Archive for the ‘Uncategorized’ Category

Watts up with residential solar power?

Wednesday, March 22nd, 2017

Since the beginning of 2017, the interest in residential solar power systems has grown among homeowners, particularly here in the Rockwall, Texas area. “What do you think about solar?” starts the conversation. Well, the question deserves and answer.

Nothing is free, not the equipment and most certainly not the installation labor. We could start with the energy density factor, but let’s instead start with the practical issue of ownership.

Few homes in my city of Rockwall were built to optimize photovoltaic (PV) solar electrical power. In fact, I should say none of the homes in my city can do so. Why?

Since we recently experienced the vernal equinox on March 20, 2017, it seems appropriate to point to the obvious: sunlight is only available during the day while typical families are away at school solarhouseand work. There is no sunlight to power the PV panels at night. Do you see how this might be a problem?

Let’s look at a home that was constructed to capture the most PV solar power. The roof is a half gable. The pitch of the roof is designed to expose the PV panels throughout the year. Do remember the sun is low in the south during winter, higher in the north during summer.

Next, do you notice the roofing material below the solar panels? The roofing material on the home pictured is not susceptible to hail damage, it won’t rot. Throughout the lifetime of the home, roof replacement is not at all likely. This particular home is in California and the solar power system was provided by Spectrum One Contract Builders. Roof construction of this home is ideal for solar panels.

I invite my Rockwall area residents to walk outside and look at their roofs. Your home may feature hip roof, gabled roof, dormers, and other geometries that don’t allow reasonable exposure to the sun. In most of our neighborhoods, the houses don’t particularly square off with the earth’s lines of latitude or longitude. One Rockwall home with solar panels has a gable roof and the panels are on the east-facing side, not exactly optimal.

The typical Rockwall area roof has composition shingles, a few with wood shingles, and even fewer with aluminum shingles. Before you decide to mount solar panels on your roof, first know there is no existing hail damage. What is the manufacturer’s warranty on the roof? If you mount solar panels on your easily hail-damaged roof, there are probably undesirable outcomes in your future.

For the roofing contractor to replace your hail-damaged roof, those solar panels must come down, then be reinstalled after the roofing contractor drives away. Do you see any roof vents, fireplace chimneys, dormers, gables, etc. that will limit the number and placement of solar panels on your roof?

Have you engaged an engineer to run the numbers? Probably not. Solar salespeople do that, but the motivation for you to buy into a solar program is all about the financial incentives. Right? May I suggest you get an objective report from a registered professional engineer before you sign on the dotted line?

Rockwall requires a building permit from the city and there are new electrical codes in play that affect wiring. You didn’t think about the wiring?

Early PV projects in which I have been involved wired panels in series. Terminal voltage would be as high as 180 volts DC. What happens when a fireman squirts water on a high-voltage electrical system? Yes, he may be electrocuted, so the latest electrical codes require lower voltage which means more wiring which means more cost to you.

Would I ever consider PV solar for my own home? Yes, but later on when more efficient PV panels will be available.

There are more factors to consider before you decide to go ahead with a solar project. The chief thing I hope you take away from my seemingly negative point of view is this. If you want a long-term practical PV solar power system for your home, engage an engineer not associated with the contractor or marketing company that will install it. Ask for a sketch on the layout of the proposed panels. The appearance alone may be the deal breaker for you.

Disclaimer: Rockwall Controls Company, Inc. is not a provider of photovoltaic solar electrical power systems and we rarely provide control systems to residential customers.

John White

Rockwall Controls Company, Inc.
(972) 771-3514

Intrinsic Value v. Low Price | Making Wise Choices When Choosing Products and Service Providers

Saturday, December 21st, 2013

Intrinsic Value v. Low Price

Making Wise Choices When Choosing Products and Service Providers

Any discussion about making wise choices of necessity must include a discussion around how we come to conclusions about which vendor or service provider we choose.

One thing I learned… is that perception matters. – Jack Welch

Perception dominates our decision making process. Without it, we would never act. It’s the only thing we can change.

According to Dr. Nicholas Romanov, perception includes “senses, feelings, ideas, thoughts, (and) theories.” He says the final point of perception is concept – a general idea.

“The bitterness of poor quality remains long after the sweetness of low price is forgotten.” – Benjamin Franklin

Purchasers of products and services for buildings tend to assume all things being equal, lowest price is the best choice, despite facts to the contrary. This isn’t to suggest that the highest price is always the better choice; but, most often it is.

Let’s look to Sears that for many years sold tools in three categories: Good, Better, Best. As you would expect, pricing followed product quality – the better the quality, the higher the price. Sears did an excellent job of managing perceptions.

So, how does one fine the best vendor or service provider? Qualified referrals remain forever the best tools for finding the best value. Do you use Yelp or Angie’s List? How about Amazon? Following recommendations of customers who have actually used a product or service, serves to refine and validate your perceptions. As a rule of thumb, the longer a vendor’s or service provider’s history with a customer, the more reliable the recommendation.

What about “street bids”?

We don’t participate. Regardless how low my price, someone will usually bid a lower price.

Keep in mind this simple truth: we all get our materials from the same manufacturers, by and large. The outcome isn’t a function necessarily of the materials applied, but the knowledge and skills that deploy those products.

Rockwall Controls Company, Inc. is a design-build service provider that assumes full responsibility for the design, materials, installation, programming, commissioning and after-market technical support, therefore, there is no “finger pointing”.

John N. White
Rockwall Controls Company, Inc.
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iLoad Manager

Tuesday, March 13th, 2012

The Problem

Conventional HVAC control systems react to temperature settings, a discharge air temperature, for example, instead of intelligently responding to the actual heating or cooling load presented by associated zones of indoor climate control (heating, cooling, relative humidity and pressurization).

Consequently, owner experiences a higher frequency of mechanical failures than should be expected. Naturally, mechanical failures affect the performance of the building which affects the persons that occupy that building, whether employees or clients.

The Solution

Rockwall Controls Company, Inc. iLoad software matches load demand of zones with equipment to intelligently provide smooth operation of mechanical systems which yields more stable indoor climate conditions as it reduces energy consumption and minimizes costly repairs. What’s the Bottom line? Better performance with a lower total cost of ownership (TOC).

How Does It Work?

Proprietary iLoad algorithms build a body of knowledge from which the algorithms optimize heating, cooling, humidification and pressurization control loops.

Conventional control systems will operate heating or cooling equipment whether zone(s) require little or much conditioning. iLoad algorithms recognize opportunities to scale back heating and/or cooling capacity without loss of comfort.

Can I Adapt iLoad to My Control System?

Rockwall Controls iLoad can be adapted to most digital (intelligent, programmable) control systems: Tridium® Niagara Framework®, Vykon®, Honeywell WEBs®, and most modern control systems deploying programmable controllers. In some cases, we have integrated existing control systems with modern programmable controllers that communicate via IP, BACnet or LonMark-compliant networks.

For More Information Contact

Rockwall Controls Company, Inc.

PO Box 836

Rockwall, Texas 75087-0836

(972) 771-3514

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Dehumidification – Why do it?

Wednesday, May 5th, 2010

Honeywell dehumidifiers remove latent heat while the air-conditioning system removes sensible heat. What does this mean? Own a piano? Is your home constructed of wood? Own any valuable works of art or even wooden-framed family photos? Click here to view a brief video on the importance of dehumidification.

Honeywell Dehumidifiers

Honeywell Dehumidifiers

Control Systems: Intelligent Management of Indoor Climate

Thursday, November 12th, 2009

More Than a Thermostat

A combination of mechanical equipment, motor-operated zone dampers, zone temperature sensors, programmable controllers and an IP-enabled server comprise a Rockwall VAV system.

Single-Zone Constant-Volume

The simplest format control system has a basic programmable thermostat that sequences heating and cooling for multiple rooms served by a single fan/furnace unit: all spaces served by this unit heat and cool equally. While its simplicity is a plus, its inefficiency is a big minus. Unless every space served by the air handling unit has the same occupancy (people in every space over the same time period), unused spaces inherently waste energy.

Multi-Zone Variable-Volume

Although a multi-zone variable-volume (VAV) control system adds cost, it yields excellent long-term benefits to the owner.

  • Perfect air balance
  • Perfect comfort
  • Lower operating cost
  • Fully automatic – needs no human adjustments, automatic seasonal changeover
  • Intelligent – resolves issues and transmits appropriate messages to service personnel
  • Conserves energy by not conditioning unoccupied spaces
  • Effectively extends capacity over same-design constant-volume systems, depending on variability of loads

Rockwall VAV monitors the use of all zones, it is self-aware, it ‘learns’ and it ‘understands’ how each zone behaves. A variety of specialized sensors and techniques inform the control system when a space occupies and when it becomes vacant. Heuristic algorithms track human use of spaces over seven days to develop profiles to assure comfortable re-entry of each space. For optimum control, Rockwall Controls applies a broad array of zone sensors that best suit respective zones.

Unoccupied Mode

While unoccupied, Rockwall VAV modulates the zone damper and sequences the central air handling unit to allow temperature naturally rise and fall between a maximum unoccupied cooling set point (nominally 85OF) and a minimum unoccupied heating set point (nominally 65OF). Depending on materials (works of art, antique furniture, sensitive plants, etc.) within the conditioned zone, logical control may include a relative humidity sensor. This mode provides maximum energy conservation.

Occupied Mode

While spaces occupy, the control system modulates respective zone dampers and activates central air handling unit equipment to precisely maintain zone temperature. An algorithm weighs cooling and heating demand by aggregate zones of a given air handling unit and sequences refrigeration, heating and supply fan to satisfy all the zones.

Automatic Air Balance

The inherent beauty of VAV is its self-balancing characteristic. Due to the way each zone damper individually modulates to precisely control temperature while automatically compensating for changes in cooling load, assures precise delivery of conditioned air to each respective zone.

Add Space without Adding Cost

Depending on load variability, the differences in demand for either heating or cooling between spaces served by a given air handling unit, it is possible to extend a five-ton air conditioning to cover a seven-ton load. What’s the catch? There is no catch. All those features enumerated under “Prescription for Success” must align for Rockwall VAV to work satisfactorily.

Assuming not all zones occupy at the same time, capacity unused by unoccupied zones can easily be diverted to occupied zones through addition of a properly designed air distribution network.

Looking Under the Hood – A Technical Discussion

Conversion of a single-zone air-handling system to VAV challenges the central mechanical equipment. Operating as a constant-volume system, the fan moves constant airflow through the cooling coil. This is important, because the air must remain in the coil long enough for the coil to absorb heat from the air, but fast enough to expel condensed moisture from the fins of the coil. If too fast, space relative humidity is high; if too slow, the cooling coil can freeze over.

VAV introduces an important issue: tight coupling between the zone dampers and the supply fan. Our solution is to de-couple fan-coil airflow from zone airflow. Refer to the following diagram. 

Rockwall VAV with 8 Zones
Rockwall VAV with 8 Zones

 In order to optimize fan airflow to the heat exchanger coil, we must de-couple zone variable-air volume (VAV) airflow from the constant-volume (C AV) airflow required of the heat exchanger. A de-coupler air valve, responding to airflow, fan discharge pressure, fan motor current and the fan curve, modulates to maintain relatively constant airflow through the coil.

It is helpful to understand this discussion relates to geothermal air conditioning systems that use super energy-efficient ECM (electrically commutated motor) fan motors. These DC motors use approximately 75% less energy than conventional AC-powered fan motors.

ECM fan systems work in a low-pressure, low-velocity configuration for optimum

GE ECM 2.3 Motor Cutaway

GE ECM 2.3 Motor Cutaway

 efficiency. GE ECM motors are programmed by the air conditioning equipment manufacturer. Rockwall VAV communicates speed commands to the motor. The System analyzes motor current, input power, fan discharge pressure and leaving-air temperature to manage the fan for greatest operating efficiency. Click here to learn more about the GE ECM motor.

Back to Residential …

Indoor Air Quality

Thursday, October 29th, 2009

HowRadonEntersTheHome¬Return to Residential Main

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. 


Sound & Noise

Tuesday, October 27th, 2009

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Soundly Annoying

Impulse noise, steady-state noise and annoying noise – you have no control over them outside the home, but in the castle, you are king or queen over your domain. We want what we want and we want home to be quiet.

Room to Room Noise

One drawback to metal air ducting is its sound transmission quality. In the case where adjoining rooms share a duct tee, noise and conversation from one room can transmit to the other through the duct work. I prefer flexible ducting because it natively attenuates sound. For this problem I suggest replacement of metal ducting with flexible ducting. One must remember to upsize the ducting one size over the metal when doing this to overcome additional air flow friction of flexible ducting.

Grill Noise

Turbulent air flow generates noise and all air flow from grills is turbulent. The key to quiet grills is to use properly sized grills and properly sized ducting.

Measuring Noise

Contemporary residential standard is 40 dba maximum A-weighted steady background sound level for simultaneous operation of HVAC, lighting andSoundLevelMeter other building services. The instrument to the right is a typical sound level meter set on 60 dba scale with A-weighting selected.

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.

Optimum Air Filtration

Tuesday, October 27th, 2009

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This Is the Air We Breathe

Indoor airborne contaminants include, but are not limited to, pet dander, pet hair, dust mite droppings, mold, dust, bacteria (pathogens), allergens (pollen and the like), viruses, radioactive radon daughters, undesirable gasses (CH4, CO, H2S, etc.), solvent vapors and odors.

This Is How We Clean the Air We Breathe

There are essentially three methods of residential air filtration:

  1. Capture Particulate Matter
  2. Destroy Pathogens
  3. Absorb Gasses

Capture Airborne Particles

First, I want to put my prejudice right out in the open: those ‘consumer quality’ disposable, and washable, cheap filters you buy from local storesDisposableAirFilterMedia do next to nothing for you except keep airborne material from damaging the inner workings of your air handling unit, coils and furnace.

“95% Efficiency” boasts the label of more costly particle filters. What does that mean, 95% efficiency? It’s a holdover from the old days when manufacturers used the “dust arrestance” method. Basically, the manufacturer first records the weight of a clean test filter, then places it in a small wind tunnel with certain airspeed and progressively dumps three different grades of dirt into the air stream. After dumping the dirt, the manufacturer records the weight of the dirty filter and compares its weight to the known weight of the three grades of test dirt. If the filter can absolutely hold 100% of all the three samples of dirt, he rates the filter efficiency at 100%. It is helpful to note that no one ever calibrates household dirt. I consider this method worthless as a measure of how effective a filter serves the homeowner. In fact, the high efficiency rating actually translates to an energy penalty, due to excessive pressure drop across the filter that contributes to overall fan system inefficiency.

HEPA-filterThe ultimate disposable filter is the HEPA filter, originally designed to protect workers in atomic energy plants, is commonly used in hospitals. They are very effective and they are very expensive – not really practical for a residence.

We still want to capture the big stuff to protect our equipment and duct work, but we want to do it as part of a larger strategy to maintain a healthy indoor environment.

Electronic Air Cleaners, my favorite, incorporate three mechanisms: a pre-filter to capture the really big stuff, an electronic filter to capture the very smallest stuff and a post filter to capture the small, but high mass, particles thatElectronicAirCleaner make it passed the electronic section. Here, I quote Honeywell: “The Honeywell Electronic Air Cleaner is an advanced and easy-to-use, whole-house solution that traps and filters up to 98% of pollutants from the air passing through your heating and cooling system. This advanced air cleaner captures microscopic impurities like dust, smoke and smog particles in addition to larger particles like mold spores and cat dander. It works by placing an electric charge on airborne particles, and then collecting the charged pollutants like a magnet.  Plus, you can say goodbye to monthly filter replacements by simply washing the air cleaner cells in your dishwasher or sink.”

Flexible air duct actually enhances the performance of electronic air cleaners because the designer selects flexible ducting for low velocity to overcome high resistance relative to metal ducting. Consequently, return air encounters theMyElectronicAirCleaner electronic air cleaner at a low face velocity.

My favorite electronic air cleaner: the Honeywell F300 series. I use it for my residence. See the picture to the right ?.

Remove Gaseous Pollutants

Activated charcoal filters do an excellent job of absorbing volatile organic compounds (VOC). This material is excellent for odor control, but it is wholly ActivatedCharcoalFilterineffective for the removal of deadly carbon monoxide (CO) produced by automotive engines, gas-burning appliances, grandma’s old kerosene lamps and wood-burning fireplaces.  Note – CO is an insidious threat only detectable by either an electronic monitor or a pathologist.

Destruction of PathogensUV Air Treatment

Ultraviolet air treatment equipment destroys pathogens with high-intensity ultraviolet. The product pictured destroys “up to 87% of certain airborne bacteria”. The only maintenance is the annual lamp replacement.


No single method is perfect and effectiveness can be diminished if equjipment is improperly installed.

Properly Designed Air Distribution Network

Tuesday, October 27th, 2009

¬Return to Residential Main

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.