Manometer Readings

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Woodcutter Tom

Burning Hunk
Apr 28, 2019
169
Northern Illinois
I had this experience yesterday.

Chimney cap 14 ft above stove top.
Outside temp: 51 F
Wind: S 15 Mph
Damper: Fully closed (full closed still allows 20% of air flow per manufacturer)
Primary Air: completely shut

Manometer reading: .10
SBI states .03 - .05 is the sweet spot. .08 is excessive draft.

One more thing: I did not have a fire going. The stove was cold. Loaded with wood but cold.
 

gthomas785

Minister of Fire
Feb 8, 2020
530
Central MA
I had this experience yesterday.

Chimney cap 14 ft above stove top.
Outside temp: 51 F
Wind: S 15 Mph
Damper: Fully closed (full closed still allows 20% of air flow per manufacturer)
Primary Air: completely shut

Manometer reading: .10
SBI states .03 - .05 is the sweet spot. .08 is excessive draft.

One more thing: I did not have a fire going. The stove was cold. Loaded with wood but cold.

And with the primary air fully open... does it go down?

Keep in mind what the draft measurement is telling you. It's just the amount of pressure difference between the chimney and the room. You can have a large pressure difference, with total blockage and no airflow, and your fire will still be controllable. Draft measurement on a cold stove with the primary air flap shut is kind of meaningless.
 

brenndatomu

Minister of Fire
Aug 21, 2013
6,574
NE Ohio

begreen

Mooderator
Staff member
Nov 18, 2005
90,628
South Puget Sound, WA
I had this experience yesterday.

Chimney cap 14 ft above stove top.
Outside temp: 51 F
Wind: S 15 Mph
Damper: Fully closed (full closed still allows 20% of air flow per manufacturer)
Primary Air: completely shut

Manometer reading: .10
SBI states .03 - .05 is the sweet spot. .08 is excessive draft.

One more thing: I did not have a fire going. The stove was cold. Loaded with wood but cold.
Does the house have an HRV or intake fan or something that is slightly pressurizing the floor that the stove is on?
 
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coaly

Fisher Moderator
Staff member
Dec 22, 2007
4,351
NE PA
What you're reading with no flow is static (or theoretical) draft developed by a chimney as a function of its height and the temperature difference between the flue gasses and the surrounding outdoor air.

This is the draft that would exist if there were no flow as would happen if the smoke pipe (or chimney) were suddenly sealed off at the connection of the stove. This static draft then depends only on the average flue gas temperature and the chimney height. This can be found in graphs that show chimney height vs. average flue gas temp to find out what the chimney will produce at 40*f outside temp and 30 inches of mercury.

Allowing any opening into stove or leaks into pipe joints allows air movement up chimney, preheating the flue causing a slight draft reading. Mechanical fans, or house pressurization from many factors can increase the reading. That will not be a factor during a static test since the stove intakes need to be closed fully.

The third factor that determines NET draft is flow. (height, temperature differential, velocity = NET draft) Notice I mention net draft since the draft at chimney exit will be zero, and the highest normally at flue collar. Velocity is determined by diameter, connection pipe configuration, and any other resistance such as spark screen, cap and internal firebox resistance. The air intake is the most resistance in the system until variable resistance is added by flue damper. The flue damper controls velocity of rising gases which decreases net draft. The setting will fluctuate with weather conditions (pressure and temperature) and flue gas temperature.

To give you an idea,
Looking at a Static draft chart, a 20 foot high chimney and 20* f temp difference, draft is .01
At 80* f temp difference draft is .04.
300* temp difference draft is .12
 

Woodcutter Tom

Burning Hunk
Apr 28, 2019
169
Northern Illinois
What you're reading with no flow is static (or theoretical) draft developed by a chimney as a function of its height and the temperature difference between the flue gasses and the surrounding outdoor air.

This is the draft that would exist if there were no flow as would happen if the smoke pipe (or chimney) were suddenly sealed off at the connection of the stove. This static draft then depends only on the average flue gas temperature and the chimney height. This can be found in graphs that show chimney height vs. average flue gas temp to find out what the chimney will produce at 40*f outside temp and 30 inches of mercury.

Allowing any opening into stove or leaks into pipe joints allows air movement up chimney, preheating the flue causing a slight draft reading. Mechanical fans, or house pressurization from many factors can increase the reading. That will not be a factor during a static test since the stove intakes need to be closed fully.

The third factor that determines NET draft is flow. (height, temperature differential, velocity = NET draft) Notice I mention net draft since the draft at chimney exit will be zero, and the highest normally at flue collar. Velocity is determined by diameter, connection pipe configuration, and any other resistance such as spark screen, cap and internal firebox resistance. The air intake is the most resistance in the system until variable resistance is added by flue damper. The flue damper controls velocity of rising gases which decreases net draft. The setting will fluctuate with weather conditions (pressure and temperature) and flue gas temperature.

To give you an idea,
Looking at a Static draft chart, a 20 foot high chimney and 20* f temp difference, draft is .01
At 80* f temp difference draft is .04.
300* temp difference draft is .12
Any thoughts on how the wind factors into this? I see increases when the wind blows. When the wind dropped to 4 mph, the manometer reading dropped way down. Even to .00 after a while.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
26,170
central pa
Any thoughts on how the wind factors into this? I see increases when the wind blows. When the wind dropped to 4 mph, the manometer reading dropped way down. Even to .00 after a while.
Get a cap with a wind band. And also use you manometer to check the pressure in the stove room vs outside. It is possible the wind is depressurizing the house leading to your high draft measurements
 
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coaly

Fisher Moderator
Staff member
Dec 22, 2007
4,351
NE PA
Depends on windward side or leeward side of roof or other ground features such as hills or mountains.

On the windward side of roof or hill, wind builds up pressure on the side of roof it hits, pushing down into chimney decreasing draft. On the opposite side, wind going over the roof causes a low pressure area, decreasing atmospheric pressure, increasing draft. So when the wind hits a mountain, it builds pressure on the windward side, slips up and over the top creating a lower pressure on the leeward side. When wind slips up and over roof peak the velocity of the moving air decreases pressure. The faster it goes, the lower the pressure. So the higher atmospheric air pressure in the home where stove intake is can push into the stove easier with more pressure differential.

Swirling currents from trees or buildings nearby create turbulence in the vicinity of the chimney outlet adding to the equation.
 
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coaly

Fisher Moderator
Staff member
Dec 22, 2007
4,351
NE PA
Get a cap with a wind band. And also use you manometer to check the pressure in the stove room vs outside. It is possible the wind is depressurizing the house leading to your high draft measurements
I think you mean pressurizing the house causing high draft measurements. Sounds like positive pressure from something. Sun beating on masonry chimney or another flue in it in use preheating it?
 
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bholler

Chimney sweep
Staff member
Jan 14, 2014
26,170
central pa
I think you mean pressurizing the house causing high draft measurements. Sounds like positive pressure from something. Sun beating on masonry chimney or another flue in it in use preheating it?
Well that depends how you define high draft. By high draft I mean actually a lower reading meaning more vacuum. Pressurizing the house would not increase vacuum reading it would decrease it. Negative pressure will make the pressure differential greater meaning a higher vacum reading.

A manometer is basically just a tube open on both ends with liquid in the middle and guage markings on that tube. One end goes in the chimney which is negative pressure. The other end is open to the house. If you blew in that open simulating positive pressure in the house it would push the liquid closer to zero. Sucking would pull it away from zero
 

Hoytman

Feeling the Heat
Jan 6, 2020
360
Ohio
My high side is open to the room. Blow on it and it registers negative pressure just like sucking on the low side hooked to the stove.
 

EbS-P

Minister of Fire
Jan 19, 2019
1,489
SE North Carolina
Really we need a flow meter on the intake of the stove. Measure the total cfm that is coming into the stove. Seems much more reliable/ accurate than WC.
 
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coaly

Fisher Moderator
Staff member
Dec 22, 2007
4,351
NE PA
Well that depends how you define high draft. By high draft I mean actually a lower reading meaning more vacuum. Pressurizing the house would not increase vacuum reading it would decrease it. Negative pressure will make the pressure differential greater meaning a higher vacum reading.

A manometer is basically just a tube open on both ends with liquid in the middle and guage markings on that tube. One end goes in the chimney which is negative pressure. The other end is open to the house. If you blew in that open simulating positive pressure in the house it would push the liquid closer to zero. Sucking would pull it away from zero
Correct, high draft is lower pressure.

A manometer is also called a U tube gauge, or slack tube gauge. (I'll dig mine out so others can see the simplicity of the instrument - I used a board with 1 inch graduation marks up and down each side. It had a hook on the top to hang it, and partially filled the tube with water to the bottom or zero line. I used that to check gas pressure for many years until buying a mechanical gauge. Later I bought a Dwyer slack tube gauge that is essentially the same thing as my home made U tube to calibrate the mechanical gauge.)

The tube is a glass, or clear vinyl tube you can see through. Bent in a U shape at bottom, and inch graduations up each side. Water placed inside the tube will be level on both sides since the same atmospheric pressure is pushing into each end. This water level line is 0 on the gauge. (or atmospheric pressure when measured as absolute) By making inch graduations from the water level up both sides, when a pressure or suction is applied to either end, the water moves down the pressure side, and up the suction side. Both measurements are added together for the total water displacement. So a drop on one side of 1/2 inch and rise on the other of 1/2 inch = 1 inch of water column. It will always be the same on each side of vertical tubes.

For measuring low pressure below 1 psi this is more accurate than a pressure gauge. As an example, working LP gas pressure in a supply line is about 11" W.C. or about 1/2 psi. That is less than your breath, so below the minute pressures found in a chimney you won't see the level change on a slack tube or U tube gauge. Geometry is used in the U shape to accentuate the curve so you can see it instead of measuring directly vertical.

One end of the tube is left open to "sense" the pressure of the atmosphere outside the chimney or stove. The other end is inserted in the chimney, pipe or stove. (perpendicular to the flow) If there is some draft, pressures will be unequal on the two sides of the U tube. In America, the inches of water refers directly to this height difference. Since the difference in water level is too small to be seen on a U tube level gauge, different geometries are used to amplify this visual affect. (the strange looking curve of the water column)

Pressurizing the open end in the building will push the water down on the open, or pressure side, and raise the level on the chimney, or negative side, resulting in a "higher" draft reading, or lower pressure in the chimney compared to the building.
 

Woodcutter Tom

Burning Hunk
Apr 28, 2019
169
Northern Illinois
Well that depends how you define high draft. By high draft I mean actually a lower reading meaning more vacuum. Pressurizing the house would not increase vacuum reading it would decrease it. Negative pressure will make the pressure differential greater meaning a higher vacum reading.

A manometer is basically just a tube open on both ends with liquid in the middle and guage markings on that tube. One end goes in the chimney which is negative pressure. The other end is open to the house. If you blew in that open simulating positive pressure in the house it would push the liquid closer to zero. Sucking would pull it away from zero
I have my manometer set up to measure draft using the instructions from Dwyer for the Mark II. Like Hoytman, the tube to the stove connects to the Low port on the manometer (right side). The High port (left side) is left open to the room. Blowing air into the left side port (simulating pressure in the room) would move the gauge to a higher reading. Thus indicating more draft.

If the tube to the stove was placed on the High port (left side), and the low port were left open to the room, the gauge would display negative numbers. The gauge scale only goes to negative .05. Therefore using the gauge in that manner would not provide accurate information. Once past -.05, the draft would be unknown.

See my picture in the opening post.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
26,170
central pa
Yeah I had it backwards guys.
 
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