Tamarack believes it is important for you to know about indoor air quality and ventilation. Indoor air quality problems have the greatest effect on children, the sick and the elderly. Ventilation is a vital part of all the mechanisms operating in a house to keep the occupants healthy, safe, and comfortable.

On This Page: Articles Relating to Ventilation

• The Meaning of Ventilation What ventilation is and what is is not.

• Ventilation Basics Learn the basic principals of ventilation and the different types of systems available today.

• Best Residential Ventilation Practices Know what your system must have and tips to get the most out of it.

• Ventilation Controllers  What they are and why they are needed.

• Exhaust Only Ventilation?  It's not just the heat loss that costs!

• Performance Testing  How do you know if the system you installed is working?

Other Informative Selections

About Cooling

About Indoor Air Quality

Complete Index

The Meaning of Ventilation

For years ventilation has been the neglected stepchild of HVAC (Heating, Ventilation and Air Conditioning). As buildings have become tighter, "ventilation" has popped up as the buzz word/cure-all. It’s not.

Ventilation used to mean moving air through a building by natural means to cool or refresh. And certainly replacing overheated air in a building with cooler, outside air can bring the internal temperature down. (This is what makes TTi’s HV1000 and TC 1000 products work.)  But "ventilation" has taken on a meaning of cleaning the air, and as ASHRAE (The American Society of Heating Refrigeration and Air Conditioning Engineers) struggles to come to terms with new ventilation standards it is important to understand the needs for air in a home.

Air movement in a home is needed for five purposes: clean air for people to breath, make-up air for the drier, make-up air for the water heater and/or furnace, make-up air for the fireplace, and make-up air for the range hood. All of these can be seen as complete paths from the outside of the house, through the house and back outside.

The engineer’s rule of thumb is to state that if the building operates at .35 air changes per hour, that all of these needs will be satisfied. The fact is that a drier can suck 200 cubic feet of air per minute (cfm) out of the house when it is running. One of these commercial range hoods that are becoming popular in higher end houses can take 1000 cfm. With that level of mechanical exhaust, naturally aspirated appliances such as the water heater or furnace can easily backdraft, bringing carbon monoxide and other harmful gases into the house. Turning another "ventilation" system or exhaust fan on at that point will only increase the problem.

Each "ventilation" component should be considered individually for both supply and exhaust. Sealed combustion appliances, for example, eliminate backdrafting because they are divorced from any other pressure considerations in the house. Dedicated make-up air sources should be supplied to the drier and the fireplace and the range hood. This approach will not only improve indoor air quality but will make these devices work better.

Once that has been accomplished, the amount of ventilation make-up air required for the people to breath comfortably becomes very small and can be effectively satisfied with building leakage.

Air is required in houses for people to breathe and for certain appliances (range hoods, clothes dryers, central vacuums, fireplaces, etc.) to function as designed. Air moving through houses can remove moisture and increase the longevity of the building materials. These various air movements are considered to be "ventilation".

Uncontrolled ventilation is driven by natural forces such as wind and stack effect, generally creating drafty, uncomfortable conditions in the house. As the cost of heating and cooling has increased and housing technology has improved, uncontrolled ventilation has been drastically reduced. In fact, construction details have reached the point where houses are commonly too tight, requiring mechanical or controlled ventilation to keep the occupants healthy.

Ventilation has some rules:

• One cubic foot of air moved out of the house equals one cubic foot of air moved in;

• Three elements are needed to produce air flow: air, a hole, and a driving force;

• High pressure always goes toward lower pressure;

• Ventilation strategies operate by dilution.

There are several basic approaches to mechanical ventilation:

1. Exhaust "only" (relies primarily on exhaust fans and passive leaks);

2. Supply "only" (relies on pressurizing the house, drawing the air in with a fan);

3. Multi-port supply or exhaust (distributes the supply or exhaust points for the fans);

4. Heat-recovery or energy recovery balanced ventilation (uses the heat, cooling or moisture in the exhaust stream to condition the supply stream).

    

Ventilation control can be as simple as "hard-wiring" the fan on so that it runs all the time or as complex as operating the system in response a variety of conditions. But if the ventilation system is sized to move a certain number of cubic feet of air per minute, the system has to run for enough minutes to make it worthwhile. If the ventilation system is only on when the bathroom is occupied, the walls will still be covered with moisture which will be given off to the space over time. If moisture condenses on the windows of the house, there is not enough ventilation. If the air is too dry, there’s too much.

Houses provide shelter from the elements. The air inside them can be controlled to provide comfortable temperatures, humidity levels, odor, particulate, and quality control. People need to breathe fresh, clean air. This is generally accomplished by bringing fresh air in from the outside and expelling polluted air.

The system must:

• Meet the flow requirements;

• Be capable of continuous operation;

• Be quiet (1.5 sones or 50dB or less);

• Be energy efficient (50 watts or less).

•  

According to ASHRAE, the air in the house should be changed .35 times per hour. The ventilation system must be sized to accomplish this. ((House Volume x .35)/60 minutes = cubic feet per minute (cfm)) This approach will require the ventilation system to run constantly at that rate. The fan must be rated to run constantly at the required flow rate. (In general, the flow rate listed on the fan box must be derated by 30 cfm for the typical installation.) Since the ventilation system is designed to be the primary fresh air source for the home, even if it is controlled by a timer, dehumidistat, or other control, it should be of a high enough quality to last at least five years. The fan specifications should indicate that it is capable of "continuous" operation. The ventilation system also needs to be extremely quiet or it will add "noise pollution" and the homeowner will shut it off.

• Exhaust airflow must take the air all the way to the outside of the building.

• Duct runs should be short and as straight as possible.

• Use smooth, rigid ducting whenever possible.

• Slope ducting to the outside to allow any condensation to weep to the outside of the building.

• Ducting running through unheated spaces must be insulated.

• Exhaust hoods must prevent birds or animals from entering.

Minimum system control should be coupled to the bathroom light switch so that the fan will run whenever the bathroom is occupied. Or the fan should be "hard wired" to run constantly. Or the fan should run on a timer or humidity control to extend the run time beyond bathroom occupancy. Inlet air will often be supplied through building leakage. Any combustion appliance, however, should be supplied directly with outside air. Range hoods, clothes dryers, and central vacuums may also need outside air sources.

July 1998

I once asked an electrician why he chose the type of bathroom fan he installed, and he replied "It depends on what kind of car the client drives." This, I fear, is the logic behind most ventilation choices. It depends on what the client can afford and has little, if anything, to do with the solution to a ventilation problem.

As the "experts" in this field, we are partially at fault. The needs for ventilation are poorly defined and often squabbled over - negative pressure or positive pressure, ducted distribution or point source exhaust, continuous running or intermittent. At some point, all of this will shake out, and we will agree on . . . something.

These contentious issues, however, are not the topic of this paper. My concerns here are: first, if the ventilation system is to be operated intermittently, what are the options for accomplishing that, and second, as the building codes move toward prescribed ventilation rates which may lead to in situ testing by building inspectors, what methods and tools are available for practical performance testing?

The traditional, residential ventilation system has three components: building leakage, windows, and the bath and kitchen fans. As the buildings have become tighter, the leakage component has diminished. Windows are rarely used except in warm weather. And so adequate ventilation of the building relies on the bath and kitchen fans. Again, traditionally the kitchen fan is on when smoky, smelly odors are being created on the range. And the bath fan, coupled to the light switch to assure some ventilation, is used when the bathroom light is on. This averages out to somewhere between seven and eleven minutes per day, but who's counting? (Note that the debate about sizing the "cfm" of the bath fan becomes moot if the fan is only operating for a few minutes. A 50-cfm fan running for an hour will move 3000 cfh. A 50-cfm fan running for ten minutes will move 500 cfh.)

This suggests that the simplest and most sensible approach is simply to let the fan run all the time. In fact, Ecotope has demonstrated that the effective ACH can only be reasonably accomplished using a continuously running fan. This, however, requires that there be a continuous source of IAQ pollution in the building (such as a large piece of Limburger cheese) and relies on the durability of the fan to run all the time and the tolerance of the homeowner to let it run all the time.

Operating the fan with the light switch is not a bad starting point for intermittent control. It is reasonable to assume that the bathroom will be occupied if the light is on. The ideal control, however, would be one that would operate when, and as long as, ventilation was needed. (It has been "tongue-in-cheek" suggested that the ventilation system should be coupled to the television as the occupancy sensor. When the TV is operating, someone is home. Quite often the TV is operational the entire time someone is home.)

Several more scientific indicators have been suggested for occupational sensing: time, humidity control, CO₂ control, motion control, Oxygen level, or "IAQ" control. A variety of these controls have been summarized in the accompanying table.

Timers- A variety of timer controls are available. Mechanical "crank" timers can be cranked around to a number of minutes and then allowed to tick away until the prescribed amount of time has elapsed and the control snaps back to its off condition. These controls are relatively inexpensive and simple to install and simple for the homeowner to understand. If the timer is cranked on just before the user leaves the bathroom, the clicking noise can be left behind. However, homeowners often forget (or want to forget) to use these controls.

"Pin timers" are twenty-four hour clock timers (similar to plug-in light timers) that can be set to operate the fan at prescribed periods throughout the day. These can be set to operate the system during anticipated periods of occupancy, and they require no input from the homeowner. Their drawbacks are that they rely on continuous electricity to maintain their real-time clock setting, and they rely on the homeowner not to pull the pins out to defeat the system.

Electronic timers come in a variety of forms. These can be set for specific intervals, real time operation, or different flow rates. With the sophisticated state of electronics, these controls can be designed to operate in a large variety of ways, and this can sometimes be confusing to the installer who has to program the device. They are more expensive than their mechanical brothers are, but they can be more flexible to the situation, solving a wider range of potential problems.

There are a variety of time-delay switches. When the switch is turned off, the fan will continue to run for a prescribed amount of time. These can be frustrating for the consumers who don't quite understand why their fan is still running after they turn it "off". Once understood, however, they do lengthen the amount of time that the fan is running to dissipate the moisture.

Humidity control - The rationale behind operating a fan in response to humidity is multi-fold. Humidity can be a damaging factor in building construction. When the indoor RH exceeds the dew point of building surfaces, the moisture in the air will condense out. But the RH of the air is relative to the temperature of the air. A fan controlled by a dehumidistat could run all the time during a period of hot, humid summer weather. This has given rise to differential dehumidistats, which can sense the difference in RH between two points, and to "rapid-change" dehumidistats that will respond to "rapid" changes in RH. The difficulty here is determining what "rapid" means.

CO₂ control - People breathing in a building can increase the CO₂ level. So the CO₂ level can be used as an occupancy sensor. This has proven to be a reasonably accurate indicator of the IAQ condition and has been effectively applied to commercial buildings. These controls and sensors have been expensive to date, and the measured level of CO₂ tends to drift over time, requiring regular recalibration, which is impractical on a residential basis. A solution to this final difficulty can be achieved with the "rapid-change" approach.

Motion control - A simple motion sensor can be used to determine if a room is occupied. These are more commonly used for alarm circuits but can be applied to a ventilation strategy. Another manifestation of this is a counter that counts the people entering the room or home and subtracts the people leaving. With multiple outside doors, etc., this can be an overly complex strategy.

Oxygen and IAQ control - There are several, fairly expensive controls available which utilize "gas" sensors. Some are claimed to measure the changes in oxygen level. By setting the static level of "good" IAQ, changes from that level (increasing or decreasing) might indicate a need to ventilate. Some of the gas sensors have been designed specifically for IAQ control particularly for sensing cigarette smoke. With an associated microprocessor, these can be tuned to respond to residential air quality conditions.

A control for increased levels of radon has also been used for ventilation control. These should be directly associated with the venting of the radon gas and not the general ventilation of the home or basement. Increasing the negative pressure in the building can actually increase the radon level by drawing in more radon gas.

 Timers in a Nutshell

Exhaust-only Ventilation?

October 1999

According to ASHRAE the air in a house should be changed .35 times per hour.  In order to be sure that a house is operating at .35 ACH in any weather condition, some mechanical ventilation is required. The simplest approach is to size the fan to meet the .35 ACH and run it all the time - hard wire it on! It’s going to cost you something to do that.

Using a "cheap" fan this way would result in an electrical cost of $169.25 per year. (1.4 Amps, 161 watts, running for 8760 hours per year (161 x 8760 = 1,410 kWh) x $.12/kWh = $169.25)  The heat loss from running this fan all the time would cost approximately $36 per year. (House with an annual heating cost of $600 with an infiltration component of 30%. The fan will increase this by approximately 20% or .20 x .30 = .06 x $600 = $36.)  Therefore the cost of ventilating this way would be $205.25 per year.

If you were to replace that inefficient fan with one which uses only 17.4 watts, the electrical cost would plunge to $18.30, and your total cost would be only $54.30 per year.

It is also clear that the ventilation system does not have to operate all the time. At certain points in a day no one is home or the windows might be open or there is simply adequate ventilation without an additional mechanical system running. The fan might have to operate only 1/3 of the time or less. Such a control on the "cheap" fan would save about $137 per year and with the more efficient system, $36.38 per year.

And there are other reasons not to run the fan all the time:

Building longevity - If the fan is running all the time, there are times when the exterior humidity will exceed the interior humidity. The extra moisture will condense on cool surfaces within the building.

Comfort - If the building is too humid or too dry, it will be uncomfortable. The noise the "cheap" fan makes will add to the discomfort.

In any case, it is clear that heat loss is not the major expense in running the fan. It’s the electrical cost. And less than $50 per year is not a lot of expense to keep breathing.

A word about Tamarack's Airetrak Control

Either way Tamarack's Airetrak control can get the job done. The Airetrak-CD can be set to running the fan at a very low speed all the time while the standard Airetrak can be setup to run the fan during the period when the homeowners are home.

July 1998

All right. So you've done all the calculations, you've done all the engineering, and you know exactly what fan system you're going to put in. And you install it. How do you know if it is working? You can tell if the fan is running because you can hear it. In the past it was assumed that the noisier it was the more air it is moving. Some people assume that that is still true.

Fan manufacturers test their fans before they start to market them. HVI (the Home Ventilating Institute) provides laboratory standard numbers on sound and air flow performance. Bathroom fans are rated by HVI at .1" static pressure. The air moved through any fan is relative to the system in which it is installed, and to standardize things, the manufacturer members of HVI decided that a typical system would have a .1" static pressure resistance to the air flow. That is the number that commonly appears on the packaging.

HVI does complete testing, however, and the fan's performance at other levels of resistance is also measured, and those points are plotted on a curve. By looking at that curve, one can tell how the fan performs at .25", for example, which is more typical.

This is an extremely brief overview of this topic just to emphasize the point that the performance of any fan is directly relative to the system in which it is installed. So the only accurate way to know what the air is doing, moving through the system is to measure it. The problem is how.

Duct-blaster - Although awkward, the duct blaster approach, using the duct blaster fan coupled to a flow box with connecting ducting and using a digital manometer, is about the most accurate field measurement process presently available. The duct blaster fan mimics the airflow generated by the bathroom fan where it is measured. It is unreasonable to believe that this process will ever be commonly used by anyone other than building scientists, but it does provide excellent results.

Flow hood - Flow hoods are somewhat simpler to set up and use, but they generally don't like the low flow rates generated by bathroom fans. There is a unique device made by Aldes called the SAM Air Flow meter that is remarkably accurate for low flows. Consisting of a 10" diameter, Plexiglas cylinder, a motorized door is driven over the opening in response to the flow of air over a vane. The final position of the door indicates the actual airflow. These devices are extremely good for exhausting bath fans, but do not produce as reliable results for HVAC systems.

The Hand, the Tissue, and the Garbage Bag - The least expensive approaches have developed folklore of their own. There are those of us who have been measuring airflow for so long that we can swear our hands are calibrated. It's sort of like the folks at a carnival that can guess peoples' weight. Like the flow hood, the accuracy of this method seems to drop off a bit at very low flow rates.

The "Tissue" test is somewhat more scientific. We have noticed that different weights of paper will offer different approximations of airflow when taped to the grille. Two pieces of tape at one end of the paper will secure it to the grille. Three different weights of paper: a standard sized, single ply tissue (peal off from a double ply if necessary), a standard sized piece of single ply paper towel, and an eight and one half by eleven piece of "20 LB" copier paper. Tape the paper to the grille of the fan. When the flow is great enough, the paper is actually drawn up to the bottom of the fan and slaps into the grille. Once the paper has completely covered the opening, the flow rate will need to drop to less than 15 cfm before it will drop away.

The Research Division of Canada Mortgage and Housing Corporation developed the Calibrated Garbage Bag approach. The idea is to stretch a "Glad" 66x 91cm-garbage bag over a hanger to hold the mouth open. Swing the bag around to inflate it, hold the opening over the bath fan, and time the deflation. CMHC calibrated this bag to deflate in 3 seconds at 50 cfm, 5 seconds at 30 cfm, and 12 - 13 seconds at 10 cfm!

When all is said and done, we don't recommend that code be written for garbage bag measurement. But performance testing is likely to come along some day. Better and cheaper approaches to testing of installed products will be critical. At least this is a place to start.

Return to IAQ and Ventilation Index.

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