Stove pipe damper

I have a question about stove pipe dampers. I have a old mill wood stove with a 6in back vent. I am getting my chimney lined and rebuilt next weekend. My chimney is 35ft high plus the tee, pipe and 90 pipe add less than 3 ft and its single wall pipe unless we run into clearance issues then double wall it is. I know my existing is way to close to the floor joists above so the chimney sweep is filling that one in and relocating the hole to the side and down lower for the clearance to combustables. Should I have a damper? I have heard 2 things 1 is no you will over fire the stove and the second is yes you should have a damper to better control the heat and help with a cleaner burn. My chimney sweep is certified and recommend that at the very least I need the stove pipe thermometer and to place it as close to the 90 as possible. That would be well above the damper. Is this a good idea? To me it makes sense since the 90 will restrict flow to a degree. In my line of work 1 90 is like adding a 21ft length of pipe as far as flow is concerned so 2 90s is in theory is 42ft. I have not burned with this stove. I have used 1 about 20 years ago so I am just getting back into it. I have always loved wood heat and I have missed it.

Here are some pictures of the weld markings on the bottom sorry guys the weld marks are upside down and the chimney.

 

I have one in my setup but dont really use it. I look at it as an emergency brake type thing. I would rather have it and not need it than need it and not have it.

Hopefully you will get some more experienced members to chime in here.

 

^ what he said.

 

Previously posted.

Using a Stove Pipe Damper

 

Thank You. I have got it on the list. I will post pictures of the work after it's finished. Thank You Again

 

Thank you for the link. I remember seeing something like this on a sight and I couldn't find it. That was very helpful. I have the damper on my list. Thank You again

 

Thank You.

 

Thank You.

 

This is the exact sentiment I was going to reply with.

 

Thank you for taking the time to answer. Thanks to all of the answers that I have gotten I will have 1 installed. Thank you again

 

Keep in mind the objective is to maintain a minimum of 250* f flue gas temp to the top of chimney flue when smoke is present.

A 35 foot high 6 inch liner will certainly require a damper at colder outdoor temperatures. 30 foot height at 28 square inch area flue capacity (6 inch) is 130,000 BTU per hour capacity. You have less than that, so the chimney draft will need to be slowed down to closer match the appliance. (under 100,000)

Only monitoring creosote build up (near the top) will tell you if you are closing it too far.

Each appliance has a required pressure measured at the stove connector where the lowest pressure in the system is.
The damper is a variable resistance to slow velocity of rising gasses which decreases draft.
Since you don't know the draft requirement, measured in inches of water column, it is a guess where to set the damper at any given time.
The larger flue without liner requires much more heat to cause the same draft you will have with an efficient liner. The larger flue (8 X 8?)would be rated for a 275,000 BTU per hour appliance. Your smaller stove doesn't loose enough heat to maintain proper chimney flue gas temps with that large of a diameter, so no damper would be used with your stove in the old larger un-insulated flue.

In this industry, resistance to flow is measured by resistance coefficients of chimney components. Draft is the cause, (chimney creates) flow is the net effect which is the most important. This is the number of cubic feet of flue gas that gets through the chimney in a given time. Damper controlling velocity.

There are tables used that give temperature differential between inside and outside of flue across the table bottom, and System Resistance Coefficient up the side. This gives a curve where any Fuel Gas Flow can be calculated.

Resistance Coefficients are added together to use the total in the system for table calculation.
As an example, here are coefficient numbers to give you an idea of resistance to flow;
6 inch pipe .05 -.08 per foot
8 inch pipe .04 -.06 per foot
90* elbow .5 - 1.5 (Notice this equals about 10 feet of 6 inch pipe)
Tee or breech into chimney causing 90* turn upward 1.0 - 3.0
Chimney top;
Open -0-
Spark screen 0.5 (clean)
Rain and wind caps 0.5 - 3.0 depending on style
Damper Open negligible
Closed 5 - 20
(Horizontal runs pitched upward min. 1/4 inch per foot also affect flow)

Remember, with flue damper open, the greatest resistance in the system is the air intake. This is the primary control. Slowing velocity with damper decreases draft which affects the stove by slowing incoming air. It is a chimney control, not stove control, so anyone telling you a certain appliance requires a damper is incorrect. (Unless it is a double door stove with screen for open door burning which requires a flue damper to control flow when doors are open in Fireplace Mode). Your flue damper only controls an over drafting chimney, which affects the stove. Coal stoves need more critical draft control, so barometric (automatic adjusting) dampers are used.

Your stove would be about right with a 15 foot height, 6 inch insulated flue and minimal resistance requiring partial flue damper use only during extreme cold outdoor temps when draft is strong. That would be closer to 100,000 BTU chimney capacity matching the stove, then dampened down to about 80,000 guessing your radiant square inch surface area of your stove. I'm going by brick measurement inside to approximate a 1500 square foot heating stove, the way heating area was calculated when built, not using BTU comparison used today for newer stoves.

 

Thank You for that very detailed answer. I am planning on using it when it is really cold out and windy. My chimney has a very strong draft. The air intake is still going to be my maine control for the stove. I will have a stove pipe thermometer after my 90 going between the 90 and thimble as this was recommended by the chimney sweep and by others.

Thank you again.