Posts Tagged ‘air currents’

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.


What do you know about your house’s nose?

January 19, 2017

What’s special about an exterior vent hood or cap or (if you want to be technical) termination fitting?  That’s like asking what’s special about a nose?  Without vent caps the air would not leave the house in an orderly fashion.  Just like the air coming out of your lungs.  When your nose is stopped up, it’s hard to breathe.  The same is true with a vent cap.  If a dryer vent cap is full of lint, the air has a hard time getting out of the dryer.  And that’s a shame because it is the movement of air that allows the clothes to dry.  Lint traps don’t always work very well despite the enthusiasm that dryer manufacturers have for them.

wc-series-wall-cap-building-envelope-rainscreen-225x225But I want to tell you about a very special wall cap made by Primex.  This one is meant to be connected to 4″ ducting.  Nothing really special there.  So what is special?  Well, for one thing the 4″ duct is meant to slide inside the throat on this fitting.  As duct pieces are fitted together, the first piece is meant to fit inside the second piece, the second piece inside the third and so on.  Why?  Because if the first piece fits outside the second piece, any gaps or cracks will spill air outside the duct because the pressure is on the upstream side.

What else is special about this vent cap?  The mounting flange and the outside collar are all made of one piece so water can’t come in.  And yet the hood itself can be unscrewed from the flange for cleaning and service.  The flange can remain permanently attached to the wall!

It also has an very good, gravity return back-draft damper and bird screen both of which can be removed (the damper snaps out, the screen has to be cut out).

But the best part is the curve of the hood itself.  This curve gently eases the air out of the end of the duct.  A lot of caps have very abrupt exits and that increases the resistance.  Resistance in these products can be simulated by the number of equivalent feet of straight,


Poor Quality Vent Caps

rigid ducting.  Some hoods can have equivalent lengths of 60 or 70 feet!  This hood has an equivalent length of just 25 feet.  Air has to trundle along the duct, bounce around corners, and rattle away over the corrugations of flex duct.  And when at last it gets to the termination fitting, it is compelled to make one last turn while pushing open the damper and then exit to freedom!  You want to make that as easy as possible.

Oh, one more thing . . . two more things: the cap is made of durable UV-protected polymer resin that lasts a really long time and, two,  it comes in a multitude of colors – white, taupe, black, light gray, tan, and (on special order) dark gray and dark brown.

Think about it. PRMX-WC401

Ice Dams and Soffit Vents

February 25, 2015
Ice Dam

Ice Dam and Soffit Vents


Right now in Massachusetts people are going crazy because of the ice dams on their roofs!  A company has arrived from Minnesota that has steam generating devices and personnel that will climb up on your roof and melt the amazing amount of ice that is collecting in the gutters, weighting them down, backing the water up the roof, and leaking into the ceilings below.  Companies have produced melting products like salt that can be thrown up on the roof to melt the ice.  Why is this happening?

Obviously because there is an enormous amount of snow that has fallen on roofs in Massachusetts!  But this really only highlights a problem that festers every year.  Before we insulated attics we didn’t have ice dam problems.  Heat from the interior of the house passed through the ceiling, heated the attic, and melted the snow evenly.

To save energy (and money) we now insulate our attics so much less heat escapes from the house which is great for a whole lot of reasons.  If air sealing was done prior to the installation of the insulation, reducing the amount of air that moves through the holes in the ceiling, transferring warm, moist air from the house into the attic AND if the insulation is installed perfectly from one edge of the ceiling to the other, there wouldn’t be ice dams either because the entire attic would be almost the same temperature as outside and the snow would melt evenly.

Soffit and ridge vents were the solution to the imperfect installation of the insulation.  The idea is that cold air pours in through the soffit vents and sweeps up under the entire underside of the roof deck and pours out of the ridge vents.  Soffit and ridge vents are there to prevent ice dams!  The building code says that you have to have attic venting.

Soffit and ridge vents are designed to solve the ice dam problem only.  They are NOT going to cool the attic.  The building code is the same in Florida as it is in Massachusetts.  They haven’t had much of a problem with ice dams in Florida.  They do have a problem with hurricanes.

For ridge and soffit vents to do their job inducing an airflow up the entire underside of the roof deck, they have to have an unimpeded path from the soffit to the ridge.  Companies make baffles or chutes to guide the air from the soffit.  These are not easy to install effectively particularly on a retrofit basis.  And when the air sweeps in from the soffit and passes through the fiberglass insulation or blows back the loose fill cellulose, it reduces the insulation value at one of the most critical points in the thermal boundary: the tops of the exterior walls.  Consequently, heat flows up the exterior walls into the attic, melting the snow on the roof immediately before the roof extends out beyond the house where it is exposed to the outside air and cold on the top and on the bottom.  The snow melts, runs down the roof, hits the cold surface and freezes.  More water from melting snow moves down the roof, and collides with the ice dam.  The dam forms a lake above it and the water works it way back in, under the shingles, and drips down onto the ceiling and into the house.

How do you fix this?

Well you can’t do much inside the house until the ice and snow is gone from the roof.  Once that wonderful day arrives, you should try to resolve the problem so that it doesn’t happen again.

  • Before adding insulation to the attic, make sure that as many of the holes between the house and the attic are sealed.  It is a lot easy to do that when they aren’t buried under lots of insulation;
  • Make sure that the soffit and ridge vents are actually open and allow air to flow through them;
  • Install effective air baffles to guide the air from the soffit vents up the underside of the roof deck.  It is very hard to see because it is a narrow, triangular space at the eave and there are nails sticking through the roof deck, but it is vital that the baffles make contact with the soffit and seal the entry air into a pathway to the ridge;
  • Then add insulation to the attic.  Blowing in insulation is like painting the walls.  The hardest part in painting and insulating is the preparation.  Once that’s done, blowing in the insulation is quick and cheap.

You could consider insulating the underside of the roof deck with spray foam.  That is a more expensive process but can be quite effective.  Any of these solutions is cheaper than hiring a company from Minnesota to keep steaming your roof time after time!



Please visit our website at

Still to come: the BPI Home Energy Professional QCI Handbook

Sleepy From Turkey CO

November 26, 2013

I have been forced to leave more houses during energy audits because of gas ovens than for any other reason.  A gas oven burns gas.  Anything that burns can generate Carbon Monoxide or CO.  The combustion fumes move up through100_2776 an opening or chimney that generally vents just below the control panel near the burners.  If you hold your hand there, you will feel the warm, moist combustion air leaving the oven.  If the oven is old, dirty or mis-adjusted, an excessive amount of CO will get produced when the oven is being used.  The CO level is particularly high when the oven is first turned on (commonly over 1,000 ppm)  and should decrease as the system achieves a steady state operation (dropping to around 100 ppm or less).  It is advisable to keep the range hood running and venting to the outside or a window open slightly while using the oven.  CO has about the same molecular weight as oxygen so it is neutrally buoyant, but I would keep the infants out of the kitchen while the oven is warming up.

A well-adjusted gas oven flame should be blue in color, symmetrically shaped, and about ½ inch tall.  A ragged, hissing flame indicates the combustion process is getting too much air.  A yellow orange flame indicates it is getting too little air.  The flame should be continuous along the length of the burner.   If it’s not, some of the ports may be clogged, but make sure the oven is turned off and cool before making any adjustments.

I have always heard that people get sleepy from the tryptophan in the turkey, but I have begun to wonder if it’s the CO from the oven!  Enjoy Thanksgiving, but make sure your kitchen is properly ventilated!

Please visit our website:

Innovations Under the Sun

August 13, 2013

I am really, truly glad that there are new minds getting into the solar and energy efficiency world.  When I got into this Colorado-Mesa-Verde-National-Park-cliff-dwellingbusiness back in 1977, everything was new . . . to me at least.  There were numerous luminaries out there who became my heroes.  The fundamentals of the second law of thermodynamics were true then and are true now, but technology has changed rapidly since I used the heat from the transmitters in a local radio station to heat the basement offices or when I glazed the cinder block south wall of a newspaper to act as a massive, passive solar collector.  When the oil embargo hit the country and we waited in line to buy gasoline, the fear was that we were going to be cut off from fuel for heating our homes and a massive amount of innovation took place.  We called the homes Passive Solar Homes, but the adjective, solar, was dropped when the house as a system concept took hold.  The thing is that there was a vast amount of innovation being done, and that technology is out there and available and we shouldn’t go about reinventing the wheel unless you want to make the same mistakes again.

There is a fantastic book on “The practice of the Art of Ventilating Buildings” written in 1891 that you can read on-line: Since this was written prior to the use of electricity in homes, William Buchan’s solutions are all passive.

More recently is the work of Bill Shurcliff.  Dr. Shurcliff was born in 1909 and worked on the atomic bomb.  He died in 2006 with 19 patents to his name.  In the later part of his life he turned his attention to solar energy and energy efficiency and would attend monthly luncheons at MIT.  As a prolific writer, he produced a number of amazing books on the subject many of which are available on Amazon:

Solar Heated Buildings of North America – 120 Outstanding Examples, 1978

New Inventions in Low-Cost Solar Heating – 100 Daring Schemes Tried and Untried, 1979

Air to Air Heat Exchangers for Houses, (personally published in 1981)

Thermal Shutters and Shades, 1981

Super Insulated Houses and Double Envelope Houses, 1981

Ned Nisson also wrote a book on super insulated houses: The Super Insulated Home Book.  Ned was the editor of Energy Design Update.  There are a bunch of other books on super insulated houses that should be reviewed.  Certainly insulation technology has changed since 1985.  We thought that urea-formaldehyde was the greatest thing going.

The Solar Home Book by Bruce Anderson the editor at Solar Age magazine was my Bible.  My copy is barely holding together, and I still use it.  The Passive Solar Energy Book by Ed Mazria (who was rumored to have played for the Knicks) concentrates on the solar aspects of the design of homes.  Check out Ed’s 2030 Challenge:

And there is an amazing book called The Solar Energy Handbook written by Henry Clyde Landa, Mariann Cox Landa, Juliet Marie Landa, and Douglas Cox Landa.  I have a hand typed copy of this that has information in it that I can’t find anywhere else.



Steve Baer of Zomeworks invented some amazing things.  His Beadwall product used polystyrene beads to fill up the cavity between two panes of glass in evening, and a vacuum extracted them in the morning.  (I still have a Beadwall tank in my garage that I never had the courage to install.)

There were Waterwalls and Earth Tubes and inflating window shades and Thermol 81 and Ecosea houses and Amory Lovins’ solar briefcase.  Some of these technologies did not survive because they were not good, but many good technologies were lost because the companies were under-funded or run by enthusiasts and not business people.  It would be great if we could develop a building science resource library so that the technologies could be tapped into and the many mistakes that were made would not be repeated.  There has always been a struggle between the passion for improving the environment and developing, producing, and selling products.  Passivhaus seems to have been able to transcend the struggle.


Please visit our website for some innovative products:

No Problem Here Under the Lampost!

September 21, 2012

I remember a story I heard when I was a kid about a man who comes upon another man obviously searching for something under the light from a lamppost on the street corner.  He asks the obvious question, “Lose something?”  The second man looks up and replies, “Yup.  I lost my keys.”  The first many asks, “Where did you lose them?”  The second man says, “Down the street a ways.”  Puzzled, the first man asks, “If you lost them down the street, why are you looking for them here?”  The second man looks up and replies, “The light’s better here.”


I was doing a duct test on a new house this week and couldn’t get the ducts up to pressure which generally means something’s

Where’s the Hole?

wide open – I missed covering a grille or the grille tape blew off or there’s a missing duct connection.  After checking all the obvious possibilities, sure enough, down in the crawl space, on the back side of the duct board trunk on the return, a piece of duct board had been left off.  It was just missing.


It wasn’t a bad contractor just being lazy.  It was just a mistake.  Mistakes happen.  Looking at the system, everything looked fine.  The missing piece was hidden, on the backside of the duct.  The system was working great, satisfactorily cooling the house since it was that time of year.  So a conscientious HVAC contractor wouldn’t have caught it by commissioning the system.  It would have been that way for the life of the system – a hole the size of a large pizza in the duct work, sucking air in from the crawl space. Without testing, that problem wouldn’t have been found.  And this is certainly not the first time I’ve seen this, and there are lots of people who test lots more ducts than I do.  It’s mind boggling.  It’s one of those weird LOVE/HATE things: I love finding this things, but I hate that they exist.


Contractors complain to us all the time about having to comply with new codes and regulations and those air heads in Washington who make all these new rules.  “But go ahead.  Do your testing.  Waste of time, though.  I don’t see any problems . . . here under the light.”

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.

The Pressures of a Stinky Mind

September 3, 2012
Condo Entrances

Condo Entrances

When you’re living right next to someone and they like to smoke and you don’t, the situation can sometimes get nasty.  When I was in college, I lived over a guy who worked as a DJ on weekends and he had some pretty powerful stereo equipment that would drive me crazy at times.  I remember one night flying feet first through his door in frustration!  The good thing about sound transfer between units, however, is that you can clearly shut it off.  Smoke is something else.  It floats around on the air currents, and the odors embed themselves into the materials of both homes.

I was asked to assist in the amelioration of a project where the neighbors were just short of coming to blows in the form of attorneys.  Two, moderately new, side-by-side condos located on the edge of a golf course.  The smokers live in a two-story end unit.

  • Finished basement with a walk out patio under a deck where a smoker likes to sit and watch the golf course;
  • An attached garage on the front where a smoker occasionally likes to sit with the garage door open;
  • A eating area next to the kitchen next to a sliding door to the outside;
  • A loft and bedroom area on the second floor.

The non-smoker unit next door is a single story with a finished basement.

  • Owner’s study backs up to the sitting area by the kitchen next door;
  • Cathedral ceiling in the master bedrooms mates to the smoker’s unit;
  • Basement bedroom, bathroom and utility area mate to the finished basement next door.

I wanted to get a handle on what causes the pressure differences between the units so I measured the airflows of the bath fans.  Both units have recirculating range hoods.  Both units are heated and cooled with conditioned air systems.  Both units have clothes dryers vented to the outside.

The smokers’ unit has four bathrooms with four fans with a total measured airflow of 153 cfm if they were all running at the same time.  The condos were built attempting meet Energy Star requirements and so the 30 cfm fan in the Master Bath is controlled by a timer to cycle on and off throughout the day.  The non-smokers’ unit has three bathrooms and three fans for a total measured airflow of 121 cfm.

The worst-case pressure difference occurs when both units are completely closed up (winter conditions) and all the exhaust systems are running in the non-smokers’ unit and none are running in the smokers’ unit.  So I set up my manometer to measure the pressure in the smokers’ unit with reference to the pressure in the non-smokers’ unit under worst-case conditions, first running the bath fans, adding the clothes dryer, and finally adding the air handler.  The pressure in the non-smokers’ unit increased to – 2 Pa WRT the smokers’ unit.

In order to determine how much pressure would be required to tip the pressure balance the other way, lowering the pressure in smokers’ unit, I installed the blower door in the smokers unit and increased the flow through it to 275 cfm, when the pressure difference was continuously negative.

To reach that level we could have simply installed a 275 cfm fan running continuously in the smokers’ unit, but I chose a combination approach.  I hardwired the upstairs bath fan in the smokers’ unit to run continuously at 45 cfm and installed a quiet 200 cfm fan in the ceiling of the loft also running continuously.  I also installed an exhaust fan in the ceiling of the garage near where the smoker likes to sit and wired it to a motion detector.

Passive HRV

In the non-smokers’ unit, I installed a passive heat exchanger (an HRV without fans).  I connected the 30 cfm exhaust from the downstairs bath to the exhaust side of the exchanger and installed 140 cfm, in-line supply fan to the intake side.  Although totally unbalanced, the heat exchange is enough to temper the air coming into the non-smokers’ home through the return side of the duct system.  As a further precaution, the condo management folks renewed their efforts to seal any penetrations in the wall between the two units.  Going back to retest the units after the equipment had been installed demonstrated that the pressures were now consistently in the right direction as long as the systems were not defeated and turned off.

The non-smoker, however, swore that he could still smell the smoke.  He stated that the air in his study hurt the back of his throat.  I suggested that what he needed to do was to empty out the small room, have the carpet cleaned or replaced, and thoroughly air the space out.  He was not thrilled with the advice.  Once that odor has embedded itself into the materials no amount of pressure difference and changes of airflow will get rid of it.  And even with the cleaning and airing out, the odors are glued to the occupant’s consciousness and may never go away.  It’s tough to change the pressure differences in a mind!