At $39, real time PM sensor can empower homeowners – and neighbors
Ever wonder if your wood stove or pellet stove elevates your risk of inhaling particles that could endanger your health? Recent advances in technology now make indoor air quality monitors small inexpensive and simple for people with wood and pellet stoves - or fireplaces. The Alliance for Green heat bought and tested several and found the Govee Smart Air Quality Monitor, at $39, to be the best.
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The Govee Air Quality Monitor (right) shows PM, temperature and humidity on a digital display. The Amazon Smart Quality Monitor on the rleft ($79) only has a small light indicating air quality. |
Not all wood and pellet stoves result in poor indoor air quality. But there is little research that compares different types of wood and pellet stove installations. The human nose is very sensitive and if you smell wood smoke in your home often, that is enough of a reason to look into the problem.
Low-cost sensors like the Govee or the Amazon sensor (that connects to Alexa) detect particulate matter (PM2.5) and can help homeowners better understand the issue and may be a motivating factor to take action, which could mean calling a chimney sweep or just cleaning your pellet stove better before lighting it.
In the future, such monitors could easily be built into stoves, which could be a significant advance in helping homeowners understand how to best run them. This year, MF Fire will be introducing an aftermarket sensor package designed to help homeowners operate their stoves better, but they do not measure indoor PM.
On Feb. 19 both the Govee (right) and Amazon (left) monitors caught the morning start-up/breakfast and elevated evening PM. The Govee had a 2 ug/m3 daily average and the Amazon had a 3 ug/m3 daily average. One big benefit of the Govee is that is shows temperature on the same screen as PM, and you can see when the pellet stove is turned off around 10PM and turned back on around 6:30 AM.
Digital displays are now everywhere in our homes and cars and they tell us everything from when to add air to our tires to when to change the filter on a furnace. But for those of us who heat our homes with wood stoves and pellet stoves, the health of our kids and our spouses should be paramount. The EPA also has good information on indoor air monitors.
Another benefit of the Govee is that it pushes a message to your phone, wherever you are, when PM spikes. At 6:40, just as I was lighting the pellet stove, it briefly spiked to 35 ug/m3, and I got a message on the cellphone when I looked at it, 15 minutes later, at 7:05. The average on Feb. 21st was 3 ug/M3.and there was also a little PM elevation around dinner.
Low-cost sensors are also useful for neighbors who may be breathing wood smoke from someone else’s wood stove. When outdoor air quality is bad, it leaks into homes and these sensors can help neighbors talk to each other using data, not emotional reactions, to wood smoke issues.
Data from my home’s pellet stove
My pellet stove emits a little smoke on start up, but I found that after the first half hour, there is no air quality impact that the sensors could detect. I turn my pellet stove off when we go to bed and when we wake up the sensors usually say the house has 1 µg/m3, meaning 1 microgram per cubic meter of air. By the time the pellets catch fire, it usually reads 6 or 7 ug/m3 but it can also go up to 30 or 40. But if we fry something for dinner, the sensor often goes to 30 or 40, and if we over fry something to the extent you can smell something is nearly burning, the sensor can read 100 or even 200. Our conclusion is that the riskiest behavior in the house is cooking, but your house may be very different, and a wood stove may emit far more indoor smoke than my pellet stove.
If I don’t clean the burn pot of my pellet stove, it takes longer for the pellets to light off, and there can be quite a bit of smoke. Currently, I have a Harman Accentra but I had the same problem with a Ravelli. It appears that the exhaust fan doesn’t go on until the stove sensors fire in the burn pot, but by then the smoke has found any available crack to leak into the room. This seems like a design flaw, but we all know that positive pressure in a firebox is a recipe for problems.
In our house, we use an older gas stove that is located in the kitchen, about 15 feet away from the PM sensors which were placed just 2 feet away from the pellet stove. Our impression is that regardless of whether you have a gas, electric or induction stove, frying will reduce air quality, particularly at higher temperatures. We don’t have a exhaust hood or fan above our cook stove, which we presume would be a big help to reduce PM from cooking. A hood with a fan exhausting to the outside may be the best investment in homes that fry food often.
Indoor air quality for wood stoves, pellet stoves and fireplaces
Conventional wisdom, based on tens of thousands of wood and pellet stove users, is that indoor smoke is usually far worse with wood stoves. Part of the problem is that wood stoves need to be reloaded, usually every 1 – 4 hours, and depending on wind conditions outside, how quickly the door is opened, etc. the negative pressure in the firebox is broken, and smoke spills into the room. When a pellet stove is reloaded, you open a door to the hopper, not to the firebox, so there is no impact on pressure in the firebox and no opportunity for smoke leakage. Also, since the exhaust flue is powered by a fan in a much smaller pipe, windy conditions outside are far less likely to impact indoor air quality.
An air quality sensor can tell you how bad your indoor air gets when you reload – and crucially – how long it takes to get back to normal. Once you start measuring it, it gives you a metric which can lead to behavior change.
With both wood and pellet stoves, improper installation can aggravate or cause leakage of smoke. With pellet stoves, the exhaust pipe has positive pressure and can leak if it's not sealed or installed properly. With wood stoves, a short exhaust pipe which can often be found in trailers, and self-installed stoves, can lack the suction needed to keep smoke from spilling out into the room.
Did I need the sensor to help me understand the relationship between cleaning my firepot better and start-up smoke? Not necessarily, but it gave me data to measure it on a daily basis, better understanding the solution, and it gave my whole family some reassurance that the stove did not cause much of a problem at all compared to cooking.
How much smoke is acceptable?
It is easy to say that no additional indoor PM is acceptable yet we all live with it when we cook, toast bread and vacuum floors and rugs. The EPA and the World Health Organization(WHO) say that indoor 24-hour averages should be below 12 – 15 ug/m3, or more recently down to 9 ug/m3. My house, with cooking meals for 3 people and running a pellet stove from 6:30 AM to 10 PM, almost always stayed well under those average limits. However, in homes were a wood stove were to result in daily averages of 20 or more, that would likely be unacceptable to many people, as I think it should be. Learning when and how to reload your stove quickly and efficiently may help.
In terms of outdoor, ambient wood smoke, we know that concentrations of wood stoves in valleys, especially those with more frequent and severe inversions can pose serious health risks. I have come to believe that restrictions on the new installation of cord wood stoves in those areas is often a step in the right direction. Wood stoves are not a good energy solution in any densely populated area, not because they are usually not operated well-enough with dry enough fuel. However, we think pellet stoves can be a far greater energy solution than they are today, and should be allowed to be installed even in areas where cord wood stoves aren’t.
The toughest question is how to compare and contrast the additional local PM from wood or pellet stoves and the disastrous global impact that oil and gas heating have. All renewables are increasingly getting local pushback even in the face of overwhelming data showing we need to ramp up renewables far faster than we have been.
What do the academic studies say?
There have been multiple academic studies of indoor PM impact of cord wood stoves but we could not find any about pellet stoves. For wood stoves, one study found an average of 20% higher PM in homes with wood stoves. Another said that wood stoves triple the PM in a homes. Another found that the age of the wood stove was not a determinant of indoor air pollution but more frequent cleaning of the flue was a factor. This was the same conclusion of a well-known study in British Columbia that found there was not a consistent relationship between stove technology and outdoor or indoor concentrations of PM2.5. Yet another studyfound higher PM in homes with wood stoves and the primary mechanism for introducing PM into the home was opening the door to the stove.
Do air purifiers work for wood smoke?
Yes, air purifiers can be effective for indoor smoke from forest fires, your neighbor’s wood stove, or your own wood stove. AGH began promoting the use of air purifiers in 2015 and since then, the rise of wildfire smoke have made them a commonplace solution. But if the problem is your own wood stove, you should fix the problem before putting a band-aid on the symptom of the problem. One of the highest rated air purifiers is Blueair Blue Pure 211i Max(around $279) or Blueair Blue Pure 211+ (around $169). Both consistently perform nearly as well or better than more expensive ones. One downside for all air purifiers is the cost of replacement filters ($25 - $35 for Blueair filters) which should be replaced once or twice a year depending on how much you use it and how dirty your air is.
Purple Air Sensors
I have used multiple Purple Air sensors extensively but did not include any of the data here in part because they are far more complicated to set up and can be glitchy. They are highly recommended, however, for a wide variety of ambient PM measuring. Data from ours led us to the same conclusion: the pellet stove caused a spike in PM when it started, but air quality returned to the same levels as when the stove was off pretty quickly. With the Purple Air, we also confirmed that by far the worst events in the house were cooking, not starting or operating the pellet stove.
Use of Govee and Amazon sensors outdoors
The Govee and Amazon sensors are indoor sensors but I was curious to see if they could detect start up smoke of my pellet stove when placed on my front porch, about 25 feet and around a corner from my pellet stove exhaust pipe. I brought both sensors outside and turned my stove off at 11 AM and turned back on at 4 PM. The result indicates that outdoor emissions of this pellet stove start-up were not detectable at 25 feet away on a relatively windless day
This shows that both the Amazon and Govee monitors did not detect elevated PM2.5 during a pellet stove start-up cycle a 4:00 on Feb. 21 when placed about 25 feet from the pellet stove exhaust pipe. If the sensors were placed directly under the exhaust pipe, the results would be very different, as there is always a strong smell of smoke there.
Conclusion
We found that our pellet stove does not significantly impact air quality in our home compared to other daily activities such as cooking and cleaning. The pellet stove does emit some PM on start-up, but on a daily or hourly average basis, it was usually still well within EPA and WHO recommended limits.
If you regularly smell wood smoke in your home, you have a problem and you should not delay in trying to fix it. A low-cost air sensor can be an excellent way to assess the problem with reliable data, or let you know that you don’t really have much of a problem at all compared to other daily household events like cooking and cleaning.
If you have a pellet stove that emits some smoke on start-up, or if you have done any air quality monitoring in your house with a wood or pellet stove, please contact us and share you experience.
