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In their stock condition, Thermacell’s Portable and Backpacker models do not work well at altitudes above 7000 ft above sea level. With an easy modification, I have been successful in regularly using my Thermacells up to 12,000 ft above sea level.
After dorking it up, I recommend:
Thermacell MR300 Portable Mosquito Repeller
and / or
Thermacell Backpacker Mosquito Repeller
- Thermacell mosquito repellers do a pretty good job creating a small bubble of space around you with far fewer mosquitoes. Don’t expect miracles, but it genuinely makes hanging out outside more enjoyable when the blood suckers are out.
- The manufacturer says it creates up to a 15 foot wide zone of protection. You should really expect about an 8 foot wide zone of fewer bugs bothering you.
- Lets you use less DEET or other insect repellent on your skin.
- Fairly compact and lightweight, easy to take with you.
- IMPORTANT: These units have an elevation limitation, as designed by the manufacturer. Thermacell says the Portable and Backpacking models won’t work well above 7000 feet above sea level. The subject of this article is to fix this limitation.
- CAUTION: Product uses allethrin, which is generally safe for people and dogs when used as directed, but is highly toxic to cats, birds, and aquatic life. I wouldn’t use this in your yard at home if your cat likes to hang outside with you. Dispose of packaging and used repellent pads properly. Wash your hands after you handle the pads.
Keep reading to Dork It Up!
The summer of 2019 was a buggy one in Colorado, following record snow levels in the mountains the previous winter. My family and I like to backpack in the mountains, and the mosquitos near streams and lakes were out in force.
I had owned a Thermacell Backpacker model for several years, but hadn’t used it much. I hauled it up a mountain in 2019 to a lake near 10,000 feet above sea level, and found that it did not want to consistently light and stay lit. I didn’t know of the elevation limitation until I came home and Googled it. When the Thermacell did stay on, it worked pretty well at repelling mosquitos, so I tried to figure out how to make it work more consistently for camping in high country.
The fuel powered Thermacells are basically tiny stoves that heat up a metal plate. The flame of the stove is really tiny. There is a viewing port where a tiny blue flame and an orange glow can be seen when the device is on. I believe the orange glow is a wire that is heated by the flame and glows red hot, to make it easier to tell when the device is on. The hot metal plate warms an absorbent pad treated with insect repellent, which then releases the repellent into the air.
The stove runs on fuel in gas form. The Portable model uses little butane cartridges. The Backpacker model runs on common backpacking stove canisters, which typically are filled with a blend of isobutane and propane.
There is no adjustment valve for the tiny stove in the Thermacell. You flip a switch to start the gas flow, press an igniter button, and the flame lights.
Combustion of fuel requires the right ratio of fuel to oxygen. If there is too much fuel (too little oxygen), combustion will be “rich”. If there is too little fuel (too much oxygen), combustion will be “lean”. The air is less dense at higher elevations, and therefore has less oxygen in a given volume of air. At high elevations, the Thermacell doesn’t get enough oxygen for proper combustion of fuel. Let’s fix that.
First I wanted to estimate how rich the fuel / air mixture is when the device is working at altitude. Then I could estimate how to adjust the Thermacell for better operation.
I needed data on the density of air at different elevations. I found an old NASA document with exactly what I needed, US Standard Atmosphere, 1976. The needed data starts on page 122 of the document (page 135 of the PDF file). A table of altitude vs. air density is below, with an extra column added for perspective. When I’m camping at 10,000 ft above sea level, the air is about 25% less dense than at sea level, meaning there is about 25% less oxygen in every breath I take. No wonder my Thermacell doesn’t work well there (and no wonder I get winded walking up the mountains!). Thermacell says their device works up to 7000 ft above sea level.
| Altitude (ft) | Density of Air (kg/m^3) | % of Sea Level Density |
| 0 | 1.225 | 100.0% |
| 500 | 1.207 | 98.5% |
| 1000 | 1.190 | 97.1% |
| 1500 | 1.176 | 96.0% |
| 2000 | 1.155 | 94.3% |
| 2500 | 1.138 | 92.9% |
| 3000 | 1.121 | 91.5% |
| 3500 | 1.104 | 90.2% |
| 4000 | 1.088 | 88.8% |
| 4500 | 1.072 | 87.5% |
| 5000 | 1.056 | 86.2% |
| 5500 | 1.040 | 84.9% |
| 6000 | 1.024 | 83.6% |
| 6500 | 1.009 | 82.3% |
| 7000 | 0.993 | 81.1% |
| 7500 | 0.978 | 79.8% |
| 8000 | 0.963 | 78.6% |
| 8500 | 0.948 | 77.4% |
| 9000 | 0.933 | 76.2% |
| 9500 | 0.919 | 75.0% |
| 10000 | 0.905 | 73.8% |
| 10500 | 0.891 | 72.7% |
| 11000 | 0.877 | 71.6% |
| 11500 | 0.868 | 70.8% |
| 12000 | 0.849 | 69.3% |
| 12500 | 0.836 | 68.2% |
| 13000 | 0.822 | 67.1% |
| 13500 | 0.809 | 66.1% |
| 14000 | 0.796 | 65.0% |
| 14500 | 0.783 | 64.0% |
| 15000 | 0.771 | 62.9% |
The middle altitude range of the Thermacell is 3500 ft. I typically camp at elevations ranging from 7000 to 12,500 ft above sea level, in the foothills and mountains of Colorado. I decided to attempt to change the middle range of the Thermacell’s altitude operation to 10,000 ft.
The composition of air is constant, and is not dependent on altitude. Air is composed of about 78% nitrogen, 21% oxygen, and 1% other gases (carbon dioxide, hydrogen, helium, argon, etc.). However, at altitude, there is less pressure on the air, and therefore the molecules of air can spread out more, making the air less dense. If the air is less dense, there is less mass of air in a given volume (like the volume of your lungs!), and therefore less oxygen in that same volume. Here’s a good explanation from National Geographic (intended for middle schoolers, but it’s the perfect level of detail).
The density of air at 3500 ft is 1.104 kg/m^3. The density of air at 10,000 ft is 0.905 kg/m^3. 1.104 / 0.905 = 1.22. So there is 22% more air at 3500 ft than at 10,000 ft. My theory is that if I get the Thermacel 22% more air while operating at 10,000 ft, it should work about the same as it normally would at 3500 ft.
Modification of the Thermacell will of course change its overall elevation operation. If I assume that increasing the air delivered to the Thermacell by 22% simulates increasing the density of air by 22%, I can estimate the Thermacell’s new elevation range. The Thermacell works at air densities from 1.225 down to 0.993 kg/m^3. Looking at the table below, I get simulated densities in that same range from about about 7000 to 13,000 ft, so the Thermacell should now work in that range. This also means that after modification the Thermacell might not work well at low elevations! If you regularly camp at both low and high elevations, there’s an easy solution: buy two units, and only modify one (and label which is which!).
| Altitude (ft) | Simulated Density Increase of 22% | % of Normal Sea Level Density |
| 0 | 1.495 | 122.0% |
| 500 | 1.473 | 120.2% |
| 1000 | 1.451 | 118.5% |
| 1500 | 1.434 | 117.1% |
| 2000 | 1.409 | 115.0% |
| 2500 | 1.388 | 113.3% |
| 3000 | 1.368 | 111.6% |
| 3500 | 1.347 | 110.0% |
| 4000 | 1.327 | 108.3% |
| 4500 | 1.307 | 106.7% |
| 5000 | 1.288 | 105.1% |
| 5500 | 1.268 | 103.5% |
| 6000 | 1.249 | 102.0% |
| 6500 | 1.230 | 100.4% |
| 7000 | 1.212 | 98.9% |
| 7500 | 1.193 | 97.4% |
| 8000 | 1.175 | 95.9% |
| 8500 | 1.157 | 94.4% |
| 9000 | 1.139 | 93.0% |
| 9500 | 1.121 | 91.5% |
| 10000 | 1.104 | 90.1% |
| 10500 | 1.086 | 88.7% |
| 11000 | 1.069 | 87.3% |
| 11500 | 1.058 | 86.4% |
| 12000 | 1.036 | 84.6% |
| 12500 | 1.020 | 83.2% |
| 13000 | 1.003 | 81.9% |
| 13500 | 0.987 | 80.6% |
| 14000 | 0.971 | 79.3% |
| 14500 | 0.956 | 78.0% |
| 15000 | 0.940 | 76.8% |
How a Gas Burner Works
Many gas burners (propane, butane, natural gas, etc.) operate similar to the figure below. Gas from the source (propane tank, gas line at your house) flows through an orifice (a tiny hole) and into a venturi (a narrowing and then expansion of a pipe) with vents or holes in it. Air is sucked into the venturi due to the velocity of the gas (and the low pressure that velocity creates), and mixes with the gas. The air and gas mixture continues to travel down and out the burner tube, where it can then be burned and heat something.
The propane burner on my Weber gas grill operates in this way.
During combustion, fuel needs oxygen to burn. In perfect combustion, all the fuel is burned, and all the oxygen is consumed. The ratio of air to fuel for perfect combustion is called the stoichiometric ratio. The stoichiometric ratio for butane combustion is 15.44 (air) to 1 (fuel), by mass, per Wikipedia. So 15.44 parts air needs to mix with 1 part fuel for perfect combustion. If too much air is present (or not enough fuel), combustion will run “lean”, and not all the oxygen will be consumed. If not enough air is present (or too much fuel), combustion will run “rich”, and not all the fuel will be burned. At some point, if the air to fuel ratio is too far off, combustion will not occur at all.
Typically to adjust a gas burner’s air to fuel ratio, either the diameter of the orifice is changed (to increase or decrease the flow and velocity of the gas), or the size of the venturi air vents is changed (to suck in more or less air). Sometimes the air vents are adjustable, and are called air shutters.
Thermacell Modification Details and Instructions
Taking a look inside the Thermacell, we can see components similar to other gas burners. The fuel travels from the fuel canister, through an orifice, past an air inlet, into a burner tube, and to the burner. To increase air available for combustion, we will increase the size of the air inlet.
WARNING: This modification likely voids any warranty on the Thermacell. Additionally, this is an appliance that operates on combustible gas. Be careful, and only operate the Thermacell outside. Proceed at your own risk!
You will need some simple tools to complete the modification:
- Small screwdriver, to disassemble the unit. This set looks decent.
- Calipers, to measure the air inlet gap. Digital calipers are easier to read.
- A tiny file, to make the air inlet gap bigger. This set should work fine.
- Cotton swab (Q-tip) and isopropyl alcohol to clean up dust created from filing.
Portable Model
- Remove six screws from the back of the unit, flip over, and take off the front cover.
- Take a picture of the inside of the unit, to help remember how everything will go back together. Remove two more screws, remove the spring, and take out the white assembly (it will need to disengage from the black gas tube). Now that I look at these pictures again, it seems that one could access the air inlet with the white assembly screwed down, so this step may be optional.
- Locate the air inlet. Using calipers, measure the length of the air inlet. When I measured the inlets on my Portable and Backpacker models, I got 0.107 and 0.113 inches, respectively.
- Determine your desired new air inlet length. I wanted 22% more air, so I enlarged the air inlet length by 22%. Portable model: 0.107 inches * 1.22 = 0.131 inches. Backpacker model: 0.113 inches * 1.22 = 0.138 inches.
- Use your file to increase the air inlet length. Hold the white assembly such that filing dust falls away and out of the assembly, to avoid blocking the tiny gas outlet holes. File a tiny bit at a time, re-measure, repeat as needed. 0.024 inches is not a lot of material to remove, so don’t file too hard or too much without measuring again.
- Remove any dust you created, so it doesn’t block the tiny gas/air paths. Shake or blow the dust off. Use a cotton swab and isopropyl alcohol to clean everything as well as possible.
- Put everything back together.
- Take your Thermacell into the mountains, and see how it does.
Backpacker Model
Modification of the Backpacker model is basically the same, but with different disassembly.
- Remove four screws from the bottom side of the unit, slide the gray cover off.
- Locate the air inlet. Use the directions above for the Portable model for modifying the air inlet. Put it back together, and enjoy.
Results
After this modification, my Thermacell works pretty consistently at elevation. I have used it many times at elevations ranging from 8,000 to 12,000 ft above sea level. It also still works at my home, at 5800 ft above sea level, so it works at least a bit below the altitude range I estimated above.
It is occasionally still a little fussy to light, taking several clicks of the igniter, and then monitoring through the peephole for 30 seconds or so to see if it stays lit. If it goes out, try again. Try to keep the unit still until it warms up, or move slowly if you need to set it down. Once the unit is warmed up a bit, it will stay lit until you turn it off.
I have found that pressing the igniter several times IMMEDIATELY after turning on the gas works pretty well. If you let the gas run too long before trying to ignite it, I think the gas displaces all the air and won’t light. If this happens, turn the gas off, let the Thermacell sit for a bit to let the gas disperse, and try again.
I find the Thermacell to be effective at making camping in bug season more pleasant. It takes 10 to 20 minutes to fully heat up and disperse the repellent into the air. It creates about an 8 ft zone of less bugs (less than the 15 ft advertised), so put it right next to you when you are sitting around and hanging out. If it’s breezy, position the Thermacell up wind so the repellent blows towards you.
Backpacker or Portable?
There are two light weight Thermacells suitable for backpacking.
One model is actually called the Backpacker. If you already use a canister stove, the Backpacker model is the lightest option (4 oz), since you already have the fuel with you. The Thermacell uses a minuscule amount of fuel, so won’t impact your overall fuel usage too much. The Backpacker will run for 90 hours on a 4 oz fuel canister (the smallest common size), so running for several hours a day will not use much fuel compared to what you will use for cooking. The disadvantage of your Thermacell sharing a fuel source with your stove is that you can’t use the Thermacell at the same time as cooking, a common time that you may want to be able to sit and enjoy the scenery around you without mosquitoes in your face.
I prefer the Portable model (MR-300). It weighs 6.7 oz with a butane cartridge installed, but I always bring at least one extra cartridge with me too. The cartridges last 12 hours and weigh 1.0 oz each. Bring as many as desired for longer trips.
The repellant pads last for 4 hours, and weigh 0.1 oz each in their packages. Bring extra!
There are other slightly different Portable models. The MR450 is armored (added weight) and has a light that tells you when the unit is warmed up (I just check with my hand to see if heat is coming off). The MR300F comes with a holster, which could be useful if you want to use this while on the move.
Alternative to Gas?
Thermacell does make a battery powered unit (called the Radius) that should work as-designed at any elevation, but it has severe limitations. The battery only lasts up to 6.5 hours, which doesn’t cut it for a backpacking trip of several days. You could also take a portable battery pack or two with you to re-charge the Thermacell, but then the weight adds up quickly. A battery pack weighs a lot more than a butane cartridge, and one charge lasts half as long. The battery powered Thermacell might be useful for car camping trips, where a power source is available, and it could be re-charged every night.
Recommendation
Get a Portable or Backpacker Thermacell, and enjoy your recreation time in the outdoors even more.
Stock up on extra repellant pads and butane cartridges too. Combo pack for Portable models, repellant pads only for Backpacker model.
I use and recommend Thermacells. I paid for them with my own money, and am sharing my personal experience with them.
Many of the links on this site are “affiliate” links. If you use the links provided to purchase the recommended items, or to navigate to the web retailer site to purchase anything at all, I may receive a small commission. These commissions will help me expand this site and provide you with additional recommendations. As an Amazon Associate I earn from qualifying purchases. Thank you!
Dork It Up Yourself:
More info on burner venturis: https://www.physicsforums.com/threads/gas-burner-venturi-effect.526432/
More info on how elevation affects oxygen levels: https://media.nationalgeographic.org/assets/file/TWD_Altitude.pdf
I didn’t think of all this on my own, a search for a solution at elevation found a random forum post that pointed me in the right direction. I think my explanation is better :-).
Looks like your in Colorado like myself. I live around 5000 feet (front range) but would like to use this while camping (typically between 10-11k) AND also in the backyard as the mosquitos get pretty bad here in the evening. After making the modification for 10k feet, how well will this work at 5k feet?
Can I send you my thermacell and pay you to modify it for that elevation?
Thanks. That’s exactly what happened (they replaced it). Unfortunately I may be adventuring nearer to sea level the rest of this summer (damn fires) but hopefully can try it out in the spring. Thanks for sharing your knowledge!
THANK YOU! I just bought my first Thermacell and I was so excited to take it on it’s maiden voyage. I had not read the details about elevation limitations prior to my trip. I was devastated when I arrived at my campsite at around 10k and could not get it to ignite. The only reason I bought the unit was for camping! I am going to give this a try!
You are welcome! Let me know how it goes at high elevation.
This is pretty awesome. Thanks for taking the time to dorkument your mods.
I live at sea level and the Thermacell (backpacker) has worked well this summer. But last week it totally failed at 4500′ and wouldn’t stay lit for for more than 30 seconds. I assumed at first there was something wrong with my fuel (I’m using up my old 1/4 full cannisters of isobutane) but when it persisted with a second can, I figured it was likely altitude (sunny weather, 70F so temp wasn’t an issue)
I’m wondering if my orifice got made too small or something as 4500′ feet should easily work, right? I may try to open it up just a tad (that’s a scientific measurement term) to see if I can get it to work at both elevations. Or maybe I’ll just send it back to costco and try another one.
I’m in in the PNW so I probably won’t be using it much above 6000′ — bugs aren’t that much of a problem here once you get that high but dang, sea level and mid-level altitude functionality would be nice.
Thanks again.
Thanks for your comment. I would expect the Thermacell to work fine at moderate elevations. Perhaps your unit is faulty. I did have an unmodified unit stop working at normal elevations, and contacted Thermacell. Even though it was well past the original 2 year warranty, they replaced the unit free of charge. Might be worth a shot in your case too.