Yes, your stove is over drafting... Blame the EPA, NFPA, UL and stove Manufacturer jackwagons!

coaly

Fisher Moderator
Staff member
Dec 22, 2007
4,141
NE PA
This might have already been answered in this post, but practically speaking can anyone tell me where to find information on:
  1. Realistic draft reductions due to horizontal runs or elbows?
  2. Realistic draft reductions due to size of chimney pipe (friction loss of smaller pipe versus larger pipe)?
  3. Realistic draft reductions due to chimney caps?
  4. Realistic draft reductions possible with a pipe damper?
I know that I am going to have a problem with a 27' chimney on a new install, but it would be nice to be able to calculate the extent of the problem I'll have relevant to the stove I will install.
The Wood Burners Encyclopedia by Jay Shelton is an excellent book that gives tons of technical info in an easy to read and understand manner. They go cheap on eBay and by far the best book I use. This book was listed as further reading in every Fisher manual that came with a stove! It's like manufacturers don't want customers to know these things making themselves dangerous today.

Your question is asking for resistance coefficients of chimney components.

There are pages that explain the variables.
There are 4 parameters that change total flow. (Temp difference between flue gasses and outdoor air, chimney height, chimney diameter, and whole system resistance coefficient)

After adding up the resistance values compiled from the first very important table (I give examples below) you then use the total system resistance coefficient in a table that calculates flow at given temperatures of the chimney size. The curve shows at what temps the flow will be.

This is how resistance values are calculated;
An open fireplace or open door stove uses inlet resistance as 2.
At the other extreme, if an air damper is air tight, its resistance would be infinite.
Everything else is given a resistance value in between.

Starting with the stove itself, different size stoves with air wide open has an air inlet damper effective resistance. Meaning door closed, air wide open. The air intake opening is the most resistance in the system.
As an example, a small stove with 6 inch outlet can be 10-40. Larger stoves with larger air opening can be 5-20.
This is the resistance coefficient of the stove itself with air damper wide open.

The items you questioned;
Each component is given an estimated range since there are large variations of resistance of various kinds of caps, pipes and elbows due to different surface smoothness, gradualness of curves, shapes of caps etc.

Round elbow, 45* --------- 0.2-0.7
Round elbow, 90*---------- 0.5-1.5
Tee or Breeching ---------- 1.0 - 3.0
Straight pipe
4 inch------- .08 - .12 per foot
6 inch ------- .05 - .08 per foot
8 inch ------- .04 - .06 per foot
Chimney top
Open --------------------------0.0
Spark Screen-----------------0.5
Rain / Wind Caps------------0.5 - 3.0
Pipe Damper
Open--------------- ---------- negligible
Closed ------------------------5-20

Notice this shows the inlet damper wide open with the resistance through stove can be about the same as a closed flue pipe damper. Modern dampers have no regulated opening when closed, they are very tight compared to antique dampers made for a calculated opening when closed.

These numbers can be used to estimate the overall capacity of a venting system, but are perhaps more useful for estimating the effect of contemplated changes to an existing system.

What a horizontal pipe does (pitched upward at least 1/4 inch per foot) is adds to the pipe resistance without adding to the total height of system. So using the tables with resistance vertical and chimney temp differential horizontal, the curve changes due to less height at the higher resistance. A century ago it was common to put the stove as far away from chimney as possible. So calculating 50 feet of pipe resistance without gaining height, you see how much draft was needed to overcome the added resistance.

There may be computer programs now to calculate flows better, but I'm old school still using the paint method to figure out square footage of radiation surface area.
 

MR. GLO

New Member
Jan 26, 2021
91
Massachusetts
I've been saying for years that half the problems people come here with are due to wet wood...the other half are draft issues....and I am just shocked that you almost never see anybody suggest checking the draft (in the stove forums) or somebody says, "they checked it when they put it in...back in September"...say what?! They checked the draft with no fire, and when it was 60* out?! :rolleyes:
I would like to see a video of somone checking a draft on a stove during the entire burn process.. I cant find any. I know the damper will change your draft but I heard others using magnets over the secondary air hole, but I don think this would change the draft readings. Has anyone tested the magnets?
 

begreen

Mooderator
Staff member
Nov 18, 2005
86,564
South Puget Sound, WA
I would like to see a video of somone checking a draft on a stove during the entire burn process.. I cant find any. I know the damper will change your draft but I heard others using magnets over the secondary air hole, but I don think this would change the draft readings. Has anyone tested the magnets?
Putting magnets over the secondary intake should be a last effort, after others fail. Doing so will upset the designed combustion balance in the firebox. Cutting off the boost air first and if necessary reducing the primary air a bit more is better for keeping the clean burning characteristics of the stove.
 
  • Like
Reactions: MR. GLO

john26

Feeling the Heat
Oct 27, 2008
463
Wildwood MO
On my Lopi I reduce the secondary air inlet in the low setting from 3/8" to 3/16" and it drastically changed the way the insert performed. I tried 2 fires with it it smoldered with little to no secondary flame, window turned very black quickly but burned time was extended some but not enough to justify a smoke dragon.
 
  • Like
Reactions: MR. GLO

DBoon

Minister of Fire
Jan 14, 2009
1,237
Central NY
Thanks for the info, Coaly. I've found some other on-line resources as well using some of the key words you mentioned and I'll be diving into this a little more over the weekend.
 

ABMax24

Minister of Fire
Drilling holes and creating orifices' to reduce airflow isn't as intuitive as it seems. take a 1/2" and 1" hole for example, the 1" hole has 4 times the area, and roughly 4 times the flow of the 1/2". I think this is where a lot of people go wrong in doing this.

I've played around with restricting air to the secondary tubes a lot, quite frankly more than any sane person should. I have got reasonable results, and managed to get my stove to operate semi-normally on a 36ft stack. That being said it was still more sensitive to outside air temps than any other stove I have run.

I'm now a big believer in flue dampers, it reduces the vacuum on the stove to that a normal height chimney. They can also be adjusted to suit the current draft situation, and allow the primary and secondary airflows to be balanced with each other.

My stove is now setup as it came from the factory, the ICC Ultrablack damper in my flue works well, although it did require a little more restriction to deal with my excessive stack height.