Rooftop units. Episode sixty-five of the Gas Service Animated series.

A packaged rooftop unit is mostly a furnace you already understand — burners, heat exchanger, induced draft, the same ignition sequences — bolted into the weather. What changes on the roof isn't the logic. It's the environment: wind, dust, vibration, and the ladder between you and the answer.

The gas heat section: tubular heat exchanger, two-stage or modulating gas valve, inducer, spark or hot-surface ignition, flame rod — inputs from sixty thousand to four hundred thousand Btu. The piping comes across the roof per the code: supported off the deck, drip leg at the unit, shutoff and union outside the panel, everything weather-rated because it lives outside.

Wind is the roof's signature fault-maker. It shoves on flue terminations and pressure-switch sensing, causing nuisance trips and short cycles on gusty days — a furnace that 'fails only in storms.' Check the flue and intake orientation against the manual, the gasketing on panels the wind pries at, and the pressure-switch hoses that vibration works loose.

Dust and vibration write the other two classics. Roof dust and oxidation glaze flame sensors — the microamp story from the electrical season, at altitude. And vibration cracks hot-surface igniters and loosens spade terminals. On a roof call, the flame-sense reading and a gentle igniter inspection come early, not late.

High-efficiency rooftops add winter's trap: condensate freezing in traps and drain lines that were fine all summer. A frozen trap backs water into the unit and trips pressure switches — another 'fails in the cold' signature. Heat tape, trap location, and slope per the manual are the fixes.

The code cares how you get there. Roof appliances come with access rules — safe access to the roof, a walkway or platform where required, service clearance around the unit, and lighting and a receptacle at the unit per the code's roof provisions. An RTU you can't safely reach is a code problem before it's a comfort problem.

The method doesn't change at altitude. Sequence first: call, purge, ignite, prove. The chain-walk works the same on a rooftop board. The analyzer still gets the last word through the flue port. Bring the residential method up the ladder — just add wind, dust, and vibration to your suspect list.

Roof discipline: ladder rules, tie-off where required, weather judgment, and panels secured before you leave — a loose RTU door is a sail. And the same gas rules as the ground: leak-test what you touched, at pressure, every time.

Rooftops in one breath. It's a furnace in the weather. Wind makes nuisance trips, dust glazes sensors, vibration cracks igniters, winter freezes traps. Access is code, not convenience. Same method, harsher courtroom. Next: the commercial kitchen.