I recently read a thought provoking article by David Richardson
in the July 7 Air Conditioning Heating and Refrigeration News. In it he argues
that sealing an undersized duct system can cause problems. The system cooled
BEFORE you sealed the ducts, and now that you have worked on it, the system
cannot move enough air to stay operating. I know I have seen many systems with
marginal ductwork that still managed to operate, even if inefficiently. The
duct leaks could be allowing just enough extra air flow to keep the system
operating. The combination of leaks on both the return and supply sides of the
system serve to reduce the static pressure difference against which the fan
must move the air. If the static pressure difference between the return and
supply is already at or past the limit the manufacturer publishes for their
system, sealing the ducts will increase the already high static pressure
difference and push the system over the edge. This is why Mr. Richardson
advises taking a reading of the total external static pressure difference
BEFORE sealing the duct to avoid this trap. If it is too high, the duct system
will need more attention than just sealing – it will need some duct
modifications as well. Better to know before you do the job. What is too high? I
don’t like to see anything higher than 0.8” wc – that is usually the top end on
most residential systems. Most residential systems will operate comfortably
around 0.5” wc. Some will go all the way to 1” wc, but at a cost. With ECM fan
motors you are burning electricity to shove air through restrictive ductwork.
With PSC blowers you lose airflow at high external static pressures. This costs
electricity in another way. The suction pressure drops, the compression ratio
increases, and system capacity decreases – causing extended run times and
inefficient operation.
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