Pressure
Switches
New modulating and multi-stage
furnaces are more complex and
require that installation instructions and
procedures be followed so that the
furnace will operate as it was designed
to do.
There is little room for error.
A common part in many furnaces is a
pressure switch. This switch is a safety
control that will not allow gas burner
ignition if the proper pressure is not
present.
This switch is one of the parts most
often replaced; many times for the
wrong reason.
Pressure switches are
safety controls
to ensure inducer motors are coming
up to speed and no restrictions are
present in the vent system.
They are normally open (N.O.) and
close only after the inducer motor is
energized.
These switches
must close
to allow the
ignition sequence
to begin.
Switch
pressures
should
be
measured with a Magnahelic®, Digital
or an inclined manometer.
A meter range of 0.0”wc to 5.0”wc is
necessary to test most pressure
switches.
The tester should be teed into the
pressure switch tubing so it is in
parallel with the switch. This way the
gauge will read the same pressure as
the switch sees. Depending on the
furnace the gauge may need to be
outside the cabinet with all furnace
panels on.
Induced Draft
Motor
Field supplied tee
Furnace
Pressure Switch
0.6”WC
Magnahelic
After attaching the gauge properly, call for
heat and observe the gauge pressure.
If the pressure reading is higher than the
makepoint of the switch and the switch is not
closed, the switch is defective.
Magnahelic
Note: There are some extreme cases where
the switch is closed but no electrical circuit is
made. This can be caused by a coating on
the switch contacts which does not allow
good conductivity. This can be verified with
your ohmmeter or voltmeter.
If the pressure reading is lower
than the makepoint, a problem
exists that does not allow
enough combustion air to flow.
Some causes for this are:
•Obstructions in the intake or
exhaust piping (snow, ice, rocks,
balls, rodents, insects, nests
•Improperly supported and pitched
exhaust piping (condensate may
build up and create a blockage
•Undersized piping
•Too long piping run or too many ells
If one of these potential problems is
suspected, the best way to determine it
is to disconnect the intake and exhaust
piping from the furnace.
If the gauge reading greatly increases,
the problem is in the venting system.
If the reading does not increase there are
other checks to be made:
•Pressure switch tubing kinked,
cracked, obstructed or disconnected
•Inducer wheel loose on the motor
shaft
•Blades missing on the inducer wheel
•Tight bearings on inducer motor
•Restricted pressure tap ports
•Blocked condensate drain pan
•Blocked or plugged heat exchanger
Blocked Drain Pressure Switch
Modulating furnaces may also have a
switch to sense if the secondary heat
exchanger is not draining properly.
They will open at a pre-set pressure
and indicate a fault code on the
controller.
The pressure is also checked with a
Magnahelic or an inclined manometer.
®
Blocked Drain Pressure Switch, cont.
A furnace that is not level or restricted
drain system can cause this problem.
Sagging hoses will collect condensate
and cause pressure changes.
If furnace is converted from upflow to
horizontal or downflow hoses may need
to be cut so they are not too long and
sag.
Remote Pressure Transducer
Some modulating furnaces have a
pressure sensor called a transducer.
A transducer converts combustion air
pressure to a voltage which is sent to
a control circuit which then varies the
gas firing rate and the inducer speed
as the pressure changes.
Remote Pressure Transducer, cont.
If the amount of combustion air available is
reduced by a restriction in the vent and
intake system the control will speed up the
inducer to provide the correct amount of
combustion air.
If the inducer is at full speed and the
pressure indicates there is not enough
combustion air to support proper combustion
at full input rate, the control will reduce the
firing rate of the gas valve to maintain proper
firing combustion.
An example of one manufacturers test method:
Warning
Never attempt to adjust a
pressure switch or use one
of a different pressure
setting than specified for the
model being serviced.
Last but not least-----
RTI
(read the instructions!)
Most furnaces come with installation manuals.
New modulating and multi-stage furnaces are
more complex and require that the installation
instructions and procedures be followed so that the
furnace will operate as it was designed to do.
There is little room for error.