I just installed one of those 5 inch furnace filters. I attached one of those vacuum differential devices as well. When furnace blower runs in propane mode is shows .20 inches of water. When furnace runs in HP mode is shows .30 inches of water. Reason being the fan runs slower in the Propane mode.
My question is how high does the resistance need to go before I replace the filter?
My question is how high does the resistance need to go before I replace the filter?