Pellet Stove Operation Explanation!

[SmokeyTheBear]

Most pellet stoves are very good at burning the fuel, where they have problems is in the use of electricity to run the system and the efficiency of the heat exchanger (the heat exchanger issue is complicated by the ash that gets deposited on all interior surfaces of the stove including the heat exchanger).

This seems like it could be a real simple mechanism to scrape the heat exchanger on a time. Heck even a manual item would be great on my harman, can't believe the do not have these on the stoves, I nkow they do on the Harman furnaces.

Those Harman stoves might have a heat exchanger geometry issue (I haven't really looked at one so don't really know).

My stove has an ash scraper for the heat exchanger, dumbest concept implementation I've ever seen, you scrap that ash right down into the burn pot.

There are several problems that would need to be solved for an effective automatic ash removal system for a pellet stove's heat exchanger, the first of which is a truly efficient heat exchanger may not lend itself to a simple system strictly because of its geometry.

Then most autmatic mechanical systems are prone to problems. Which fly in the face of keep things simple and safe.

While Snowy and MOS, seem to be on the off of the shelf mode of operation which for some parts is fine. I might point out that it may not be possible if they are trying to get the last BTU out of the combustion process let alone the overall pellet stove system.

It might be a useful exercise to look at what is required in the manner of controls for the existing high end stoves in terms of efficiency.

I never had a chance to work on efficiency issues in stoves, but I sure have in other systems and sometimes what appears to be reasonable doesn't always turn out to be the best way let alone the simplest way.

Nobody has mentioned anything about getting more heat for less electricity yet. But that is also part of the pellet stoves efficiency numbers.

 

[Stovensen]

Hi folks,

Interesting thread this. It's really boiling with a lot of detailed and precise information on the topic, so I'm sure OP yomoto must feel lucky that s/he made the inquiry here... hearth.com is the best of it's kind on the entire internet!

OP's inguiry also made me curious on the topic.

Improving the overall efficency of pellet stoves certainly is important, since pellet prices only seem to go up and up. :bug:

But achieving this with minimalistic technology as suggested by Snowy seems very difficult.
When the pellet stove for residential use was invented by Jerry Whitfield in 1983 the world saw a huge improvement ( low emission, high efficiency ) compared to traditional wood stoves, but even the first Whitfields certainly were more complex and thus more fragile than wood stoves.
It seems to be a common rule related to the nature of beast that the more efficient the stove is made the more complex and fragile it gets.
Something similar is happening within the automotive industry
They are constantly working to improve the gasoline combustion engine... latest improvement is the direct injected gasoline engine, which has an efficiency that comes close to that of a diesel engine, but in order to control this super lean burn at low throttle, massive use of CPU-power and sensors is needed.
Super lean burn is a very hot burn that tends to melt the exhaust valves. The four stroke engine may prove incompetent here.
Evinrude uses the two stroke principle with great succes in their E-tec direct injected outboard engines.

So is minimalistic design of pellet stoves possible in the future? Well, I certainly hope so or else we, the stoveowners are facing some nightmares as for reliability and serviceability with the next generations of pellet stoves.

Please let me draw your attention to this newly developed pellet stove from Snoqualmie Stove Works in Snoqualmie near Seattle WA.
The design is very different and futuristic... it looks like one of the warp engines of the starship Enterprise from the Startrek series.
Perhaps the looks is what Snowy would describe as something that "sticks out like a pimple on a ducks butt" LOL, but judge for yourself. It appears very promising.
The well known issue of room fan noise is gone, since the whole stove in itself is one big advanced heat exchanger. ;-)

Link to snqstoves home site:
(broken link removed)

YouTube clip of the SNQ-1 Radiant Heat Pellet Stove in action:

http://www.youtube.com/watch?v=lwYNcZPvdr0&feature=related


Bo

 

[krooser]

I thought pellet stoves worked by loading pellets into a hopper and pressing a button... hmmmmm.

 

[Snowy Rivers]

Yup, Ducks Buttt, including the feathers no less ;-P

Old saying. "The more they overthink the plumbing, the easier it is to plug up the drain"

You are right, the more complex the stove becomes, the more fragile it is.

Basically with some simple controls, you can have a nice clean fire and reliability too.

A good flow of clean air through the fire pot will get things going well.
A simple draft adjustment is all it takes to adjust the burn for any given amount of pellets being fed to the fire.

The issue is that the common village idiot (Not pointing the finger at anyone) has come to expect the stove, the car, the dish washer or whatever to do everything at the press of a button.

We currently have enough computing power in our stoves to go to the moon.

Society has gotten to the point that they not only don't want to think about how to run anything but in far too many cases these folks are not able to handle the task at hand either.

If the cash register at Mickey D's stops working, the server can't make change.

If the power goes off, the people scream to high heaven that the food in their fridge is spoiling.


Hmmmm Last winter we had power out for several days. Stuffed the food from the fridge into a cooler and sat it out in the snow.

Put the frozen foods from the deep freeze in boxes and placed them in the back of the car we were not using 10F outside. Nope not gonna thaw any time soon.


Ran the pellet stoves on the generator and did quite well.

Did not lose one thing from the freezer or fridge.

The issue is not that making it simple is not possible, but instead making the thing so simple people can run it without getting hurt or burning the house down.

Its far to complicated for some folks to turn more than one knob or to adjust the air ratio so the flame burns bright and clean.

The same reason that the Old Coleman gas laterns are almost gone from the scene now and replaced by the simple screw on propane bottle.

I even own several of the refillable propane latern bottles with the adapter that you screwed onto the 5 gallon bottle then inverted it to fill the little ones.

OMG can you imagine something so difficult.


Ahhh well I digress.


Snowy

 

[SmokeyTheBear]

Stovensen,

That is a neat stove, I wonder what the actual heat output is and what the air flow around it is like.

I'm all for quiet, one of the reasons I'm no longer in a town or city.

Sounds like they may have a better way of handling possible burn back issues.

Now if they actually described the controller for that stove, dual air injectors (top and bottom) would likely mean there is a balance to maintain. I also wonder how they extract the 20 plus or minus pounds of ash per ton of pellets.

Snowy,

Give the kids at Micky D's somewhat of a break they had at least two strikes against them.

1. The cash drawers likely won't open so the kids that actually could do change can't get to it to complete the transaction.
2. They were never really taught even simple arithmetic (You don't want to go there my youngest is still fuming about that one).

Other than that discharge the other side of the ships cannons ;-).

 

[Snowy Rivers]

Sorry Smokey

The fact that so many of our youth are embarking on life with far less than half a boatload of smarts is a real tragedy, for them and the country.

On occasion I will find a youngster that knows how to count change, I ask, where did you learn that, the reply is usually Grandma, Grandpa, never in school though.


The little stove shown here is certainlly a lovely looking piece for sure.

Like what was mentioned earlier though, the more complex it is, the more fragile it becomes.

I am currently working as many know on redesigning the controls on my Advantage II T

Several issues to think over carefully before spending the $$$$ for the parts.

I do believe that our young student here can do well if he keeps simplicity in mind and concentrates on less electronics and more on airflow, ease of operation and maintenance.


Having been directly involved with pellet appliances for 20 years has given me a very good insight into their idiosyncracies.

By my nature I tend to be very synical abouyt most things.
My luck with electronic devices tends to be very poor. As I always say, its not if it will fail, just WHEN.

Electronics are way cool, but I really dont like to see so much of it in "MISSION CRITCAL" places.

To give an example, My big truck engine is all "fly by wire" and has been almost trouble free for 15 years and near 600,000 miles.

The quality of the materials is far and away better than the stuff used on consumer vehicles.

Same applies to our stoves, the electronics are not the same quality as what is being sent off to war or into space or even sent down the road in a PeterBilt.


What I hope to see here is that our young friend will digest all this smack and come up with a very good design for his project that really works well and is simple to build and operate.

Build a better mouse trap ya know.

Snowy

 

[SmokeyTheBear]

I hope that the OP hasn't waited until the last minute to start the project.

I used to see some otherwise smart kids put off what seems to them to be a minor project for way too long and then discover that it isn't so minor.

Used to see it all the time when I was doing duty at the computer center in days long ago.

 

[yomoto]

Guys,

Thanks for all the great responses. This is exactly the kind of responses I was looking for.

I'm not put off at all. I think we will definetly be using a PCB with a micro processor to control the unit. We want to increase the overall efficiency of the product so are interesteed in all design issues with current stoves.

Please keep the good info coming

Steve.

 

[Snowy Rivers]

Have fun and keep us posted as to how things are coming along.

Snowy

 

[jtakeman]

Guys,

Thanks for all the great responses. This is exactly the kind of responses I was looking for.

I'm not put off at all. I think we will definetly be using a PCB with a micro processor to control the unit. We want to increase the overall efficiency of the product so are interesteed in all design issues with current stoves.

Please keep the good info coming

Steve.

You need to check out the Bixby. Its the most unique stove I have seen and has the most high tech control in the business. I really like the fact you can adjust pretty much any thing on the stove. Great for tweaking hard to burn fuels. This stove self cleans and you can even empty the ash pan without stopping the stove.

With biomass not just limited to wood pellet I would also consider a multifuel unit. You will need a stirrer,agitator, or mixer in the burnpot to handle the higher ash fuels. Or a fully self cleaning burnpot. Fuels available are grass pellets, wood pellets, cherry pits, olive pits, corn, wheat, barley, straw pellets, high ash wood pellets from reclaimed wood and nutshells. I am sure there are more. I have even burned used coffee grounds myself. Even if the future buyer is only planing on burning wood pellets. With the stove basically cleaning itself, The user will get longer running time between the tedious cleanings! Plus even the worst wood pellet will be able to be burned with ease. I don't like picky stoves that can only burn the best pellets.

With biomass being the best outlook as our future fuel, I would also consider a multifuel burning furnace in the lineup of stoves. If we are going to replace the fossil fuels. We need more than just space heaters to heat larger homes with biomass fuels.

Personally, I want all the bells and whistles I can get. But still trying to keep it simple and user friendly so anyone can use it!

Enjoy and keep us posted on what you come up with.

 

[lmjr]

Guys,

Thanks for all the great responses. This is exactly the kind of responses I was looking for.

I'm not put off at all. I think we will definetly be using a PCB with a micro processor to control the unit. We want to increase the overall efficiency of the product so are interesteed in all design issues with current stoves.

Please keep the good info coming

Steve.

I did start a lengthy post about a direction to go in and features to add to make these stoves more "User Friendly" (aka "Idiot Proof")
But...the problem always, seems to me, come back to "Too Much Technology" required to accomplish it. (thanks Snowy)
By it's very nature, any "Automated Solid Fuel Burner" has a "steep uphill climb" w/ by-products issues.

So, it is my opinion, discover the perfect combustion process which also deals w/ by-products effectively...this is the Biggie
and once this is accomplished...build the "Necessary" control system around it. Can it be done???

I work construction and have made a good living working in many Electric Generating Power Plants some burning "Solid Fuel".

As you might imagine, a lot of controls and a lot of by-products...and regularly scheduled outages as well as some UN-scheduled outages. (Sound familiar???)

Electric Producers/Utilities would LOVE to reduce or eliminate any of what I mentioned above...so far it hasn't happened.

I know it's not fair to compare your Bio-Fuel burner to a powerhouse but you should see the problems are not really that different.

There you have it....build a stove that automatically deals w/ by-products and will do all the scheduled (and Un scheduled) Maintenance Outages and you'll have the Market Cornered!

I forget which utility says....."Your Bills Pay Ours"

Wishing you the Best!
I'm pull'in for ya.

 

[Snowy Rivers]

MOS

Exactly
If fuel quality is a finite without any real solid waste products to speak of, controls can be very sofisticated and work well (Clean fuel oil, natural gas, Propane )

When we are dealing with a solid fuel such as pellets, fire wood or other biomass fuel, the going gets dicey at best.

There are no constants and adjustments must be made accordingly to maintain optimum burn characteristics.

With the varying amounts of fly ash and such the airflow and the heat transfer are not even a close constant.

With a clean fuel, the controls can even get to the point of running a closed loop system that monitors the flue gas and adjusts fuel/air mix accordingly.

Basically a pellet stove is a somewhat cleaner way to cage up a camp fire and be free of the dirt and little critters that come with fire wood.

Even with a pellet stove, the little critters seem to find their way in. Found a trail of tiny ants crawling from my OAK yesterday

Tiny squirt of raid took care of that crap. for sure.

For whatever its worth, unless the fuel is made to conform to a constant and not vary (Not gonna happen with pellets or bio fuel) the quality of the burn is going to need to be adjusted by the operator.

My new batch of shells are much larger pieces and burn different than the finer material that I set the Advantage up with earlier.

No biggy, just have to pull out the air handle a bit to keep the flame brisk and with some Blue in the base area og the fire.

As soon as I get back into the material that ahs more fines, the process will change again.


The Pellet scene is no different.

I never gave much thought to different brands of pellets being all that much different, OHHHH MY, after reading the stories here they seem to vary as much as the shells that I use as far as how they burn.


I still maintain that too much overthinking of the plumbing on these pellet stoves is not going to net a great return.


I suppose an exhaust sensor tied into a loop circuit could allow the controller to either add or subtract combustion air to maintain a more "perfect" burn.

This said, it just makes no sense to go this far on a device that was designed to be simple and reliable.

The worst possible scenario is to have the onboard electronics fail in the middle of a dark cold and windy night.

Not only does the owner hate the stove he hates those who designed it and is not pleased that he has to wait until morning or if its a weekend, Monday to see about getting the thing fixed.

Now add to this the repair shop is booked up and the service tech can't come for a week.

OMG you are really on the list.

Now to add more fuel to the fire ( no pun intended) find out halfway through a season that your latest crop of control boards (you are the builder) has a batch of faulty Triacs that fail and burn out the exhaust blowers.

The warranty implications rise exponentially the more complex the design.


If the design incorporates a totally proprietary control system then you really have troubles.

One bad batch of control boards can theoretically put a fledgling stove company out of business.


The concept may have been wonderful but in actuality the results can be quite different.


Let me change the subject for just a second to emphasize a point.

GM back in 1994 introduced a fully electronic injection system for their light duty V8 diesel engines in Pickups and SUV's

They were so much in a hurry to get this drive by wire system into the public hands that they made some serious mistakes.

The vendor (Stanadyne corp) told the Engineers that the system was flawed and still needed more R&D before it should be released.

NOPE get it into production.

Well the fallout, the failures and the unhappy customers were on a grand scale.

The service techs were not up to speed at all not even.

It took from 1994 up until about 2 years ago for the aftermarket to finally come up with a GOOD FIX for the problems.

The system though light years ahead of where it started is still not bullet proof.

Bottom line
The quest to make a better mouse trap failed due to too much complexity.

There is a point of diminishing returns and then the curve falls off and failure is the next stop.

I still say keep it simple.

Spend your engineering time on things that can make the product more user friendly and as effeicient as possible as well as easy to maintain.
`
Make it so it lasts too.

Cheeezy things that fall apart quickly is not the answer.

Far too many products are made to last long enough to get out of warranty and then they start costing the owner $$$$$ on a regular basis to maintain.

Small things like gaskets and such needed during some routine maintenance is fine, but major parts should last.

A pellet stove is a "HARD ITEM" this is, a long term investment that should be pretty much trouble free as long as its maintained well for MANY years.

20 years on a Pellet stove and still see it work well is a good spot to shoot for.

My Old Eath Stove WP50 was 20 years old when I removed it a few weeks ago.
Aside from the feed/cup motors being a tad noisy when first started up the thing worked as good as when it was new.

This socielty has become far too accustomed to buy it today, fiddle with it, get it fixed and in a year or two throw it away.

Washers, driers, cars ?????????/ its all the same these days. Mostly Junk that will fail shortly after the warranty runs out.

The advent of the "extended warranty scam" OMG if a company offers me an extended warranty right up front, I am likely going to look elsewhere for the product.

The scam is this, the seller knows that many folks will buy the Stove or ?? run it a little bit and probably not need the warranty. BUTTTTTTTT there are those of us who run the wheels off whatever we buy.

Just some thoughts.


Snowy

 

[SmokeyTheBear]

Steve I have yet another set of pet peeves about pellet stoves. Thought I'd share it with you.

Electronic packages and motors don't really do well in enclosed spaces that are subject to high temperature.

This likely would go a long way in explaining both control board failures and convection fan failures.

You might want to take a look at proper thermo isolation of the controls and convection blower assemblies.

The other thing that I've run across is poor wire runs. There have been some posters on this forum that have had magic blue smoke emitted from their controllers due to shorts caused by this and improper connections.

There is also a tendency for certain controllers to malfunction due to cross talk issues.

 

[yomoto]

Hi All,

The quality of replies that you have all contributed is absolutely fantastic!! Thankyou one and all!!

I have another few questions that you might be able to collectively answer.

1. As I live in Ireland we do not have alternatives to wood pellets such as corn pellets, grass pellets etc. What is different in the burn characteristics with these fuels. I am presuming that most of these have a higher ash content and in the case of grasses leave a sticky residue. Grass pellet stoves must require a lot of maintenance. I believe that corn pellets have a higher calorific value? We have some new fuels in Ireland such as people briquetting straw for burning in solid fuel / wood stoves. These vary in consistency depending on what the farmer has used to fertilise the crops. It has been recorded that straw briquettes can contain a significant amount of sulphur and nitrogen which tends to eat away at metal surfaces inside solid fuel stoves and chimneys. Do you have the same problems with corn stoves? I must admit this is just more out of curiosity than relating to the design of our project.

2. I think I am correct in assuming from several of the posts that bottom feed horizontal augers are used to de-ash at the same time as adding fuel to the burn pot, however these systems are more prone to burnback. Is this correct? I am assuming that some manufacturers use secondary motors / augers / electrically controlled mechanisms to have independent cleaning of the burn pot? However this does lead to complexity and extra costs.

3. Our system goals are as follows: -
a) Control the correct air / fuel ratio combined with the expulsion of the exhaust gasses at different stages of the burn cycle to improve pellet combustion. For this we have to use a micro processor and sensors. As it is a university research project we hope we can commercialise some of the technology we develop (If any)
b) Improve on the Fan noise issues prevalent on a lot of Stoves.
c) Improve the flame "picture" so it mimics a wood stove more than current pellet stoves.
d) Make it easy to clean and service. For this all your advice has been invaluable!!
e) Maximise heat exchanger transfer. I would never have guessed on the practical implications of keeping the heat exchanger clean, which makes perfect sense. Again thankyou!

May I be so bold as to ask what you all think of our goals as set out above?

 

[hossthehermit]

IMHO, c) is not a good idea, you can already get that effect if you just let your stove get dirty.

 

[yomoto]

IMHO, c) is not a good idea, you can already get that effect if you just let your stove get dirty

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As the proverb says " there is more than one way to skin a cat" and there is more than one way to burn pellets!!

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P.S. Who makes automatic pellet ignitors?

 

[lmjr]

FireRod Cartridge Heaters

Chromalox Cartridge Heaters

Omega. com Cartridge Heaters

Gaumer Process Cartridge Heaters

 

[SmokeyTheBear]

Steve I'll take a stab at some of your questions and would like to do a X2 on hossthehermit's suggestion that 3 c) will likely be a performance killing endeavor.

The issue with various solid bio fuels (which by the way coal and oil are despite all of the bull being put forth by various entities) runs a fairly wide spectrum from massive ash content to some extremely corrosive combustion by products.

One must pay attention to the corrosion handling capabilities of the metals used to build the stoves and all along the exhaust path including the vent piping.

There is another item that isn't brought up that should be, some solid bio fuels also contain various pesticide residue and break down products.

For example the hazelnut shells that Snowy burns may produce some really nasty byproducts when burned that would depend upon how the various orchards mange their filbert worm problems.

Corn is notorious for producing a lot of nitric acid residue, more than a few people discovered that leads to small pin holes in their venting if the incorrect metal is used.

 

[SmokeyTheBear]

On the horizontal auger systems, they remove ash in at least two ways via air flow and pushing the ash over the burn pot lip.

The pellet drop stoves usually use the air flow to eject the ash.

There are however stoves that have various ash extraction systems. Some via opening and dumping the fire pot contents, these stove go through a clean cycle and others through ash augers that move ash to the side and over the lip of the burn pot.

The pushers (horizontal auger systems) and auto clean systems can handle a higher ash content fuel.

The puffers (air flow only systems) can handle a lot of ash as well provided it isn't dense.

 

[SmokeyTheBear]

Oh and everyone would like a nice large ash pan.

 

[lmjr]

Steve,

In regards to your ...

3. Our system goals are as follows: -
a) Control the correct air / fuel ratio combined with the expulsion of the exhaust gasses at different stages of the burn cycle to improve pellet combustion. For this we have to use a micro processor and sensors. As it is a university research project we hope we can commercialise some of the technology we develop (If any)
b) Improve on the Fan noise issues prevalent on a lot of Stoves.

I would steer you in the direction of using DC controls, as opposed to AC controls.

Varying all the fan and auger motors speeds w/ DC would be more reliable as well as quieter.

There are currently some stoves using DC motors / controls...but very few!

 

[Snowy Rivers]

Never have had any problems with corrosion in any of my stoves due to the NUT WORMS.

I do however have to clean out the burn pot far more often that with commercially produced pellets. I also had to install a filler plate in the burn grate. This has small round holes that keep the shell pieces from falling through into the ash pan.

Yes indeed, a very large ash pan that is located such that it is easy to get the ashes from around the burn pot and into the pan.

An ash pan that can be pulled out with the fire going would be a real treat.

The pull rod heat exchanger cleaner is way cool but what is even better is a fully replaceable heat exchanger setup like the Quads have had.

Many stoves have welded in heat tubes. If for instance one of these springs a leak, you have big troubles

The Quadrafire has a double sided aluminum heat exchanger that has fins on both sides and bolts into an opening between the fire box and the convection bonnet.

The side of the heat exchanger that goes to the fire has a stainless cover that slips over to protect the aluminum from direct contact with the flames.


All sort of cool stuff


Snowy

 

[SmokeyTheBear]

In the case of hazelnuts Snowy it isn't a corrosion issue.

It is methyl isocyanate in the exhaust if a certain pesticide was used to control filbert worms.

It depends on what was used, how much was used, how long ago it was used, and so forth.

Combustion by products produce a very complicated chemical soup. Luckily almost all of this exits the area being heated and quickly becomes diluted once it exits the vent outside.

This is true for anything that is burned to provide heat.

This is one aspect that is rarely discussed.