The Importance of Wood Pellet Length

Wood pellet length is important because it directly impacts your pellet heater burn rate and how long a bag of pellets lasts. Pellets should not be too long or too short. Pellet length will usually vary and ranges from 10mm to 40mm in length. Ideally the average length of wood pellets should be around 20mm. However, this doesn’t mean that every single pellet will be the same length. What it does mean is if you eyeball a bag of pellets you should see different lengths but overall the average length would be around 20mm. Pellet length is more important than hardwood vs softwood pellets as it impacts pellet consumption, heat output and reliability of pellet heaters.

wood pellet length

Here are the key points on why pellet length is important. Read on below for more detail:

  • Short pellets feed faster so the fire can be too big, burning more pellets. This increases running costs.
  • Long pellets feed slower so the fire may be too small or die out. Long pellets also increase the risk of auger jams.

Ideal Wood Pellet Length

The ideal wood pellet length is around 20mm. The way pellets are made, it’s near impossible for them to be the exact same length. There will always be variation. However, what you need to be on the lookout for is pellets that are primarily too short or too long. If you look at a bag of pellets you can quickly guage the overall length of pellets in the bag. If they look short, 15mm or less, then those pellets will be shorter than normal. If they look to be 30mm or longer, then that bag of pellets would be longer than recommended. You don’t want pellets that are too short or too long.

Too Short vs Too Long Wood Pellets

Pellets that are too short, on average less than 20mm per bag, will feed through your pellet heater faster than normal. The auger feed rate on pellet heaters is set a different speed for each heat setting. The more heat you want, the higher or faster the auger spins. Likewise, the slower the auger spins the fewer pellets are fed into a burn pot, so there’s less heat put out by the heater. However, shorter pellets are fed into the burn pot faster due to their short length. This means on low, your pellet heater will burn more pellets than it normally would if using standard length wood pellets.

Because your heater is burning more wood pellets than normal, even on low, this means your pellet heater running costs will be higher. You’ll know if there’s a problem because a bag of pellets won’t last as long as normal. Likewise, if pellets are longer than normal, say 40mm or more, fewer pellets will be fed into the burn pot by the auger. Fewer longer pellets go through the auger. This causes the reverse of what happens with short pellets. Fewer pellets in the burn pot means heat output will be lower than normal. This might sound good to some people thinking they might save some money. However, long pellets are worse than short ones.

The problem with wood pellets that are too long is they can cause your pellet heater fire to die out, may cause excessive clinkers or worse can cause an auger jam. Long pellets move slower through the auger than short pellets, meaning the fire in the burn pot is smaller. If your pellet heater is running on high, then this may not be a problem because the feed rate is higher. However, running on low the fire may get so small it simply dies out. Your pellet heater will detect the fire has died out and shut down. With the pellet heater burning poorly, you may also get excessive clinkers.

Even worse than the fire going out is long wood pellets can cause an auger jam. The chance of this happening is small but it can happen. What usually happens is if a pellet is too long it is simply broken as it moves into the auger before it comes out the other end and drops into the burn pot. However, sometimes as long pellets go into the auger it breaks down to the required length but then it becomes stuck in the auger. With the pellet stuck in the auger, other pellets behind it become jammed and the auger becomes blocked. Your pellet heater won’t start because no pellets can be fed into the burn pot.

How to Burn Short Wood Pellets

If you’ve got several bags or a pallet of wood pellets that are too short then what you need to do is adjust the feed rate of your pellet heater. How you do this depends on the model of pellet heater you have. However, what you need to do is decrease the “off” setting. Augers have an “on” and “off” setting. “On” means how many seconds the auger is spinning feeding pellets. “Off” obviously means how many seconds it is off. So, if the auger feed rate on your pellet heater for low is 2 on 14 off, then this means it turns for 2 seconds feeding pellets, then stops for 14 seconds before going on again.

What I would recommend is to increase the “off” setting. Decrease it by 1 or 2 seconds and see how it goes. If your pellets are only a little bit shorter than normal then increase the off number by a small amount, such as 1. Try it out and see the size of the fire decreases in size to what it should be. You can decrease the “on” setting but normally that number is already very low, so you won’t have much to work with. If you notice the fire is still too big then just increase the “off” number by a little bit and try again. This will only work if you have access to feed rate settings.

Finally, and this mentioned below, consider mixing short pellets with longer ones. If you have another brand of wood pellets at home that are normal length, then mix the short ones with long ones and try burning them. You may get away with a 50/50 mix or might need more normal ones than short ones to improve the burn. Doing this means you won’t have to change the feed rate, good if you don’t have access to it or just don’t want to adjust the feed rate. The best thing to do is experiment and see how it goes mixing short with long pellets.

How to Burn Long Wood Pellets

If you’ve got a bag of long pellets, then you need to do the opposite of what you do with short pellets. You need to decrease the auger “off” setting. If your pellet heater setting is 2/14 then you try decreasing the “off” down by 1 or 2, so maybe 2/13 or 2/12. This will increase the speed pellets are fed into the burn pot. If you notice the fire still goes out then repeat and decrease the “off” setting a little bit more. Keep in mind changing this on/off setting will not decrease the chances of getting an auger jam. However, it will help keep your heater running if long pellets are causing the fire to go out.

If you have other “good” pellets available to burn, you can also mix the long and normal pellets together. This way you’ll get a much more balanced mix. This won’t stop the potential auger jam problem but it will improve the how well the fire burns. Mixing short and long pellets together may mean you don’t have to adjust the feed rate. It really depends on how long the pellets are and what kind of problem you are having with your heater. The best thing to do is experiment and see how it goes. You may need to mix more normal pellets than long ones to balance the length out.

Call Tech Support

If you don’t have access to the feed rate settings on your pellet heater or don’t feel comfortable making the changes then please call your retailer for support. The best thing you can do is before you buy pellets just eyeball the bag. Do the pellets generally look OK or does you see a majority of short or majority of long pellets? Pellet mills sometimes have bad batches, meaning too short or too long, and while it is rare it is still something should keep an eye out for. Adjusting the feed rate of your pellet heater to deal with long or short pellets will improve how well they burn but remember long wood pellets can cause auger jams.

European Pellet Heaters and Long Pellets

Some European pellet heaters, such as Palazzetti pellet heaters, may come with an auger that is designed to ensure the right amount of pellets, as in weight, goes into the burn pot. Their “star” valve design essentially cuts pellets that are too long and ensure only the correct maximum length of pellets are fed into the burn pot. On all pellet heaters, if a pellet is too long it will snap off as it goes into the auger and be broken down to the correct maximum length. The difference here is the Palazzetti heaters “cut” the pellet down to length, resulting in a clean cut and reduced chance of an auger jam. However, not all pellet heaters have this feature.

How to Check Pellet Length

The quickest and best way to check pellet length if just by eye balling the plastic bag for you buy them. With a bit of practice it becomes very easy to do. Take a look at the bag and see if the pellets look too short, medium or too long in length. You can also check for consistency. If you see that the length looks fine, somewhere in the middle, but occasionally you see some short or very long pellets, then it shouldn’t be a problem. The key point here is that the majority of pellets look OK. If you see that the majority look too long or too short then that’s going to be a problem.

Many retailers also check pellets they sell are of the correct length, as selling pellets that are too long might cause problems for their customers, something they obviously don’t want. PFTas in Tasmania actively checks all pellets they sell and so do many other retailers, so ask your retailer for help if you need it when trying to ensure the pellets you buy are the correct length. Also, if there’s a new brand of pellet you’ve never burn before, make sure you buy a few bags first to check length and how they burn.

There’s nothing worse than buying an entire pallet of pellets only to get them home, rip open a bag and find out that they are way too short or way too long. You could return them but it would be a major hassle and might not even be worth it if you had to get them shipped from another city. So, I always recommend you eye ball bags before buying them and if you are trying a new brand of pellet don’t buy a pallet the first time.

Influence of pellet length on performance of pellet room heaters under real life operation conditions
Influence of pellet length, content of fines, and moisture content on emission behavior of wood pellets in a residential pellet stove and pellet boiler