Posts Tagged ‘pressure’

Angry Air!

June 7, 2017

John Tooley said, “Air is like crooked rivers, crooked people, teenagers, and cheap labor.  It always seeks the path of least resistance.”  He didn’t say that Angry air is Noisy air.   Air doesn’t like being forced through corrugated, flexible ducting, pushed around corners, and made to force open dampers.  It resists being made to perform in a way that it doesn’t want to.  It takes more and more force as the resistance increases.  Air is just fine when you just let it move at will.  It can become amazingly strong as any building that has met a hurricane or tornado can attest to.  And as objects like asteroids and space capsules hurtle through the atmosphere they burn up!

ASHRAE 62.2 requires bathroom fans to make no more noise than a quiet refrigerator in a quiet kitchen: 1 sone or less.  And if you put an Energy Star bathroom fan on the bench and plug it in, you can barely hear it.  It’s amazingly quiet.  “Is it running?” people ask.  And it is.  So how come once you install the fan in the ceiling it gets uncomfortably loud?

Fan manufacturers not only made these fans quiet, they put DC motors in them that are extremely tolerant ofchanges in pressure.  As the pressure increases in the installation, the fan motor compensates by using more power to increase the speed of the spinning wheel that is pushing the air.  (Notice the curve on this graph that starts on on the left side and then drops off the cliff at about 75 cfm.  It has about the same airflow from 0.45 iwg as it does at 0.0 iwg!)  That’s a wonderful thing because people can install the fans horribly and step on the duct and lots of other nasty things and still come out with the same airflow . . . but not the same sound level.  What was really, really quiet is now uncomfortably loud.  And as houses get tighter they get quieter and a noisy fan is annoying which is why so much effort was made to get them quiet so they could run all the time without bothering anyone!

I have found that builders get aggravated because these quiet and expensive fans that they have been compelled to install really aren’t all that quiet.  And they should be quiet.  They have been designed to be quiet.  Tested to be quiet.  And if you disconnect them from the installation, they are quiet.

So here’s a simple way to determine if the fan is working right: listen to it.  If the air is angry, it will be noisy and noisy DC fans equal bad installation.  The air is yelling at you.  I have found ducts filled with the foam that was sprayed on the house for insulation.  Backdraft dampers remain taped closed.  Ducts terminated against a wall or floor in the attic and don’t actually get to the outside.  If a bathroom fan that is rated to be < 0.3 sones is noisy, its a bad installation.  Period.  Fix it.  It may still be moving enough air to meet the ventilation requirements, but if it is noisy the homeowner will find a way to turn it off and stuff it full of socks.  Then the air in the house will get bad and people will get sick.  And the occupants will get angrier than the air!  And the really dumb thing is that all these codes and standards and mathematical computations and formulas to size the fan correctly mean absolutely nothing if the fan is turned off.

Homeowner’s Energy Workbook – Part 6 (Definitions)

December 24, 2012

Stack effect on a Fireplace

Infiltration and ex-filtration relate to convective air movement.  Infiltration is uncontrolled air leaking in and ex-filtration, logically, is uncontrolled air leaking out.  How and where it leaks in has a lot to do with the pressures in the house.  (Remember, high pressure moves to low pressure.)  If the pressure is higher in the house than it is outside, air will move out of the house.  Warm air is lighter than cool air so it rises, increasing the pressure at the top of the house due to the Stack Effect when it is warmer in the house than it is outside.  The top floor ceiling of a house can have an enormous amount of pressure pushing on it, pushing air through any crack or hole or gap it can find.

A wise building science guy, John Tooley, once said, “Air is like crooked rivers, crooked people, teenagers and cheap labor.  It always seeks the path of least resistance!”  He also said, “Air doesn’t care where you want it to go.  It will always move through the closest and biggest hole.”

We have to have ways to measure things like pressure and temperature and airflow.  We measure temperature with a thermometer.  In the U.S. we measure it in degrees Fahrenheit named after Daniel Gabriel Fahrenheit in 1724.  Most of the rest of the world adopted the Celsius scale (after Anders Celsius) in the mid to late 20th Century.  The Fahrenheit scale has water freezing at 32 °F and boiling at 212 °F.  On the Celsius scale, water freezes at 0 °C and boils at 100 °C.  Seems a bit more logical somehow, but there we are.  We don’t deal with change well.

The pressure in buildings is commonly measured in Pascals, because houses have gotten so tight and the pressures are so small that using the inches of water gauge or iwg or inches of water column scale was too gross.  It would be like measuring a house in fractions of a mile!  Inches of water gauge is an indication of how far a certain amount of pressure can raise a column of water.  Iwg is the most common measurement for indicating pressures in heating and cooling systems and the operation of fans.  One iwg is equal to about 250 Pascals.  The important thing to remember here is to visualize how small a Pascal is.  It’s really, really small.  If we could measure a gnat’s burp, it would be about that small.  It’s a tiny amount of pressure, but it can make an enormous difference in how a building works and the health of the occupants.  We measure pressure using a manometer.  These days it’s common to use an electronic, digital manometer, but there are analog manometers as well.  The availability of digital manometers probably has done more for the advancement of building science than any other tool.  It has allowed us to measure very small pressure differences.  It has ushered us into the world of CSI for homes.

Testo 417

Large Van Anemometer

Airflow speed is measured in feet per minute (fpm), like miles per hour.  FPM is the velocity of the airflow.  The volume of air movement is measured in cubic feet per minute (cfm).  The volume of air moving through a duct or fan is indicative of a rate of heating or cooling.  Airflow is commonly measured with anemometers which come in a wide variety of styles.  There are large vane anemometers and mini-vane anemometers and hot wire anemometers.  There is a tool called a Balometer which is commonly used to measure airflows in commercial applications.  Airflow can be measured with a garbage bag or a bathroom tissue, but the numbers are not easily repeatable with these approaches.  Airflow can be measured using a manometer as well, by using the difference in pressure across a known sized hole.

You should also be aware of some of the nasty elements that can be found in homes.  Carbon monoxide (CO), for example, can kill people.  It results from incomplete combustion.  If the flue gases don’t flow up the chimney the way they are supposed to, they can “spill” down into the house and make the occupants sick, exhibiting symptoms similar to the flu.  More about CO later.

Radon is a gas that emanates from granite in the ground.  Radon can leak into the house as a gas, and rapidly decay into particulates that can get lodged in the occupants’ lungs.  It can also be found in water.  It is much worse for smokers to breathe radon gases than non-smokers, but that is true of many things!  Certainly not every house has radon issues, and it can be tested and it can be reduced to acceptable levels.  And just because your neighbors’ house has a radon problem it doesn’t mean that yours does.

Humidity is another significant pollutant for a wide variety of reasons.  Relative Humidity (RH) is indicative of the amount of moisture a volume of air can hold at a certain temperature.  Warmer air can hold more moisture than cooler air.  As the temperature of the air rises, its RH goes down.  That’s why cold winter air drawn into a house makes the air in the house drier.  Cold air in a closet has higher relative humidity and mold grows on the cooler surfaces.  RH is not the easiest thing to visualize, but we can certainly feel it.  “It’s not the heat.  It’s the humidity!”  The dew point is the temperature of a surface that will cause moisture to condense.  As the dew point rises in warm weather, our discomfort increases.  When the dew point is only a few degrees below the outside temperature, it is almost hard to breathe.  It is interesting to note that there are microclimates all around us.  The air around a glass of ice water can be cooler and at the dew point and moisture droplets form on the sides of the glass.  This is similar to the microclimate convective loops in insulation gaps and on the surface of walls.  All this stuff is going on around you.  Right now!  And you thought things were just sitting there!

Just to be consistent, we measure relative humidity with a device called a hygrometer.  A humidistat is a control that turns something on when the humidity goes down.  A dehumidistat is a control that turns something on when the humidity goes up.

Next time: The Skeleton!


Imagine Yourself as an Air Molecule

September 17, 2012

There is a problem solving technique called synectics.  It refers to problem solving by analogy.  It is a technique that can be amazingly effective when trying to visualize a complex situation such as the air moving through a pipe or duct.  In my classes, I try to get the participants to imagine themselves as an air molecule being tossed around by a fan and thrown out into a duct, being pushed and shoved by the surrounding molecules, much like sports fans moving into a stadium for a game.  They have to squeeze together and slow down going through the entrance gate, and then they can move more freely in the space on the other side.  As they move through ramps and hallways toward their seats, they have to slow down moving around corners.  Moving from a narrower hallway to a wider one, all the congestion seems to almost disappear.

People as Air Molecules

Air moves through ducting the same way, but how much resistance do components like elbows and vent caps create?  If we want to get the air to move through the duct at a predictable rate, we need to know stuff like that.  Grille manufacturers are good at providing useful information, providing static pressure and throw at different velocities.  But I don’t know if any vent cap or hood manufacturer that provides that sort of information.  There are some interesting tables (one of which is available in my book Residential Ventilation Handbook) in places like the HRAI training program.  I decided I needed to verify that information.  I needed to do some testing on some hoods.  (I have listed those results on our site with each of the hoods/caps that we sell.)

There are three components to designing a duct run: the actual length of the ducting, the equivalent length of the fittings, and the effective length of the system.  The actual length is the measured distance from beginning to end.  The equivalent length is an approximation of the resistance of each fitting in terms of duct length. And the effective length is the sum of the actual length and the equivalent length.  It is the distance that the air feels as it moves through the system.  So if you are standing there in the attic looking at where the bath fan is installed and where you want it to leave the building, it may not look all that far.  But when you start adding up all the fittings and stuff, 20 feet of actual length approaches 100 feet of effective length in a hurry.

And looking at a table like this one, it’s no wonder that it takes so long for clothes to dry in a clothes dryer.  If you’re trying to push 200 cfm through a 4” diameter duct, the air is looking at 2.5 iwg or 625 Pascals for an effective 100 foot run!  Longer drying times mean more energy consumption and greater impact on the fabrics.

Airflow (cfm)

Duct diameter

Pressure in 100 feet duct  iwg/Pa






















Bath fans are certified at 0.1 iwg so it is little wonder that they are not running at the rated flows once they are installed.  But check out what happens to the resistance when you increase the size of the ducting.  A hundred cfm moving through 100 feet of 4” duct experiences 175 Pascals of pressure.  Increasing the ducting to a 6” diameter drops the pressure to 22.5 Pascals!  So if an existing bath fan is tolerably quiet in a home that needs to meet ASHRAE 62.2, it may get there by increasing the duct diameter and improving the path to the outside.  (Note that the sound produced by the fan will decrease as the resistance decreases.)

It is important to realize that these numbers are for rigid, smooth ducting and not flex duct.  Flex ducting is 33 times rougher than galvanized pipe and 100 times rougher than PVC piping.  Fan manufacturers have gotten pretty good at addressing these performance problems and some of the new fans with the EC motors automatically adjust their performance to meet the resistance of the ducting.  (I wish crowds at sporting events would do that!)  But the sound level of even these sophisticated products will increase as the resistance increases, so it is still a good idea to make the duct run as short, straight, and smooth as possible.

Make it easy for the air to get through the ducting all the way to the outside and you’ll have better airflow.  Just think of yourself as an unhappy air molecule the next time you are stuck in traffic with all the other air molecules trying to get to the same place at the same time.