Packaged Gas/Electric Unit (Gas Pack): Anatomy and Service

What it is

A packaged gas/electric unit — a "gas pack" — combines a gas furnace and an electric air conditioner in a single outdoor cabinet. Gas burners and a heat exchanger provide the heat; a normal compressor/condenser refrigeration circuit provides the cooling; one blower serves both. It's the gas-heat cousin of the packaged heat pump: same all-in-one packaging and single-point power, but heat comes from combustion instead of the refrigeration cycle.

You'll find them on residential rooftops and ground pads (especially where there's gas but the layout favors a package unit) and all over light commercial as the workhorse rooftop unit. The refrigeration side is the AC you already know; the heating side is a gas furnace you already know — the new part is that they share a cabinet outdoors, draw one power feed, and the combustion has to be vented and managed in a unit sitting in the weather.

How it's built

One cabinet, divided into sections:

• Return/filter section — return air enters and passes the filter rack (commercial racks are big; check them every visit).
• Blower section — a squirrel-cage blower (belt-drive on bigger commercial units, direct-drive PSC/ECM on residential and many newer units). Serves both heating and cooling.
• Indoor (evaporator) coil — cools and dehumidifies; drains condensate to a pan and out of the cabinet.
• Gas heat section — burners, manifold and gas valve, a heat exchanger, an inducer, hot-surface igniter, flame sensor, limit and rollout switches, and a flue that vents out of the cabinet. This is a full gas furnace built into the box.
• Compressor(s) and condenser section — the refrigeration circuit and the outdoor coil/fan rejecting heat, same as any condensing unit.
• Controls — control board(s), contactors, transformer, gas-heat safeties, and the low-voltage terminal block.
• Single-point power and the curb/duct interface — one power feed; ducts connect through the cabinet (down-flow through a roof curb, or horizontal on a ground/through-the-wall setup).

How it's powered, gassed, and ducted

Power: one electrical feed (240V single-phase residential, often three-phase commercial) lands at the unit. The single-point MCA/MOCP on the plate covers the whole unit. Gas: a gas line runs to the unit's gas valve with a sediment trap and shutoff; manifold pressure is set to the plate (≈3.5" w.c. NG, ≈10–11" w.c. LP). Combustion air and venting: the burners need combustion air and the flue products have to vent out of the cabinet to the weather — the unit is built and listed for this, but the flue/vent hood has to be clear and intact. Ducts: supply and return tie to the cabinet openings; a rooftop gas pack drops its ducts through the curb.

In the field — sequence of operation

Cooling: thermostat calls Y (and G) → blower runs → contactor pulls in → compressor and condenser fan start → the AC cools and dehumidifies the return air → runs until satisfied. Identical to a split AC, just packaged.

Heating: thermostat calls W → the gas section runs its furnace sequence — inducer energizes, pressure switch proves draft, igniter warms, gas valve opens, burners light, flame proves on the sensor, igniter drops out → after a delay the blower starts and pushes heated air into the supply → on shutdown gas closes, inducer post-purges, blower runs its off-delay. (Component-level detail is in the gas-heat sequence material.) Second-stage heat (W2) adds capacity on two-stage units.

Normal values & targets

• Temperature rise (heat): must land in the rating-plate rise band, commonly ~35–65°F. Measure return and supply and confirm.
• Manifold pressure: to the plate — ≈3.5" w.c. NG / ≈10–11" w.c. LP nominal.
• Flame-sense current: ~1–6 µA DC healthy.
• Cooling side: subcool/superheat to the application and conditions; condenser split ~15–25°F at design on R-410A.
• Airflow: ~350–450 CFM per ton cooling; heat airflow set to hit the rise band. Measure external static pressure — packaged units are rated against a design ESP and high static (dirty filter/coil, bad ducts, closed dampers) drops capacity.
• Three-phase (commercial): all legs present and balanced; check scroll rotation.

Common faults & what they mean

• No heat. Same gas-furnace stalls as any furnace — pressure switch not proving (hose, port, condensate on condensing models, restricted flue), igniter, flame sense, gas supply, or a tripped limit/rollout. Watch the sequence and note where it dies.
• No cooling. Compressor/contactor/capacitor failure, dirty condenser, or a charge problem — diagnose like any condensing unit.
• Short cycling on the limit in heat. Airflow loss — dirty filter, slipping belt (belt-drive), closed registers, weak blower — overheats the exchanger and trips the high-limit. Fix the airflow; don't jumper the limit.
• Cracked heat exchanger. Flame distortion when the blower starts, CO in the supply, or visible cracking — condemn and shut down. A package unit's exchanger lives through brutal outdoor thermal cycling.
• Water/air leaking at the curb or duct seal. A bad curb gasket or duct connection on a rooftop unit dumps conditioned air and lets weather in — a package-specific failure that mimics low capacity. Condensate must drain off the unit, not pool in the base or hit the roof membrane.
• Whole unit dead. Single-point power — disconnect, breaker, or the one feeder. One tripped breaker kills both heat and cool.
• Spillage / blocked flue. Outdoor flue hoods get blocked by debris, nests, or weather; verify draft and test CO.

Tech tips & gotchas

• It's a gas furnace and an AC sharing a box — diagnose each side with the tools you already have. Heat side: furnace sequence, temperature rise, manifold pressure, flame sense, CO. Cool side: superheat/subcool, condenser split, capacitor, contactor.
• Check the curb and duct seal on rooftop gas packs. Air/water leakage there is a classic package complaint that looks like low capacity. A split system never has this failure point.
• Single feeder = single point of total failure. A completely dead unit (no heat, no cool, no lights) is usually power — check the disconnect and the one breaker before opening anything.
• CO and exchanger checks matter more outdoors. Package-unit exchangers cycle through huge outdoor temperature swings and the flue hood is exposed to weather and critters. Test combustion and inspect the exchanger and flue on every heat-season visit.
• Belt-drive units need PM the residential world skips (on commercial gas packs): belt tension/condition, sheave alignment, bearings, and CFM setting. A slipping belt quietly kills both heating and cooling capacity.
• Measure static pressure. It catches airflow problems on a package unit that otherwise masquerade as refrigerant or heat-exchanger problems.

Safety / code notes

• Combustion air and venting follow the appliance listing and the fuel-gas/mechanical code — the unit is built for outdoor combustion, but the flue/vent hood must be clear and intact; never shortcut it.
• Any spillage, blocked flue, or suspected cracked exchanger: verify draft and test CO with an analyzer before returning to service. Never jumper limits or rollouts.
• Single-point power: data-plate MCA/MOCP governs the feeder and overcurrent protection; a disconnecting means within sight is required per NEC §440. Lock out and discharge capacitors before service.
• Gas piping, shutoff, and sediment trap per the fuel-gas code; leak-check joints you open.
• Rooftop work is fall-hazard work — follow fall protection, mind curbs and edges. The curb must support the unit and seal the duct; condensate drains per code, not onto the roof membrane.