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.This might have already been answered in this post, but practically speaking can anyone tell me where to find information on:
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.
- Realistic draft reductions due to horizontal runs or elbows?
- Realistic draft reductions due to size of chimney pipe (friction loss of smaller pipe versus larger pipe)?
- Realistic draft reductions due to chimney caps?
- Realistic draft reductions possible with a pipe damper?
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
4 inch------- .08 - .12 per foot
6 inch ------- .05 - .08 per foot
8 inch ------- .04 - .06 per foot
Rain / Wind Caps------------0.5 - 3.0
Open--------------- ---------- negligible
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.