Chimney analytics

[Poindexter]

Looking for informed discussion with usable data included.

All of the material in the "all about chimneys" section at woodheat dot org, including the various ancillary articles on the left margin of the page, is incorporated herein by reference:

http://woodheat.org/all-about-chimneys.html

and is explicitly expected to be material either within the grasp of posters in this thread, or the genesis of specific questions for discussion.

Thank you.

This table, which appears in many articles related to chimney performance, is available in the "How chimneys work" article at woodheat dot org and also makes a noteworthy appearance in an article titled "The Florida Bungalow Syndrome."



Having spent some effort in research here, I prefer to work in Pascals as the table above. It is a unit appropriate to the task. If you wish to convert to Inches Water Column, 1 Pascal is 0.004 iwc.

 

[Poindexter]

Two data points.

1. One morning last weekend I had a cold stove, upstairs temp was +72dF. Downstairs temp was +65dF. Outdoor temp was +20dF. There should have been a small positive pressure on the air intake to my stove from the stack effect of inside my house, and a larger negative pressure on the stove collar from the end of the chimney being out in the cold air.

I don't "know" where my neutral pressure plane was, but I could feel a distinct thermocline walking downstairs in my bare feet.

Looking at the table above, I should have had about +1Pascals pushing on the intake and about -5 Pascals sucking on the stove collar. Assuming my pipe has been cleaned recently and the air in the chimney is moving fast enough to still be +72 when it reaches the top of the stack....

Anyway, I should have had about 6 Pascals working for me when I lit the cold stove yes? 6 Pascals x 0.004 is 0.024 iwc.


2. From that cold start I ran on high for 30 minutes, turned the stove down to medium for an hour or two, and then back up to full throttle for ten minutes before taking another set of observations. It was +80dF in my stove room and +60dF downstairs. My thermocline on the stairs is almost but not quite ready to spill on the flooring of the upper level, which is about right for me, maybe a little too close for comfort. But still, 20 degree spread over ten vertical feet, the intake on my stove should be seeing about +2 pascals compared to atmospheric pressure.

My flue gas probe (19" above the stove collar weld line) is showing +500dF. Tstat is on high, combustor engaged, deck fans off. Combustor probe indicator is mid to high active. Out door temp is +21dF. My steam plume is very thin and took a while to locate with confidence, but it is detached. Winds are calm.

I am comfortable calling my exit temp at +212dF or higher since the steam plume is detached, and giving away 19" of stack and calling it +500dF at the firebox exit. That should give me (assuming linear relationships) about 23.75 pascals of suction at the stove collar given 15 feet of stack at +350dF average.

23.75 + 2 = 25.75 pascals working for me, *.004 conversion factor, I should be at about 0.103 inches water column, well above the published max in the manual for my stove.

A few observations.

For item one, the cold start, 6 Pascals is the theoretical max possible draw. That assumes my chimney pipe is pristine. Mine isn't. I did brush it out less than half a cord ago, but I am not licking the interior. So I got some friction in there not accounted for in the table in post one. Second, I ass/u/me'd the air in my stack with the loading door open was +72dF top to bottom, but it probably wasn't. I got 4-5 feet of pipe sticking out of the snow accumulated on my roof top, and it was +21dF out there so, nope, I probably really had a bit, I dunno how big a bit, but a bit less than 6 Pascals.

Likewise, i think my 25.75 Pascals estimate for the hot read was likely on the low side. I don't know how far above +212dF my exhaust plume was when it left the stack, and I calculated the average temp in the flue using +200dF as the exit temp, so I gave away some calculated draw there, and I gave away 19" of pipe measuring above the stove collar like I did.

I fell pretty confident saying at cold start i had less than 6 Pascals (0.024iwc) of draft available, and at my hot read I had at least 25.75 Pascals (.103 iwc) of draft.

So I emailed my stove manufacturer with my observations and said more or less the 0.04 to 0.05 iwc draft specification in the manual for my stove is not useful to me because it does not specify under which conditions (or where on the stove) the measurement should be taken. Clearly my stove operates in that range sometimes, blah blah. The response I got was the company was working on getting existing models through EPA II (cord wood emissions certification) and the spec might be a hold over from who knows when and my PM would be forwarded to "the geeks" but their priority was the 2020 cordwood deadline.

Makes sense to me, and I got a trouble free intstall anyway. But I have been stewing on this.

 

[Poindexter]

One question I got that is the door to the EPA rabbit hole. As near as I can tell (I am neither insomniac or maniac enough to read large amounts of government regs in one sitting) the crib method, EPA 28, specifies a chimney with 1" of solid insulation on the outside of the pipe.

@begreen or @bholler might know, chime in if you do....
1. Is that still current? Is the EPA still requiring stoves to use 1988 tech when certifying stoves in 2017?
2. How does this pipe I had never heard of compare to modern double wall?

It is abundantly clear to me from my limited data the most important factor in building draft is exhaust gas temperature at the flue exit. You gotta carry as much heat as you can as far up the pipe as you can, or draft suffers.

 

[Poindexter]

I am tagging @kf6hap so he can tell me where I went astray... we have traded a few PMs. He is using an actual manometer or three on his stove and stack and house.

I do not want to discourage anybody from similar pursuits, but I have a couple cautions before you go drilling holes in expensive things. I did a marine sedimentology minor as an undergrad. If you really want to describe the flow of fluids in rectangular shaped channels or rectangular shaped boxes you will need to know the entire Greek alphabet and you should be able to evaluate partial differential equations with less than 12 elements in under 30 seconds without writing anything down. This gets into serious math in a hurry, and serious profanity shortly thereafter.

My intent with this pursuit is to look at how much draft is being applied to the stove collar by the stack effect of the chimney and the stack effect of the building. What is going on inside the stove, from my perspective, is best left to folks with master's degrees in plain old just math with 5-10 years experience in stove design. Current technology is orders of magnitude beyond improving Ben Franklin's design.

The more sensitive your test equipment, the more stuff you are going to find in corners.

Here ends the public service announcement.

If I understood kf6hap correctly, he found no pressure drop across his catalytic combustor. This is a good thing for a couple reasons. One, it means his combustor probably doesn't need to be cleaned, and it means (to me) his combustor left the factory with enough airflow to support the firebox. Most likely the rate limiting step in his install is the air intake, which is good and useful.

We talked a little bit about the various things he has measured. Mind you, he is using a tube bent into a shape filled with colored water and a ruler on it. When I do a manometry on a human esophagus my probe has 32 nano strain gauges in it. Medical grade probe and supporting electronics is north of $50k, a Dwyer manometer calibrated in iwc is $43 on Amazon.

He is the expert on chimney manometry here and I don't want to take that away from him. If I have understood the limitations of these devices correctly (remember in that one Star Trek movie where Scottie tried talking into a computer mouse?) if I got this right, I _think_ the hot setup would be to drill a hole in an exterior wall and stick one mano probe outdoors to reference atmospheric pressure, and then stick the other probe in the flue some inches above the stove collar. That reading should then show both the stack effect of the building and the stack effect of the stack combined, as a pressure reading, and be useful.

The limitation here is the tubing for these things is really small (3/16" I think?) and tubing runs should be kept short to minimize laminar flow effects.

Another issue is where in the stack to put the dang probe.

On freestanders I would be in favor of using the already drilled hole for the gas temp probe. Two, two and a half feet up from the collar turbulence should be pretty well settled down, just get the probe into the middle of the pipe and see what you got.

That won't work for insert users. We are already talking about drilling a hole in the insulation envelope to reference atmospheric pressure... breaking a bunch of bricks out of the living room to get at the insulated pipe 24" above the insert is a non starter.

If "we" standardize on measuring 4-6" above the stove collar insert owners can play too, freestander owners will have another hole in their double wall telescope and everybody will need a longer data set to average because turbulence will likely still be evident in the data sets. So i dunno what to suggest on this one.

FWIW one atmopshere of pressure, 29.92 inches of mercury is 101,325 Pascals, 407.189 inches water column, 760 mm Hg.

 

[Poindexter]

And I have some speculation. I promise, really and truly, knowing I will someday stand in judgement before my personal Lord and Savior Jesus Christ, I do not have any inside information. What I am about to post is a SWAG based on having three related knowledge bases and some life experience.

If I was a stove manufacturer today, I would not be bringing out new models until the cordwood test, 28R I guess, is nailed down.

If I was a stove manufacturer today I would be on the phone with chimney manufacturers, weekly if not daily, screaming, I mean SCREAMING for the best insulated pipe I could get that consumers could afford. And I would be telling the EPA monthly if not weekly that their 1988 technology is a non-starter for the 2020 test.

A 75% efficient stove puts 25% of the BTUs in the firebox into the chimney, that makes draft. A 88% efficient stove puts 12% of the BTUs into the firebox into the chimney, good for the house, bad for the chimney. How you going to make draft with half your energy missing? Going from smoke dragon to EPA-I, you put an insulated liner in your masonry thing. Going from EPA-I to EPA-II , everbody got to keep every single iota of heat they can inside the chimney for as long as possible. Without heat, no draft.

 

[Poindexter]

Looking forward, I am likely to either put a thermocouple on my chimney exit or an outdoors referenced manometer at my stove collar.

Sometime in late April, when outdoor temps are in the +50dF range, I should be able to turn my stove down to low, take another set of readings, smell smoke in my stove room at however many Pascals draft and then we can compare draft numbers to figure out who has the more sensitive nose.

Still a fascinating subject though.

NB: The table in post one, I believe, ass/u/me's you have no elbows in your pipe.

 

[Blazing]

Interesting!

 

[Woody Stover]

Interesting!

Yep, good stuff! I hadn't considered observing weather the plume is detached but that should tell you when you are carrying sufficient heat to the top to avoid condensation in the stack. I'm just wondering if, at higher outdoor humidity, is the distance to where the steam condenses going to become so short that you won't be able to tell from the ground weather the plume is detached..