Let me begin this post by saying that thus far in my career in wood pellets if there has been one single component of a stove’s operation that has stood out the most for being misunderstood, skipped over (through ignorance of its existence or otherwise), and crucially important, it is the outdoor air in-take. There actually IS a reason manufacturers strongly recommend their use. Here we’ll tackle a few of the reasons why you need to install one and how it can potentially fix a few of the issues you’re having with your stove.
So what is an outdoor in-take anyway?
Let’s start this article of stove anatomy with what an outdoor in-take even is. An outdoor in-take is an apparatus that connects the air in-take of your pellet stove (or any stove for that matter) with the outside air. This is usually accomplished by drilling a hole into the wall. The gist of this apparatus is that it allows your stove to draw cold air from outside into the combustion chamber and thereby turns your stove into a closed heating system, sealing it from the rest of your house. More on this part later.
On the back of your stove (typically), you will find a small vent opening usually between 1″ and 2″ in diameter. That is where your stove brings air into the combustion chamber via the combustion fan. The combustion fan is the fan that blows air through the pellets in the burn pot to help facilitate complete, clean combustion. The combustion fan is more commonly known as the silly thing you’ve probably gotten sick of adjusting every time the big box store runs out of pellets and switches brands – which is every year if you actually liked them this time (another great reason to start using our pellets, but I digress). The outdoor in-take is any combination of materials that connects to that vent on your stove and then runs through your wall to an opening on the outside of your house. You can find dedicated kits online for around $50 at the time of this writing, but we here at MWP like economy and have crafted our own. The dedicated and specifically designed ones you find online or at your local chimney shop are build with looks and performance in mind. If you think about the increase in stove efficiency, $50 will pay for itself in the first year for most users, but YMMV. If you’re crafty and want a cheap fix, you can make one like we did here out of PVC and flex piping. I believe it cost us around $4 to make ours, but it could be a tad more if you need to buy the requisite drill bit to cut a perfect hole in your wall. At the end of the day, all you have to keep in mind is that this system just has to be air tight, not fancy. Regardless the materials or medium used, I strongly recommend putting some metallic covering over the end of the piping that is outside. Mice are crafty and not-so-fun to remove from a small, long opening in your wall.
What are the benefits of this piping?
To answer this question, let’s start by looking at how a closed heating system is created with one of these kits added on.
When you connect your in-take to the outside air, you create a closed heating system and you seal your stove from the rest of your house. One end of your stove draws air in from outside and that air is used to cause oxygen to rip electrons from an organic (aka carbon-based) substrate. That chemical reaction causes a plasma to form in an exothermic combustion reaction. After said electron theft by oxygen (shout out to my chemistry friends who thought that O.I.L. R.I.G. was never going to be useful in real life; score one for your teachers this time), you have created heat and CO2, amongst other things, that now fill your burn chamber. Your stove piping then creates an opening for which this CO2 and smoke can now exit your stove. That exit leads to the outside of your home. At no point is any part of the air used in the stove’s burn process in contact with the air in your home and therefore it is closed off from it.
***On the way out, a heat exchanger in the top of your stove captures the heat created and uses it to warm air inside your home before that heat is blown out of your stove piping and lost to the great outdoors. This differs from a traditional wood stove which radiates heat into your home after the body of said wood stove gets incredibly hot from the fire within it. The efficiency of that heat exchanger is the primary cause for varying price points on a stove, typically. A good rule of thumb is that the more expensive the stove, the more efficient the heat exchanger, the more heat you get to use in your home per pellet combusted. This part about heat exchangers comes back later on, so keep that in mind.***
So now what we know what a closed heating system is and how an outdoor in-take helps us create one, let’s look at some of the benefits.
1.) What, are we trying to heat the outside?!
Dad jokes aside, global warming doesn’t need our extra help by belching heated air out into the atmosphere. When you don’t have the in-take leading outside, you are pulling air from the room your stove occupies to burn your pellets. You are literally taking air you just paid good money to heat and blasting it out of your exhaust stack. Don’t be hypocritical and yell at your kids for letting all the heat out when they don’t close the front door quickly enough, their error isn’t running 24/7 during the winter heating season like your stove is.
2.) Man… this has been a DRY winter!
One of the issues of drawing air from inside your home into your combustion chamber has is that it creates a slight negative pressure within your house. Nature abhors vacuums, as Aristotle said, so when you’re taking air from your house and force-drafting it out of your exhaust, you’re creating something that the environment wants to correct. Since you’re not pulling in air from anywhere else, it has to leak into your home from outside via windows, loose walls, and when you open the door every day. The bottom line? Warm, humid, expensive-to-heat air leaves your house and cold, dry, outdoor air enters back in. That’s the definition of two steps forward, one step back. No joke, we’ve had customers call in and tell us how rough this winter has been with their kids getting bloody noses all the time. We tell them to get an outdoor in-take kit on their stove and the problem is solved overnight. Some of our customers with really tight houses and no in-take kit have even reported a strong gust of wind every time they open the door to leave in the morning. Think of all the movies where a space ship gets breached and everything is sucked out into the abyss but this time use papers flying around instead of people. The problem is the same thing, you’re reducing humidity around the clock, sucking cold air into your house that causes your stove to run more, and you’re creating a low-pressure system within your home. It doesn’t sound like much, but it does truly add up over 6-8 months of the heating season.
3.) The three legged stool of fire. Fuel, Heat, and Oxygen.
Perhaps you’ve found yourself looking for the best pellet you can buy only to be saddened when carbon-rich, sooty crud is caked to your burn chamber or almost completely unburned pellets litter your ash pan. Soot on your heat exchanger is bad because it’s an insulator and keeps precious heat from transferring from the burn chamber to your room with any efficiency. But it also tells a tale of incomplete combustion. Unburned pellets in your ash pan often signifies your feed rate is too high and smothering out some flame. Turning down the feed rate can make this problem go away. Making sure you heating element isn’t past its lifespan is also a good thing to check at this time. In both the cases you have incomplete combustion and the fix could be as simple as air.
All of us remember the Cub/Girl Scouts or a camping trip with the family when we were young. We had a lesson in the virtues of fire, its danger, and its necessities. My father posed it to me as the legs of a three-legged stool. If one goes away or gets out of balance, the stool falls over. Too much Oxygen is like blowing out a candle. Too much Fuel will smother the fire. Too little Heat and you have no ignition. There are other nuances like dry fuel and the timing of added oxygen, but the point of all of this is that a pellet stove runs on these same principals and simply is just smarter about it. A computer runs all the systems to allow for more efficient combustion and if you have a thermostat controlled stove, that’ll only be when combustion is needed. As our stove can control room temp with smarter controls, so too can we be smarter about what we do with our stove set up.
Just about all pellets are between the 5%-10% moisture range, so dry and consistent fuel isn’t much of an issue (unless you accidentally leave your bags outside) and the stove’s built-in heating element or your use of fire gel provide the right heat to start the party, but the story of the oxygen from your combustion fan isn’t so cut and dry. You see, air at a colder temperature has less energy pushing molecules apart from one another. It is more dense. As such, you’re going to have a larger number of oxygen molecules per cubic foot of air that is 20 degrees than you would in air that is 100 degrees. The colder the air, the denser the concentration of oxygen is within it. Pellet stoves I’ve come into contact with maximize their burning efficiency around 14 degrees F. Warmer air has less oxygen in it to rip electrons away from the wood you’re burning and anything colder than 14 deg isn’t going to offer much of an increase in efficiency. So unless the room you’re heating is 14 degrees, you need to use air from outside to maximize burn efficiency. And if the air inside your house is 14 degrees F, then you will certainly need a plumber. Theoretically, even if your air inside the house is 14 degrees, it won’t stay that way for very long once your stove starts puking heat out. Your “indoor advantage” would cannibalize itself.
So the solution to the problem posited at the beginning of this section could simply be that you’re too low in oxygen for a healthy fire to burn. The difference in oxygen content of your air is the same difference between short, excited, and dancing flames that say your stove is tuned up like a NASCAR engine and the long, lazy flames that scream, “Inefficiency!” right in your face. That means the it’s also the difference between complete combustion of your pellets and mounds of cleaning your burn chamber and heat exchanger to accompany all of the frustration of not “finding the right pellet” that you’ve created for yourself. Now granted, we can’t control the air temp when it’s in the shoulder seasons and 40 degrees outside, but that air is definitely still better than the 70 degree air inside.
***I’ve heard some people claim that they don’t care about the oxygen content of air, they don’t want to have cold air in their burn chamber because they have to waist heat to get that colder air up to temp. I say that’s a silly statement. While it is definitely true that chemical reactions take place more rapidly in a warmer solution, that doesn’t apply directly here. This reaction is taking place only when oxygen comes into contact with carbon from the wood with enough heat to activate the process and warmer air would certainly circulate around the wood more quickly. But since a fan is blowing the air over the wood, air is circulating at a fixed rate and the concentration of oxygen matters far, far more than the temperature of said oxygen. Even if the fan was not a part of this conversation, have you ever had a campfire outside in winter? Bet it burned a heck of a lot faster than in summer. And it wasn’t because you had to heat up the cold wood either. A wood fire in your fireplace clocks in between 1200 and 1500 degrees F. Do you really think that your stove fire temp cares if the air it’s eating is 50 degrees cooler than in your home? As they say every Monday night during football season, “C’Mon, Maaan!”***
4.) This Old House
Some folks have claimed that their house is too loose and that it will seep in cold air anyways because it is a tough space to insulate. I sympathize with you on that front, but as I’ve already covered this point above I’m going to keep this answer brief. You’re already going to have cold air seep into your house as is. Don’t double down by blasting heated air out of your exhaust vent too. That makes it even more expensive to keep warm than it already was going to be!
The Pros and the Cons
In the end, the benefits of an outdoor in-take kit will vastly outweigh the cons in 99% of cases. I have yet to encounter one where it doesn’t make sense, but I’m still a firm believer that I don’t know everything in this world and one day I may find myself with one such case. The only real con I can think of is not wanting to cut a hole in your wall. I totally understand that sentiment, but you should have a pretty good idea where your stove will be sitting for the next forever before you install it anyway. If you don’t, then I recommend taking a second and thinking about it. You’ve made great progress in thinking by simply finding this article already, and look how far you’ve come in reading this tome! Anyway, even if you do move your stove, patching that hole isn’t an impossible, super-elaborate task either. You can even hire it out pretty easily. (Got kids? Great learning opportunity here for them. In fact, get them in on cleaning the ash and have them change your car’s oil while their hands are already dirty. A dinner well earned tastes better anyway.)
What you’re trading for here is the full use of your stove. Remember when we talked about heat exchangers earlier? More complete combustion means that you can use that heat exchanger to its fullest potential. If you bought an expensive stove, you absolutely need to take advantage of its capabilities or you’ve just wasted part of the $3,000+ you spent. You get more BTUs harvested per pound of pellets as your heat exchanger captures more of that heat to send out into your home. That means you burn harder with less pellets and still get the same heat you required. That is saving you money year after year after year. As an example, let’s just say that you’ve increased your combustion by 5%. Assuming you have to fire up the stove for half the year, which on average isn’t a bad guestimate with cold nights and the occasional long/early winter, that means you’ve effectively removed 9 days worth of your normal heating costs. That means for the rest of your stove’s useful life, you’ve reduced your heating season by over a week. And that’s just a relatively small improvement in efficiency. Imagine if you went from burning 60lbs each day to 40lbs. That’s 1.825 tons of pellets a year!
Now this is where many would claim, “But I bought a $900 New Englander down at the big box store in town, so that means none of this really matters to me.” It is true that you can more easily create more heat than your stove is capable of capturing with a less efficient heat exchanger, but you should still aim for a healthy flame and overall fire. Your stove company had to make some compromises in order to keep the cost of your stove down and as such you’ll need to use as much of an advantage as possible to get the best burn out of those pellets you spent so much money on. I argue that it is a better problem to burn pellets hotter than your stove can transfer and get complete combustion than it is to burn them inefficiently to avoid lost heat in the transfer stage. In either case you’re still going to throw away energy be it via hot air or via charred pellets. The difference is that one is a lot easier to clean each week.
Inefficiently burned pellets lead to soot build up and larger, denser material in your ash pan. This in turn leads to more time spent cleaning your stove to prevent your heat exchanger from losing even more efficiency to soot and ash covering it. It’s a vicious cycle. Just because you made an economical choice in pellet stoves doesn’t mean your time isn’t valuable, folks. No one likes cleaning their stove and even fewer like cleaning the heat exchange tubes. Save yourself some time and effort by running a healthy fire in your stove and spend those saved extra hours sitting by a good fire with a book or with the game on.
In conclusion
You can take home a few bullet points from this whole conversation. They are as follows:
-An outdoor in-take kit is the often ignored piece of your stove that is crucial to full operation and efficiency.
-Many problems with pellet burning can be traced back to the third leg of the fire stool, air supply.
-Your home WILL be more comfortable when you add in the outdoor in-take kit
-Incomplete combustion/more maintenance is a potential price to pay for not using one.
-I am not a certified HVAC nor a solid fuels installation expert. This article is purely educational in nature and is intended to help you ask better questions when consulting with you HVAC professional. They can help you make a sound decision in the set up of you home heating system, not me. Be sure to follow all manufacturer recommendations and have a licensed professional handle all the work involved. Your insurance agent will certainly agree with me on that note.
-SMR