Heating alarms

HS1 lockout due to limit switch

The HS1 Lockout occurs due to a loss of 24 VAC to the SEC board LIMIT terminal. The LIMIT circuit

will have two or more limit switches wired in series. One switch is an automatic limit switch and

the other is a manual limit switch. If the SEC board recognizes three limit switch trips in one hour a

hard lockout of unit operation occurs and no operation occurs other than blower operation.

If a HS1 Lockout occurs complete the following steps:

1. Use a volt meter to check that 24 VAC power is present at the LIMIT terminal located on at the

upper left corner of the SEC board.

a. If no voltage is present, check the wiring diagram and trace where the 24 VAC power

is lost.

b. If voltage is present, continue to Step 2.

2. Determine if a limit switch is open and check the possible cause of the limit trip.

3. Check the circuit wiring for damage or a loose connection.

4. Check if the limit switch is faulty. If the switch is tripping without reaching tripping

temperature, this can indicate a faulty LIMIT switch.

5. Check that all the filters are clean.

6. Check for correct air flow according to the unit installation manual. Return static should not

be higher than 0.2 in. w.c. Factory equipment should be air balanced to operate in a cfm

range from 350 cfm to 450 cfm per ton.

Note:

a. If the unit AHU or RTU is a VAV application during the heating operation, the fan

should run at an adequate speed to allow correct air flow that will prevent the

unit from tripping on HS1 lockout. This means in most cases the VAV boxes must

be open when a call for heat is commanded.

b. On occasions where the system has two transformers, proper low voltage

phasing must be verified or HS1 could occur.