ASI's Learn About HVAC Duct Work and Grille Sizing
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Duct Work and Grille Sizing Information with some rules of thumb
Duct work sizing on most homes is not correct and this is due to a couple of factors, the primary reason is cost and the other factor is misused "rules of thumb" in the industry.Supply Ducts
Many HVAC contractors have used a rule of thumb for duct sizing for many years and in many cases no actual duct calculations ever really took place outside this rule of thumb. The rule of thumb has to do with calculating the friction losses in any given duct.
Many contractors use a friction loss factor of 0.1 inches of water column for both the supply and return duct size calculations and in most if not all cases this leads to poorly sized ducts and causes uncomfortable areas as well as noisy grilles because the ducts end up too small. In our experience 0.06 inches of water column for supply ducts provides better circulation with minimal noise.
Return ducts are sized incorrectly on a regular basis and many homeowners could save a considerable amount of energy by checking their return duct size. The industry standard for system total air flow is usually based upon the amount of cooling required, this is due to the fact that central air conditioning systems require a specific amount of air flow at all times to perform at their maximum efficiency. This industry standard is 400 cubic feet per minute (CFM) of air flow for each ton (Btu/hr) of cooling.
Again many contractors use the 0.1 inches of water column for the "rule of thumb" on their return ducts when in fact the friction losses should be closer to half that at 0.05 inches of water column. This, in effect starves the system for air flow when it needs it most, when it is running the air conditioning.
As an example for a standard 3 ton AC unit on a given home the return duct should be at a minimum 14.5 inches in diameter, granted the next standard size duct is 16 inches and that should be the size of the duct for proper air flow. Many homes will have a 12 inch or 14 inch duct for the return duct connected to what in most cases is the wrong size return grille.
Return grilles themselves are another critical factor for air flow and again many in the industry use in an incorrect "rule of thumb" for sizing the return grilles of 1 square foot per ton or 100 square inches per ton. When in fact a grille will generally flow air through a grille at about 2 cubic feet per minute per square inch.
As an example lets look at that same house with 3 tons of cooling and assume it has a 20 inch x 25 inch return grille which has a total of 500 square inches of gross area. We then multiply the 500 square inches by the standard 2 cubic feet per square inch of air flow and we get 1000 cubic feet per minute (CFM). That is 200 cubic feet per minute (CFM) short of what is actually needed for a 3 ton system which is a minimum of 1200 CFM.
As a result many systems return ducts and return grilles are undersized by as much as 30% which means that you give up 30% of your air conditioning systems capacity.
Local Code Requirements
Recently in Southwestern Idaho new and improved HVAC code requirements provide that duct work be designed using standard methods of design, the "Manual D" among others. If enforced correctly this should lead to much better performance and efficiencies on new homes, however this does not correct all those homes out there now that were designed using these "rules of thumb" that have been passed down from generation to generation in the HVAC industry.
ASI's HVAC and Home Improvement Articles HVAC Contractor Air Conditioning Refrigerant Rip Off One of the more common things we see are air conditioning systems that have been overcharged with refrigerant. This common rip off is usually done when you call for a simple maintenance service and they... HVAC Heating Air Conditioning Capacitor Rip Off Another common practice by dishonest contractors is to replace a simple small part that many times is not necessary. The capacitor is one of those parts that is relatively inexpensive... The Geo-Thermal System Explained in a Nut Shell Geothermal is by far the highest efficiency heating and cooling system available today. While the systems require up to three times more investment upfront than a conventional heating and cooling system... Ameri-Serv, Inc. Heating and Cooling News Ameri-Serv Inc Helping the Treasure Valley Community Ameri-Serv, Inc. Heating and Cooling has successfully helped dozens of Treasure Valley Families with thousands of dollars worth of free maintenance services and repairs through this holiday... |
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ASI's Cooling Related Articles
  What
should I Look for Before I call for services on my air conditioning
system? The number one issue we find on air conditioning systems is
clogged filters. First look to see if your air filter is clean and not
clogged up, if it is clogged change the air filter and try your system
again but keep in mind with the air conditioning system running with a
clogged air filter you could have also iced up your indoor coil due to
low air flow.
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  Why
does my Air Conditioning run all day in hot weather? Assuming this is
not a new symptom... this is a common complaint and can be attributed to
two possible things. The first is that your system may have been
designed correctly to meet the latest energy codes to perform at the
highest efficiency. The other possibility is that it was not designed at
all or was not sized properly in the first place. |
  Things you can do to maintain your air conditioning system. You can keep the outdoor air conditioner clean. You can
vacuum out the inside of the unit and get all those leaves and stuff
out. On most units you can gain access through the service panel. Be
sure the power is off before opening the panel. Be careful not to knock
off any wires and do not hit the coil as you will cause permanent
damage.
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  In
the case of the thermostat calling for cooling the thermostat switches
on the power from the “R” or in some cases the “RC” terminal which is
the 24 volt power source to the “Y” terminal on the thermostat which is
in essence the call for cooling. These terminals are located on the back
or on the wall mounted face plate of the thermostat. All the wires
connected to the thermostat will usually always go directly to the
furnace and another set of wires will then go out to the outdoor air... |
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13601 W McMillan Rd Suite 102-120 Boise, Idaho 83713 Office (208) 342-4328 Fax (208) 906-1282
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Copyright 2011. ASI Heating and Cooling a Division of Ameri-Serv, Inc. All rights reserved