Ever since the adoption of the 2009 International Residential Energy Conservation Code (IECC), duct sealing efforts have been tested using the total leakage or leakage to outdoors methods.  It really is amazing how much duct gains effect the operation of your equipment and the comfort of your customer.  The missing part of "duct testing" appears to be how well the ducts are insulated.  Sure, you are required to use R-8 insulation on supply ductwork when located outside the building envelope, but how do we know it was installed correctly?  We're not required to test like the duct leakage codes, so is a check in the box sufficient?  How about we use some simple math to figure the gains, or lost capacities in cooling, and hold everyone to a standard - check out the equation below.

Sensible Btu/hr = CFM x Delta T x 1.08
   So, based on this math, one can take a few simple temperature measurements and calculate the lost sensible capacities due to duct leakage and insulation.   You should measure the temp of the air entering the return grille and at the unit to figure the return gain in sensible temperature.  I would also recommend averaging the temp readings at the furthest supply register, an average supply register, and a close one to figure the delta T from the air exiting the coil or air handler.  Do the same for an extensive return system.

  When you start measuring, it is very scary what you start to find.  As you can see, like the industry accepted national average, this system is losing 30% of the 3-ton capacity through it's well sealed, but poorly insulated duct system.  Unfortunately, the method is not perfect since a hotter day in the attic could cause greater gains.  But, that would be the time to sell duct sealing and insulation to the homeowner!  Any of you energy auditors out there could probably comment on how much attic ducts effect home infiltration as well.  So, why do we hate R-8 insulation?  Is it really just the hassle of wrapping the equivalent of a blanket around the ducts?  I don't recommend retrofitting a distribution system in an attic in the middle of July, but maybe it makes sense to offer to come back in the Fall?  Please, don't just leave it be!  Doing so makes it hard for this tree hugger to sleep at night!


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Comment by Bob Blanchette on June 29, 2012 at 6:44pm

The best bet is to reduce duct surface area if you can't get the ductwork into conditioned space. Have fewer registers and larger ductwork instead of lots of registers with small ductwork. Use truck/branch instead of '"ductopus". Keep runs short as possible. Eliminate runs to small/lightly used spaces. Put registers on sidewalls closer to the air handler, set sidewall registers to blow out into room.

The return isn't as big of an offender as the supply. As long as return is tightly sealed the wetbulb temperature won't change much. Wetbulb determines evaporator load. Then there is doing what you can to reduce attic temps, the best solution is typically large area passive vents. If you can see light coming through while in the attic, it's working...

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