FURNACE TROUBLESHOOTING pressure switches - HVAC Repair Tips

Uploaded by NORDYNEU on 18.11.2010

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.