No heat - Daikin DR96TN/DD96TN

What is going on

Thermostat calls for heat but the furnace does not start, light or deliver heat. Step-by-step check of the heating sequence from the manufacturer flowchart.

Likely causes

• No power or blown board fuse
• Open thermostat circuit
• Inducer or pressure switch problem (vent, drain, hose)
• Igniter or control board output failure
• No gas, low gas pressure or failed gas valve
• Dirty flame sensor or poor ground
• Blower motor or harness failure
• Furnace in R-32 mitigation mode (EAF/EAL/EAS/EAr shown)

Check in this order

1. If the display shows EAF, EAL, EAS or EAr the furnace is in R-32 mitigation mode (fan only, no gas heat) - fix that code first.
2. Check the furnace power switch and breaker are on and the thermostat is set to heat above room temperature.
3. Measure 115 VAC between line H and N on the control board. None: check the breaker, service switch and blower door switch.
4. Check 115 VAC at the transformer primary and 24 VAC at the secondary. Primary OK but no secondary: check for a short, then replace the transformer.
5. 24 VAC at the transformer but not between R and C on the board: replace the board fuse (find the short first).
6. No 24 VAC between W (W1) and C with a heat call: the thermostat or its wiring is open.
7. Inducer not running check 115 VAC at the inducer. No voltage = control board or inducer wiring; voltage but no run = inducer bad or off on its thermal protector (check for over-voltage).
8. Pressure switch not closing measure the negative pressure at the switch. Enough vacuum = replace the switch; not enough = check venting, drainage, water in the inducer and a plugged switch port.
9. No 115 VAC to the igniter after pre-purge: replace the control board. Voltage but no glow: replace the igniter. Igniter glows only part of the warm-up: check for the pressure switch opening during warm-up.
10. Gas valve must open and hold proper manifold pressure for at least 4 seconds. If not: confirm house gas pressure, 24 VAC at the valve wires, and 24 VAC between HI/M and common; on 90% models check the front cover pressure switch in series with the valve, on 80% models check for an open auxiliary limit in series with the valve; then replace the valve.
11. Burners do not light or will not stay on check burner dirt and alignment, orifice plate position, flooded or blocked heat exchanger, flame signal, ground wires, and that all pressure switches stay closed.
12. Blower does not start about 30 seconds after ignition: check 115 VAC between CIRC-H and neutral and at the motor L/N, then the speed tap signal between motor common and a T terminal; check the harness, then the motor.
13. Burners shut off before the thermostat is satisfied: read the fault code and compare the actual temperature rise with the rating plate.

More furnace guides

• Control display blank, furnace dead - Daikin DR96TN/DD96TN
• Furnace not paired with R-32 cooling - disable A2L function - Daikin DR96TN/DD96TN