Measuring the chimney draft on a pellet stove

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Brokk

Member
Dec 9, 2008
126
Central/Eastern Mass
My understanding is that draft is basically measured in WC units as the difference in pressure between your room and the pressure in your exhaust pipe near where it leaves the room. This should be a negative number, as the draft is really a vacuum.

A pellet stove typically has an exhaust fan which is pushing the exhaust out of the stove and into your flue pipe. This is positive pressure.

I've read that the ideal draft is -.05 WC. However, I can't recall if that was for wood stoves or pellet stoves. Is it the same? Different?

Assuming I'm trying to reach -.05 WC, how should I calculate my draft? Is it just the raw pressure reading, or do I take into consideration the positive pressure from the fan?

Example: Exhaust fan creates .035 WC. After the pellet stove has been up and running for 30 min the exhaust pipe is measured at -.055WC.

Is my draft -.055 WC or -.09 WC or something else entirely?
 

johneh

Minister of Fire
Dec 19, 2009
3,755
Eastern Ontario
Does not matter you want the best draft possible in the stove to get the cleanest hottest burn you can from any given pellet. You measure the draft just before the combustion fan and without the fan running there should be just about 0 draft my stove has enough to clear any smoke if the power goes out
some people are not so lucky.
 
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Brokk

Member
Dec 9, 2008
126
Central/Eastern Mass
Does not matter you want the best draft possible in the stove to get the cleanest hottest burn you can from any given pellet. You measure the draft just before the combustion fan and without the fan running there should be just about 0 draft my stove has enough to clear any smoke if the power goes out
some people are not so lucky.
My effective heat is terrible. I have the air intake almost completely shut and the exhaust pipe gets a lot hotter than it should. I believe the heat/energy from the pellet stove is just being sucked up the chimney. My idle draft with a cold stove (hadn't been run in days) and it only being 53F outside with a 5mph wind, was -.01WC. I expect as temps get colder the draft will increase. Both idle and even more when the stove is running.

I am waiting for a damper that I ordered, but I am trying to figure out how to measure my draft taking the exhaust fan into consideration so that I can set it properly when the stove is running.
 

johneh

Minister of Fire
Dec 19, 2009
3,755
Eastern Ontario
Your manual will tell you what the Magnehelic gauge
should read for your most efficient burn
I can not see the draft of your chimney pulling all the heat out
in order to do that there has to be an air leak in the stove and
if there were an air leak your stove would be burning very lean
because the air for proper combustion has to flow throw the
burn pot

 

Brokk

Member
Dec 9, 2008
126
Central/Eastern Mass
Your manual will tell you what the Magnehelic gauge
should read for your most efficient burn
I can not see the draft of your chimney pulling all the heat out
in order to do that there has to be an air leak in the stove and
if there were an air leak your stove would be burning very lean
because the air for proper combustion has to flow throw the
burn pot

I have just looked through the manual I got with the stove, as well as the online manual that the newer models have and in neither location is there any mention of what the reading should be. In fact, the online manual only mentions the word "draft" two times. I have now written to the mfg to ask for this information. Hopefully I will get a response back in a reasonable time frame.

I don't believe it's pulling "all" of the heat out. The stove is still delivering heat into the room, but much less than one would expect for the advertised BTUs of the stove. Perhaps I'm only losing 30%-50% of the heat up the chimney.

Stoves are not air tight. Even a properly installed seal around the door will still allow some air in. They aren't rubber. I expect there are a number of spots where air can be pulled into the stove if the vacuum is strong enough. I would assume they are designed so that smoke/CO does not get out into the room under most circumstances, rather than preventing all possible paths of air entering the stove. That being said, your air intake should be the path of least resistance for a normal burn.
 

johneh

Minister of Fire
Dec 19, 2009
3,755
Eastern Ontario
This may help it is an Enviro stove
No, the door is not open just looks like it is. Decorative grill work will open
so as to look like an open fire
Best explanation of what the flame should look like Note Gauge upper right hand
 

Brokk

Member
Dec 9, 2008
126
Central/Eastern Mass
Your manual will tell you what the Magnehelic gauge
should read for your most efficient burn
I can not see the draft of your chimney pulling all the heat out
in order to do that there has to be an air leak in the stove and
if there were an air leak your stove would be burning very lean
because the air for proper combustion has to flow throw the
burn pot

Well, I got a response back from the mfg. Completely useless response. Let me know what you think...

"We do not measure static or running vent draft for pellet stoves, we have to measure draft at the stove. Install a vacuum Tee between the vacuum switch and drop tube. Install the vacuum gauge on the Tee. Turn thermostat on Dial Control. **Make sure hopper has fuel in it this can effect the vacuum reading**. It takes a minimum of .04 inches of water column to close the vacuum switch. Your vacuum gauge should be reading .19 t0.30 inches of water column while the exhaust blower is running on high. (on a cold appliance and no fire)"
 

johneh

Minister of Fire
Dec 19, 2009
3,755
Eastern Ontario
Maybe you are going about this the wrong way !
A pellet stove needs a min draft to operate properly.
The draft needed has to be enough to have the stove
work at its best burning /heat level You may find your
pipe warmer than you would like but there may be
another cause . If there is a lair of ash sitting on the
heat exchanger pipes that ash is an effective insulator .
which means the heat goes up the exhaust instead of
being cooled by the incoming air to be heated.
Is your chimney single or double wall? When my stove
is running at medium heat output I can rest my hand
on the double-walled pipe At high the pipe gets too hot
for anyone to hold there hand against the pipe.
I would not worry about the draft number at the top of the chimney
draft readings are taken just before the combustion fan
and adjusted from the number by the air intake slide/flapper
whatever your stove has. After the combustion fan it
becomes positive and if it leaks you smell smoke
Any leaks before effect burning and the draft number
 
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Brokk

Member
Dec 9, 2008
126
Central/Eastern Mass
Maybe you are going about this the wrong way !
A pellet stove needs a min draft to operate properly.
The draft needed has to be enough to have the stove
work at its best burning /heat level You may find your
pipe warmer than you would like but there may be
another cause . If there is a lair of ash sitting on the
heat exchanger pipes that ash is an effective insulator .
which means the heat goes up the exhaust instead of
being cooled by the incoming air to be heated.
Is your chimney single or double wall? When my stove
is running at medium heat output I can rest my hand
on the double-walled pipe At high the pipe gets too hot
for anyone to hold there hand against the pipe.
I would not worry about the draft number at the top of the chimney
draft readings are taken just before the combustion fan
and adjusted from the number by the air intake slide/flapper
whatever your stove has. After the combustion fan it
becomes positive and if it leaks you smell smoke
Any leaks before effect burning and the draft number

Well, yeah, I was going off of wood burning stoves for numbers and techniques for measuring. I guess it's just more common with those. I hadn't seen much about pellet stoves measuring draft. I certainly hadn't run into any description of pulling the stove apart to take measurements before the fan. Back to the drawing board I guess.

To be fair, it does make sense to take measurements there, as you will automatically include the negative pressure from the fan.

Now to figure out how to pull apart the stove to install a way to measure the pressure. While I understand they want to measure it cold, I really think there must be an upper limit to the draw for efficiency when it is running hot.

I hear you about the ash and agree. However this pellet stove has a scraper that is used every time I start the stove. It runs back/forth over the heating exchange tubes to clear off any ash. It's just part of the startup procedure for me.
 

johneh

Minister of Fire
Dec 19, 2009
3,755
Eastern Ontario
If using the Magnehelic gauge
just remove the vac. switch hose
and measure the stove vac from there
(the port into the stove by the combustion fan)
 

Brokk

Member
Dec 9, 2008
126
Central/Eastern Mass
If using the Magnehelic gauge
just remove the vac. switch hose
and measure the stove vac from there
(the port into the stove by the combustion fan)
I followed their instructions, first removing all the work I did to install the hookup on the exhaust vent and closing that off. I then removed the hose connection where it exits the stove, installed a T and ran another line out to my gauge. I gave everything a good cleaning while I had it open.

With the stove off and the door open, I zero'd my gauge. When I close the door, the gauge goes up to .01 WC. When I turn on the stove, the exhaust fan starts up immediately and the the gauge jumps to .80 WC. The MFG said it should be between .19 and .30, so it seems like even cold (and thus nothing much for a chimney draft), the stove exhaust fan seems to be running way too high based on their own numbers.
 

johneh

Minister of Fire
Dec 19, 2009
3,755
Eastern Ontario
Your draft fan speed is adjustable on the panel
slow down the fan reduce the draft
Watch your fire don't make it lazy
 
Last edited:

Brokk

Member
Dec 9, 2008
126
Central/Eastern Mass
Your draft fan speed is adjustable on the panel
slow down the fan reduce the draft
Watch your fire don't make it lazy
I tried that before. I believe I have it turned down as far as it will go.

The mfg responded again, suggesting I open the air intake more. I retested just now with the air intake fully open and the vacuum is at .40 WC, which is still greater than the mfg range and would also result in my pellets jumping out of the grate like popcorn. Opening the door to the stove reduces the vacuum to near zero. Not really a solution. Just a data point.
 

Brokk

Member
Dec 9, 2008
126
Central/Eastern Mass
Your draft fan speed is adjustable on the panel
slow down the fan reduce the draft
Watch your fire don't make it lazy
I grabbed a small screwdriver and played with the fan trim. The manual said to set it when the feed rate is #1 (low). I get the feeling that is the only time it has an actual impact on the exhaust fan speed. Changing it in either direction as far as possible, made zero difference in the vacuum during the startup/shutdown cycle (which is when the mfg said I should be gauging the vacuum).
 

Brokk

Member
Dec 9, 2008
126
Central/Eastern Mass
I found this webpage talking about testing a corn pellet stove. Seems to be very similar to what the mfg suggested I do with similar readings expected. I have a very long vertical run (33') so I don't think that is the issue. This draws the same conclusion that the fan is too strong.



"The Vacuum reading inside the combustion chamber of a NEGATIVE-DRAFT stove, insert, or furnace should be around .25 to .35 w.c. If there is a lower reading it may suggest there is improper combustion, too little air for combustion, a lazy flame, a clog in your Flue Exhaust Pipe, or too long a run for your flue exhaust motor to push the exhaust out the pipe run. A higher reading may suggest too much air for combustion, a hyper-active flame, a leaping flame with an inch blue-bottom, too strong of a flue exhaust fan, or flue pipe run is too short and does not create a proper back-pressure or draft. Most Flue Exhaust Kits are at least 5 feet long, go outside through a wall thimble, have a right-angle Tee with clean-out, and go up the side of the wall at least 3 feet with a 3 inch wall-siding spacer. Flue exhausting direct to the outside may make your stove run too fast and not create the proper vacuum and back-pressure for good combustion. If your stove runs too fast, it will use more fuel than it has to use and will be inefficient."
 

johneh

Minister of Fire
Dec 19, 2009
3,755
Eastern Ontario
Make or have made a restrictor plate (sliding plate) for the air intake (before the fire )
made and fitted to the air intake.
Then you can adjust the air to whatever draft you want
 

Brokk

Member
Dec 9, 2008
126
Central/Eastern Mass
Make or have made a restrictor plate (sliding plate) for the air intake (before the fire )
made and fitted to the air intake.
Then you can adjust the air to whatever draft you want

"The mfg responded again, suggesting I open the air intake more. I retested just now with the air intake fully open..."

Yes, my stove has a restrictor plate that you slide back/forth to open/close the air intake feed. With it open all the way, the draft is still more than the mfg says it should be (.19-.30). It's over .40 WC and with that much airflow through the burn pot, it can literally blow the fire out and send the burning pellets flying through the air. Yes, someone else on this forum came over to help me troubleshoot and when he opened the air intake all the way, that is in fact what happened.

I seem to be reaching a dead end with this issue. It feels like I need some way to reduce the blower speed/flow. That seems to be the only way to reduce the vacuum without turning my burn pot into a wind tunnel.
 

johneh

Minister of Fire
Dec 19, 2009
3,755
Eastern Ontario
just the opposite of what you want More air = less draft (wc)
less air means stronger draft (wc)
 

Clarkbug

Minister of Fire
Dec 20, 2010
1,247
Upstate NY
"The mfg responded again, suggesting I open the air intake more. I retested just now with the air intake fully open..."

Yes, my stove has a restrictor plate that you slide back/forth to open/close the air intake feed. With it open all the way, the draft is still more than the mfg says it should be (.19-.30). It's over .40 WC and with that much airflow through the burn pot, it can literally blow the fire out and send the burning pellets flying through the air. Yes, someone else on this forum came over to help me troubleshoot and when he opened the air intake all the way, that is in fact what happened.

I seem to be reaching a dead end with this issue. It feels like I need some way to reduce the blower speed/flow. That seems to be the only way to reduce the vacuum without turning my burn pot into a wind tunnel.
Do you think you could put a rheostat in line with the feed to the exhaust fan to turn it down a little?

Is your run of pipe 3" or 4"?
 

Brokk

Member
Dec 9, 2008
126
Central/Eastern Mass
Do you think you could put a rheostat in line with the feed to the exhaust fan to turn it down a little?

Is your run of pipe 3" or 4"?

I will look into the rheostat option.

The stove is made for 3" or 4", so I believe the pipe coming out is 3", but I have 4" vent running from there. Double wall to the chimney, then 4" single wall liner up the chimney.
 

johneh

Minister of Fire
Dec 19, 2009
3,755
Eastern Ontario
Just to see I had my youngest son came over with his Gauge .
He is an HVAC tech here in Ontario. He tested my draft and found where I
have it set is higher than recommended. We adjusted it as close as it
would go to recommended ( really not even close with draft almost closed)
Flame very dirty black tips. So we adjusted the fire for its best burn
clean and efficient. That's all I ask for from my stove other than heat
 
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