McQuay & Daikin Rooftop Won't Heat? Limits, Ignition & Freeze Stats

A Daikin-McQuay rooftop that will not heat almost always stops at one of four checkpoints: the unit is not enabled to heat, a limit has opened on airflow, the ignition sequence is dying before flame proves, or freeze protection and outdoor-air logic are shutting the unit down to protect a coil. Work those four in order and most no-heat calls are resolved before a part is ordered.

This guide covers the heating-season failure points that the summer-focused Daikin-McQuay Parts Field Guide leaves out. That guide handles boards, condenser fan blades, and cooling-season freeze stat misreads. This one picks up where the weather turns.

Why Does Mild Winter Weather Still Cause No-Heat Calls on Rooftops?

NOAA's September seasonal outlook, built around a developing and potentially very strong El Niño, leans warmer than normal across much of the northern tier and the West for December through February. Treat that as directional: seasonal outlooks describe probabilities over three months, and individual cold snaps still arrive.

The mechanical consequence is worth planning around. A rooftop in a mild winter spends more hours at part load, with the burner cycling on and off through the shoulder band between roughly 35°F and 50°F outdoor air. Igniters, electrodes, and flame rods are cycle-limited parts, so more heating starts per week means more wear on exactly the components that fail first. The first hard freeze then arrives at a unit whose ignition parts have been quietly aging all fall. For the broader pre-season sweep, the Pre-Winter HVAC Readiness Checklist covers the shared components; this post goes deep on the rooftop heat section.

Is the Unit Actually Allowed to Heat?

Before opening a burner compartment, confirm the controller is permitted to call for heat. Two checks account for more "dead furnace" calls than any failed part.

Heat-enable inputs. On the C-vintage RoofPak with MicroTech II described in Daikin's IM 738-2 manual, a field heat-enable input can disable heating entirely, and the unit ships with a factory jumper in that circuit. A removed jumper, or a building-automation enable that has dropped out, looks identical to a failed furnace. Check the enable terminals first. Wiring and terminal numbers differ by unit generation, so work from the diagram supplied with the unit.

Airflow proving. On the same unit, the controller expects the airflow-proving switch to close within about 120 seconds of fan start, and shuts the unit down with an alarm if it does not. No airflow proof means no heat, and the cause is upstream of the furnace: a slipping belt, a loaded filter bank, a VFD fault, or a tubing leak at the switch. Confirm the actual alarm before condemning anything in the heat section.

Why Does a High Limit Open, and How Do You Know the Limit Itself Is Bad?

A limit opens because the heat exchanger is not shedding heat into moving air. The limit is the messenger. The usual causes are loaded filters, a failing supply fan or belt, closed or stuck dampers, and blocked return paths. Replace the limit without finding the airflow problem and the new one trips on the same schedule.

To separate a tripped limit from a failed one, let the plenum cool to room temperature and check continuity across the limit with power removed. A limit that reads open at room temperature has failed. A limit that tests closed cold and opens only while firing is doing its job, and the fault is airflow.

Two limit controls are confirmed in the GSIstore catalog:

Never substitute a limit with a higher temperature rating to stop nuisance trips. The rating protects the heat exchanger, and a higher setting trades a callback for a cracked exchanger.

Which Ignition System Is on the Unit: Spark, Hot Surface, or a Combined Assembly?

Daikin-McQuay heat sections are not uniform. The catalog carries three distinct ignition styles, and they are not interchangeable. Identify which one the burner uses from the unit's burner assembly and nameplate before ordering anything.

Direct spark with a separate flame rod. The 059867801 ignition electrode creates the spark, and the 059867701 flame rod proves flame by rectification, returning a microamp DC signal to the control. The electrode listing notes that the two are commonly paired.

Hot surface ignition. The 300032902 ignitor is listed as a hot surface element. These fail by cracking or by going open. An open-circuit reading across the leads confirms a dead element. Handle by the ceramic base, since skin oils create hot spots that shorten element life.

Combined igniter/flame-rod assembly. The 036557700 is listed as a combined igniter and flame-sensing assembly. Confirm the part against the original before ordering, since this style does not match either of the two above.

Where in the Sequence Does the Burner Stop?

The stopping point identifies the failed component better than any parts guess:

• No spark or no glow after the draft is proven. Suspect the electrode or ignitor, then the control's output. Draft must be confirmed first, so see Furnace Won't Ignite? How to Diagnose a Draft Inducer Failure.
• Spark or glow, gas valve opens, flame lights, then drops out within a few seconds. The burner is lighting, but the control is not seeing flame. Suspect the flame rod first. A rod coated with soot or oxidation, a cracked ceramic insulator, or a poor burner ground weakens the rectified signal. Measure the microamp signal in series with the sensor lead during a call for heat and compare it to the control's specification, which varies by control.
• Spark but no flame. Check gas supply, manifold pressure, and valve operation before touching the electrode.

Light oxidation on a flame rod can be cleaned with a non-abrasive pad. A cracked insulator or a pitted probe means replacement. For the single-stage versus staged ignition logic found on furnaces generally, see Gas Furnace Ignition Lockouts: Single-Stage vs. Staged Systems.

Why Does a Freeze Stat Trip in Winter, and What Does It Shut Down?

A freeze stat trip is a response to a real low coil-side air temperature, not proof that the stat has failed. The older field guide covers the cooling-season version. The heating-season version is different: it is usually protecting a hot water or steam coil.

On the C-vintage RoofPak in IM 738-2, the freezestat is an automatic-reset device in the heating section. When it senses low air temperature at the coil, it shuts off the fans, opens the heating valve, and closes the outdoor air damper. The manual lists a factory setting around 38°F, adjustable from 35°F to 45°F, with a fixed 12°F differential. These values are model-dependent, so confirm them in the manual for the unit on the roof.

The same manual is blunt about the limits of protection. Hot water coils are not recommended where entering air falls below 40°F, no control system can guarantee against coil freeze-up, and glycol solutions are the only freeze-safe medium at low entering air temperature. The manual also names air stratification and failed outdoor dampers or preheat coils as freeze-up causes.

When a freeze stat keeps tripping, work this sequence:

1. Outdoor air damper position. A damper stuck open or leaking pulls subfreezing air across the coil.
2. Mixed-air stratification. Cold streaks across a coil face trip a stat that is measuring correctly.
3. Heating valve operation. A valve that fails to open on the trip leaves the coil unprotected.
4. The capillary. The freeze stat uses a sensing capillary filled with temperature-sensitive vapor, so a kink or crack compromises accuracy. A stat that trips with no cold spot anywhere on the coil usually has a damaged sensing element.

The catalog carries the 107193101 automatic freeze stat, listed as an SPDT low-limit control with automatic reset and a 240-inch vapor-tension capillary that senses the coldest point along any one-foot section. Route the capillary in a serpentine pattern across the coil face so the full face is covered, and avoid sharp bends at the mounting points.

How Does a Drifting Outdoor Air Sensor Cause Heating and Economizer Problems?

The controller bases economizer enable, lockout decisions, and some heating logic on outdoor air temperature. A sensor that has drifted gives the controller wrong information while logging nothing obvious. The unit lets in too much cold air, locks out when it should not, or shuts the damper at the wrong time.

The GEF1948K outdoor air/water temperature sensor is listed as a 10k ohm NTC thermistor for Daikin Applied and legacy McQuay rooftops. Test it by measuring resistance at the sensor and comparing it to the resistance-temperature chart for the current ambient temperature, then compare the controller's displayed reading to a calibrated thermometer. A reading that disagrees by more than a few degrees points to the sensor or its connector. Corroded or moisture-damaged connectors cause both drift and intermittent opens, so reseat and inspect the connector as part of any fall check.

What Is the Fastest Symptom-to-Cause Reference?

What Should Technicians Stock Before the First Cold Snap?

The parts above cover the most common Daikin-McQuay no-heat failures on the heat side. Confirm the ignition style first, since spark, hot surface, and combined assemblies are not interchangeable, then stock the matching ignition parts alongside the limit controls, because limit trips and ignition lockouts are the two most common reasons a rooftop heat section stops. Browse the full Daikin-McQuay catalog at GSIstore to confirm current availability.

Frequently Asked Questions: Daikin-McQuay Rooftop Heat Failures

Q: Why does a Daikin-McQuay rooftop shut down shortly after the fans start with no heat? A: On RoofPak-style controls, the controller expects the airflow-proving switch to close within about two minutes of fan start. If it does not, the unit shuts down and alarms, and heating never begins. Check belt condition, filters, VFD faults, and the switch tubing before opening the burner section.

Q: How can a technician tell a bad limit switch from a tripped one? A: With power removed and the plenum at room temperature, a good limit shows continuity. A limit open at room temperature has failed. A limit that is closed cold and opens only while firing is tripping on airflow, and the cause is a restriction, a failing fan, or a damper problem.

Q: What causes a flame rod to read a weak signal even when the burner lights normally? A: Soot or oxidation on the probe, a cracked ceramic insulator, or a poor burner ground. Clean light oxidation with a non-abrasive pad and recheck the microamp signal. A cracked insulator or pitted probe needs replacement.

Q: Does an automatic-reset freeze stat mean the unit restarts by itself after a trip? A: The stat resets itself once temperature recovers past its differential, but the unit may stay locked out if the control is programmed to hold an alarm after a freeze condition. Clear any latched alarm, and find the cold-air cause before assuming the stat is at fault.

Q: Can a limit with a higher temperature rating be used to stop nuisance trips? A: No. The rating is set to protect the heat exchanger, and a higher rating removes that protection. Find the airflow restriction causing the trips.

Compatibility note: Daikin-McQuay part numbers vary by unit model, serial range, and burner assembly. A part number listed above may not fit your particular unit. Confirm part-number-to-model-number compatibility against the unit nameplate and manual before purchase.