RMH and PM2.5

Hello Permies,

I have not been here for a while so i hope you are all well.
There seems to be a concerted effort recently to talk about wood stoves for heating and the general jist is that a wood stove puts out a lot of pm2.5 particles. (attached diagram i saw today)
A lot of the stuff on line seems to just be attack pieces as they conflate open wood fires with modern home stoves (there is zero talk of german Kachelöfen or RMHs).

I had a search on here about RMHs and PM2.5 particles but i couldn't find what i want looking for.
Is this something that has ever been done as a part of the testing?

thanks in advance.

Mark

Hi Mark,

As someone who uses wood as one component of home heating, I feel rather threatened each time I read one of those "attack pieces" online. There are many reasons that I burn wood (in-cycle carbon; collected and processed by myself from our woodland; not dependent upon fossil fuels or other unknowns, beyond my sphere of influence) and I would hate for the emissions debate, which seems to be flaring up, to deprive me of that option.

I confess that I haven't got an air quality meter and so much of what I am about to type is mere conjecture, however, I do feel that using a decent quality stove (or RMH!) that is well-maintained and burning dry fuel has a pretty insignificant effect on indoor air quality.

I've lived in properties with open fires and the soot that gets everywhere is pretty good evidence that the air is full of carbon particulates. The only problem I see with my stove is the ash, which can escape during cleaning, and the soot generated when sweeping the chimney. I use a vacuum cleaner when both cleaning and sweeping to try and minimise these concerns.

I have also heard that opening the door to a sealed wood stove will suck some of the flue gasses into the living area. I've definitely seen a curl of smoke escape if I've opened the door too quickly. By gently cracking the door open and opening it slowly, after a moment of hesitation, I've found this to be avoidable.

With RMH, there often isn't a door to open and I don't feel this would be a concern. If the fuel is fed into an open fuel box then the draw into the flue pipe should be continuous - taking the particulates with it. The same should be true for open fires but I think the large, open area detracts from its ability to draw cleanly - most open fires I have known will puff smoke back into the living area when the wind blows hard outside, and often sporadically until the flue has warmed sufficiently when the fire is first lit.

This leads me to mention, again, proper maintenance and burning practices. If the flue/chimney is well swept (avoiding restriction of air flow), the fire is kept hot enough to full combust the fuel, the air vents open enough to allow the fuel to be fully burnt AND the fuel is of good quality to begin with, I feel that the in-house emissions would be much lower than the reported figures.

Maybe I should look for an air quality indicator and run some experiments...

I hope someone with more experience than me can chime in on RMHs.

I think the PM2.5 is about total emissions, not just indoor air quality. It is relevant at least to neighborhood pollution if not global effects.

Also, the "zero" particulates for electric is disingenuous, as it ignores the emissions from any fossil-fueled generating stations.

This thread was interesting - Huge gaps in US rules for wood-stove smoke exposed

I believe that the Testo meters measure PM 2.5 and that there are a number of graphs from different RMH burns that show ultra-low PM 2.5. This article from Aprovecho says:

Experiments have shown that elevated temperatures shorten the combustion time for CO and PM 2.5. At 900°C the combustion time required for complete combustion is less than half the time needed at 700°C for biomass particles (Li, 2016). At 900°C, a residence period of 0.5 seconds resulted in close to complete combustion of well mixed CO and PM 2.5 (Grieco and Baldi, 2011; Lu et al., 2008; Yang et al., 2008).

EDIT: I might be wrong about the Testo Meters. Attached is one of Peter van den Burg's testo charts>

Jeremy VanGelder wrote:I believe that the Testo meters measure PM 2.5 and that there are a number of graphs from different RMH burns that show ultra-low PM 2.5.
EDIT: I might be wrong about the Testo Meters. Attached is one of Peter van den Burg's testo charts.

You are right, my Testo doesn't measure PM 2.5 nor PM 10.
But there are some things that can be said about all particulate matter: coming from a "normal" woodstove most of it is organic, so it's soot. In other words, better combustion will produce less PM. There's a second effect: in a bell construction, after combustion is done, the forward velocity of the gases are slowed down a great deal. Just because of this, the fine dust will settle at the bottom of the bell quite easily. Most of the fine dust in a well-behaving mass heater do consist of the minerals that the tree took up from the soil. This is anorganic and mostly harmless.

In my own batch box rocket heater, the bell below the core is 23 times wider than the chimney cross section area. As the result, a layer of very fine dust has been accumulating at the bottom over the years. Last year the heater seemed to be not so enthusiastic anymore, the lower ridge of the exhaust opening was reached. I managed to scoop and vacuum most of it out and after that was done it behaved as its old self again.

So, in order to substantiate the above statements, here are two test reports from two very different batch box rockets, both housed inside a bell.





The first one being a DSR2, the second a straight batchrocket with altered air inlet.

As far as I know, the EU norm is at the moment on 40 mg/m³ at 13% O².

That's more great information, Peter. Does the Testo use filters for the sample intake? If you had a really sensitive balance, you could weigh them before and after a session for total particulate content in a given sample volume.

The Testo use white filters for the sample intake, that's correct. But those won't catch all PM in the volatiles, further down I have other filters and those are catching more of the, admitted sparingly,  dust and soot.
Moreover, I use these filters at least four times in development stage. Otherwise, it would be a costly affair, the number of test runs are easily into the hundreds for each core development.

I wish I could give a really good answer to this one. I have some inklings because my husband was involved in automotive diesel engine development up till about 15 years ago. As Luke says as well, this presses one of my buttons as a wood fuel user, so it is difficult for me to be objective about it. If they are talking about pm2.5 inside the building I'm a bit surprised any is due to gas or oil fired boilers (or at least can be directly attributed to the one in any particular home) and can only assume this is fumes drawn back in from outside.
There is no perfect answer (other than reducing energy waste?...) and a balance between the different pollutants (and other factors) is always necessary - you have to be careful what you ask for sometimes.

Thanks everyone for the replies. As i have thought about it more, it seems to actually be something quite complicated.
PM 2.5 is just a measure of particle size, however i'm pretty sure the chemical composition of what is burning will also matter.
The other things i have seen online also ignore how much you need to burn. So they quote simple log burners that are not that efficient. RHM are not only cleaner, but you need  to burn less.

Burning Biomass is a significant part of energy production (at least where i live). I am not sure how clean those power plants are but it would be interesting to know.
Obviously saying electricity is 100% clean is questionable.

The analysis posted seems to make a lot more sense since it measures efficiency and also combustion by products like CO.

Mainly the stuff I have been reading about is more about the wider environmental effect rather than the in home air quality.
I'm now just wondering why people are quoting PM2.5 number so much when talking about home heating. It doesn't seem to be something that really looked at by anyone who's looked at this in detail.