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A Blower Door Is the Hydraulic Jack of Building Science

Blaise Pascal discovered the physics that connects the blower door and the hydraulic jack

Posted on Jul 2 2014 by Allison A. Bailes III, PhD, GBA Advisor

Remember the first time you ran a blower door? OK, maybe that's not the best way to get where I'm going because most first-timers turn the pressure up like they're practicing tai chi on Jupiter. After you've done a few tests, though, you learn to crank it up to 50 Pascals of pressure difference in just a few seconds. And that's where you may have discovered the mystery that Blaise Pascal solved nearly four centuries ago when he invented the hydraulic press.

When you run a blower door and move air from indoors to outdoors, you're decreasing the air pressure inside the house. At least you know that's true right by the fan, because that's where you set up the manometer to measure the pressure difference. If you've done any blower-door testing, you may have wondered if the pressure is really the same in other parts of the house and how long it takes for the pressure in the farthest reaches of the house to drop to the same level.

Of course, you could always do the experiment. Set up your blower door as usual on one side of the house. Set up a second manometer on the far side of the house with a way to see the results right there at the blower door. You could even use data loggers to record the blow-by-blow if you were so inclined. The solution lies in understanding how a hydraulic jack works.

How a hydraulic jack works

If you've ever raised your car with a hydraulic jack, you probably noticed how easy it was to lift a lot of weight. That's because the jack multiplies your force. The diagram below illustrates how it works. You push down on Piston A and it moves a relatively large distance with a relatively small force. The physics of fluids allows you to trade off those two quantities.

On the other side, Piston B rises only a small distance but does so with great force. The amount of work done, force times distance, is the same on each side, though. It works just like a lever, but with a fluid instead of a stick.

Pascal's Principle and blower doors

This is where our friend Monsieur Pascal comes in. He discovered what is now called Pascal's Principle:

A change in pressure at any point in an enclosed fluid at rest is transmitted undiminished to all points in the fluid.

The hydraulic jack works because when you press down on Piston A, you increase the pressure in the fluid under the piston. That pressure is then transmitted right across to Piston B. And it happens right away because that pressure wave transmits to the other side at the speed of sound. It is a sound wave, in a sense, since both are pressure waves.

So when you turn on the blower door, Pascal's Principle is what tells you the pressure you measure at the blower door will be the same as the pressure on the other side of the house. And it happens almost immediately... unless you have an unusually long house. If your house were, say, a mile long, it would take about 5 seconds for the pressure to reach equilibrium.

OK, strictly speaking, Pascal's Principle applies to incompressible fluids that are completely enclosed. Air is compressible, but the pressures we're working with in a blower door test don't do much compressing. Also, houses aren't airtight, so the leakier they are, the more pressure variation you'll find, especially when you're close to the leaks. Even in a leaky house, though, the pressure change you induce with the blower door transmits across the house at the speed of sound.

Happy birthday, Blaise Pascal!

Pascal's birthday was last month, on June 19th. Although he didn't make it even to 40 years old, he accomplished a lot in his short time. In addition to discovering Pascal's Principle, he invented the hydraulic press and the syringe. He was one of the few who rejected the Aristotelian notion that a vacuum couldn't exist, contradicting luminaries like René Descartes. He also helped advance the development of the scientific method by writing that falsifiability is a distinguishing feature of scientific theories. And then there's every high school algebra student's favorite Pascalism: Pascal's Triangle.

Happy (belated) birthday, Blaise Pascal!

Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. Check out his in-depth course, Mastering Building Science at Heatspring Learning Institute, and follow him on Twitter at @EnergyVanguard.

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Image Credits:

  1. Image #1: Public domain
  2. Images #2 and #3: Energy Vanguard
Wed, 07/02/2014 - 05:25

Edited Wed, 07/02/2014 - 07:27.

Blaise Pascal was also a writer and philosopher
by Martin Holladay, GBA Advisor

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Thanks for highlighting Blaise Pascal's scientific accomplishments.

As every French high school student knows, Blaise Pascal was also a writer and a philosopher. Some of his sentences were so well-crafted that they are still regularly quoted.

To illustrate the butterfly effect, Pascal made a reference to one of Mark Antony's love affairs:
"Le nez de Cléopâtre, s'il eût été plus court, toute la face de la terre aurait changé."

Or, in English: "Had Cleopatra's nose been shorter, the whole face of the world would have been different."

On intolerance:
"Les hommes ne font jamais le mal si complètement et joyeusement que lorsqu'ils le font pour des raisons religieuses. "

Or, in English: “Men never do evil so completely and cheerfully as when they do it from religious conviction.”

An example of accidental (or coincidental) Buddhism:
"J'ai découvert que tout le malheur des hommes vient d'une seule chose, qui est de ne savoir pas demeurer en repos dans une chambre."
("All men's miseries derive from not being able to sit in a quiet room alone.")

On the ways of the heart:
"Le cœur a ses raisons que la raison ne connaît point."
("The heart has its reasons of which reason knows nothing.")

Concrete contractors, however, will probably be drawn to this quote from L'Art de Persuader:
"Si l'on n'assure le fondement on ne peut assurer l'édifice."
("If we do not secure the foundation, we cannot secure the edifice.")

Wed, 07/02/2014 - 21:22

by Malcolm Taylor

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Don't forget Pascal's Wager

Thu, 07/03/2014 - 06:49

Response to Malcolm Taylor
by Martin Holladay, GBA Advisor

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Here is a link to more information on Pascal's Wager.

The wager centered on the question of whether belief in God is a sensible choice in light of the fact that the existence of God is unknowable. Pascal's conclusions depend on Christian beliefs that include the concept of spending eternity in heaven and hell.

Thu, 07/03/2014 - 22:20

Blaise and the stack effect
by Derek Roff

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It's interesting that GBA posted this article, and one from Tamasin Sterner, within a few hours of each other. Tamasin asserts that pressures are not the same throughout the house, and seems to say that air will move from higher pressure to lower pressure, under some conditions. Allison, and his buddy Blaise, present a contrasting argument. It may take an additional article to clarify, in a way that is useful and practical for builders and designers, just how Blaise's Pascal Principle gets along with the stack effect.

Mon, 07/07/2014 - 12:28

Response to Derek Roff
by Allison A. Bailes III, PhD, GBA Advisor

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I can see how it might be confusing, Derek, but the two are actually not in conflict. The pressure exerted by the blower door on the air throughout the house is transmitted a la Pascal's Principle. In an airtight house, the 50 Pa pressure difference is felt throughout. In a leaky house, the pressure difference won't be quite as high as you move away from the blower door, especially when you're near the leaks.

The stack effect was there before you turn the blower door on, though, and is still there afterward. If the stack pressure is 0 Pa at the bottom of the house and 5 Pa at the top of the house before you turn on the blower door, you'll end up with 50 Pa at the bottom and 55 Pa at the top with the blower door running (assuming an airtight house).

One interesting thing to note is that you cannot measure the stack effect pressure difference with a manometer. The reason is that the air in the tube is subject to the same pressure difference.

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