Posts Tagged ‘ASHRAE 62.2’

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.

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Remembering Common Sense

March 23, 2015

A house is meant to be a shelter from the weather, a small, controlled subsection of the planet earth where a family can live safely and comfortably. Caves worked but they were hard to keep warm. But they mostly kept the rain off and blocked some of the wind. House design has advanced over the years becoming safer and more protective. Most of the time. There have been problems with chimneys, for example. A hole in the roof works to let some of the smoke out, but it was an improvement to enclose more of the smoke and guide more of it out. But there was a problem with wooden chimneys. Common sense dictated that chimneys be built of fireproof materials. In fact, many of the improvements in building science were dictated by common sense, wisdom, and skill. The problem came into it when unskilled builders decided that it couldn’t be all that hard and there was money to be made by ignoring some of the details. So rules and codes and standards were created.

Now there isn’t anything inherently wrong with having rules and codes and standards. The problem is that the focus tends to drift from why the rule or code or standard was created in the first place to developing rules and codes and standards just to regulate the rules and codes and standards. Let’s face it: we’re not perfect. And our rules and codes and standards won’t ever be universally perfect either no matter how hard we tweak and tinker and debate. Some people like a airconditioned thatchlittle more salt on their meat and some a little less. And some don’t like meat at all. One rule that covers all the ways to eat a steak simply wouldn’t work. We could have committees and conferences and technical papers ad nauseam but we would still never come up with the perfect rule. When a committee or a society or a club self-perpetuates by simply constantly making changes to a set of rules, the original point is lost. No doubt we are learning more and things change, but we’ve lost the link to common sense. There is no room in our rules or codes or standards for the application of common sense! And we need to just stop and try to remember why the rule or code or standard was written in the first place!

What is the fundamental, bottom line point for the existence of the ASHRAE 62 Standard, for example? (Having been on that committee for over ten years now, I feel that I have a right to use it as an example.) The Standard says, “This standard defines the roles and minimum requirements for mechanical and natural ventilation systems and the building envelope intended to provide acceptable indoor air quality (IAQ) in low-rise residential buildings.” That sounds pretty reasonable. The basics of the standard are great – segmented and detailed to define important stuff. Why can’t we just finish it? Maybe tweak it a little once in a while as we learn more and technology improves. But a huge amount of brain power and hours of discussion and tons of paper go into the constant adjustment of the standard.

When a 747 is landing, it is important for the pilot to line the plane up with the runway accurately so that that little or no adjustment is need to keep the plane rolling straight when it touches the ground. At those speeds, any moderately radical change of direction would be disastrous. A consensus standard is the result of general agreement about diverse views. Can you imagine what would happen if a 747 was landed by a committee? A compendium of diverse views doesn’t always allow room for common sense.


If you are planning to challenge the BPI Quality Control Inspector’s certification, you might find the Quality Control Inspector’s Residential Handbook helpful. Publishing date is June 1, 2015.  Add your name to stay in touch.  Thanks.

QCI Handbook Cover copy

Should a homeowner have control of the ventilation system?

January 25, 2014
Brightened Circuit 2

Sophisticated Control

Allison Bailes started this discussion on his Energy Vanguard site.  (Go to http://bit.ly/LRL43Q)  I was going to respond there, but there wasn’t enough room.  I used to build sophisticated controls that would do all sorts of wonderful things, but they got complicated and expensive.

You can feel the heat from a heating system.  You can feel the coolth from the air conditioning system.  You can see the change in daylight and know when you should turn on the electric light.  You can’t see or smell radon or carbon monoxide or PM 2.5 particles.

Heat is needed when it’s cold.  Cooling is needed when it’s hot.  Ventilation is needed . . . when?  When the bathroom is smelly?  When the bacon burns?  There is no one, single marker or flag for mechanical ventilation.  If there was, it would be simple to answer the question, “Should a homeowner have control of the ventilation system?”

So if a homeowner is going to control his or her ventilation system, how would he or she do it?  Manual control through an on/off switch perhaps coupled to a light in a bathroom?  This approach is equivalent to an occupancy sensor.  I did some tests of ventilation controls a number of years ago, and a manual control like that had exactly the same impact on the humidity in the bathroom as having no fan at all.  No impact.  Might as well not have a bath fan as far as humidity is concerned if you’re going to control it with a light switch.  It might have some impact on methane, but I don’t have the data on that.

Manual ventilation control will not work well because we can’t tell people when they should turn the fan on and when they should turn it off.  And when (or if) they ever turn it on again.

So that leaves the alternative of automatic control.

A standard humidity control will turn the fan on when the humidity rises above the set point.  What’s the set point? 70% RH (like 70 degrees F)?  55% or 30%?  Do you change the set point seasonally?  Do you change it on the same days every year like the change in daylight savings time (or putting fresh batteries in the smoke detectors)?  Will the fan run all the time in hot humid weather?  In my control tests, a humidity control that was set to turn the fan on at 43% RH and off at 41% RH ran for 20 minutes on the day that I tested it.  If I had set it to turn on at 41% RH and off at 38% RH on the same day, it would have run for 9 hours.  Relative humidity is difficult to explain under any conditions, but constantly adjusting the RH set point is not an effective way to control the ventilation system.

CO2 might be good for occupancy, but it is certainly not the only reason to ventilate a house.  I built a ventilation control that used a mixed gas sensor.  We called the “Flatustat”.  Works great.  The one in my bathroom has been operational for the past 20 years or so.  We could create a control that responded to a any number of IAQ conditions, but they would be expensive, and it is difficult enough to get people just to invest in mechanical ventilation in the first place.  Price is definitely a barrier.

So how about quasi-occupant control with a timer?  How should it be set?  The ASHRAE 62.2-2013 Standard says that if you’re going to run the fan half the time you need twice the airflow.  If you’re going to run the fan one third of the time, you need three times the airflow.  If you’re going to go beyond a three hour on/off period, you’re going to need to do some more calculations which depends on the ventilation effectiveness and air turnover and the fan gets really big.

But why do that?  The energy saved for most systems by shutting them off for part of an hour or even three hours, is small.  You could save energy by shutting off your clock when you weren’t looking at it. You could save energy by shutting off your doorbell when you weren’t expecting company.  Doorbell transformers use power just sitting there.

So why not just size the fan to meet the 62.2-2013 Standard and let it run all the time?  There is some weird psychological barrier to this really simple, basic, least expensive and logical solution.  The Standard says you have to give the occupant control so they can shut it off.  It’s their house.  They should be able to shut things off that they don’t want running, but there probably should be a sign warning of the consequences if they do that.

Someone once told me that the first thing many people do when they walk in the door of their home is to turn the TV on.  Maybe the ventilation system should be controlled by the same switch.  Turn on the TV.  Turn on the ventilation system.  I’m glad that wouldn’t work for everybody.

You could think about the ventilation system as a scuba tank.  When you’re under water, you wouldn’t want to shut your air off for any period of time.  When you’re in a house (a contained volume of air that is continuously being polluted by waste air from people and possessions), you’re effectively under water.  Don’t shut off your air.  Keep it simple.  Take a deep breath.  It’s okay to let it run.

Check out our website: http://www.heyokasolutions.com/

Coming soon: Average and Effective Air Change Rates: One Limburger at a Time

Should a homeowner have control of the ventilation system?

January 24, 2014
Brightened Circuit 2

Sophisticated Control

Allison Bailes started this discussion on his Energy Vanguard site.  (Go to http://bit.ly/LRL43Q)  I was going to respond there, but there wasn’t enough room.  I used to build sophisticated controls that would do all sorts of wonderful things, but they got complicated and expensive.

You can feel the heat from a heating system.  You can feel the coolth from the air conditioning system.  You can see the change in daylight and know when you should turn on the electric light.  You can’t see or smell radon or carbon monoxide or PM 2.5 particles.

Heat is needed when it’s cold.  Cooling is needed when it’s hot.  Ventilation is needed . . . when?  When the bathroom is smelly?  When the bacon burns?  There is no one, single marker or flag for mechanical ventilation.  If there was, it would be simple to answer the question, “Should a homeowner have control of the ventilation system?”

So if a homeowner is going to control his or her ventilation system, how would he or she do it?  Manual control through an on/off switch perhaps coupled to a light in a bathroom?  This approach is equivalent to an occupancy sensor.  I did some tests of ventilation controls a number of years ago, and a manual control like that had exactly the same impact on the humidity in the bathroom as having no fan at all.  No impact.  Might as well not have a bath fan as far as humidity is concerned if you’re going to control it with a light switch.  It might have some impact on methane, but I don’t have the data on that.

Manual ventilation control will not work well because we can’t tell people when they should turn the fan on and when they should turn it off.  And when (or if) they ever turn it on again.

So that leaves the alternative of automatic control.

A standard humidity control will turn the fan on when the humidity rises above the set point.  What’s the set point? 70% RH (like 70 degrees F)?  55% or 30%?  Do you change the set point seasonally?  Do you change it on the same days every year like the change in daylight savings time (or putting fresh batteries in the smoke detectors)?  Will the fan run all the time in hot humid weather?  In my control tests, a humidity control that was set to turn the fan on at 43% RH and off at 41% RH ran for 20 minutes on the day that I tested it.  If I had set it to turn on at 41% RH and off at 38% RH on the same day, it would have run for 9 hours.  Relative humidity is difficult to explain under any conditions, but constantly adjusting the RH set point is not an effective way to control the ventilation system.

CO2 might be good for occupancy, but it is certainly not the only reason to ventilate a house.  I built a ventilation control that used a mixed gas sensor.  We called the “Flatustat”.  Works great.  The one in my bathroom has been operational for the past 20 years or so.  We could create a control that responded to a any number of IAQ conditions, but they would be expensive, and it is difficult enough to get people just to invest in mechanical ventilation in the first place.  Price is definitely a barrier.

So how about quasi-occupant control with a timer?  How should it be set?  The ASHRAE 62.2-2013 Standard says that if you’re going to run the fan half the time you need twice the airflow.  If you’re going to run the fan one third of the time, you need three times the airflow.  If you’re going to go beyond a three hour on/off period, you’re going to need to do some more calculations which depends on the ventilation effectiveness and air turnover and the fan gets really big.

But why do that?  The energy saved for most systems by shutting them off for part of an hour or even three hours, is small.  You could save energy by shutting off your clock when you weren’t looking at it. You could save energy by shutting off your doorbell when you weren’t expecting company.  Doorbell transformers use power just sitting there.

So why not just size the fan to meet the 62.2-2013 Standard and let it run all the time?  There is some weird psychological barrier to this really simple, basic, least expensive and logical solution.  The Standard says you have to give the occupant control so they can shut it off.  It’s their house.  They should be able to shut things off that they don’t want running, but there probably should be a sign warning of the consequences if they do that.

Someone once told me that the first thing many people do when they walk in the door of their home is to turn the TV on.  Maybe the ventilation system should be controlled by the same switch.  Turn on the TV.  Turn on the ventilation system.  I’m glad that wouldn’t work for everybody.

You could think about the ventilation system as a scuba tank.  When you’re under water, you wouldn’t want to shut your air off for any period of time.  When you’re in a house (a contained volume of air that is continuously being polluted by waste air from people and possessions), you’re effectively under water.  Don’t shut off your air.  Keep it simple.  Take a deep breath.  It’s okay to let it run.

Check out our website: http://www.heyokasolutions.com/

Coming soon: Average and Effective Air Change Rates: One Limburger at a Time

Measuring Airflow through a Return Side Tap

March 16, 2013
Image

Averaging Flow Sensor

I’m putting together an advanced on-line residential ventilation course for GreenTrainingUSA, and in the process a couple of issues came up.  One of these is testing airflow on the supply pipes to air handlers.  This popular approach includes a pipe from the outside of the building attached to the return side of the air handler.  The opening can be controlled by a damper that opens when the air handler turns on and air is drawn into the air handler along with the return air from the house.  It gets circulated around the house, blended with the other air.  Some of these systems include a control that monitors the run time of the air handler.  If the air handler does not run long enough to satisfy the ventilation requirement, the control restarts the air handler just for ventilation.  Some of these controls also monitor the outside air humidity and temperature, overriding the ventilation operation if it is too cold or too humid outside.

This approach effectively puts the house under positive pressure.  The pipe to the outside is like a hole in the return side of the HVAC system and since more air is being sent to the house than is being removed from the house by the air handler, the house is effectively under positive pressure.  The question is: How much air is being drawn through that pipe into the house?

It is often difficult to measure the flow into the system from the outside of the house either because of the location of the exterior hood or because of the irregular surface of shingles or clapboard or brick.  A measuring instrument that is sensitive to air motion is likely to be impacted by air movement on the outside of the house.

Drilling a hole in the ducting and measuring the pressure when the air handler is running, will provide the static pressure in the ducting.  Let’s say that produces – 5 Pascals of pressure.  The duct can then be disconnected from the exterior hood and a duct tester can be attached to the pipe.  Turn the air handler on again and then turn on the duct tester fan and increase the flow until there is – 5 Pascals of pressure.  At that point, the air moving through the duct test fan will be equal to the air moving through the ducting when the system is operating under normal conditions.

A simpler way to approach this is to get a small flow station and insert it into the ducting.  I recently found some remarkably inexpensive ones from Dwyer (Series PAFS-1000).  The one for 4” ducting is $7.25 and the one for 6” ducting is $8.50.  These work like simple pitot tubes.  You need to drill a 7/8” diameter hole for the sensor.  Insert the sensor into the duct and attach Channel A of your manometer to the two taps and read the pressure.  Use this process:

 

Air movement through a duct measured with a pitot tube (or averaging flow sensor):

VP = TP – SP (ΔP in Pascals)

FPM = SqRt(VP) x 253.29

CFM = A x FPM

Where:

VP = velocity pressure in Pascals

TP = total pressure in Pascals

SP = static pressure in Pascals

FPM = feet per minute

CFM = cubic feet per minute

A = area of the duct in square feet

So, for example, if you get a pressure reading of 1.3 Pascals and it is a 4” diameter duct, because you are looking at the pressure difference between the two ports, that is the velocity pressure or VP.  The FPM velocity is the square root of 1.3 times 253.29 or 329 feet per minute.  The area of the duct is 0.087 square feet, so the CFM equals 29.  This process is not perfect, but it’s a lot easier than trying to measure the flow from the outside or the duct tester approach.

We need to get better data on the performance of this ventilation approach.   29 cfm wouldn’t meet many of the ASHRAE 62.2-2010 requirements even if the air handler was running 24/7.

Visit our Website at http://www.HeyokaSolutions.com

Ventilation and Health

January 31, 2013

Sleeping StudentThere are a lot of grumblings about a requirement to add mechanical ventilation to homes.  When we gather around the campfire on a starry night, there isn’t a need for mechanical ventilation.  When we bring the campfire into the building, we a flue to take the smoke and pollutants out.  And we also bring a lot of other things into the house including the dogs and cats.  And we cook in the house and generate a lot of moisture by taking showers.  We even bring plants inside to make it feel like we were still outside.  And we tighten up the house to keep it warmer in winter and cooler in summer.  So we’re not out under the starry, starry night any longer.

Okay, but is the indoor environment really hurting us if we don’t have mechanical ventilation?  In a dry climate like Nevada, is there really enough of a humidity problem to need mechanical ventilation?  Although it is difficult to study the relationship between mechanical ventilation and health, studies do exist.  Just this morning on MSN there was an article about “20 things that are making us dumber”.  Along with “Honey Boo Boo” and excessive email that we have to check on our “SmartPhones” every 90 seconds, was “Poor Ventilation”.  The article said, “Excess carbon dioxide is doing more than wrecking our climate – it’s also making us stupid.  Researchers at Lawrence Berkeley Livermore slowly increased the amount of CO2 in a poorly ventilated room while college students attempted to solve a series of complex, strategic problems.  Not surprisingly, the more CO2 in the atmosphere, the more mistakes the test subjects made.”

In a paper in Indoor Air entitled “Ventilation rates and health: multidisciplinary review of scientific literature”, a group of very knowledgeable researchers reviewed 27 papers published in peer-reviewed scientific journals.  Their conclusion was that “there is biological plausibility for an association of health outcomes with ventilation rates”.  They also said that, “Home ventilation rates above 0.5 air changes per hour (h-1) have been associated with reduced risk of allergic manifestations among children in a Nordic climate.”

In another Indoor Air paper entitled “Association between substandard classroom ventilation rates and students’ academic achievement”, researchers from The University of Tulsa, the Illinois Institute of Technology, and the National Institute for Health and Welfare in Kuopio, Finland studied the relationship between classroom ventilation rates and academic achievement.  They determined that, “There is a linear association between classroom ventilation rates and students’ academic achievement”.

Pine Tree air freshenerSomething else to think about.  The global market for air fresheners is forecast to reach US$8.3 billion by the year 2015. Growing consumer inclination towards fragrance products such as candles for decorating their homes is poised to propel the air fresheners market.  [Global Industry Analysts, Inc.]  (Candles also put out CO.  I’m just saying.)  Air freshener advertising puts people in rooms with wrestlers and brags about how fresh it smells!  If there are no pollutants in our air, why are we spending so much money to cover them up?  In an article in the University of Washington News entitled “Toxic chemicals found in common scented laundry products, air fresheners” author Hannah Hickey says, “”Be careful if you buy products with fragrance, because you really don’t know what’s in them. I’d like to see better labeling. In the meantime, I’d recommend that instead of air fresheners people use ventilation, and with laundry products, choose fragrance-free versions.”  Her study showed, “58 different volatile organic compounds above a concentration of 300 micrograms per cubic meter, many of which were present in more than one of the six products. For instance, a plug-in air freshener contained more than 20 different volatile organic compounds. Of these, seven are regulated as toxic or hazardous under federal laws.”

I have a control on my own ventilation system that uses a sensor that was developed in Japan to react to the chemical soup emitted by cigarettes.  No one smokes in my house, but there have been numerous times when the control has activated the fan when I can’t smell anything foul in the air and I have to wonder why it is operating, but I’m glad it is.  Our homes are full of stuff that we shouldn’t be breathing.  Is it worth betting your long term health and that of your children to save the cost of a fan?

(Copies of these articles are available upon request.)

http://www.HeyokaSolutions.com

Don’t Blame the ASHRAE 62.2-2010 Standard

January 17, 2013

I have to interrupt my series on Homeowner’s Energy efficiency to say a couple of things about the ASHRAE 62.2 Standard.ASHRAE Guy

Change is always a problem.  The fact is that any change requires some rethinking and relearning.  Building science is changing all the time.  We are learning more.  We have better tools and better materials.  Buildings are getting more energy efficient and tighter.

The fact is that the ASHRAE residential ventilation standards have changed regularly.  The Standard is on a three year update schedule.  What is being referred to as the “simpler” ASHRAE 62-89 Standard is one, very small component of that standard referring to the sizing of a whole building ventilation system.  It is sort of like saying a house is simple when you only look at the insulation in the attic.  The fact is that the ASHRAE 62-89 Standard is 26 pages long.  The ASHRAE 62.2-2010 Standard is 14 pages long.

The ASHRAE 62-89 Standard is no longer supported by ASHRAE partly because of its complexity, but if you were to truly follow that procedure you have two choices in sizing the system: The Ventilation Rate Procedure or the Indoor Air Quality Procedure.
There are three steps involved in the first part of the Ventilation Rate Procedure, determining if the outdoor air is acceptable for ventilation:
1.    The contaminants in the air outside do not exceed the levels in an accompanying table and consider the size of the local community whose population is less than 20,000 and there is adequate air monitoring for three consecutive months.
2.    If the outdoor air contains any of the contaminants in the table, you can refer to another table.
3.    If you still can’t determine the quality of the air, you can perform air sampling based on NIOSH procedures.
The next part allows you to treat the ventilation air and suggests how you might accomplish that, and allows you to vary the ventilation rates during certain periods, like rush-hour traffic.
Once you have reached that point, you can refer to Table 2.3 from which you can extract the magical “0.35 air changes per hour but not less than 15 cfm per person” along with a few notes.  Also in this table are the continuous and intermittent ventilation rates for kitchens and bathrooms (which haven’t changed) along with 100 cfm per car in a garage (which is also in the IMC).

If you want to go through the second procedure – The Indoor Air Quality Procedure – you’ll need to find a copy of the 62-89 Standard.

Note there is nothing in the 62-89 Standard about calculating a Building Airflow Standard, Building Tightness Limit, Minimum Ventilation Level, etc.

The two main points of the 62.2-2010 Standard are:
1.    A whole building ventilation system to refresh the air in the house;
2.    Local exhaust ventilation to take pollutants out at the source – bathrooms and kitchens.
To calculate the whole building ventilation rate you need to know the floor area and the number of bedrooms, and you can look at the chart and figure out the CFM to meet the whole building ventilation rate.  Or if you want you can use a pretty simple formula:
0.01 x floor area + 7.5 x (number of bedrooms + 1)

Local exhaust is the same as it was in the 62-89 Standard:
Room            Continuous      Intermittent
Bathroom     20 cfm              50 cfm
Kitchen         5 ACH              100 cfm

Both Standards talk about reducing the impact on atmospherically vented combustion appliances – nothing new there.

When you buy a new combustion analyzer, it has all sorts of capabilities, but maybe all you need it for is to measure CO in the flue and you can stop there.  It comes with a detailed manual and you might even be able to take a course on how to use it to do lots of other things that are built into its sophisticated electronics.  The ASHRAE 62.2-2010 Standard can be used to simply size mechanical ventilation to cover two basic functions and you can stop there.  At the same time it has a lot of flexibility built in to fine tune the ventilation rates for a variety of applications.

If anyone can guarantee me that he or she can build me a house with the right materials and perfect indoor air quality in any location in the U.S. to surround and protect my grandchildren that will never have bad indoor air quality that will affect their health any day of the year as long as they are living there, then I will agree that you don’t need to apply any mechanical ventilation standard.  The ASHRAE 62.2-2010 Standard doesn’t guarantee perfect indoor air quality and states that clearly, but it isn’t complex to apply, and it is constantly under consistent and predictable review and welcomes input for improvement.

http://www.HeyokaSolutions.com

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

50

3”

0.8/200

4”

0.2/50

6”

0.025/6.25

100

3”

3.0/750

4”

0.7/175

6”

0.09/22.5

200

3”

>10.0/>2500

4”

2.5/625

6”

0.3/75

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.

And Another Thing about Residential Ventilation

April 1, 2012

Now that the ASHRAE 62.2-2010 residential ventilation standard is so much in the fore front of residential energy efficiency, I have been trying to explain some of the more complex parts like the Infiltration Credit and the Existing Building Deficit from Appendix A.  I got so fed up with all the bits and pieces of this process that I have developed a spreadsheet that is available on our www.HeyokaSolutions.com website.  All you have to do is plug in the state or province where the house is located, select a city, enter the floor area, the number of bedrooms, and the number of stories.  The spreadsheet can then calculate the basic whole building ventilation rate from that, give you an ‘N’ factor based on the ASHRAE 136 weather tables, and give you a target ACH50 for .35 air changes per hour.  If you enter a starting CFM50, it will then use that information to calculate the adjusted whole building ventilation rate including the infiltration credit, and provide the ACH50 and the ACHnatural.  You can then plug in a Finish CFM50, and will give you the readjusted whole building ventilation rate.

If it is an existing building, you can plug in information about the kitchen and bathroom windows and fans, and it will give you a further adjusted whole building ventilation rate that will allow you to leave the existing fans in place.  It will also calculate the house depressurization based on the fan airflows.

And since the whole building ventilation system can be used intermittently, I have added a tab that can be selected to determine either the size of the fan required or the run-time depending on the desired operating cycle.  This spreadsheet is free because I want people to be able to apply the standard effectively without spending a huge amount of time on the math.  Check it out.

Also available on our website is a ASHRAE 62.2 checklist for analysts and auditors that will allow you to go through a home and determine what needs to be done to meet all the elements of the Standard.  It’s sort of like the whole Standard on one page.  There is also a system documentation sheet that will allow the designer of the system to meet the documentation requirements of the Standard and provide really useful information to anyone that comes along afterwards to check the system as well as information for the homeowner to know what their system is supposed to do and maybe understand why they should shut it off and stuff it full of socks!