Magnehelic Question

Can I keep a magnehelic connected at all times to my stove without having to pull out the hose, reset it, ect as I need to do for my manometer?

I am looking for a permenant connection so my wife can just look at the draft anytime without worrying about pulling the hose or resetting anything. Thanks!

The magnehelics are more stable than the MkIIs since there is no oil to be affected by room temperature. Make sure you have it secure, plumb, and level in your mounting setup. I have found them to be pretty stable, just zero it out and forget it, of course if you have the need you can occasionally take a peak to see how it is doing.

Matheus, what do you mean by "Zero it out" ? and also, which connector should I use the top one or bottom one on the magnehelic? Thanks!

The screw on the face is for zeroing the movement after you get it plumb and level. You will need a small flat blade screwdriver. If you use the inlet marked low side (the top one) that should work to allow use of the positive scale to read negative inches of water. Make sure you plug the ports in the back of the gauge with the 1/8" pipe plugs that come with the unit.

Matthaus, thank you!

I have my magnehelic installed and mounted, all flush and level. I won this on ebay last week for $25. It seems like new but no instructions.

The questions I have (please see pics)

1. The low pressure pipe is connected to the top of the unit which should show negative pressure on the scale as a positive reading left to right, correct?
2. Do I need to plug or tape over the bottom connection since I am only using the top?
3. To zero out the unit. Do I zero it when the stove is off and all doors open? When the stove is closed with blower fan on? Confused on how to exactly zero it out and how often.

Anything else I am missing?

Thanks much!

jrn8265 wrote:snip...
1. The low pressure pipe is connected to the top of the unit which should show negative pressure on the scale as a positive reading left to right, correct?
2. Do I need to plug or tape over the bottom connection since I am only using the top?
3. To zero out the unit. Do I zero it when the stove is off and all doors open? When the stove is closed with blower fan on? Confused on how to exactly zero it out and how often.

1. No, my mistake, should be the lower one, that is the low side, sorry.
2. No if you do that it will not work, needs ambient pressure as reference
3. Perform zeroing operation with hose disconnected from stove, just turn the screw on the gauge face until it is at zero, hook the hose back up and you should be in business forever. A spot check can be done if anything seems out of the ordinary with the readings.

Enjoy!

Edit, oops just noticed you have the hose going into the stove, this is a reference location that is used for checking over fire draft (should be around -.02" WC) normal hook up should be in the flue pipe before the barometric damper close to the flue outlet on the stove.

I have the same Magnahelic that Matthaus uses. Mine is mounted to an electrical box as a stand. I use a piece of plastic tubing connected to a short piece of stainless tubing that normally lives in the flue pipejust above the lower elbow. When I want to check the overfire draft, I simply pull the tube out of the chimney and insert it in the 1/4" hole in the fire door.

These are fairly expensive gauges, but I found mine on the Bay for about $20. I leave it portable so I can use it on my stove upstairs.

Highlander, nice setup! I would not have bought mine if it were not for ebay...$25....when they usually go for $80.....Matthaus, thanks for your help...moved the tube connection to the bottom port...i zeroed it out outside of the stove. this morning the needle has moved slightly when the tube is outside of the stove...interesting...maybe the difference in air pressure between days?

jrn8265 wrote:i zeroed it out outside of the stove. this morning the needle has moved slightly when the tube is outside of the stove...interesting...maybe the difference in air pressure between days?

This is probably the case. You will see slight changes due to changes in air pressure as you stated. Looks good.

Jeff

Just a side question for Matthaus or anyone else, does anyone prefer the magnehelic over the oil filled jobs. If so what are the reasons? Just curious. Thanks.

Jeff

ceccil wrote:Just a side question for Matthaus or anyone else, does anyone prefer the magnehelic over the oil filled jobs. If so what are the reasons? Just curious. Thanks.

Jeff

I have no preference but would point out that they are two different types of something that does the same thing. What it boils down to is they do it two different ways. The MAG is simple to transport and use instantly but can fail mechanically. The liquid device is simple and reliable but is a clumsy kind of thing. Rabbit ears come to mind.

ceccil wrote:Just a side question for Matthaus or anyone else, does anyone prefer the magnehelic over the oil filled jobs. If so what are the reasons? Just curious. Thanks.

Jeff

For the same dollar spent on a new pressure measuring instrument the manometer type instruments are usually more accurate. I'm talking about absolute accuracy, i.e., the instrument actually measuring correctly. Of little importance for the relative measurements we are normally making.

Think about how a manometer is constructed. In it's simplest form it's just a U shaped tube with linear ruler markings on it. It's easy to manufacture a tube with an uniform inside diameter (the only dimension that matters) and a mark scale with linear numbers (inches H2O or mg). It's much more difficult to make an accurate instrument that measures the pressure and transform that measurement to a needle arc movement.

I had a dwyer II manometer that was .03% accurate to scale. The Magnehelic says .02% accurate to scale.

Thanks, I was just wondering. I have a Mark II and have no plans to switch when this one works just fine.

Jeff