Hate to do this but I am pulling out the soap box, Fuel makes the BTUs not a "bigger stove".

[SmokeyTheBear]

I am unconvinced over the argument that bigger is always more efficient. It's in the design...you have to keep enough heat in the exhaust to avoid condensation in the stove and flue..there are lots of small every efficient stoves (pellet and otherwise). Also in most cases a smaller stove running harder will be more efficient than a larger one loafing along.

Nobody said that bigger was always more efficient.

 

[stoveguy2esw]

i think one should look at the claims on some stoves and wonder, "X" btu's per hour is based on how many lbs per hour are burned and what the transfer efficiency is. most pellet stoves are going to run in the 70's to low 80's in efficiency (at least thats what im lead to believe)

so using one stove mentioned in this post (not picking on that stove specifically,but its a prime example), this unit claims 70Kbtu now if you base that on the 8500 btu per lb average in the pellet world that means the stove would have to feed about 8 1/4 pound of pellets an hour to achieve this. now using the same figures and looking at output at 78% (minimum default efficiency) thats 6630 btu/hr thats a staggering 11 lbs per hour to output 70Kbtu (or 1 bag every 3.6 hours) kind expensive to run through that kinda fuel.

one should question things such as consumption rates (lbs/hr) both low and high end, and estimated (usually best case) sq footage of coverage, bear in mind also that best case "X" sq ft, means tight well insulated open floor plan heating from outside temps of 32F, if your house isnt super insulated the max may not be what it will cover. for example my home is not very well insulated and its about 1200 sq ft, stoves ive run in it have heated it reasonably easily until weather gets below mid teens, it keeps it comfy with more fuel use but both ive run lately have been rated higher in square footage than i own. OTOH if i taped off the same square footage in my yard , the stove aint gonna heat it. think about your heat loss when sizing a stove especially pellet stoves, unless you have a brand new well insulated house or have done work to get there go bigger by sq foootage.



but the best advice i can give is ignore the "BTU ratings" unless its stated on the EPA cert tag on the stove its not likley to be accurate.

 

[Countryboymo]

I am still waiting for my shipment of hydrogen encapsulated pellets that have more btu's impregnated into the wood fibers. They are also easier to handle because the hydrogen is lighter than air. A normal sized 40lb bag weighs around 25lbs. *sigh* oO(I think I need to check the carbon monoxide detector again)

Edit.. I might have just came up with BTU's next marketing angle!

 

[AVIVIII]

ESW, your math is right, but it doesn't HAVE to burn at that rate for a full hour.

Mr. Takeman said the magic word, RECOVERY.

The point is that for whatever time necessary a larger stove can put out enough BTUs to heat up the given area from a certain previous temperature, quicker.

My P68 is capable of consuming large amounts of pellets for a time (brief or extended) in order to warm up the giant slug of cold air in my house. I'll go back to the P68 vs the Empress example. If I can figure it out precisely enough, I could set the P68 to feed at the same rate as the MAX on the Empress burning the same pellets and I would effectively be putting out the same amount of heat in both stoves.

If I wanted to actually make 68,000 BTU at ~75% efficiency, yes, I would be burning 11 lbs per hour. So potentially I could burn 6 bags a day, if my stove didn't melt and I wanted it 130*f inside my house.... But if I burn 11 pph for 15 minutes and the stat rolls her back to 'normal' I effectively burned LESS pellets in the P68 than my buddy did in his empress (given the same space) to achieve the same rise in temperature. He would have had to burn wide open (38k BTU = ~6-ish pph) for a longer time, say 30-40mins and continue burning at the accelerated rate to keep up. Add in the fact that the OAT will cool as the night goes on puts the Empress at a further disadvantage as it will top out at some point, where the P68 will continue to increase as necessary.

Now, to be clear, I'm not crapping on the Empress, its a great little stove and my friends house is about 1200sqft smaller than mine, so it is perfect for him.

 

[AVIVIII]

but the best advice i can give is ignore the "BTU ratings" unless its stated on the EPA cert tag on the stove its not likley to be accurate.

So by that statement, there is no reason for your company to make the 25-PDV when everyone can heat their house with the 25-PDVC.....

 

[kinsmanstoves]

And now we got everyone thinking.... some very good arguments were made and supported. I did not want to mention any brands of stoves but it seemed to go into the Ford vs. Chevy fight. Everyone have a good and warm night

Thanks
Eric

 

[AVIVIII]

I wasn't trying to make a one vs. another argument, I used those two stoves as they are on the reaching ends of the heat spectrum and both good stoves. I am a firm believer that every stove has a place, owner and application this it is best at.

 

[jtakeman]

but the best advice i can give is ignore the "BTU ratings" unless its stated on the EPA cert tag on the stove its not likley to be accurate.

Well, Thats the guide most use to size the stove to the sq ft needed to heat. I used the rated input BTU's when I bought my bigE. It was rated at time of purchase at 55K BTU's. I never looked at the rate of fuel consumed. Its best(max) was 5 LBS per hour. So there was no way in heck it could produce 55K BTU's. A more realistic number would have been 40K max. I was replacing a 48K max Quad at the time. If I had done my home work and looked deeper.I would not have bothered with the bigE. My gripe with the bigE was they over rated the crap out of the thing! From the get go it was too small for my needs! Why would I want to replace a 48k stove that wasn't keeping me warm with a actual smaller unit? Never again will I be fooled by BS from a manufacturer's over claimed inflated ratings!

If Look into what the thing eats and use a more realistic BTU value of 7800 to 8000 BTU's per pound. Deduct the 75% for effiency loss and you can get a decent output for the stove.

By the way Breckwell has now rated the bigE at 45K max input, Still a bit over rated as you would need 9000 BTU's per pound pellets to achieve the rating.

 

[MCPO]

but the best advice i can give is ignore the "BTU ratings" unless its stated on the EPA cert tag on the stove its not likley to be accurate.

So by that statement, there is no reason for your company to make the 25-PDV when everyone can heat their house with the 25-PDVC.....

I have the 25 PDVC and next door is the 25 PDV. There`s a enormous difference in heat output between those two. There`s a good reason they make both .

 

[SmokeyTheBear]

Now to add to the flames, the figures in pounds given for firing rates at specific heat levels are also a load of chicken litter.

The stove does not feed based upon weight. So no matter how you go about using those fine figures you are going down a yellow brick road without any real idea of where you are going to wind up.

Mr. Wizard would you be so kind to render your opinion on this matter?

 

[turbotech]

I re-read the original post to see what it was trying to say. I think I understand it now. I had to laugh........it seems that some people think just because a stove is bigger it gets more heat out vs pellets burned than a smaller stove burning the same amount of pellets. The only way that is going to happen is a much improved heat exchanger design & material type. It is all about feed rate / how fast the auger turns dropping pellets in and the quality of pellets. I wish all testing was done to list feed rates and heat exchanger efficiency or at least specify the BTU content of the fuel used for testing.

The only gain of a bigger stove is it can handle a higher feed rate and suck up more pellets and spit out more heat. Is there a "standard" used for calculating heating sq ft advertised by manufacturers? I see a number listed as heating sq ft, but never see what the actual conditions are.

 

[AVIVIII]

The only gain of a bigger stove is it can handle a higher feed rate and suck up more pellets and spit out more heat.

My point EXACTLY!

Is there a "standard" used for calculating heating sq ft advertised by manufacturers? I see a number listed as heating sq ft, but never see what the actual conditions are.

From what I have looked at, the sqft rating is based on the BTUh requirements of the house. This is going to vary greatly on the climate that the house is in and the construction of the house (insulation, doors, windows, ceiling height etc). With the climate in the northeast, an older, draftier home could use up to 40 BTUh for every square foot, where a smaller, tighter, more efficient home may use as little as 20 BTUh for every square foot. It seems that some manufactures would like to think that their stoves are only going to be placed in new, high-efficiency homes. Well, we all know that that is definitely not the case. Take, for instance, a stove that is rated, on its box, by its manufacturer for 2200 sqft, also say that further investigation yields that that stove is also rated at 48k BTU. A little math shows that the manufacturer is planning on that stove heating at a rate of ~21 BTUh per square foot. Now that would be a pretty tight house. To be fair, the box probably says "UP TO 2200sqft" Another manufacturer may use a different rate. You can take the guess work out by doing a little math for yourself.

The majority of houses in the NorthEast built in the last 20-30 or so years probably fall into the 30 BTUh/sqft range. This means that with a properly placed stove, you would be looking at needing about 30,000 BTU output for every 1000sqft. Start adding second stories on and its going to get a little more difficult, take into account imperfect installation locations (like most of us probably have) and natural air-flow and it gets harder yet.

Readers digest: The sqft ratings my be an idealistic number based on a perfect situation.

 

[chris288]

I dont think people are suggesting that becuae a stove is rated at 50,00 BTU's rather than 25,000 BTU's that the bigger stove extracts more heat from a lb of pellets than the small stove. It's simple, the 55K stove is able to burn more pounds / hr and produce more BTU's / hr without melting the stove down the hearth pad. In general, If your heating 1,000 sq. ft. than just about any 25 - 30,000 BTU stove will heat your home on ~ 1 bag / day on a low - medium setting, where the big stove out performs the little stove is when your trying to heat a much larger area, and need to sustain a high heat output level without running the stove full tilt and melting something to do it.

 

[SmokeyTheBear]

The only gain of a bigger stove is it can handle a higher feed rate and suck up more pellets and spit out more heat.

My point EXACTLY! ....

A bigger stove can do a bit better than just that, bigger doesn't always mean burns more fuel. Some can't. I'll leave that as an exercise for the interested reader.

 

[MCPO]

There`s more to this than the stoves ability to burn a given lbs per hour. The larger stove would normally have a larger heat exchanger and mass which would allow it to absorb more and deliver more of that heat from those lbs burnt per hour and less of it up the chimney.
I think it`s more in the heat exchanger design , it`s size, and the material used than in the stoves ability to burn up pellets at a given rate.

 

[AVIVIII]

That would have more to do with the efficiency of a stove rather than its output.

 

[Snowy Rivers]

As has ben mentioned, the secret is extracting that magic number of BTU's and getting it into the room.

My Old WP50 Earthstove was a metal monster, weighed about 400 pounds and had a voratious appetite.
Now this said, its ability to heat the house was far less than the Whitfield stove I now have in its place.

Most stove manufactures rate their stove by BTU input rather than BTU output.

The stove is set to burn X amount of fuel and that fuel is tested under ideal conditions and yields X amount of heat (BTU's)

The stoves usually consume the rated amount of fuel per hour, but do they actually deliver the heat that is generated into the room.

Sadly even as efficient as most pellet stoves are, there is still a lot of wasted heat energy that goes out the exhaust.


A secondary heat exchanger placed in the exhaust pipe can recover some of this lost energy.


The key to this whole scenario is the stoves ability to get the heat into the room and not just to burn the fuel.

I have seen some very tiny little pellet stoves that do a far better job of heating than some of the larger stoves.


Bigger is not always the answer, efficiency is the key word.

The use of heat exchangers that can capture and transfer the most heat from the source is the ticket.


Snowy

 

[AVIVIII]

The only gain of a bigger stove is it can handle a higher feed rate and suck up more pellets and spit out more heat.

My point EXACTLY! ....

A bigger stove can do a bit better than just that, bigger doesn't always mean burns more fuel. Some can't. I'll leave that as an exercise for the interested reader.

Some, not all.

And if they are incapable of it, how did they receive that rating in the first place? Even if it is not duplicated in a realistic environment, that consumption had to be achieved at some point, some how to get to that number.

I refuse to believe that stove/grill/furnace/boiler manufactures have been pulling BTU ratings out of thin air for all these years.

 

[SmokeyTheBear]

There`s more to this than the stoves ability to burn a given lbs per hour. The larger stove would normally have a larger heat exchanger and mass which would allow it to absorb more and deliver more of that heat from those lbs burnt per hour and less of it up the chimney.
I think it`s more in the heat exchanger design , it`s size, and the material used than in the stoves ability to burn up pellets at a given rate.

It is one thing to have a heat exchanger that is extremely good at sucking the heat out of the exhaust stream, you have to have a means of removing it and getting it into the room as well.

The goal is to get the most heat into the room and still allow the bad stuff[tm} to exit the flue at the lowest possible temperature that doesn't cause issues.

There are differences (sometimes major) between most stoves in every area that counts.

Even between stoves that burn the mythical pound of pellets at the same rate.

Sizing any heating system is very different than simply measuring square footage and buying accordingly. Mike Holton even provided a big clue in his posts in this thread.

Something about 32 degrees, go look at his posts.

 

[kinsmanstoves]

Now to add to the flames, the figures in pounds given for firing rates at specific heat levels are also a load of chicken litter.

The stove does not feed based upon weight. So no matter how you go about using those fine figures you are going down a yellow brick road without any real idea of where you are going to wind up.

Mr. Wizard would you be so kind to render your opinion on this matter?

I guess I have a new nickname, I been called worse.



On the Breckwell they do use the 1-2-3-4-5 settings as a ROUGH estimate as level one is approx 1 lbs per hour and so forth with level 5 being approx 5 lbs. Remember ESTIMATE not that auger is connected to a scale. This came from one of the four factory seminars I been too.

Eric

 

[kinsmanstoves]

I re-read the original post to see what it was trying to say. I think I understand it now. I had to laugh........it seems that some people think just because a stove is bigger it gets more heat out vs pellets burned than a smaller stove burning the same amount of pellets. The only way that is going to happen is a much improved heat exchanger design & material type. It is all about feed rate / how fast the auger turns dropping pellets in and the quality of pellets. I wish all testing was done to list feed rates and heat exchanger efficiency or at least specify the BTU content of the fuel used for testing.

The only gain of a bigger stove is it can handle a higher feed rate and suck up more pellets and spit out more heat. Is there a "standard" used for calculating heating sq ft advertised by manufacturers? I see a number listed as heating sq ft, but never see what the actual conditions are.

Yes Captain we are off course, Hard to port!

Eric

 

[chris288]

Talk about pulling #'s outta their butts, when I bought my whitfield the literature said 80 lb hopper for up to an 80 hr burn time, I understand it says up to, but that stove couldn't burn 1lb / hr if it's life depended on it, best I EVER achieved is 30-31 hrs. out of a 40lb bag.

 

[Rooscooter]

I may be a little late to the thread but the OP is describing a basic principle of Physics (well 2 or 3 maybe). There is a finite amount of energy in a bag of pellets and it doesn't matter how slow or fast you burn them you will only release that original finite amount of energy. Variable in the mechanics of the stove and material properties are where some difference in efficiency can be produced.

Stainless Steel's specific heat is slightly higher than Carbon Steel which in turn is slightly higher than cast iron. The higher the value the more energy it can absorb or emit which is how the heat exchanger works. The design of the heat exchanger (tube length, size etc.) can increase this slightly but not significantly due to the nature of of air. Air has a finite ability to absorb and emit heat as well. Unless the exhaust/heat exchanger system is 20' long there isn't going to be a meaningful increase or decrease in the heat exchanged either inside the stove or in the exchanger tubes.

The final variable is volume of air. The longer the air is in contact with the exchanger tubes (either inside the stove or in the tubes) the more POTENTIAL there is for heat exchange. So fan speed is also a factor for both the combustion and convection fans. Humidity is one final factor that can change this a little as humid air can hold more heat than dryer air.

All of this will give you your BTU's exchanged through conduction of heat from the air in the combustion chamber to the heat exchangers to the air that comes out heated from the stove.

There is a remaining significant portion of the energy created by the burning of the pellets that is transferred by radiation. So the overall mass of the stove can have a direct effect on the overall efficiency of the transfer of energy from the pellets to the living space. The more mass the more heat will be absorbed and vise versa.

So in the end I see a finite amount of potential heat in pellets and a somewhat variable system to extract that heat resulting in higher heating outputs for better designed stoves.....however I would bet the differences are smaller than you would expect.

I would like to see stove designers make pellet stoves more efficient in radiating heat as much more heat can be retained through radiation than conduction to a low mass substance like air.

 

[SmokeyTheBear]

Actually the finite amount of energy in even one pellet is far more than any pellet stove can extract.

In fact the same mass of water contains exactly the same amount of energy.

That too is a well understood and accepted "principle" of Physics.