HVACopediaLennox · Heat pump
Green OFF / Red OFF No power to the demand defrost control
What the control is telling you
Both LEDs on the defrost control (DS2 green, DS1 red) are dark: the board has no 24 VAC between R and C, or the board has failed.
Applies to: Lennox EL16KP1 (Elite) and ML14KP1 (Merit) R-454B heat pumps, sizes -018 to -060, demand defrost control A108 (CMC1)
Likely causes
- No 24 VAC at defrost control terminals R and C
- Indoor transformer, fuse or low-voltage wiring problem
- Failed demand defrost control
Test and repair steps
- Measure 24 VAC between R and C on the defrost control.
- If there is no voltage, check the indoor unit transformer (24 VAC, 40 VA minimum), its fuse and the thermostat wire between indoor and outdoor unit.
- Check the low-voltage connections at the outdoor terminal strip for loose or broken wires.
- If 24 VAC is present at R and C and the LEDs still do not light, replace the demand defrost control.
Settings for these units
Outdoor unit clearances · Lennox EL16KP1
| Location | Clearance |
|---|---|
| Service side (one side next to control box) | 30 in. (762 mm) |
| One of the other three sides | 36 in. (914 mm) |
| One of the remaining two sides | 12 in. (305 mm) |
| Last side | 6 in. (152 mm) |
| Between two units | 24 in. (610 mm) |
| Top | 48 in. (1219 mm) |
| Above roof surface (roof mount) | 6 in. (152 mm) minimum |
Increase clearances where snow/ice can fall on the unit or air can recirculate (decks, tight spaces).
Wind barrier, if used, 24 in. (610 mm) from the unit.
Outdoor unit clearances · Lennox ML14KP1
| Location | Clearance |
|---|---|
| Service side (one side next to control box) | 24 in. (610 mm) |
| One of the other three sides | 36 in. (914 mm) |
| One of the remaining two sides | 12 in. (305 mm) |
| Last side | 6 in. (152 mm) |
| Top | 48 in. (1219 mm) |
| Multi-unit, between units | 12 in. (305 mm) minimum [18 in. (457 mm) recommended] or 24 in. (610 mm) depending on layout |
| Multi-unit service side | 24 in. (610 mm) recommended |
| Above roof surface (roof mount) | 6 in. (152 mm) minimum |
Multi-unit spacing (A/B) depends on single row, wall or multiple rows layout - see figure on p.7.
Increase clearances where snow/ice can fall on the unit or air can recirculate.
Models that show this code
14 models in Lennox EL16KP1, Lennox ML14KP1
Other codes on the same units
- Green & Red simultaneous FAST flash Ambient (outdoor air) sensor problem
- Green FAST flash / Red OFF Discharge sensor lockout
- Green FAST flash / Red ON Discharge line temperature lockout
- Green OFF / Red FAST flash Discharge sensor fault
- Green OFF / Red ON Low pressure lockout
- Green OFF / Red SLOW flash Low pressure fault
- Green ON / Red OFF High pressure lockout
- Green ON / Red ON Demand defrost control failure
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Source: Lennox EL16KP1 Unit Information 100195 / ML14KP1 Unit Information 100108, Table 4 Demand Defrost Control (A108) Diagnostic LEDs, p.32. Summarized in our own words; check the original manual for your model.
Homeowner looking at this code? See what it means for you and when to call a technician.
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.