Arkansas HVACR NewsMagazine January 2024

HVACR NewsMagazine January 2024

Tech News

Sequence of Operations

We can get a clue where to begin diagnosing a 90%+ high efficiency furnace by going through the sequence of operations. The overall sequence of operations is practically identical to that of an 80% furnace. You’ll start in the same place: with a W call from the thermostat. The control board then has to verify that the pressure switch is open . From there, the inducer fan motor starts, closing the pressure switch.

Pressure Switches

It’s worth noting that 90%+ furnaces come in single-stage, two-stage, and modulating models, each with different ratings and types of pressure switches. Different manufacturers may also have different equipment designs. You’ll probably notice that many single -speed furnace pressure switches aren’t that much different from those on 80% furnaces. Two stage furnaces often have a pressure switch for each stage, and modulating furnaces might use a pressure transducer.

The the secondary heat exchanger may have multiple pressure switches, which can be overwhelming for techs who aren’t used to seeing that. Those pressure switches are designed to respond to pressure drops between the primary and secondary heat exchangers. The manufacturer’s literature, including schematics, may include detailed instructions on testing those. The instructions usually include the pressures needed for the switches to open and close. If you’re not achieving those pressures, it’s time to ask why and investigate. The problem could be as simple as a cracked or dirty hose, or the problem could lie within intake and or exhaust piping. If you are indeed achieving the pressures needed to open or close the switch, then you can narrow the problem down to the pressure switch. Some tools also test pressure switches, like this Fieldpiece manometer. (click on the link to view)