Adding a manometer/magnehelic gauge to a flue pipe

FixedGearFlyer

Burning Hunk
Oct 8, 2010
207
Michigan's Upper Peninsula
I've had a couple of PMs about how I added a Magnehelic gauge to our wood furnace and thought I'd post it here as it's own thread for easy reference.

To connect to the flue pipe, I got a brass fitting that has a 1/4" O.D. tubing compression fitting and a 1/8" male pipe thread (MIP). The hole in the flue pipe is a 3/8" and the pipe thread end of the fitting is simply screwed into it (no tap needed). You could make it extra secure by adding a brass 1/8" FIP lock nut, if you wanted. The flue pipe hole should also be as close to the pipe collar on the furnace as possible and definitely before any baro or key dampers.

1/4" copper tubing runs from the compression side of the fitting to my Magnehelic gauge, which uses an identical fitting to go from the tubing to the 1/8" pipe thread port on the back of the gauge.

If it was a manometer with flexible tubing, I would have terminated the copper tubing with a fitting that had a 1/4" OD compression fitting on one end and a hose barb to fit the manometer tubing on the other.

I made a sheet metal bracket for my gauge to permanently mount it on the furnace.

That's it. Less than $10 in connector parts for either the magnehelic gauge or manometer option. Pictures of my set up are attached.

20201205_183608.jpg 20201205_183613.jpg 20201205_183552.jpg
 

brenndatomu

Minister of Fire
Aug 21, 2013
6,154
NE Ohio
Your setup sure looks nicer than the "S hook" that I made out of the copper line (just hangs in the hole in the pipe) but I like easily being able to pop it out and clean the end off...it gets soot/carboned up after a while...that lil hole has to stay open. ;)
 
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FixedGearFlyer

Burning Hunk
Oct 8, 2010
207
Michigan's Upper Peninsula
Your setup sure looks nicer than the "S hook" that I made out of the copper line (just hangs in the hole in the pipe) but I like easily being able to pop it out and clean the end off...it gets soot/carboned up after a while...that lil hole has to stay open. ;)
An S-hook of tubing is a great idea! Even easier and cheaper than my brass fittings.

I agree that you have to keep it clean. I clean mine each month when I change my furnace air filter - I just pop the tubing out of the flue pipe fitting, run a small bristle brush through the fitting's hole, and put it back together. I don't think it even takes 60 seconds.
 

brenndatomu

Minister of Fire
Aug 21, 2013
6,154
NE Ohio
I agree that you have to keep it clean. I clean mine each month when I change my furnace air filter - I just pop the tubing out of the flue pipe fitting, run a small bristle brush through the fitting's hole, and put it back together. I don't think it even takes 60 seconds.
Mine only takes 30 seconds... :p ;) ;lol
 

brenndatomu

Minister of Fire
Aug 21, 2013
6,154
NE Ohio
The Dwyer Mark II model 25 manometer that I am using comes with rubber tubing...I just have that connected to the copper "S hook" then...just needed a foot or two of copper to get out far enough to isolate the rubber from the heat...
 
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MR. GLO

Member
Jan 26, 2021
126
Massachusetts
I've had a couple of PMs about how I added a Magnehelic gauge to our wood furnace and thought I'd post it here as it's own thread for easy reference.

To connect to the flue pipe, I got a brass fitting that has a 1/4" O.D. tubing compression fitting and a 1/8" male pipe thread (MIP). The hole in the flue pipe is a 3/8" and the pipe thread end of the fitting is simply screwed into it (no tap needed). You could make it extra secure by adding a brass 1/8" FIP lock nut, if you wanted. The flue pipe hole should also be as close to the pipe collar on the furnace as possible and definitely before any baro or key dampers.

1/4" copper tubing runs from the compression side of the fitting to my Magnehelic gauge, which uses an identical fitting to go from the tubing to the 1/8" pipe thread port on the back of the gauge.

If it was a manometer with flexible tubing, I would have terminated the copper tubing with a fitting that had a 1/4" OD compression fitting on one end and a hose barb to fit the manometer tubing on the other.

I made a sheet metal bracket for my gauge to permanently mount it on the furnace.

That's it. Less than $10 in connector parts for either the magnehelic gauge or manometer option. Pictures of my set up are attached.

View attachment 268705 View attachment 268706 View attachment 268707
Look great...What model is that Magnehelic gauge ...Dwyer says use the 2003 for wood stove. Can you send us a picture of the rear gauge connection. Thanks
 

FixedGearFlyer

Burning Hunk
Oct 8, 2010
207
Michigan's Upper Peninsula
Look great...What model is that Magnehelic gauge ...Dwyer says use the 2003 for wood stove. Can you send us a picture of the rear gauge connection. Thanks
Mine is a 2000-00 (0-0.25” W.C.). You can hook the pressure line from your flue pipe to either a high pressure or low pressure port, then leave the other one open to the atmosphere to measure + or - pressure values. So, in my case, the pressure line from the flue pipe connects to the low pressure port, since I’m trying to measure a vaccum.

I’ll grab a picture tonight or early in the morning.
 
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MR. GLO

Member
Jan 26, 2021
126
Massachusetts
Thanks have you ever max the gauge out. at .25? And I am assuming if you had a backdraft/puff back it will just read zero. Thanks
 

FixedGearFlyer

Burning Hunk
Oct 8, 2010
207
Michigan's Upper Peninsula
Thanks have you ever max the gauge out. at .25? And I am assuming if you had a backdraft/puff back it will just read zero. Thanks
Oh, my. No. Both our Tundra and our new Heat Commander furnaces are considered to be overdrafting at -.08, so hitting -.25 would be thermonuclear! With our 36 foot, 7x7 masonry chimney and a Fields Type RC baro, I can get it as low as -.04 and have been keeping it around -.06

And, yes, the magnehelic can handle the switch in pressure. It has a listed pressure limit of 15 PSIG, which is over 400" water column. So, it would go off the scale, but be fine.
 
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MR. GLO

Member
Jan 26, 2021
126
Massachusetts
would you get that reading at all cycles of the burn? or just off....and didn you get any puffbacks? especially at lighting the fire?

Thanks
 

FixedGearFlyer

Burning Hunk
Oct 8, 2010
207
Michigan's Upper Peninsula
would you get that reading at all cycles of the burn? or just off....and didn you get any puffbacks? especially at lighting the fire?

Thanks
Our draft is pretty stable. This is from this morning's fire, which has been burning for about 30 minutes. The air is calm and outside temps are around 0*F. If it was windy and gusty, you'd see the draft hopping up to -.07 or -.08 momentarily at the gust peaks. In a full gale (which we get more than you'd think!), it might peak to -.10 occasionally.

When lighting a cold furnace and chimney, I usually burn 3 or 4 knotted sheets of newspaper by themselves in the firebox to get the draft moving, then lay and light my fire. Within a minute or two, the draft is stabilized at -.06.

I was having some puff back (and some light creosote buildup) as recently as last week, but I have since realized that it was because I was closing the door too quickly after lighting. If I leave the door cracked for 5 minutes so that the fire is really roaring, then close it, it settles right into a nice burn and I avoid both the puff backs and smokey starts.

Both of those issues were from closing the door on a not-hot-enough fire and making the Heat Commander chase the fire with it's smart controls. If I close the door when it's hot enough to avoid the chasing, everything goes very smoothly.

 

FixedGearFlyer

Burning Hunk
Oct 8, 2010
207
Michigan's Upper Peninsula
Look great...What model is that Magnehelic gauge ...Dwyer says use the 2003 for wood stove. Can you send us a picture of the rear gauge connection. Thanks
Pictures of the connection at the gauge. Note that there are connections for both high and low on the left side and the back of the gauge. You have to plug the set you're not using, hence the brass plugs on the side ports.

The copper tubing is connected to the rear low pressure port and the rear high pressure is open to the atmosphere.

20210129_084522.jpg


20210129_084533.jpg
 

brenndatomu

Minister of Fire
Aug 21, 2013
6,154
NE Ohio
If I close the door when it's hot enough to avoid the chasing, everything goes very smoothly.
So if you close the door too soon and it starts this "chase" you refer to, will it eventually get things headed the right way, or sometimes yes, sometimes no?
And if it does recover, smoking, some creosote buildup, and possible puff backs are the main consequences?
 
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FixedGearFlyer

Burning Hunk
Oct 8, 2010
207
Michigan's Upper Peninsula
So if you close the door too soon and it starts this "chase" you refer to, will it eventually get things headed the right way, or sometimes yes, sometimes no?
And if it does recover, smoking, some creosote buildup, and possible puff backs are the main consequences?
The HC always got things stabilized and burning well, but if I closed the door before the splits were engaged, it would have to hunt for a bit. It's literally the difference between closing the door when the kindling is burning and the splits are just starting to burn (too soon) and closing the door a few minutes later when the splits are active, lightly charred in places, and ready to cruise.

I was just rushing it. The fire would smoulder, the HC would sense it and work the air supply shutters to save it, and it would create some creosote and an occassional back puff in the process, all while I happily and ignorantly found a proper fire a few hours later when I'd check on things.

So, it CAN manage a fire completely from striking a match to the very end of the burn, but it's a much better experience if I take an extra 3 minutes to ensure the fire is well established before I close the door and hand off control to the furnace.

(So, now I clean the cats' litter box between lighting and closing the door instead of after I close the door. Really, it's no extra time or effort in my day, just a small and impactful shift in timing.)
 

MR. GLO

Member
Jan 26, 2021
126
Massachusetts
Pictures of the connection at the gauge. Note that there are connections for both high and low on the left side and the back of the gauge. You have to plug the set you're not using, hence the brass plugs on the side ports.

The copper tubing is connected to the rear low pressure port and the rear high pressure is open to the atmosphere.

View attachment 273046

View attachment 273047
Finally got the magnehelic installed. Today draft before lighting stove was zero and draft during a burn it was .11
Open the door to the stove during a burn and the draft dropped to .07 .08 . My draft rite gauge was reading .14

Jotul F 50 TL Rangeley manual states draft should be between .05 - .10, with .07 ideal, with magnehelic guage. Also Jotul suggests 2 year season wood between .12 and .20 vs kiln dry, which my kiln dry is .065. Going to try a damper and see if it stops eating wood.
Thanks for all your help.
 
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Apr 28, 2019
156
Northern Illinois
Your setup sure looks nicer than the "S hook" that I made out of the copper line (just hangs in the hole in the pipe) but I like easily being able to pop it out and clean the end off...it gets soot/carboned up after a while...that lil hole has to stay open. ;)
Does this connection need to be sealed in order to get a proper reading. Or is your copper tubing just 'stuck' into a hole in the stove pipe? When you say 'hangs in the hole in the pipe' are you meaning just loosely stuck in? Thanks.
 

MR. GLO

Member
Jan 26, 2021
126
Massachusetts
My opinions....It doesnt need to be sealed but I would want it to be.. puff backs and you could help or hurt the cold pipe syndrome. You can just stick the tubing into the fitting like [B]FixedGearFlyer[/B] did or you can tap out the center of the fitting and push the pipe in further, which isn't really necessary...you dont need a lock nut but It will be stronger with one and I had a tough time finding a nut. So I went to the HD electrical section and found it in the lamp repair section.. Id wait to put the nut on until you clean the pipes. just let the threads hold it.
I needed to wiggle the drill bit in a circle or two at the end to make the holes slightly bigger.

https://www.homedepot.com/p/Commercial-Electric-Solid-Brass-Hex-Nuts-12-Pack-81965/306198163

The pipe just stuck in the hold and dangling... I think it needs to always be perpendicular to air flow. Unless its designed to be left in hole and hang. Not sure how much of a reading error you would get.

You need it before the damper and recommend near stove but I dont think that really makes a difference as long as its before any damper. I think I will get the Dwyer 2300-0 model next time to see if it shows any readings under zero.
 

brenndatomu

Minister of Fire
Aug 21, 2013
6,154
NE Ohio
Does this connection need to be sealed in order to get a proper reading. Or is your copper tubing just 'stuck' into a hole in the stove pipe? When you say 'hangs in the hole in the pipe' are you meaning just loosely stuck in? Thanks.
No...but the hole I drilled in the pipe is almost exactly the size of the metal tubing though too...if fits snugly. I'm sure its "sealed" better than the joints of most stove pipes. Ans yes, just loosely stuck in and hanging...like an S hook.
I think it needs to always be perpendicular to air flow.
I've played around with that...didn't make any difference. I think if the draft speed were very high, then it might more so...like the velocities one would see in a forced air HVAC duct.
 
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