Combustion Air Switch Failure

The furnace sounds like its going to kick on and then an indicator light comes on that says the combustion air switch failed. It is a Red T Products gas furnace, can anyone tell me how to fix?

I have 4 industrial boilers that range from 40klbs/hr to 60 klbs/hr. Currently, we have 2 types of air proving switches: Cleveland controls AFS-951 and an old Honeywell that I cannot read the part number. My operating combustion air pressures run from 8-20"H2O. The Cleveland controls switch...

Evcon Central Furnace fault light flashes 3 times, combustion air switch failed to close, how do I make repairs?

I need to know, what is size required (duct size) for combustion on a furnace. It is a formula that you use, it is how many sguare inches for some many BTUs.