My home is equipped with a make-up air line intended to bring some outside air to mix it with indoors air prior conditioning by the furnace. I am not sure why this is necessary but my question is regarding calibration. The line is connected to a Skuttle Model 216 make-up-air control unit. See:



In summary, a damper inside the duct is moved when the furnace blower operates allowing outside air in. When the blower stops, the damper moves again to obstruct the flow of outside air. The damper can be calibrated with a pair of counterweights in the outside of the unit (see patent). I have followed the manufacturers instructions for calibration and failed every time. Either the damper wants to stay closed or it wants to stay open. I cannot get it to adjust according to the blower power. Does anybody has any experience with these units? Any ideas on proper calibration or on what could go wrong with these devices?


The calibration instructions (on page 2) require a specific set of conditions which may be challenging to adequately create and control—especially if anyone else is in the building.

I would begin by making sure the mechanism operates smoothly and freely. Also be sure that the device is installed so that the pipe is exactly parallel (or vertical) to the floor and the wire penetration is centered so that its movement isn't affected by going through odd angles with respect to gravity. Almost certainly the damper being stuck is caused by either mechanical friction or disorientation to gravity.

After that, repositioning the weight for proper operation should be easy.

  • Thanks! What exactly do you mean by "the wire penetration is centered". I guess we are talking about the wire that holds the weights? – LasEspuelas Apr 25 '14 at 23:49

All fresh air intakes do is provide a slight positive pressure within the home. This means inside air is pushed out through non-airtight spaces in the building envelope as there is slightly more air pressure inside than outside and nature abhors a pressure gradient.

Without an intake there is normally slight negative pressure where outside air is sucked inside from all over the place creating cold drafts during winter.

With positive pressure you don't feel drafts, outside air gets sent through furnace air filter slightly cutting down on dust accumulation. Air consuming devices—bathroom and kitchen vents, gas furnaces, gas water heaters, fireplaces—work better/safer.

Unless your furnace/ducting/returns are sized/designed incorrectly and the envelope of your home is being over-pressured, only same amount of air that would normally be leaking in from cracks is sucked into the fresh air intake to replace that escaping air.

This situation is all very different from a mental picture of a fan operating in a large open space where the amount of "sucking" is always proportional to amount of "blowing" where the higher you turn up the fan the more air is sucked in.

In your home an intake works more like a fan blowing into an enclosed box: no matter how much you turn up the fan "sucking" is unchanged if there is nowhere for the air to go.

Within reason (e.g. no over-pressure) no matter how much faster the fan turns sucking of outside air can only be proportional to the amount of air leaving the home. Most of the energy goes into moving inside air around between supply and return ducting.

  • I think what you are saying is that my make-up air controller might be properly calibrated. I just don't see it opening because air is not leaking out of the house at an appreciable rate? – LasEspuelas May 2 '14 at 17:39

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