Airflow adjustment check

What is going on

Before changing airflow on an AUH1B furnace, check the air temperatures across the heat exchanger first. Airflow is changed at the blower motor speed taps, not in the control panel. Disconnect power before opening the blower door for any work. The blower door switch cuts power to the blower and gas valve when the door is removed, so keep all panels closed while you are measuring and do not defeat that switch.

Likely causes

• Temperature rise outside the nameplate range
• Blower speed set wrong for the duct system
• Dirty filter or restricted ducts making it look like a blower speed issue
• Panels open or door switch not engaging

Check in this order

1. Measure inlet and outlet air temperatures with the furnace running steady, with panels closed.
2. Confirm both are within the ranges printed on the rating nameplate.
3. Before changing speed, check the filter and registers, because restriction is the usual cause.
4. If airflow must change, use the wiring diagram to find the blower motor speed change.
5. Disconnect power before opening the blower door for any work.
6. Keep all panels closed during operation; the blower door switch cuts power to the blower and gas valve when the door is removed.
7. Recheck temperature rise after any change.

More furnace guides

• Abnormal conditions and protection
• Adjust gas valve manifold pressure
• Air for combustion and ventilation in non-direct vent installs
• Airflow and temperature rise check
• Bottom return filter rack installation (upflow)
• Carbon monoxide detectors and signage for Massachusetts side-wall venting