I am getting the parts together to ventilate a 100 ft3 coat closet so I can put my rack in there. I have pretty much decided on a T6 inline duct fan from AC Infinity. I am mounting the fan in the attic above the closet and ducting it into the laundry/mudroom behind it. Just enough ducting to go from the fan to 2 ceiling diffusers. AC's thermal controller looks pretty slick and has lots of programming options so I will use that. The fan is rated for a max 351cfm.

The closet has a standard door undercut of about an inch give or take. I am trying to find out if that will be able to supply enough makeup air to balance the load on the fan. Is there a formula to calculate that? What other factors to I need to consider? I could always install an intake vent in the door or wall near the bottom of the rack, or cut another slice off the bottom of the door. Any help is appreciated. Thanks.

The T6 is a 6" fan, so that's 28.2744 square inches of opening.

You didn't say what width the closet door is. If I assume it is at least 30" or more, then a 1" gap at the bottom should work fine. As it will not create any additional static pressure.

First, this will probably work just fine, even with assumptions. People like us who are not true experts on things like airflow usually overengineer things, so I wouldn't worry....

But where's that German guy with the Indian sounding name (Gurhard?) when you need him? I ask because THIS is a situation where his information might get us an exact answer, if we had all the unknowns to plug into his formulas. And that's a big IF.

One fact is that every opening does "create... additional static pressure.*" A HUGE opening would create very little static pressure, perhaps negligible; a small opening will create more.

Another note from an article on this is "Fans of the same diameter are not all created equal. Airflow capacity at different static pressures varies depending upon fan make and model."



*"static pressure" is an odd term as it refers to the push that a fan gives to air as well as the resistance to air flow given by a constriction. It's as though voltage and resistance were replaced by a single word.

Openings don't create static pressure, they cause it- devices that move air create it. Openings provide a path for controlling the pressure (positive or negative) in the space once the device starts moving the air. Negative static pressure exists when the air (or other gas or fluid) passing through the opening is less than the volume moved by the device. If the gap is smaller than what will pass the cfm of the fan without causing excessive negative pressure, the velocity of the air will increase so that the volume WILL pass, but its velocity may cause more noise than is acceptable. One thing is certain- if the negative pressure is sufficient, the door will open until the negative pressure is reduced to zero if it wasn't latched unless the bolt/striker or hinge friction is high or the door was hung so it naturally closes via gravity and the cfm isn't sufficient to move it.

Static pressure is pressure measured at one point and doesn't regard atmospheric pressure. Pitot-static pressure measures both and the difference is called 'dynamic pressure'. If you see ducts with tape or a little piece of sheet metal screwed over a small hole in a duct near a damper on the blower side of a vent (rather than between the damper and the vent/diffuser), it's where the pressure test was taken.

In ducts, the pressure test results will be affected by many things, including frictional losses due to joints, changes in direction, various types of couplings, air velocity (under-sized ducts and improperly set dampers cause imbalances) and lack of radiused bends and mixture of the air at various locations in the path. sharp corners cause a blockage and disrupts the air flow- the goal is to achieve 'laminar flow'.

If anyone cares to determine static pressure for any reason, here's a link with a calculator-

[Link: bry-air.com]

WRT your comment about voltage and resistance being replaced by one word- the static test is performed in the space between the device that moves the air and the supply end (the place where it may be restricted, or not) when exhaust fans are used and like testing for voltage, the supply side test is done in several other places in the circuit, to make sure the pressure isn't excessive, positive OR negative.

So a good test could be to put some tape over the latch, lube the hinges, and close the door. Run the fan. If the door stays closed the gap is enough?

Craig

What's the load (BTU)?
100CuFt is small space, like 4'x4'x6' ?

Let's say it has a 30" opening and it's 8 ft high inside. It would then be 5 feet deep. Not so small, really.

2.5 x 8 x 5 = 100

You could do that. I would also listen while holding the door closed- if you hear the air rushing through the gap, it could be made larger.

In the OP's case, I think 351cfm is overkill, though.

What if the gap were not large enough for air to escape through this gap when the hoax is running? Will it push back into your duct allowing no ventilation? Typicality does not run long enough, but something to think about

The room isn't going to be totally sealed and the fan's blades don't seal against the sleeve- if it did, it would be an impeller. The air that can, will be blown out and once the negative pressure is sufficient, the fan will be basically useless because it will be trying to push against the partial vacuum.

What hoax?

Closet is 6 X 2 X 8. 351 CFM is at max speed which I hope not too need very much...... maybe on the hottest days of summer when the closet gets pretty warm.

I will just put it in and crank it up and see how it sucks and blows. :) It is not rocket science. Just thermo and fluid dynamics. No biggie.

I do not know the BTU load of each component. I could try finding those numbers I suppose. But it seems that there are different ideas when it comes to how much air you need to move for a given BTU load. Middle Atlantic, ATM, Cool Components, ACInfinity, all have different figures and explanations on what is the right way to do it.

I hope this is not a hoax!

Check with the equipment manufacturers and ask for architectural specs- if they have those, they'll have the btu output and normal operating temperature. If the closet is centrally located, I don't know why it would be much hotter during Summer- If you keep the rack off of the back wall, it should stay cool, as long as the hot pieces are at the top, so they won't overheat the cooler pieces. Is it on the first floor, or higher? Residential?

Audiophiliac is on the right track. See if it works. That trumps all the calculations in the world, since this is neither actual rocket science, nor influences whether someone lives or dies.

Let's say you get all that information -- will that include the amount of spacing needed between components? Or whether air should also be blown front to back or back to front between each pair of components?

BTU outputs are not as important as whether that heat energy gets moved away from the items putting out the BTUs! Prove this by going into an 80 degree attic -- not thought of as a normally HOT temperature -- with zero air flow... see how long it takes, just sitting there, before you start to sweat.

Stick a fan in there and see what happens. Then, as a radio amateur friend of mine used to say, tune for minimum smoke.

Closet has 1 exterior wall and 1 wall shared with garage. That probably explains the temperature differences. Cheap cookie cutter house probably has the minimum R value to meet code. It gets hot in the summer and cold in winter. I could probably just remove the door and alleviate the issue. But the point of doing the rack in the closet is to keep the kids OFF! Removing the door cancels that out.


I have ordered the ceiling diffusers already. I just have to wait until the T6 is in stock now. They say 12/29 in their site. Anyone know if another low voltage 6" duct fan? :)

This stuff?