Pellet Stove Efficiency

Can be a vague question indeed. Many things need to be clarified.
One stove can burn fuel more efficiently than another but be hampered by an inefficient heat exchanger and maybe even vice versa. The distribution system needs to be evaluated too along with the heated air flow design/path is another thing to consider.
Overall efficiency could as well be measured not only by heat output in degrees but by volume too. Then we have to take into account the amount of ash and it`s content.
Overall efficiencies are far too difficult for me to calculate so I basically rely on the Mfrs brochure and allow for some exxageration since their figures are in optimal conditions that we end users aren`t likely gonna duplicate..

An efficient heat exchanger is where I'd start. Also convection blower. Bigger may have a cooler discharge temp, But could circulate the air in the home quicker.

jtakeman said:

An efficient heat exchanger is where I'd start. Also convection blower. Bigger may have a cooler discharge temp, But could circulate the air in the home quicker.

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Yes. Heat exchanger is the largest piece of the puzzle by far. More surface area is better.

I agree with above statements on the heat exchange system. If you can't extract the heat, it ain't worth a dime.

Different Manufacturers have different types. The tube type is most common and is more efficient when the exhaust path runs down the channel the tubes reside in.

Harman has a VERY NICE heat exchange system. Probably one of the best.

So in short. Some burn better than others. But may not heat as well as others. Others may heat better, but need more maintenance (daily scrape) or may need less maintenance. Its all a toss up.

Having used several stoves myself and also knowing of quite a few different models that family and friends have, some model's simply heat better. Some require a fair amount of babysitting. Some burn for weeks on end.

There are some stoves that are the Full Package. My Fahrenheit is the closest I have seen. Quad plays a close 2nd (heats like the Sun!) But does not have High/Low. If Quad could run High/Low, it would be a Super efficient stove that is Super Simple to operate. Would be my last stove (not happening).

Efficiency has to be taken for the stove as a whole system. Nothing else is meaningful.
Efficiency expressed as a percentage is BTU OUT divided by BTU IN times 100.
Manufacturers should be able to tell us the efficiency of their product (under test conditions).

Real world is never the same as test conditions...

mepellet said:

Real world is never the same as test conditions.

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Of course not. They will work hard to get the best numbers they can. YMMV

From what I have seen most of the #'s are made up on pellet stoves. Who is really checking? I think a lot of manufacturers use combined efficiency numbers. Most stoves burn at 99% combustion efficiency so if they have had a thermal efficiency of 65% the average between the 2 would be 82%. I think that is the accepted way of rating pellet stoves.

That said, I think most of them just lie about there #'s. I see stoves with single pass through heat exchangers that rate higher #'s than ones like the CB1200 that have a baffled system that makes the exhaust gases pass the entire length of their tubes. It seems that the longer the contact time the more heat the exhanger will put in the room.

I know when I went from a Breckwell p-24 to a Mt. Vernon AE I chopped my pellet consumption from 5.5 Tons to 4 Ton per year. And I always thought that the Breckwells were very efficient.

pip3398 said:

I think a lot of manufacturers use combined efficiency numbers. Most stoves burn at 99% combustion efficiency so if they have had a thermal efficiency of 65% the average between the 2 would be 82%. I think that is the accepted way of rating pellet stoves.

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The correct way to calculate efficiency is to multiply the individual efficiencies.
A combined efficiency In your example would be that 65% of 99% of the heat value of the fuel (64.35%) is delivered as heat output. I can't see any competent engineer mis-stating that.
Saying that it is 82% efficient would be an outright lie and would leave the company liable for misrepresenting their product.

If the heat exchanger isn't at its peak, There is no other place for the heat to go but out the tail pipe. A slight percetage(anyones guess) may radiate off the unit.

There were some EPA numbers posted, a list is out there. Some units had the numbers posted on the respective website's. Since there isn't any rebates these numbers aren't as readably available. AFAIK the testing was done at independent labs and not the manufacture's test results. Don't know how acurate, But I'd believe EPA's might be less of a fudging.

They include the electricity used to power the unit as well, the EPA came up with the test and there was a list maintained by them of the units that met the required number for the gubment rebate.

ETA: Just remember those figures do not take into account the effects of ash in the works and as anyone on here can tell you the heat output into a room decreases over time at a steady firing rate.

SmokeyTheBear said:

ETA: Just remember those figures do not take into account the effects of ash in the works and as anyone on here can tell you the heat output into a room decreases over time at a steady firing rate.

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Ash is a good insulator you say.

I got a few calls where the complaint was little or no heat. My first question was "when was it last cleaned", Second thing was getting the vac and brushes to completely clean the unit. Usually the heat was night and day. I'll admit one was blamed on Inferno's. Had to make a pellet run for em. Then I saw the smile of relieve.

jtakeman said:

Ash is a good insulator you say.

.... snip

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Well it ain't a good conductor .

pip3398 said:

That said, I think most of them just lie about there #'s. I see stoves with single pass through heat exchangers that rate higher #'s than ones like the CB1200 that have a baffled system that makes the exhaust gases pass the entire length of their tubes. It seems that the longer the contact time the more heat the exhanger will put in the room.

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The CB 1200's system is one of the better ones I have seen. The Castille and Santa Fe don't channel the exhaust the same way. Its still a tube type, but the combustion blower doesn't pull the hot exhaust air down the length of the tubes.

Is the Mt. Vernon like the Classic Bay? Where the exhaust runs down the channel the tubes reside in?

Like I said above. The CB dont have a lot of Bells and Whistles. But it melts faces

After every new stove I see or work on (friends, family, etc) I like my old Quad even more.

Burned everything I have thrown at it and can go weeks between cleaning (All on Low)

jtakeman said:

An efficient heat exchanger is where I'd start. Also convection blower. Bigger may have a cooler discharge temp, But could circulate the air in the home quicker.

Click to expand...

Excellent comment... but in reality in a showroom setting the customer wants to have their buns burned by a pellet stove. This is only accomplished with a 'radiant' heat exchanger ie. one with tubes.

We were able to prove this because many EuroMax dealers complained their customers felt in a show room setting the unit was not throwing enough heat even thought it is an efficient unit. The heat exchanger on this is very large and 'convective' with a variable speed 465 cfm blower.

We even went so far as sending 50 prototype of a new unit to real home install and the majority of users felt it was not as efficient as their old unit because "when I put my hand in front of the exchanger it's not as hot". So basically it's a problem of perception.

The convective unit would make the thermostat rise faster, but the user impression was they almost needed to burn their hand in front of the unit in order to be convinced this was a more efficient unit. We ended up reverting to a standard heat exchanger for the new unit (reduce efficiencies somewhat) because that's what the consumers wanted.

Think of a high efficiency furnace. You would not expect to burn your hand on the plenum... same difference.

Isnt there a new standard test for this the epa is trying out....? 415B or something?

DexterDay said:

Is the Mt. Vernon like the Classic Bay? Where the exhaust runs down the channel the tubes reside in?

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No tubes in an AE.

FyreBug said:

Excellent comment... but in reality in a showroom setting the customer wants to have their buns burned by a pellet stove. This is only accomplished with a 'radiant' heat exchanger ie. one with tubes.

We were able to prove this because many EuroMax dealers complained their customers felt in a show room setting the unit was not throwing enough heat even thought it is an efficient unit. The heat exchanger on this is very large and 'convective' with a variable speed 465 cfm blower.

We even went so far as sending 50 prototype of a new unit to real home install and the majority of users felt it was not as efficient as their old unit because "when I put my hand in front of the exchanger it's not as hot". So basically it's a problem of perception.

The convective unit would make the thermostat rise faster, but the user impression was they almost needed to burn their hand in front of the unit in order to be convinced this was a more efficient unit. We ended up reverting to a standard heat exchanger for the new unit (reduce efficiencies somewhat) because that's what the consumers wanted.

Think of a high efficiency furnace. You would not expect to burn your hand on the plenum... same difference.

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I see what your saying. I'm guilty too, First thing I do is stick my hand in front of it to see how hot it is. If it didn't about burn my hand I'd probably ask to see another unit.

In my case, more of a furnace senario seeing I have cheated and added duct to my unit. I wanted to lower my convection temps. With the stock blower (265 CFM's) My temps were in the 250ºF range. A bit too hot for the ducts(and my toes). I Added a 455 cfm blower and I droped it to 115ºF(roughly). Yes lower temps now, But man does it turn the air over in the house quickly. My temp rise went from a 1ºF/hr to over 2ºF/hr(using exact same pellet, only change was the blower). Meaning I can bring the temp from 68ºF to 70ºF in less than an hour on an average winter day. This will hold us over until I can afford a pellet furnace.

jrsdws said:

Can you describe the "convective" heat exchanger, please?

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Should have said not just the heat exchanger but the whole machine is more 'radiating' than 'convective'.

In effect, a 'convective' stove is designed to extract as much heat as possible away from the machine. This is done with a heat exchanger with a large volume area and a blower to match the BTU output at different feed levels. The internal temp of such machine is much lower as well since it tries to distribute most of the heat through air movement.

A typical, or radiant stove has much higher internal temp. Therefore the whole stove 'radiate' the heat. Not just the exchanger.

Also, pushing heat through a straw feels hotter on the skin than the same heat over a wider area. The same principles applies to the heat exchanger and customer perceptions.

Neither is better or worse, just different way of doing the same thing. Although at Tjakeman pointed out you will get more efficiencies from a convective stove.

I think the Enerzone uses a design similar to the CPMs. I know the Drolet Eco 65 is like a convective box (small firebox housed inside a larger box and air blows between them). The Drolet is the Cousin of Enerzone

The 10 CPM is the same way. A box within a box.

I think thats what Fyrebug means by "convective" box and not a radiant box.

jtakeman said:

I wanted to lower my convection temps. With the stock blower (265 CFM's) My temps were in the 250ºF range. A bit too hot for the ducts(and my toes). I Added a 455 cfm blower and I droped it to 115ºF(roughly).

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Yes.
Btuh = 1.085 x cfm x deltaT
Or
DeltaT = Btuh / (1.085 x cfm)