Seasonal Changeover From Cooling to Heating

Purpose

The heating side sits unpowered and unfired for roughly half the year, and the faults it develops in that time are absence faults rather than wear faults: a rodent nest in a warm burner compartment, an inducer seized by summer humidity, a condensing trap dried out and no longer sealed, a vent connector knocked loose by another trade, a rollout switch that opened in March and was never reset. All present identically as no heat, on the same night, which is the first hard cold night when call volume triples.

This procedure moves that discovery to a daylight visit and guarantees one thing: the appliance is fired under a technician's eyes, with its safety chain intact and its readings inside the rating plate, before the weather asks it to run unattended.

Safety actions that gate this procedure

• Gas odor at any point: everyone leaves immediately. No switches, no lights, no phone used inside, no hunt for the source. Call the gas utility's emergency line and the shop from outside and upwind, and re-enter only when the utility releases the building.
• Personal carbon monoxide monitor on before entering any space where a fuel-fired appliance will run, ambient reading recorded appliance-off so the later comparison is against this building's baseline rather than an assumed zero.
• Panel, cover and disconnect work is done by a qualified person under 29 CFR 1910.333(b)(2), de-energized where the check can be made dead, with the meter proved on a known live source before and after per NFPA 70E-2021, 120.5. Without that before-and-after check a meter with a blown fuse reads a clean, believable zero.
• Vacuuming or brushing a burner compartment liberates settled dust and, where rodents have been in it, dried droppings. That route is inhalation: wet the material before disturbing it and work in respiratory protection matched to that route under a written program per 29 CFR 1910.134. No program means no dry cleanout; wet-wipe or rebook.
• First fire after a summer idle burns off dust and smells like it. Warn the occupant before lighting and ventilate the space.

Scope

Covers switching a residential or light-commercial system from cooling to heating: securing the cooling side, inspecting and servicing the heating side before energy goes back in, first fire, running readings, and control changeover.

Does not cover the technical method inside any single task. Furnace annual maintenance, combustion analysis and tuning, flue and venting inspection, and ignitor and flame sensor replacement own those and are called rather than restated. Dual-fuel and heat-pump changeover runs under the dual-fuel SOP; a boiler under boiler annual service; an appliance that proves unsafe under the red-tag SOP.

Roles and responsibilities

Procedure

1. Identify the appliance, its fuel and how changeover is commanded, before a tool comes out. Read the plate for type, input, rise range and manifold pressure; read the thermostat for whether changeover is manual, outdoor-temperature driven, or a dual-fuel lockout. Acceptance: type, fuel, rise range and manifold value on the ticket from the plate before anything opens. What wrong looks like: assuming a furnace because there is ductwork, on a job that is a fan coil with a boiler behind a closet door. Stop rule: a dual-fuel or heat-pump changeover leaves this procedure for the dual-fuel SOP. Hazard: none here, it is reading plates in the open.

2. Secure the cooling side without stranding the compressor. Decide the outdoor disconnect deliberately rather than by habit: where the manufacturer's installation manual specifies a crankcase heater energization period before a compressor start, either the disconnect stays closed all season or that period is served before spring startup. Acceptance: the disconnect state written on the ticket with the manual's stated period beside it. What wrong looks like: a disconnect pulled in October out of tidiness, then a spring start with refrigerant migrated into a cold crankcase. Stop rule: no manual on site, leave it closed and note why. Hazard in this step: the condenser fan can start on a call nobody made, so open the disconnect and watch the blade stop before a hand crosses the guard.

3. Inspect the burner compartment, the vent and the clearances with gas and power still off. Look for nesting material, rust flakes on the burner floor, a connector backed out of an elbow, a blocked termination, and combustibles inside the plate's clearance. Acceptance: every vent joint fully engaged and fastened as its listing requires, termination clear to the manufacturer's dimension, compartment free of foreign material. What wrong looks like: a vent seated at the appliance collar and separated at the elbow above it, invisible from the front. Stop rule: any vent defect, rollout evidence or scorching means the appliance is not fired, gas closes at the appliance shutoff, and the service manager is called from site. Hazard in this step: disturbing nesting material puts an inhalation hazard in the air, so wet it before removal under the respiratory control above.

4. Service the ignition and flame-proving path while the appliance is cold and isolated. Clean the flame sensor to bare metal with an abrasive pad and no lubricant, inspect the ignitor for cracks or chalking, confirm pressure switch tubing is clear and not water-logged, and confirm a condensing appliance's trap is primed rather than dry. Acceptance: sensor cleaned, ignitor intact, tubing clear, trap filled to its mark. What wrong looks like: an ignitor handled with bare fingers, leaving a hot spot that fails weeks into the season. Stop rule: a cracked ignitor or a sensor that will not clean up is replaced under its own SOP before firing, never fired and watched. Hazard in this step: gas stays closed and the disconnect open while hands are in the compartment.

5. Restore gas and power, warn the occupant, and fire under observation. Before restoring, confirm limit and rollout are closed on their own, and establish why any tripped device opened rather than resetting and lighting. Acceptance: the published sequence in order, inducer to pressure switch proof to ignitor to gas valve to flame on the first attempt, blower on at its own timing, and no rise above the step 1 ambient on the monitor. What wrong looks like: a device found tripped, reset, fired anyway, reaching the same failure one cycle later with a part number on the invoice. Stop rule: any lockout, retry or monitor rise shuts the appliance at its gas valve and ventilates before the next measurement. Hazard in this step: fuel, ignition energy and motion all go back in here with the customer in the building, so keep the occupant out of the mechanical space, stand clear of the blower door, and jumper no safety device.

6. Take the running readings and hold them against the plate, not a remembered range. Temperature rise measured in the plenums clear of line of sight to the heat exchanger, manifold pressure at the tap, total external static across the air handler. Acceptance: rise inside the plate's printed range, manifold at the plate value for the fuel supplied, static at or under the plate maximum, each as a number with its unit. What wrong looks like: a rise probe seeing the heat exchanger, reading high and sending a tech after an airflow fault that does not exist. Stop rule: rise above the plate range is resolved before the visit closes, not noted for next year. Hazard in this step: readings come off a firing appliance, so the monitor stays on the tech, probe holes are sealed after use, and no panel is left off while it runs.

7. Change the control over, confirm the occupant can work it, and close the record. Set the stat to heat, cycle it once from the stat rather than the board, set the fan mode the customer wants, and show them the filter they will change mid-season. Acceptance: one full call started and satisfied from the thermostat, operated once by the customer, every reading and disposition on the ticket. What wrong looks like: a system proved at the board and never at the stat, where the stat wiring was disturbed over the summer. Stop rule: an appliance left off leaves in writing, with the reason and the heat state the building is in. Hazard: none, panels are closed and the equipment is in its final state.

The record this produces

One changeover record per system: plate data from step 1, the disconnect decision with the manual's pre-energization period, the vent and compartment result, ignition path work performed, both ambient monitor values, the three running readings against their plate limits, and every open item with a disposition.

Two readers use it. The tech dispatched at 9pm in January, who can see whether this rise was already at the top of its range in October and whether the vent was signed off. The service manager next autumn, who books the accounts whose numbers drifted rather than the ones that shout.

Worked pass: 24-year-old induced-draft furnace, October changeover

Step 1 read the plate: rise range 35 to 65 F, manifold 3.5 in w.c. on natural gas, maximum total external static 0.50 in w.c. The thermostat was a manual heat and cool stat with no outdoor sensor, so the dual-fuel exclusion did not apply. Step 2 found the manual in the cabinet pocket and left the disconnect closed for the season on its instruction.

Step 3 failed. The vent connector was seated at the appliance collar and separated roughly a finger's width at the elbow above it, hidden from the front by a joist. Under the stop rule the appliance was not fired, gas closed at the appliance shutoff, and the service manager called from site. Repair was authorized inside the visit, the connector re-engaged and fastened at each joint per its listing, and step 3's acceptance re-run and passed before anything else.

Step 4 cleaned the flame sensor, found the ignitor intact and the pressure switch tubing clear, and this appliance being non-condensing there was no trap to prime. Step 5 fired on the first attempt with the sequence in order. Ambient carbon monoxide read 0 ppm before firing and 0 ppm through the run, so there was no rise.

Step 6 read return air 70 F and supply 132 F. 132 minus 70 leaves a 62 F rise, inside the plate's 35 to 65 F range and 3 F under its top. Manifold read 3.4 in w.c. against the plate's 3.5. Static read 0.44 in w.c. against a maximum of 0.50. All three cleared their gates, so the rise triggered no stop rule, but 3 F of headroom on a 30 F wide range was written as a recommendation-class finding on the return-air path rather than left as a passing tick. Step 7 then ran one full call from the thermostat, satisfied it, and had the homeowner start it once themselves.

What the failure teaches: that joint had been spilling combustion products into the basement for part of the previous season, and it is invisible from the angle a hurried tech looks from. It was found because step 3 runs on a cold appliance with a light behind the elbow. Fired first and inspected after, the same visit ends with a running furnace, a clean reading at head height, and a defect nobody looked for.

References

• Equipment rating plate for temperature rise range, manifold pressure, maximum total external static pressure and clearance to combustibles
• The manufacturer's manual for that model, for the crankcase heater pre-energization period in step 2 and the sequence of operation used in step 5
• 29 CFR 1910.333(b)(2), electrical safe work practices for general industry, with NFPA 70E-2021, 120.5 for the proving sequence
• 29 CFR 1910.134, respiratory protection program requirements gating the cleanout in step 3
• See related: Furnace Annual Maintenance; Flue and Venting Inspection on a Service Call; Dual Fuel Changeover Setup and Verification