should be putting out good useable heat. The auto feature of the blower takes too long to kick on especially if you have some depth of ash in the firebox. I will use switch to the auto feature when I go to bed so that blower will turn off once stove top has gone relatively cool. This prevents the blowers from running all night for no reason.
You don’t have to torch the firebox to limit creosote in the chimney/liner. If you have wood that is let’s say at 20% moisture content, you can turn the air down as much as possible so that you are still getting good secondary combustion at top of stove. You know you’ve turned the air down too much when you see visible smoke from chimney. How dry is your wood?

 

[bholler]

Maybe this is simple, maybe I'm over thinking it, but here goes:

I've read many places that you want to get your insert's firebox pretty dang hot, the hotter the better really, to put out more heat, and to prevent creosote. Makes sense.

My insert has a blower, with a thermo-actuated switch, that you can manually bypass and turn on. If I leave it in 'auto', the fan only kicks on when the thing is pretty dang hot, but most of the burn cycle is already done at that point, so the majority of the heat has gone up the chimney.

If I put the fan on at the beginning of the burn cycle, the moving air keeps the firebox cooler, and it doesn't get as hot.

HOWEVER.... doesn't the law of thermodynamics say that energy transfer happens faster when two things are farther apart in temperature? By that rational, shouldn't I run my blower fan on full speed, even at the beginning of the burn cycle? This would prevent the firebox from ever getting really hot, but it should also keep the entire insert the coolest.... which is further away from the temps of the fire... which means more heat transfer.

So which is it? Leave it on auto, and it barely ever kicks on? Or leave it on 100% the entire burn cycle and keep the whole insert fairly cool, knowing that by keeping it cool I've extracted the most amount of heat?

Really hot and fairly cool don't help much. You need to keep the flue gasses above 220 at the top of the chimney. You are going to have to run a little hotter to accomplish that because your liner isn't insulated.

 

[Highbeam]

Heat exchanger efficiency is highest when the temperatures are farthest apart but combustion efficiency varies greatly with firebox temperature. Do you want maximum output or maximum overall efficiency? Almost never get them at the same time.

 

[NoGoodAtScreenNames]

I use an Auber controller to bypass my insert’s switch. The insert is technically in always on mode but the controller controls the power through the cord. It measures the flue collar temp. It comes on at 400F and will turn the fan off if it dips below 370F. During startup that allows the fan to run earlier without it excessively cooling the stove in case I’ve turned the air down too aggressively.

This also turns the fan off my earlier in the late stages of the burn. I don’t see much difference in heat output with the fan at that point and I prefer the silent operation.

 

[EbS-P]

I think an easier approach might be to get a magnetic thermal switch 120-150 degrees and some extra wire and find a spot on the insert that heats up more quickly but won’t get really hot. Don’t want to demagnetized or melt insulation . I am about to do this with my insert. The location of the thermal switch just isn’t great. I like to run my blower on low all the time and then turn it up when i have a big fire or really need the heat.

 

[ADDvanced]

Heat exchanger efficiency is highest when the temperatures are farthest apart but combustion efficiency varies greatly with firebox temperature. Do you want maximum output or maximum overall efficiency? Almost never get them at the same time.

I want the most BTUs of heat into my house. Is that efficiency or max output?

 

[ADDvanced]

My opinion after using my insert for about 8 years- I turn on the blower manually after roughly 20 minutes after starting from kindling. By that time, the insert’s stove top should be putting out good useable heat. The auto feature of the blower takes too long to kick on especially if you have some depth of ash in the firebox. I will use switch to the auto feature when I go to bed so that blower will turn off once stove top has gone relatively cool. This prevents the blowers from running all night for no reason.
You don’t have to torch the firebox to limit creosote in the chimney/liner. If you have wood that is let’s say at 20% moisture content, you can turn the air down as much as possible so that you are still getting good secondary combustion at top of stove. You know you’ve turned the air down too much when you see visible smoke from chimney. How dry is your wood?

Wood is pretty dry, maybe 12-15% moisture content.

 

[bholler]

I want the most BTUs of heat into my house. Is that efficiency or max output?

That depends. If you want the highest btu output into your house or if you want the most BTUs from each piece of wood into your house.

 

[Highbeam]

I want the most BTUs of heat into my house. Is that efficiency or max output?

I interpret your statement as you want the most heat regardless of whether you’re burning efficiently or not. Is that the case? Then you need to shoot for highest stove top temperature.

 

[ADDvanced]

That depends. If you want the highest btu output into your house or if you want the most BTUs from each piece of wood into your house.

The most, total BTUs extracted.

I have a finite amount of wood. I want the most heat I can get from that fuel into my house.

 

[velvetfoot]

The thermo switch on my insert also sucked. A magnet thermo switch, which I had on a previous insert would work better.

I also am using an Auber pid controller to turn the fan on initially, switch to high speed at a higher temp (it has 2 speeds), and the reverse. My thermocouple in on the stove top. Probably not as good as a hole drilled into the collar, but easier. It seems to react quick enough, and I like the digital readout.

 

[EBFIRE]

Maybe this is simple, maybe I'm over thinking it, but here goes:

I've read many places that you want to get your insert's firebox pretty dang hot, the hotter the better really, to put out more heat, and to prevent creosote. Makes sense.

My insert has a blower, with a thermo-actuated switch, that you can manually bypass and turn on. If I leave it in 'auto', the fan only kicks on when the thing is pretty dang hot, but most of the burn cycle is already done at that point, so the majority of the heat has gone up the chimney.

If I put the fan on at the beginning of the burn cycle, the moving air keeps the firebox cooler, and it doesn't get as hot.

HOWEVER.... doesn't the law of thermodynamics say that energy transfer happens faster when two things are farther apart in temperature? By that rational, shouldn't I run my blower fan on full speed, even at the beginning of the burn cycle? This would prevent the firebox from ever getting really hot, but it should also keep the entire insert the coolest.... which is further away from the temps of the fire... which means more heat transfer.

So which is it? Leave it on auto, and it barely ever kicks on? Or leave it on 100% the entire burn cycle and keep the whole insert fairly cool, knowing that by keeping it cool I've extracted the most amount of heat?

I have an insert with auto switch that turns on certain temp (never cared to see what it is) so I'll chime in with my opinion and what has worked for me. I start my fire with a small three splits and kindling starter fire after I have heated up the chimney to prevent a down draft (filled my house twice before I learned crack a window for 5 minutes) I start this fire as close to switch sensor as possible to heat it up as quickly as I can. I then reload if needed once fan kicks on. Some days it 10 minutes or some days it's twenty depending on intensity and correct placement of starter fire. I have not found I have ever needed to by- pass the factory switch in my insert if I burn dry wood, keep the insert clean, and use common sense. I have also packed it to the gills to start and have used the theory of full load then reload full load burn cycle and it hasn't worked for me except for overnight. During the day or evening I replace with a couple good size splits and keeps my burn temp where I want it. Try building your starter fire as close to the thermo plates or whatever they are called and see if that works. Also I have found bigger splits work better in my insert. Yours may be different. I have an insulated liner, insulated block off plate, and 1/4 of chimney is indoors. I read all the info on inserts here before I did the install myself. I hope this may help.

 

[bholler]

The most, total BTUs extracted.

I have a finite amount of wood. I want the most heat I can get from that fuel into my house.

In that case you want to get up to temp as fast as possible so fan off. Then once up to temp fan on high and air shut as far as you can while maintaining 220 at the top of the chimney. And make sure your air is actually shutting all the way. Getting the insert further out of the firebox will also help get more heat off of it.

 

[ADDvanced]

So in terms of the fan.... I guess if I want most BTUs extracted, why does heating the piss out of the entire insert first, and then putting the fan on high... why is that the better idea?

Again due to that law of thermodynamics that says the bigger the temp differential, the faster the heat transfer..... if I keep the insert itself cool, like, get a fire going pretty good, and throw the fan on high, walk away. Wouldn't that put more heat into the house, vs, heating up the insert to 4-500, then turning on the fan? Once the insert is 4-500 it is absorbing less heat created by the fire, since the metal is closer to the temp of the fire itself. So keeping that metal cool, and never allowing it to heat up, would likely extract the most heat, no?

 

[bholler]

So in terms of the fan.... I guess if I want most BTUs extracted, why does heating the piss out of the entire insert first, and then putting the fan on high... why is that the better idea?

Again due to that law of thermodynamics that says the bigger the temp differential, the faster the heat transfer..... if I keep the insert itself cool, like, get a fire going pretty good, and throw the fan on high, walk away. Wouldn't that put more heat into the house, vs, heating up the insert to 4-500, then turning on the fan? Once the insert is 4-500 it is absorbing less heat created by the fire, since the metal is closer to the temp of the fire itself. So keeping that metal cool, and never allowing it to heat up, would likely extract the most heat, no?

No. And keeping the metal cool will fill your chimney with creosote. Btw 400 to 500 is not all that hot for a stove really.

 

[NoGoodAtScreenNames]

I can say if I run my fan too aggressively too soon it has an impact on the draft and could create a downward spiral. High fan cools off insert, cooler insert gets less draft. Lower draft reduces the burn rate which gets a cooler insert which lessens draft - etc. You could not turn it down so much but then a lot of that air is just warming the outside by sending more hot air up the chimney.

Basically you want to find the temp and burn rate where it will cruise with active flame (assuming a non-cat) with the fan on for a few hours.

 

[Highbeam]

I’m an engineer, spent plenty of time in thermodynamics classes. That is theory and applies simplified calculations where you assume certain things are constant. To get the most heat into your home from a fixed amount of wood you want highest system efficiency. There is more than one component of system efficiency in this system. The big two are combustion efficiency and heat exchanger efficiency. You could plot both on a graph and the lines are much different. Where the lines cross is approximately where you want to run your stove.

It’s going to be at relatively low stove temperatures but not too low to support clean combustion.

 

[EatenByLimestone]

If somebody ever plotted that, it would be awesome. It'd be different for every stove and flue system, location, but if you could direct people to burn at around 400° stove top it would be awesome.

On my T5, on a startup, I can keep the stovetop around 300 and have secondaries firing. I doubt the thermometer is accurate, and is only there as a curiosity, but that's what it says.

 

[ADDvanced]

I’m an engineer, spent plenty of time in thermodynamics classes. That is theory and applies simplified calculations where you assume certain things are constant. To get the most heat into your home from a fixed amount of wood you want highest system efficiency. There is more than one component of system efficiency in this system. The big two are combustion efficiency and heat exchanger efficiency. You could plot both on a graph and the lines are much different. Where the lines cross is approximately where you want to run your stove.

It’s going to be at relatively low stove temperatures but not too low to support clean combustion.

I'm not an engineer but I work with a lot of them, and what you said makes sense. There's like two opposing ideas here... and on top of that point where those two lines overlap, you also then have the fan being on/off, and how fast the fan is running. You'd think someone would have this all figured out by now. Maybe I'm just over thinking it.

 

[bholler]

I'm not an engineer but I work with a lot of them, and what you said makes sense. There's like two opposing ideas here... and on top of that point where those two lines overlap, you also then have the fan being on/off, and how fast the fan is running. You'd think someone would have this all figured out by now. Maybe I'm just over thinking it.

It is going to be different for every stove on every install. And even vary depending upon outside temps and barometric pressure. You are dealing with a natural draft appliance nothing is really a constant

 

[Highbeam]

A good place to start for high efficiency is to keep flue temps low but high enough to not condense in your chimney. It’s hard to measure flue temps on most inserts.

This could all be just splitting hairs. These modern stoves are all pretty efficient when burning appropriate fuel at stove temperatures high enough to not smoke but low enough to be safe.

Inserts can be especially challenging inside that masonry mass.

 

[TheDootler]

I agree with everyone here - you want to wait for the metal to get hot.

Fire is a heat reaction that happens to also release heat. The more heat you have the better that reaction continues occurring -> more % of your wood burnt and not flying out the chimney.

The basic problem each operator has to figure out with each wood stove install is how to balance:

• Wood burning efficiency
• Heat lost to the chimney
• Creosote

Generally it is a solved problem - run your stove between 500F and 600F Stove-top-temp and check outside occasionally to make sure you don't see any unburnt gasses (smoke.) If you see smoke, you're doing it wrong and are going to get a mix of creosote and less efficient burning. If you have overly high temps (700+) you're probably wasting heat up the chimney.

By keeping the metal cold by running the fan early, you're prolonging the inefficient burning stage by not allowing the heat-reaction to get hot enough to burn all the gasses. You should find that if you turn on the fan too early that it's harder to keep your fire going.

It's exactly why you build a pocket of hot stuff when you build a fire and not a bunch of spread out kindling all over the place. 2-3 pieces of kindling with a small bit of egg carton or w/e is enough in a small area of the wood stove to catch everything; but if you spread it all out then you'll just singe some wood and start over.

 

[ADDvanced]

I’m an engineer, spent plenty of time in thermodynamics classes. That is theory and applies simplified calculations where you assume certain things are constant. To get the most heat into your home from a fixed amount of wood you want highest system efficiency. There is more than one component of system efficiency in this system. The big two are combustion efficiency and heat exchanger efficiency. You could plot both on a graph and the lines are much different. Where the lines cross is approximately where you want to run your stove.

It’s going to be at relatively low stove temperatures but not too low to support clean combustion.

Reviving an old topic, I've been pondering this for a while, and I'm not sold anymore.
I want THE MOST HEAT IN MY HOUSE, right? Right.
So let's think of the woodstove as a heat generating device, and the fan, which blows through the wood stove and the outer skin, a heat extraction device.
Law of thermodynamics says the greater the temp difference, the faster the energy transfer, right? Right.

So I'd ideally want to keep my stove as cool/fan on high as possible, while still having a good burn. If it struggles to say lit, or I see smoke, that is obviously too cold for the reaction to take place, but if the fire is burning, wouldn't keeping that inner skin as cool as possibly pull the MOST heat into the house? Seems like it.

 

[ABMax24]

Heating appliance efficiency has been calculated forever by the stack loss method. The higher the temperature of the flue gases the lower the efficiency, also the higher the residual O2 concentration in the flue gas the lower the efficiency (because you are heating excess air and sending it up the chimney).

Cooling the shell of the stove increases efficiency, whether that be through fans or another means, because a cooler stove is able to absorb more heat from the flue gases. Alternatively on some stoves, redirecting the flue gases to cooler parts of the stove before sending them up the stack could also increase efficiency.

I'm a perfect world you'd put a great big heat exchanger on the flue, capture all the heat, have a drain for the water that condenses and send the rest outside at about 150f. In the real world this exchanger would plug within days because wood combustion is rarely complete, and the remaining wood gas would condense and plug the exchanger. My natural gas furnace operates like this, but it produces less PM emissions in a year than my wood stove does in an hour.

Long story short, yes crank up your blower, get as much air over the stove as possible and get the heat into the room.