Low-cost air quality monitors excel at detecting levels of indoor wood smoke

At $39 - $45, real time PM sensors 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 Tempo S1+ Smart Air Quality Monitor is the most versatile, with Govee Smart Air Quality Monitor,  to be a close second.

 

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 TemTop S1+, 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.  In 2025, MF Fire plans to introduce an aftermarket sensor package designed to help homeowners operate their stoves better, but they do not measure indoor PM.

Feb. 20. The Govee (left) and Amazon (right) are both easy to set up on your phone and see temperature, humidity, PM and CO on an hourly, daily, weekly or monthly basis. We found they correlated relatively well. They both spiked around 7 PM, cooking dinner but the Amazon did not record much of a morning spike when I lit the stove and cooked breakfast. Both calculated the daily average at exactly 3ug/m3.

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. 21^st 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 PM_2.5. 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. 

In 2019, I set up both indoor and outdoor Purple Air sensors at my house, and my neighbors’ who had a cord wood stove. Sensors with a circle around them are indoor. The data was made public, and of interest to the Purple Air community. 

 

Here the Purple Air sensor at the bottom, mostly in green was indoor and the upper one, mostly in yellow was outdoor. Thus, the 7 PM spike from cooking did not impact outdoor PM, but the stove lighting around 8 AM caused a spike indoor and outdoor. (This is data from 2 units but each unit is a combination of two sensors, and you can see how closely they track each other.) 

 

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.

 

 

EPA fines stove retailer, pursues informal enforcement with many others

In 2019, the EPA issued a $8,250 fine to a Seattle based company, Keller Supply Company,  for selling five uncertified residential wood stoves in Alaska, California, Oregon and Washington. That appears to be the only formal enforcement action in recent years amid an ongoing effort by the EPA to ensure the wood heater regulations are followed.

Almost all of the recent efforts of the EPA enforcement office fall under the term “informal enforcement,” consisting of phone calls, emails, on site visits and other communications that regularly result in corrective measures that bring manufacturers and retailers into compliance.   

In some cases, companies put uncertified stoves on the U.S. market, in violation of the EPA

The uncertified Lion pellet stove has
on the market for up to a year.

requirement that all residential wood and pellet stoves be certified by an EPA approved lab.  One example is Lion Energy, who has been importing a small pellet stove from China since at least March of 2019.  The Lion Pellet Heater is listed at $699 on the company’s website and is also sold by many online retailers for lower prices.  Amazon sold it for several months but their page on the product now says “currently unavailable.”  The stove has five reviews on Amazon including one that gave the stove one out of five stars and said, “This stove doesn’t appear to be state certified and our homeowners insurance will cancel us if we use this stove.”  

This uncertified Wanme stove is made
 in China and available to import to
the US.

The EPA office of enforcement has known about the Lion stove since at least August of 2019. The Lion Pellet Heater is the cheapest pellet stove on the US market, undercutting manufacturers who follow EPA regulations and still available from many online retailers, including the importer, Lion Energy, based in Utah.  A representative of the company told AGH that they had not heard about this issue before.  Emails to the company’s owners have not been answered.

Another pellet stove that appears to be uncertified under the name Wanme was being sold on internet sites for many months.  The online retail giant Wayfair recently removed the stove after being contacted by the Alliance for Green Heat.  However, it may still be on the market via smaller internet retailers.

Uncertified products were often
adverstised as using wood and coal after
the 2015 regulations took effect, requiring
certification for stoves using wood.

In December 2019, on a conference call with staff from the Office of Enforcement and Compliance Assurance (OECA), the Alliance for Green Heat urged the EPA to do more to make public their enforcement activity and assure stakeholders that it was following up on tips and infractions.  EPA staff relayed instances of numerous site visits and interactions with manufacturers and retailers in 2018 and 2019 that led to changes in advertising and other activities.  EPA staff explained that the EPA actively sends emails and make phone calls to manufacturers, retailers and test labs when deficiencies are identified through audits or public complaints/tips.  They also send non-compliance letters.

Tips from industry and the public resulted in scrutiny of and changes by several companies who were

Some fireplaces were advertised as "EPA
compliant" and having greater heating
capacities than many certified
wood stoves.

advertising fireplaces as heater-rated wood stoves in 2019 or uncertified stoves that could use wood or coal.  Under EPA regulations, any stove that advertises wood as a fuel must be certified.  Companies were also asked not to display the full barrel stove, when only certain parts of it were being sold.

The EPA confirmed that it had sent out what is known as “Section 114 letters,” to five or six wood heater manufacturers in 2016.   Section 114 of the Clean Air Act allow EPA to require regulated entities to submit information on a one-time basis.  It is not clear if or when the content of those letters and the responses would be public or could be through a Freedom of Information Action (FOIA) request.  However, letter requests to collect information that can be used in a potential rulemaking, investigation or enforcement action. Many of the submissions often end up being used by third parties in litigation. Responding to these requests can require the recipient to conduct testing and generate new data.

The Alliance for Green Heat often posts examples of misleading adverstisements in our free monthly e-newsletter.  If you are concerned about an enforcement issue with residential wood heaters, you can contact Rafael Sanchez at the EPA's Office of Enforcement at Sanchez.Rafael@epa.gov.

More than 40,000 new, uncertified stoves sold each year says EPA

The EPA recently released a figure of 40,000 uncertified, exempt stoves being sold each year.  An estimate on the volume of sales of these stoves had long been sought by the EPA and others, but manufacturers would not supply them.  Exempt wood stoves are not required by the EPA to meet any emissions standards and they are likely to emit five to ten times the smoke and particulates of an EPA certified stove.  These exempt, uncertified stoves usually sell for $200 to $500 a piece.  In contrast, EPA certified stoves typically start around $650. More and more used EPA stoves are coming onto the second hand market in the $300 - $800 price range.

Lit'l Sweetie, a typical exempt,
uncertified wood stove that has been
a popular seller in the US

Since the year 2000, about 135,000 EPA certified wood stoves and inserts have been sold each year, making the 40,000 new, uncertified stove market a considerable part of overall stoves sales in the US.  To get a sense of the robust exempt wood stove market, consumers can search for “wood stove” at Amazon.com.  Usually the majority of stoves shown for sale on the site are exempt, including barrel stove kits.

Next year the EPA is proposing to ban the production and sale of exempt wood stoves.  There is little doubt that the final rule will put an end to the sale of stoves that do not need to meet emission standards.  At the EPA’s pubic hearing in Boston on the proposed regulations, Steve Vogelzang, founder of the company that may have sold the most exempt wood stoves in the US, called on the EPA to reconsider their proposal, arguing that there is still a need for stoves like these to go into cabins and workshops.  He added that, if the EPA moves forward on the ban, “we are going to remove the most affordable stoves from the market.”  To read the full analysis of the impact of the EPA’s proposed regulations, see pp. 110 – 144 of the proposed rule.

One problem with exempt stoves is trying to keep them out of jurisdictions that they are not allowed to be sold in.  Washington and California have long banned new installations of them, and many retailers list those states as places they cannot be shipped to.  They are also not allowed to be sold or installed in Oregon and the Denver and Salt Lake City areas.  Vogelzang, probably the largest manufacturer of exempt stoves, does not provide any information to consumers on its website about where they can be shipped or installed.  US Stove, which now owns Vogelzang and is also a major manufacturer of exempt stoves, tells its consumers that they can’t ship to Washington and California, but does not mention Oregon or parts of Utah and Colorado.  According to Paul Williams, National Sales Manager for US Stove, while they may not have the information on their websites, when they get they calls and emails for exempt stoves, “our answer ... is they are not shipped to CA, OR, WA, CO and UT.”

Northern Tool + Equipment, a major in-store and online retailer that sells exempt wood stoves, advertises that the Vogelzang Lit’l Sweetie “meets or exceeds EPA requirements for exempt wood stoves.”  In reality, the Lit’l Sweetie does not meet or exceed any EPA emissions requirement, but is exempt from them.  The Alliance for Green Heat spotlighted this misleading advertising in September 2012 but misleading language is still common in advertisements for exempt stoves.  Northern Tool’s website is programmed to not take orders for exempt stoves from addresses in states where exempt stoves cannot be shipped.

Amazon.com, a venue many consumers turn to for everyday needs, may be the best major retail outlet for informing consumers that exempt stoves can’t be shipped to WA, OR, CA, UT or CO.  The site has a variety of sellers with product descriptions and also allows for consumers to directly ask and answer questions about the product.  Similarly, Home-Depot is good on both providing customer information and regulating the purchasing of exempt stoves.  They clearly label the stove as EPA exempt, say that they cannot ship to WA, OR, CA, UT or CO and the website will not let consumers purchase from those states.