Facility Managers Must Measure Temp Rise When an RTU Stops Heating

If an RTU is not heating, the most likely causes are a loss of power or control signal, a failed ignition or gas supply problem, blocked airflow or fouled heat-transfer surfaces, or an economizer stuck open. Treat any gas smell or carbon monoxide symptom as an emergency and evacuate immediately. Run the rapid checklist below before assuming the unit itself has failed.

TL;DR:

• Most heating failures are caused by a loss of power, airflow blockages, or control signal issues, not the unit itself.
• Confirm power, thermostat call, and airflow before suspecting ignition or gas problems, as these are the next most common causes.
• Gas pressure, flame sensor condition, and proper venting are critical; disruptions here often trigger safety lockouts or flame failure.
• Airflow restrictions from dirty filters or coils typically cause insufficient heat delivery, which can often be diagnosed by temperature rise measurements.
• Emergency safety responses must prioritize immediate evacuation and gas detection, with professional repair for ongoing combustion or safety faults.

Table of Contents

• Quick checks to run in the first 10 to 20 minutes
• How to verify electrical supply, disconnects, fuses, and control indicators
• Diagnosing gas ignition, pilot, valve, and gas-pressure problems
• How airflow restrictions and dirty coils stop heating
• Thermostat calls, safeties, pressure switches, and control logic
• When the economizer or outdoor air dampers are the real cause
• Tools, measurements, and a diagnostic sequence that produces evidence
• CO, combustion venting, and emergency safety steps
• Planned maintenance, ASHRAE expectations, and when to call a pro
• Field perspective from Brighton Air Corp technicians
• Getting an RTU back to heating fast
• Brighton Air Corp services and how to get help fast
• FAQ
• Sources

Quick checks to run in the first 10 to 20 minutes

Before pulling panels or calling for service, a few fast checks rule out the simplest failures and tell you how urgent the problem really is.

1. Verify power at the rooftop disconnect, the branch breaker, and any indicator lights on the control board.
2. Confirm the thermostat is actually calling for heat; a trained technician can test this by briefly jumpering the R and W terminals.
3. Check filters, supply registers, and the outdoor hood for anything blocking airflow.
4. Do a smell check for natural gas and note any unusual odors, sounds, or visible venting problems.
5. Look at the economizer and outdoor air damper position before assuming the heating components are at fault.

Pro Tip: Take a phone photo of the control board and any LED pattern before you touch anything. It saves the technician a trip up the ladder and speeds up phone diagnosis.

A unit that has power, a confirmed thermostat call, clean filters, and a closed economizer but still will not heat has almost certainly got an ignition, gas, or control fault, which points you straight to the next sections.

How to verify electrical supply, disconnects, fuses, and control indicators

Electrical faults are the most common reason an RTU sits dead with no response at all, and they are usually the safest category to check first.

• Confirm the rooftop disconnect switch is fully in the “on” position and has not been bumped or locked out during other roof work.
• Check the branch breaker at the electrical panel; a tripped breaker that re-trips immediately points to a short, not a nuisance fault.
• Inspect the low-voltage fuse on the control board; a blown fuse often means a pinched wire or a shorted component downstream.
• Read the control board LED pattern against the unit’s wiring diagram, since most boards flash a code for open safeties, miswiring, or communication faults.

Technicians use an isolation approach on low-voltage circuits, disconnecting legs one at a time and checking for a short before reconnecting, rather than replacing a fuse and hoping it holds. This isolation diagnosis method finds the exact failed component instead of burning through fuses one at a time. A simple jumper test from R to W can confirm whether the thermostat call itself is reaching the board, but this step involves live low-voltage wiring and is best left to someone trained on the unit’s wiring diagram.

Pro Tip: If a new fuse blows the instant power is restored, stop. That is a dead short, and repeated fuse replacement without isolation can damage the control board.

Stop troubleshooting and call a licensed electrician or HVAC contractor if you see scorched wiring, a breaker that will not reset, or any sign of arcing at the disconnect.

Diagnosing gas ignition, pilot, valve, and gas-pressure problems

When power and thermostat calls check out but the burner never lights, the fault sits somewhere between the gas valve and the flame itself.

1. Note the exact symptom: no flame attempt at all, ignition attempt with no sustained flame, or ignition followed by a lockout after a few seconds.
2. Count the LED flash code on the control board and compare it against the unit’s door-mounted fault chart.
3. Check for repeated lockouts, since a unit that fires once and then shuts down repeatedly usually points to a flame sensor or pressure switch issue rather than a dead valve.
4. Rule out an upstream gas supply problem by checking whether another gas appliance on the same line is working.

A manometer is the standard tool for confirming actual gas pressure against the manufacturer’s specification plate, and a digital thermometer is used alongside it to measure temperature rise once the burner lights. If the measured pressure does not match the manufacturer’s listed range, the cause is often a cracked hose or a restriction in the intake or exhaust piping, according to HVAC School’s troubleshooting guidance, not the gas valve itself. Water, debris, or insect nests in the combustion chamber are common and can block the pilot or flame sensor entirely. Combustion diagnostics have a hard limit for anyone without combustion analyzer training: if pressure readings are outside spec, if you smell gas, or if the burner lights unevenly, stop and call a licensed technician rather than adjusting a gas valve.

How airflow restrictions and dirty coils stop heating

Even a perfectly healthy burner cannot deliver warm air to the space if airflow through the unit is restricted. A dirty filter, a fouled heat exchanger surface, or a weak blower all show up as the same complaint: tepid or cold air at the registers.

• Pull the filter and hold it up to a light; if you cannot see light through it, replace it before doing anything else.
• Check the coil fins for dust buildup or debris, since a caked coil cuts airflow the same way a clogged filter does.
• Inspect the belt tension and look for a slipping or glazed belt on belt-drive units, and listen for a blower motor that sounds like it is straining.
• Check supply and return registers inside the building for furniture, pallets, or storage blocking airflow.

The clearest diagnostic for this category is a temperature rise measurement: take the temperature of the air entering the unit and the air leaving the supply side, then subtract. A temperature rise that falls outside the range printed on the unit’s data plate signals either an airflow problem (too low a rise usually means too much air) or a heat-transfer problem (too high a rise usually means too little air). A dirty filter or coil is routine preventive maintenance anyone on a facilities team can handle. A failed blower motor, a seized bearing, or a slipping belt on a unit that still will not restore proper airflow after cleaning calls for a service visit.

Thermostat calls, safeties, pressure switches, and control logic

The control board exists to protect the unit, and modern RTU controllers lock out heating on purpose whenever a safety input looks wrong rather than risk running the burner unsafely.

1. Verify thermostat wiring at both ends, confirming the call for heat is reaching the board; a jumper test from R to W confirms this, though it should be done by someone comfortable with low-voltage thermostat wiring.
2. Check the pressure switch, which confirms the inducer motor is pulling enough draft before it allows gas to flow; a switch that will not close often means a blocked vent or a weak inducer, not a bad switch.
3. Check the flame sensor for a buildup of oxidation, since a dirty sensor is one of the most frequent causes of ignition followed by immediate shutdown.
4. Record the fault code, the LED pattern, the measured voltages, and the time the fault occurred, since this timeline is what a technician needs to diagnose a unit quickly over the phone or on arrival.

Cycling power to the unit sometimes gets heat running again temporarily, but a controller that keeps locking out is telling you a safety condition is real, not nuisance. Repeated power cycling without finding the underlying cause tends to mask a deeper electrical or mechanical fault rather than fix it.

When the economizer or outdoor air dampers are the real cause

An RTU can have a perfectly good burner and still deliver cold air if the economizer damper is stuck open or mis-sequenced, since outside air mixing into the supply stream defeats the heat call entirely.

• Check the damper’s physical position; a damper stuck fully or partially open lets cold outside air bypass the heating process.
• Confirm the damper’s minimum position setting and check whether a building automation system is overriding it, since a faulty enthalpy or CO2 sensor can hold the damper open even in heating mode.
• Treat damper actuator failures or sequencing errors as a controls or economizer issue rather than assuming the rooftop equipment itself needs repair.

Tools, measurements, and a diagnostic sequence that produces evidence

A short, deliberate sequence of measurements turns a guess into a documented fault that a technician or warranty claim can act on.

1. Gather a manometer, a digital thermometer, a clamp ammeter, and a multimeter before starting.
2. Isolate low-voltage legs one at a time rather than testing everything live at once, which avoids blowing multiple fuses during diagnosis.
3. Record gas pressure against the data plate, temperature rise across the unit, and amp draw on the blower motor.
4. Compare each reading against the manufacturer’s specified range, since a reading within spec points to a supply-side issue while an out-of-spec reading points to a failed component.
5. Package the readings, the fault code, and timestamped notes into a single summary before calling a contractor.

Pro Tip: Photograph the unit’s data plate along with your readings. It gives the technician the exact spec range on the first call instead of a second trip to look it up.

CO, combustion venting, and emergency safety steps

Any combustion anomaly on a gas-fired RTU carries a carbon monoxide risk, and the response to that risk should never wait for a scheduled service visit.

• OSHA sets the permissible exposure limit for carbon monoxide at a low level averaged over an 8-hour period, with immediate removal of workers recommended at higher peak exposures, according to OSHA’s carbon monoxide fact sheet.
• If you smell gas or anyone reports CO symptoms such as headache, dizziness, or nausea, evacuate the area immediately.
• Ventilate only if it is safe to do so, then call emergency services and the gas utility from outside the building.
• Place CO detectors near occupied spaces and mechanical rooms, and keep short-term monitoring in place until a technician confirms the combustion system is safe.

Document the incident, including timestamps, symptoms reported, and detector readings, for compliance records and for the technician handling the repair.

Planned maintenance, ASHRAE expectations, and when to call a pro

Most “no heat” calls trace back to a maintenance item that was deferred, not a sudden mechanical failure.

• Schedule filter changes and coil cleaning on a fixed interval rather than waiting for a complaint.
• Include a combustion inspection, control board check, and motor and fan servicing in every seasonal visit.
• Follow a documented maintenance program, since ASHRAE’s guidance on operation and maintenance points to systematic, scheduled maintenance as the most reliable way to avoid emergency breakdowns and preserve equipment performance. A measurable rooftop maintenance routine, like the one outlined in E320 Air’s rooftop unit checklist, gives facilities teams a repeatable set of checks between professional visits.
• Call a licensed contractor immediately for any gas smell, repeated lockouts, or a fault that requires tools or authority beyond your team’s scope.
• Before the call, prepare photos of the unit and data plate, your measured readings, and a timeline of when the fault started and what you tried.

Field perspective from Brighton Air Corp technicians

In the field, the same handful of causes show up repeatedly: a dirty flame sensor, a clogged filter nobody tracked, and a pressure switch hose that cracked over a hot summer. Keeping spare flame sensors and filters on hand, along with clear roof access, can help shorten a service call. Logging fault codes and temperature rise readings over time often helps spot a developing problem before it leads to a loss of heat. On an emergency call, technicians typically ask for the fault code, recent maintenance history, and roof access details up front.

Getting an RTU back to heating fast

— John

Brighton Air Corp services and how to get help fast

A cold RTU on a winter morning costs more the longer it sits, and our technicians carry the diagnostic tools and parts inventory to find the fault on the first visit rather than guessing twice. We handle emergency RTU repair, commercial maintenance plans, and on-site diagnostics, with the same measurement-driven approach outlined above.

• Have your fault codes, recent maintenance records, and roof access ready before service technicians arrive.
• Photos of the control board and data plate speed up our initial assessment over the phone.
• A maintenance plan can catch filter, coil, and pressure switch issues before they cause a no-heat emergency.

Schedule a service visit or request a commercial estimate and we will walk you through what to check before the technician arrives.

FAQ

How do you reset a heating unit?

Most RTUs reset by cycling the thermostat off, then switching power off at the disconnect before restoring it. A reset clears a single nuisance lockout, but a unit that locks out again right away has a real fault that a reset will not fix.

Why is an HVAC system blowing cold air when the heat is set on?

The most common causes are a thermostat not properly calling for heat, a failed ignition or gas supply issue, or an economizer damper stuck open and mixing in outside air. Checking the furnace blowing cold air scenario against the quick checklist above usually narrows it down within minutes.

Why does a heat pump blow cold air in heat mode?

A heat pump in heat mode can blow cold air during a normal defrost cycle, a reversing valve stuck in cooling position, or frozen outdoor coils restricting the refrigerant cycle. If the air stays cold well beyond a brief defrost period, the reversing valve or refrigerant charge needs a technician’s inspection.

Why does an HVAC unit run but not blow cool air?

A unit that runs but delivers little or no conditioned air usually has a restricted filter, a frozen coil, or a failing blower motor limiting airflow. Measuring the temperature difference between the return and supply air quickly confirms whether the problem is airflow or the heat-transfer process itself.

What do RTU error codes usually mean for heating failures?

RTU control boards use flash codes to indicate specific faults such as an open high-limit switch, a failed pressure switch, or a flame sensor that is not detecting flame. The exact meaning of each code is printed on the unit’s control board label or wiring diagram, and matching the code there is the fastest way to narrow down the cause before calling for service.