Magnehelic Guage use ?

Pete1983 said:

I called another stove dealer who thinks I have too much pressure in the chimney and wants me to test it with a Magnehelic Guage.

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Actually, it's a partial vacuum you're trying to measure. Under normal conditions, the air pressure found in a chimney will be slightly less than that outside the chimney. Any holes in the chimney or stove pipe will let air into the chimney (again, under normal conditions - when not in use or very windy days, the chimney might have a reversal of flow).

The hole for the gauge will not ruin your pipe. You need to drill through both pipe layers and insert the hose. Hopefully it is a high temperature hose and it shouldn't be left in too long, nor inserted too far. You'll want to cap the hole with a sheet metal screw (not sure if both layers need to be plugged - others will probably let you know) at least in the outer layer. The screw will deform the inner hole somewhat, so if you need to measure again at some point, the hose might not seal very well. Shouldn't make a huge difference.

Another option is to go up about 18" on the flue pipe and put in a temperature probe. The holes aren't as big as a 1/4", but you might be able to get a smaller diameter hose or small tubing through probe hole. The reading will probably be a bit lower on the gauge than right above the collar, but should be close enough to tell if you have excessive draft.

The gauge will have two ports - one for normal air pressure for reference and one for your sample air pressure. If the needle on the gauge goes the wrong way, switch the hose to the other port on the gauge.

Your double wall is stainless inside and black smoke pipe outside. I do not know the outer pipe gauge. If I was to drill it, I'd drill a 1/4" hole with a nice new bit. Upon finishing the test I'd put in a 1/4" x1/2" stainless bolt and try to secure it with JB Weld cement...neatly! You do want to seal inner and outer. I can't say for sure that it would create a problem with only the outer wall plugged, but you have to remember that every system has a personality and you want yours to be more Jeckyl than Hyde. That means keeping the flue gases where they are supposed to be rather than where they can get

I've used Magnahelics to test flue sizes up to 36". Simple reliable instrument. Make sure the metal probe is perpendicular to the flue gas flow so the venturi effect creates the correct draft reading. Good luck!

Curious...what issues are you having?

As good as JB Weld is for some applications, this isn't one of them. It is only good for up to 500F continuous, 600F briefly. The outside layer of your pipe will probably not get that hot (but it could down by the collar), but a bolt that penetrates into the inner flue will easily get hotter than that. That heat will then be conducted to the bolt head and burn the JB Weld under it.

JB Weld faq

pgmr said:

As good as JB Weld is for some applications, this isn't one of them. It is only good for up to 500F continuous, 600F briefly. The outside layer of your pipe will probably not get that hot (but it could down by the collar), but a bolt that penetrates into the inner flue will easily get hotter than that. That heat will then be conducted to the bolt head and burn the JB Weld under it.

JB Weld faq

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Thanks for the correction. Given the temps, what do you think would work to simply make it stay in position. Perhaps a slightly larger bolt that would hold tight.

Jackfre said:

pgmr said:

As good as JB Weld is for some applications, this isn't one of them. It is only good for up to 500F continuous, 600F briefly. The outside layer of your pipe will probably not get that hot (but it could down by the collar), but a bolt that penetrates into the inner flue will easily get hotter than that. That heat will then be conducted to the bolt head and burn the JB Weld under it.

JB Weld faq

Click to expand...

Thanks for the correction. Given the temps, what do you think would work to simply make it stay in position. Perhaps a slightly larger bolt that would hold tight.

Click to expand...

I'd go with the temp probe mentioned above, then no need to worry about sealing the hole. If you'd rather not do that, then any screw slightly larger than the hole will work. If you can reach up through the stove or disassemble the stove pipe, you could just put a nut on the inside as well.

I think I'd wait and learn the stove for a little longer before drilling holes. Didn't you just get it last week?

On the JB weld, why not simply use furnace cement for the seal?

Seems like geeky overkill to this Mechanical Engineer. A solid fuel-burning appliance and associated flue to daylight is a very simple system. Problems with draft can typically be solved by simply standing back and thinking about what could be causing it, and taking appropriate action (most typically adding some length of chimney pipe up top). Knowing whether the pressure differential between the inside of your flue and atmospheric is 1.8" of water or 2.1" of water one way or another isn't going to tell you anything you don't already know. Besides that, there are so many variables involved (firing rate of stove, inside & outside temperatures, etc.) that your data collected from the precision instrument will be different every time you read it, and you'll basically learn nothing. Tell us what it is about the stove's performance that got you talking to folks about it. Also tell us all about the installation...especially the flue from stove to daylight. Pics are always helpful...holes drilled through double-wall stovepipe for no apparent reason seldom are. Rick

Excellent! Good troubleshooting & action. A thorough visual inspection should always be the first step before doing anything more invasive. Burn on! Rick