Pressure Switches

New modulating furnaces are more and multi-stage 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 makepoint higher than the 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.