# Thread: Fun fact of the day

1. ## Fun fact of the day

A guy who teaches at National Comfort Institute was a guest speaker in my HVACIII class. He is very big on AIRFLOW. One thing I took from it was, he said a layer of film, gunk, dust or whatever, as small as 0.005" will decrease a fan motors efficiency by 9%. I thought that was pretty amazing. From now on I probably would feel guilty not completely cleaning all surfaces of motors..

2. He also asked the class if return air was pulled through the return, or pushed through the return. Guess what the class said

3. Regular Guest
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I would say pulled. But by tone of your thread i going guess im wrong. So please go on

4. Everybody said pulled, he seemed discussted that nobody knew the displacement of air pushes through the return rather than being pulled.

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I heard it both ways.

6. ## Pedantic

pushed, pulled -- one is a pedantic explanation and the other is practical and useful. It's very much like the discussion about centrifugal force. Academics will argue that there's no such thing. However, it's a useful and practical way of thinking about what happens, and it is correct in that it can predict with sufficient accuracy what's going to happen to the objects of interest. Saying the air is being pulled is also correct from a predictive power point of view, which is all that matters in practice. You're not designing turbo jet engines, for crying out loud.

7. This is like the question my instructor asked our class one day,"If you open the door at your house in the summer time,does the heat come in the structure or does the cool go outside?

Most of the class said the heat comes in, a few said the cool goes out.

His answer was "Neither, the cool air is absorbing the sensible heat.

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I tend to enjoy the scientific (not necessarily practical) explanation, so for those that are interested, here is one way of looking at pushes vs. pulled air:

Imagine a length of ductwork open at both ends. Is there any pressure in this duct? You bet - atmospheric pressure - about 14.7 psi at sea level. Why doesn't the air move? because the pressure at both ends is the same.

Liken this situation to two people, Pete and Val pushing on a car. Pete tries to push it forward, Val tries to push it backward with equal force. Of course, the car doesnt move either.

Now I grab Val by the arm and move him off to the side, away from the front of the car. What happens? Obviously the car moves forward, because Pete is still pushing. Who is making it move? Val isn't even touching the car, he is off to the side. Pete's pushing is what is actually making the car move (although it certainly wouldn't had not Val gotten out of the way). Would we say Val is pulling? Nope, he is just not able to push, or oppose Pete, any longer.

Back at the duct, we attach a fan to one end. It removes air from the duct and air flows in the other end. As you can see, the fan's job is the same as when I moved Val aside, the fan just moves air out of that end of the duct, creating an area of lower pressure. What really makes the air flow into the other end? Pete, er, I mean atmospheric air pressure, that column of air above our heads is pushing the air in. The evacuated air molecules don't pull, they simply aren't present to oppose the flow any longer.

Does this make any difference in your day to day activities? Probably not, but it is an accurate way of describing what takes place.

Dan

9. Originally Posted by ComfortService
This is like the question my instructor asked our class one day,"If you open the door at your house in the summer time,does the heat come in the structure or does the cool go outside?

Most of the class said the heat comes in, a few said the cool goes out.

His answer was "Neither, the cool air is absorbing the sensible heat.
I woulda said depends on the pressure differential.

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