Posts Tagged ‘ducting’

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

Shades of Gray in Residential Construction Morality

April 27, 2015

I was called in to perform a last minute duct test for a modular home builder.  He was all in a dither to have a duct test and a blower door test done on a Friday so that he could get his Certificate of Occupancy (CO) so the homeowner could move in the following Monday.  He said that he’d just found out that he needed these tests.  The building inspector asked for them at the last minute!

I was glad to do it partially because it’s good to have builders aware of what is going on despite the fact that he might have been warned for the past year or more that the code had changed.  General awareness of these changes take time.  After all, this wasn’t the first house that he had built since the new codes went into effect, but this was obviously the first building inspector who had made him do it.  The 2012 IECC is quite demanding in contrast to the 2009 version, and it is clear that builders can’t just build the way they used to.  The IECC requires 3 ACH50 and ducts that leak no more than 4 cfm per 100 square feet.  This house leaked at over 7 ACH50 and the ducts were at just under 9 cfm per 100 square feet.

So the builder ran around with a caulking gun.  He stuffed paper towel under the basement door.  He pulled off electrical receptacle covers and installed those little foam pads.  And then he looked at me.  This is the point where the rubber meets the road as a Quality Control Inspector.  The house performed better than many houses that have been built over the years.  It wasn’t likely to explode or rot away in a year.  After all, it had been mostly assembled in a factory – indoors where it never rained.  So why didn’t the modular manufacturer get it right?  They could have sealed up the tops of all the wire chases in the attic.  There was air coming up from the marriage wall gap.  Whose responsibility was that?

We called the factory.  They were apparently shocked!  How could air be leaking at all the outlets?  Using a pressure pan I showed the builder which ones were connected to the outside and which ones weren’t.  It wasn’t all of them.  Attitude in the factory came into play.  Maybe someone had been assigned the task of sealing all those holes but ran out of . . . foam?  attitude? time?  Maybe it was Friday afternoon.

Open Panned Return1

Panned Return

Then there were the ducts.  The only return in the house was a large opening in the living room floor where the joists had been panned  down below.  There was a wind blowing up from the basement (outside the conditioned space) when the blower door was running.

We called the HVAC contractor.  “I sealed every joint with mastic!  We do that every time.  I don’t know what could have happened.”  Using the theatrical fogger, it was pretty obvious that they hadn’t sealed every joint.  The filter slot was uncovered and beyond that, it was located in such a manner that the gas pipe and some wires would always make it extremely difficult to change the filter.

By this point, the builder recognized that the house was not going to pass and he was not going to get his certificate of occupancy for Monday.   He told me that he would arrange to have the HVAC contractor back and would spend time sealing and tightening up the house.

Open Panned Return

Vision of the Living Room

A week went by before he called me back.  Now, all of this is unfortunately too common, but it was the second visit that really disturbed me.  On the phone the builder said the ducts had been retested and they were fine.  All he need from me was the blower door test.  I asked to see the duct test results.  He said that the HVAC guy was having trouble with his email, but he sent me a photograph of the test results.  I noticed that the building size was wrong.  The results were remarkably good.  I couldn’t read the signature or the name and there wasn’t a BPI or HERS number.   No, the builder said, you don’t need to retest the ducts.  Just do the blow test.

When I got to the house, the builder was running around with his caulking gun again.  Proudly he showed me how the marriage wall had been foamed in the basement.  He said he had talked to the factory but they really hadn’t done much.  I looked at the ducting.  The section of the floor joist panning was wide open at the end.  You could see the daylight of the grille in the living room.  There was absolutely no way that the testing could have had the results that it did.

While we were in the basement, the HVAC contractor showed  up and started caulking around the floor boots.  If the ducts were so tight, why was he still trying to make them tighter?  I showed him the open panned return.  “Don’t know how that could have happened!  We had a guy who was doing bad work.  I had to let him go.”

I asked him about the guy who tested the ducts.  “Oh, he’s just a guy that works for me.  Does this once in awhile.”

So the duct testing was a lie.  It was a lie by an employee who worked for the HVAC contractor.  The builder accepted it and refused to let me retest the ducts once the HVAC company had worked on them.  He wanted the CO and he wanted to be done with the job.

This situation was obvious: the end of the ducting was wide open.  Without my testing, they would never have known.  The system would have been running that way for its entire existence.  Even with my testing, the builder was willing to accept the results and walk away.  The HVAC contractor was willing to accept the results and walk away and complain about onerous rules and regulations.  The homeowner would have gotten a shoddy product and the building inspector would have received invalid information and had to accept it because he couldn’t recheck the result due to lack of time and money.

If we are going to make this system work and have any value, at the very least there ought to be simple ways to verify the credentials of the people doing the testing.  There ought to be a way for QCI inspectors to ding the contractor or the builder for making stuff up.  I want to believe that this was a learning experience for both the builder and the HVAC contractor and that they will do better next time.  But when I saw those original duct testing results from the HVAC contractor, I didn’t believe them.  Should I have compelled the builder to let me retest?  Obviously the ducting system would have failed miserably.  If it had been a health and safety situation, there would have been no question.  But it was a performance and long term durability question.  Are there shades of gray in residential construction morality?


If you are planning to challenge the BPI Quality Control Inspector’s certification, you might find the Quality Control Inspector’s Residential Handbook helpful.  Scheduled for publication on June 1, 2015.  For updates and a discount on publication, please add your name and email address by clicking on the book below.

QCI Handbook Cover copy

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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.

Unbalanced HRV/ERVs

August 23, 2012


I have been disturbed to find that many HRV/ERVs are installed without any concern about balancing the system.  (Click on HRV Survey for a CMHC study of system installations.)  When I first got started with these things, it was a mandatory step in the process.  They also had to be ducted independently from the HVAC system ductwork.  That is seemingly a rare installation these days.  But if you think about it, in the extreme situation when there is no airflow coming into an HRV/ERV from the outside, the system is working as an exhaust fan.  There is no heat or enthalpy recovery.  In the opposite extreme, when there is no airflow coming into an HRV/ERV from the house, the system is working as a supply fan and there is zero heat or enthalpy recovery.

The efficiency numbers that are provided through Home Ventilating Institute (HVI) testing are at the balanced condition – same amount of airflow in each direction.  If the efficiency of the heat or enthalpy recovery matters, then it should be operated in a balanced condition.  The vast majority of these units have internal fans.  If the HRV/ERV output to the house is connected to the return side of the air handler, when the air handler turns on, it will depressurize the return duct and suck the air from the HRV/ERV increasing the supply flow through the unit.  It turns out that this unbalanced condition has more impact on the house than it does on the efficiency of the HRV or ERV.  If the house is in a heating dominated climate, it may not be advisable to operate it in a higher supply volume configuration because it may force humidity in the house into the building system components.

It is certainly more expensive to provide independent ductwork for HRV/ERVs.  It takes extra work to actually design the ductwork and calculate the resistance.  It takes extra work to measure the flows and balance the system.  It takes extra work to commission the system.  But if houses, especially tight houses, are going to depend on their mechanical ventilation system for good indoor air quality, these are steps that should be taken.

Duct Design and Static Pressure

As the static pressure of the system increases, the fan/blower has to work harder, and the airflow decreases.  For example, blowing 100 cfm through 100 feet of 4” diameter duct has a static pressure of Image0.7 iwg or 175 Pascals.  Increasing the duct diameter to 6” drops the pressure to 0.082 or 20.5 Pascals.  The Effective Length of the ducting is the sum of the Actual Length and the Equivalent Length of the fittings like the elbows and grilles and exterior hoods.

The exchanger unit itself has a high static pressure because of the resistance of the core and the filters.  But that is the pressure that it was designed for and tested at.  If that resistance is much higher than the resistance of the duct work, then the resistance of the duct work won’t make much difference in the performance of the system.  In the case of simple bath fans, for example, if the duct run is so bad getting to the hood, it really doesn’t have a great deal of impact on the flow if it is a restrictive hood!   The damage, as they say, has already been done.

For a “back-of-the-envelope” calculation you can figure that a 90 degree elbow has an equivalent length of 10 feet, a wye fitting with equal takeoffs is also about 10 feet, a tee fitting is about 50 feet, a tapered increaser about 4 feet, a typical exterior supply hood with no back draft damper about 35 feet, and an exhaust hood with a damper about 60 feet.  So if you have an installation with 30 feet of actual length, 4 elbows, and an exterior exhaust hood, you would have an effective length of 130 feet or approximately 0.11 iwg for 6” duct work.  If you have about the same run on the intake side, the hood is less restrictive, so the run is very close to 100 feet.  And then you have the connections to the rooms.  Here is a quick duct calculator from Hart & Cooley, the grille manufacturers: Duct Calculator.  (There is more information on this in my book Residential Ventilation Handbook.)

The effective length can add up quickly, so the installation needs to be thought through for performance not just for convenience.  Since the system needs to be maintained – filter and core cleaning, at least – the exchanger should be located someplace very accessible and not buried in an attic or crawl space.  If you’re going to pay this kind of money for a ventilation system, you want it to work right.  You want it to work as expected.