Honeywell L7124U, Installation Instructions Manual

L7124U UNIVERSAL OIL ELECTRONIC AQUASTAT® CONTROLLER

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TROUBLESHOOTING

When attempting to diagnose system performance, 
references to the LED can help to identify specific areas 
not working properly. The LED are:

PWR—ON indicates 24 Vac power present.

C—ON indicates circulator relay energized.

TSTAT—ON indicates thermostat call for heat.

ZC—ON indicates power to zoning.

B—ON indicates burner relay energized.

LL/DSBL—

• ON indicates Low Limit function selected.
• OFF indicates Low Limit disabled.
• Flashing indicates error. See Table 3 and 

Troubleshooting Guide, Table 4.

Table 3. LED Error Codes.

.

Number 

of Blinks

Cause

2

Sensor Error (Open or shorted sensor.)

3

System error(s); e.g., setpoint POT failure.

4

Safety relay has welded, device requires 
replacement.

Table 4. Corrective Action .

System Condition

Check

Corrective Action

Boiler is cold, house is cold.

PWR LED Off

Check: 

— 120 Vac system power.
— 1A fuse on L7124U.

PWR LED On
TSTAT LED Off

Check:

— Thermostat
— Thermostat wiring

PWR LED On
TSTAT LED On
B (burner) LED On

— Check:
— 120  Vac  B1-B2.
— If no, replace burner relay.
— If yes, check burner and wiring.

PWR LED On
TSTAT LED On
LL/DSBL LED Flashing

Refer to LED diagnostics, Table 3.

Boiler is hot, house is cold.

PWR LED On
TSTAT LED On
C (circulator) LED On

Check:

— 120 Vac, C1-C2.
— If no, replace circulator relay.
— Check for circulation, air entrap-

ment, water flow.

— If yes, check circulator and wiring.
— Check for circulation, air entrap-

ment, water flow.

PWR LED On 
TSTAT LED On
C (circulator), ZC (zone circulator) 
LED Off

Check:

— Boiler below low limit temperature, 

wait for boiler to above low limit 
temperature.

PWR LED On
TSTAT LED On
ZC LED on

Check:

— Boiler above low limit.
— If yes, check for 120 Vac between 

ZC and L2.

— If no, replace ZC relay .
— If yes, check switch relays, zone 

circulators and wiring.