Posts Tagged ‘Collin County’

Indoor Air Quality

Thursday, October 29th, 2009

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Indoor Air Quality

Indoor Air Quality (IAQ) is more than mitigation of unsavory odors. In times of cheap energy when most homes were not air conditioned, natural ventilation diluted pollutants. Modern energy-efficient homes minimize infiltration and exfiltration through airtight construction and consequently accumulate all manner of unsavory and undesirable things in the indoor environment. Keep in mind, simple controlled ventilation will mitigate poor IAQ, but with an energy penalty. Let’s consider common indoor pollutants.

  • Radio Active Contamination – Radon (Radon-222) , an odorless, inert, naturally occurringradioactive gas is a ‘daughter’ of radium-226. Though not considered by authorities to be a particular threat to health, it can cause bronchial irritation and some authorities say it poses a risk of lung cancer. Despite its half-life of 3.82 days, one must remember that its material source continues to produce it forever. Learn from the EPA how to inexpensively test your home for the presence of Radon: A Citizen’s Guide to Radon.
  • Particulate Contamination – Dust, including pollen, animal dander, dust mite droppings, are commonly occurring pollutants that contribute to odors, respiratory problems and asthma.
  • Moisture Contamination – High relative humidity promotes stale odors and the growth of fungi, mold and bacteria. Acceptable relative humidity lies between thirty and sixty percent; ideally fifty percent.
  • Gaseous Contamination – Carbon Dioxide (CO2) is not a pollutant, per se, but undesirable when concentration exceeds 800 ppm (parts per million), Methane (CH4), Propane (C3H8), Hydrogen Sulfide (H2S): all but the last are undetectable to olfactory senses. CH4 and C3H8 come from gas stoves. H2S is a product of well water, sewer gas and flatulence.
  • Volatile Organic Compound Contamination – common household solvents, hobby supplies, dry-erase markers and others.

Learn more about indoor air pollution from the Environmental Protection Agency (the EPA).

IAQ Dilution Methods

Demand-Control Ventilation (DCV)

Carbon-dioxide (CO2) sensors continuously measure concentration (in ppm). The control systemCo2-Sensors enables outdoor air ventilation when indoor concentration exceeds 800 ppm. Why 800 ppm? ASHARE (the American Society of Heating Refrigeration and Air-conditioning Engineers) recommends limits of 1,000 ppm for schools and 800 for office buildings. Adults exhale CO2 concentration around 35,000 ppm. Over several years, we have determined that the optimum balance between indoor air quality and energy efficiency balances well with 800 ppm.

A suitable exhaust fan and motor-operated dampers form a simple ventilation subsystem. DependingY8150_ResidentialVentilationSystem on return-air duct pressure dynamics, the exhaust fan may be unnecessary. Naturally, induction of untempered outdoor air has an energy penalty. The Honeywell Y8150 Fresh Air Ventilation System is an inexpensive solution when used with a reliable and accurate carbon-dioxide sensor.

Energy-Recovery Fresh Air Ventilation (ERU)

Specialized heat-recover ventilation equipment dilutes all the aforementioned undesirable components without energy penalty. It transfers heat and moisture between fresh and exhaust airER150_HeatRecoveryVentilatorUnit streams. Used in conjunction with a reliable and accurate carbon-dioxide sensor, it provides optimum IAQ at the lowest cost. Perhaps the best product on the market is the Honeywell ER150B and ER200B Energy Recovery Fresh Air Ventilation Systems.

Intelligent IAQ Management

Intelligence is the key to best practices for management of indoor air quality. Rockwall Controls designs and installs intelligent residential control systems that coordinate all mechanical air conditioning equipment with the actual use of the home for optimum IAQ and energy efficiency. 


Balanced Air Distribution

Tuesday, October 27th, 2009

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You’ll Know If It’s Not Balanced

One room runs too cold, another too warm, while another seems to have a stale odor to it. Assuming adequate air conditioning capacity and correctly selected and installed air ducting and grills, the obvious clue to disparate temperatures is unbalanced air distribution.

Air Balance Mechanisms

 ManualHandQuadrantBalancingDamperManually-adjustable dampers either at the grills or upstream in the air ducting control air flow so as to equalize air distribution throughout a zone of control. In the case of a single-zone system that supplies conditioned air to multiple rooms, air balance is critical to even comfort. The test and balance (TAB) technician uses a flow hood instrument to measure discharge air flow. The TAB technician sets air flow at all grills according to a preset TAB schedule prepared by the system designer. Grills with integral dampers are a less costlyAdjustableResidentialGrill solution than manual hand quadrants in the duct, but the latter will generate less noise and it is located as close to the air handling unit as possible.

In the case of a variable-air-volume (VAV) air distribution network, motor-operated modulating dampers inherently balance comfort to match set points of the several zones. To learn more about ModulatingDamper-1VAV, click here.

Properly Designed Air Distribution Network

Tuesday, October 27th, 2009

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Working Back to the Air Handling Unit

All load calculations begin with the spaces served for which the designer calculates the airflow required. An appropriate terminal device, the register or grill, determines the direction of airflow. If too small, it constricts airflow and causes ‘grill noise’.  If too large, insufficient ‘throw’ means short-path airflow patterns that cause hot and cold spots.

Grill Selection

Source: Snips Magazine

(Editor’s note: The following is taken from the Sheet Metal and Air conditioning Contractors’ National Association’s “Residential Comfort System Installation Standards Manual,” seventh edition.)

A residential heating and air conditioning system is only as efficient as its air delivery component. TheGrills quantity and velocity of air movement within space and the proper mixing of supply air with space affect comfort levels.

Supply air should be directed to the sources of greatest heat loss or heat gain to offset their effects. Registers and grilles for the supply and return systems should accommodate all aspects of the supply distribution patterns such as throw, spread and drop; also, the outlet and return grille velocities must be held within reasonable limits. Any noise generated at the grille is of equal or greater importance than duct noise. The diagrams show recommended grille and register locations.

Vertical Spreading Pattern

Vertical Spreading Pattern

The principles of air distribution are discussed in the SMACNA “HVACs Systems – Duct Design” manual and the American Society of Heating, Refrigerating and Air-Conditioning Engineers’ “ASHRAE Fundamentals.” In residential system design, simplified methods of selecting outlet size and location generally are used.

Supply outlets fall into four general groups, defined by air discharge patterns: horizontal high, vertical non-spreading, vertical spreading; and horizontal low. The chart below lists the general characteristics of supply outlets. It includes the performance of various outlet types for cooling as well as heating, since one of the advantages of forced air systems is that they may be used for both heating and cooling. However, no outlet type is best for heating and cooling.

The best outlet types for heating are located near the floor along outside walls and provide a vertical-spreading air flow, preferably under windows, to blanket cold areas and counteract cold drafts. This arrangement, called perimeter heating, causes mixing of the warm air supply with both the cool air from area of high heat loss and the cold air from infiltration which prevents drafts.

High sidewall outlets should deliver the air horizontally or slightly upward during cooling. The throw of a high sidewall outlet should be equal to or not over 30% more than the distance between the outlet and the opposite wall (or effective obstruction to a free air stream) of the room.

The best outlet types for cooling are located in the ceiling and have a horizontal air discharge pattern. For year-round operation, the correct choice of a system depends on the principal application. If heating is of major equal importance, perimeter diffusers should be selected. The system should be designed for the optimum supply velocity during cooling. If cooling is the primary application and heating is of secondary  importance due to mild winters, ceiling diffusers will perform most Ceiling Return Grillsatisfactorily.

Return air grilles should be located in hallways, near entrance doors, or on inside walls to ensure a lowresistance return air path between every room and the return side of the blower cabinet. Return air systems use either central or multiple grille locations.

A central return occupies a minimal amount of space with a short return duct, creating a small return-side pressure drop. In multilevel homes, a central return should be installed on each level. A multiple return system provides for a return opening in every major room and transfer grilles for secondary rooms.

Return air grilles should not be located in bathrooms, kitchens, garages, utility spaces, a space used for storage of fuel or flammable materials, or a confined space in which a draft diverter or draft regulator is located or to which combustion air is supplied.

Return air grilles shall be sized to return 100% air being supplied with air velocities not to exceed 4000 fpm face velocity in order to minimize system noise.


Older residential systems have sheet metal ducting wrapped with insulating material. In today’s economy, tough, durable flexible ducting is my choice. It has far superior sound characteristics than sheet metal ducting and its installation cost is a fraction of sheet metal. Metal ducting must be separately insulated after the duct has been mounted, sealed and suspended, but flexible ducting comes in 25-foot lengths, pre-insulated and ready to go.

Sizing Flexible Ducting

Source: Hart & Cooley

Flexible duct has many advantages in the HVAC environment. Its ease of use DSC04857and timesaving (money) speed of installation compared to hard duct is inviting. But using it as a direct size replacement for smooth, galvanized duct is not one of its advantages due to a difference in performance. Because of flex duct’s unique corrugated construction and flexibility, there is a higher airflow friction loss compared to the same size smooth-walled galvanized duct. Performance equivalent to hard duct requires a larger diameter flex duct.

Friction loss in straight duct is dependent on the relationships of duct diameter, air velocity in the duct, and duct roughness as major components, and to a much lesser degree on air density. As one can imagine, flex duct with its helical corrugations is going to be much “rougher” or less smooth than galvanized duct. This is especially true if it is not stretched out to the extent possible during installation. Slack duct allows the coils of reinforcing wire to relax, which bunches up the polyester and pushes it into the interior of the core, adding more resistance to airflow.

Sizing charts and calculators for duct sizing are available from many sources. Hart & Cooley has a Sheet Metal Duct Friction Loss Calculator on one side of a slide chart with a Flexible Duct Friction Loss Calculator on the other side that we make available. We also have an interactive flex duct calculator on our web site. Spending a few minutes with these aids can quickly demonstrate the differences between the friction losses for galvanized verses flexible duct. It is worth noting that for a fixed duct diameter, as the velocity in the duct increases, the friction loss increases twice as fast. So if the velocity were to double, the friction loss would be four times greater! A handy rule that is very effective and reliable is to increase the size of flex duct one diameter to neutralize the added friction loss compared to that of galvanized duct for the same CFM.

A further penalty in performance will occur if flexible duct is compressed from its round shape to an ovaDSC04856l shape, say by squeezing it into a joist space. Just because it can doesn’t mean it should. We do allow for up to approximately a 20% reduction in diameter only if it occurs in one spot, but not over any distance or repeatedly. The friction loss for flex squeezed into an elliptical shape over any distance is severe, and the loss of airflow will be significant.

Cubic feet per minute airflow rate still equals the air velocity times the area of the duct in which the air is flowing. Increasing the area of the duct will slow the velocity of the air and reduce pressure loss. Keep in mind that the long-term system performance will be affected by the up-front, one-time cost of the flex duct. Increasing flex duct one size to offset its higher pressure loss compared to smooth duct is prudent.

Adequate Capacity – Air Conditioning Systems

Monday, October 26th, 2009

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Simple ‘Rule of Thumb’ Sizing

Obviously, conditioned floor space, expressed in square feet, is a chief factor when calculating air conditioning capacity (tons). Depending on local experience, contractors may allow a certain amount of airflow per square foot. In this example of a 10,000 square-foot residence, let’s set a ‘rule of thumb’ at one cubic foot per minute (cfm) for.

10,000 ft2 X 1 cfm/ft2 = 10,000 cfm

Next, we set a second ‘rule of thumb’ of 400 cfm per ton refrigeration.

10,000 cfm/400 cfm = 25 tons

A much more conservative value of 300 cfm/ton can be used to evaluate the margin.

10,000 cfm/300 cfm = 33 tons

A real-life example: a 26,000 ft2 Texas residence features a 100-ton ‘chiller’ that circulates refrigerated water to 24 fan-coil units. (Read more about fan-coil units)

At 400 cfm/ton, an ambitious low-bid contractor might estimate 65 tons. A more conservative contractor, using 300 cfm/ton might come up with 86 tons. In this real-life case, the ratio is 260 ft2/ton.

What’s the correct tonnage? Would you believe even the most conservative calculation proves to be inadequate?


Simple calculations may work for the small system situation where it is actually hard to go wrong. Structures, residences over 3,000 ft2 need to be scientifically evaluated. Various software products exist for this purpose and these software products are intended to be used by professional air conditioning contractors and mechanical engineers. You will not find a “How To” book at Home Depot to solve this problem – “you can’t do it and they can’t help”.

What to Do – Leave it to the Professional

Professionals measure dimensions, create a floor plan and evaluate several important features of the structure.

  • Windows – type, size, relative compass direction they face, solar screens, reflective film, etc.
  • Insulation – type (glass, cellulose, foam), thickness, condition
  • Roof – type shingle, radiant barrier, attic ventilation, construction (gable, flat, cathedral, etc.)
  • Ceiling height
  • Geographical location (latitude, altitude, ASHRAE design temperatures)
  • Use of spaces – game rooms, gymnasiums, indoor spas and pools, trophy rooms, bedrooms, media centers, etc.
  • Computer rooms, home office, etc.
  • Garage spaces
  • Vestibules and passage ways

Symptoms of Poor Design

Some problem signs are patently obvious – one register in an 800 square-foot space is a good one. Loss of control on hot summer days is another. If it’s 100 degrees outside, I expect my system to hold the inside around 73 degrees, and it does, because my system was professionally designed and professionally installed.

My Choice for Air Conditioning

Geothermal easily comes in first over air-cooled conventional residential equipment and it definitely surpasses commercial ‘chillers’ for cost of ownership and overall reliability. (Read more about geothermal…)