Hello, this is Dana Clark and I’m with the
NORDYNE Technical Service Group. Here is your troubleshooting tip for the week:
How many times have you shown up to a service call on a no-heat for a gas furnace and discovered
the pressure switch won’t close, or that the pressure switch will open during the cycle
for heat? A pressure switch problem is commonly misdiagnosed
as a failed switch. Two key areas regarding the installation of the furnace need to be
investigated first.
The first area would be the construction and length of the combustion
air and flue piping. The second would be the condensate plumbing for the furnace.
The length of run on the flue pipe could be too great given the BTU input of the furnace.
The pipe diameter size for the combustion air or flue piping may need to be increased
from two inches to three inches. Please remember the installation tables give the technician
a guideline to the maximum number of feet that can be installed.
Let’s use 90 feet as an example. Ninety feet is determined using actual linear feet
of pipe and the number of PVC fittings used in the construction of that pipe run.
The build-up of condensate inside the furnace is the most common culprit for causing a pressure
switch to open.
Too often a technician will unknowingly construct an installed condensate
piping that will have what we refer to as a double trap.
Be sure your flue and combustion air piping as well as your condensate piping is correct
before attempting to replace a pressure switch. Thanks for tuning in. Come back next time
for another troubleshooting tip..