T022_F: Main Limit/HALC Switch Open
Status
This is a critical fault. The furnace will not operate in
gas heat modes but all other modes (e.g. cooling) should function.
Description
Alert "T022_F Main Limit/HALC Switch Open"
is logged if the main limit or heat-assisted limit control (HALC) trips.
Expected Operation
The limit circuit is monitored during gas heating (including
defrost) and post-purge. If the main limit/HALC trips, control logs "T022_F
Main Limit/HALC Switch Open", shuts off the inducer and gas valves,
and runs the blower at the highest heat speed.
If the limit re-closes in less than 150 seconds, the control
will clear alert "T022_F Main Limit/HALC Switch Open" and
transition to blower off delay without any need for inducer to go through
post-purge. If a call for heat remains after the off-delay is
complete, the furnace will go into a normal ignition cycle. If the limit
opens four times in one heat call, alert "T022_F Main Limit/HALC Switch
Open" will be elevated to an alarm "A022_F Main Limit/HALC
Switch Open".
Cause
1. No airflow
2. Insufficient airflow
3. Faulty Limit Control
4. Loose or Faulty Wiring
5. Dead (non-functional) blower
6. Input too high
Solution
1. Check for proper blower operation. Is the blower turning
during heat (or any other) mode? If not, a blower motor fault may also be
present. Check the wiring to the motor then check the motor. It may need
replacing.
2. Check duct work and filters. Determine the static
pressure and make sure it is not above the published values for the furnace.
Check the rate and outlet air temperature for high and low heat using a jumper
wire between R and W/W1 respectively. Compare them to the nameplate maximum
values and adjust if necessary. If a supply air sensor is used cycle power to
the furnace.
3. Replace the faulty limit control.
4. Check wiring and connections. Replace and/or repair as
necessary.
5. Insure properly sized burner orifices are installed.
Check the manifold pressure at high fire and compare to the nameplate values.
Adjust as needed.
6. Input too high, adjust as needed.
