The Gas Furnace Start-Up Sign-Off SOP

Purpose

A new furnace that lights and blows warm air has passed nothing. Over-fired input cracks heat exchangers, a rise outside the plate range trips limits or condenses in an 80 percent appliance, a vent pitched wrong locks a condensing furnace out on the first cold night, and a pressure switch nobody tested is the one that fails closed. This procedure makes every new furnace leave with the same set of measured numbers on one sheet, filled by the installing tech and reviewed by the lead before the job closes, so a callback starts from a baseline instead of from memory.

It names the owning card for each reading. It does not teach the methods; it fixes which readings are taken, in what order, what passes, and who signs.

Scope

Covers the start-up of every new or replacement natural gas or propane forced-air furnace this shop installs, single or two-stage, 80 or 90-plus percent, before the job is closed in the system.

Does not cover the cooling side, which the condensing unit commissioning SOP owns, or a fuel conversion, which follows How to Convert a Furnace From Natural Gas to Propane before this sheet starts. Does not cover the annual maintenance visit, which uses the maintenance checklist and not this sheet. New gas piping is pressure tested under the plumbing or gas permit before step 3; this SOP checks joints at the appliance, not the whole run.

Roles and responsibilities

Procedure

1. Read the rating plate and record it on the sheet before anything else. Copy fuel type, input (high and low fire on a two-stage), temperature rise range, maximum external static, and the inlet and manifold pressure figures. Acceptance: plate fuel matches the gas at the meter and every plate field on the sheet is filled. What wrong looks like: a natural gas plate on a propane supply, or a blank rise range. Stop rule: fuel mismatch means the furnace does not fire until converted. Hazard: none at this step beyond the cabinet's sharp sheet metal edges; gloves on. See related: Furnace Rating Plate Read Before Start-Up.

2. Inspect the vent and combustion air before the first fire. Walk the whole vent: material matches the furnace's vent category, slope back toward the furnace on a condensing unit as the manual specifies, joints cemented or locked, supports at the manual's spacing, terminations at their required clearances from windows, doors and grade. Acceptance: no sag, no open joint, every termination clearance measured and recorded. What wrong looks like: a belly in a PVC run, a B-vent connector with screws missing. Stop rule: any vent defect is corrected before the burner lights, because the first fire puts flue gas through it. Hazard: attic and roof work is a fall and heat hazard; ladders tied off and no attic travel off the framing. See related: Furnace Vent Categories and What Each Allows.

3. Leak check the appliance gas connection and read static inlet pressure. Open the appliance shutoff, soap every joint from that valve to the gas valve inlet with leak solution or sweep with a combustible gas detector, then connect a manometer to the inlet tap and read static. Acceptance: no bubbles and no detector response; static inlet inside the plate's minimum and maximum. What wrong looks like: a slow bubble at the union or flex nut. Stop rule: any leak, close the appliance shutoff, remake the joint and recheck before continuing; a gas smell through the house means everyone leaves, nobody touches a switch, and the call goes to the utility from outside. Hazard: an open test port leaks gas; it gets its screw back the moment the gauge comes off.

4. Fire the furnace, read inlet under load, and set manifold pressure. With the furnace on high fire and other gas appliances running, read inlet again, then read manifold and adjust only at the gas valve regulator to the plate figure. Acceptance: inlet under load still inside the plate range, and manifold at the plate figure for each stage. What wrong looks like: inlet dropping below the plate minimum with the water heater on, which is an undersized supply. Stop rule: low inlet under load stops the start-up and goes to the lead as a piping issue, not a valve adjustment. Hazard: the burner compartment is open and firing; hands clear of the flame path and a personal CO monitor running at breathing height. See related: Gas Pressure Set-Up Numbers Field Card.

5. Clock the meter to confirm input. With every other gas appliance off, time one revolution of the smallest test dial, convert to cubic feet per hour, and multiply by the utility's heating value. Acceptance: clocked input at or below plate input and no more than 5 percent under it, which is this shop's default band. What wrong looks like: input over plate, which over-fires the heat exchanger. Stop rule: over plate, adjust manifold within the range the manual permits and re-clock; outside that range, orifices are wrong for the gas or altitude and the lead is called before any orifice comes out. Hazard: turning off the water heater and range is a relight and a customer notice, written on the sheet so step 10 restores them. See related: How to Clock a Gas Meter to Check Furnace Input.

6. Measure temperature rise at steady state. After at least 10 minutes on high fire, read return air before the filter and supply air out of the line of sight of the heat exchanger. Acceptance: rise inside the plate range, recorded for each stage on a two-stage furnace. What wrong looks like: a rise at or past the top of the range, which is low airflow, or under the bottom on an 80 percent furnace, which risks condensing in the heat exchanger. Stop rule: outside the range, correct the blower tap and re-read before going on. Hazard: probe holes are drilled in the duct, not the cabinet, and every hole gets a plug. See related: How to Measure Temperature Rise on a Gas Furnace.

7. Measure total external static. Read static between the filter and the blower and in the supply plenum ahead of any coil, and add them as magnitudes. Acceptance: total at or below the plate maximum. What wrong looks like: a total over plate, which means the blower is moving less air than its table promises even when the rise in step 6 passed. Stop rule: over plate, the sheet goes to the lead with both readings; the furnace can heat while duct correction is scheduled, but the sheet stays unapproved. Hazard: none beyond step 6, the same plugged test holes. See related: How to Measure Total External Static Pressure on a Furnace.

8. Take combustion readings in the flue and the room. Sample the vent at the manual's test location after steady state; record O2, CO2 and CO air-free, then ambient CO at the furnace and in the nearest living space. Acceptance: O2 and CO2 inside the maker's stated range, flue CO air-free under the maker's limit or this shop's 100 ppm action level, whichever is lower, and ambient CO at 0 ppm. What wrong looks like: rising CO as the furnace warms. Stop rule: flue CO over the action level, shut the furnace off, red-tag it and call the lead; any ambient CO, the furnace goes off and the space is ventilated before diagnosis. Hazard: the flue pipe is hot enough to burn; the probe goes in and out by its handle. See related: Gas Furnace Combustion Analysis and Tuning.

9. Prove the safety circuit. Prove the high limit opens by the method the manual names, usually removing blower power while firing and timing burner shutdown; confirm the rollout switch shows continuity and is in the circuit; stop the inducer and confirm the pressure switch drops the call. Acceptance: limit opens and burners shut off; inducer off means no ignition. What wrong looks like: burners still lit with the inducer stopped. Stop rule: any safety that does not open, red-tag and call the lead; nothing is jumpered at any point, and rollout is never induced by blocking the burner or vent. Hazard: line voltage on the blower and inducer leads; every lead is moved with power off at the service switch and proven dead with a meter checked on a known live source before and after, and the blower lead is reconnected the same way and the blower seen running on the next call before step 10. See related: Inducer and Pressure Switch Diagnosis.

10. Restore and hand over. Panels on, door switch closed, other gas appliances relit and checked, a CO alarm present on that level, thermostat set and cycled twice through a full call. Acceptance: two clean cycles with no lockout code and every appliance from step 5 relit. What wrong looks like: a water heater left off. Stop rule: any lockout during handover reopens the step that produced it. Hazard: relighting a standing pilot appliance follows its own label, and the tech stands clear of the burner opening when it lights.

The record this produces

The start-up sheet: plate data, vent findings with termination clearances, inlet static and under load, manifold for each stage, clock time and computed input, rise for each stage, total external static with both component readings, O2, CO2, flue CO air-free and ambient CO, the three safety results, every triggered stop rule with what was done, the installing tech's name, and the lead's approval with the date. It is attached to the job in the system. The first maintenance tech compares against it, and on a heat exchanger warranty claim it is the evidence the furnace was set up to its plate.

Worked pass: a two-stage 96 percent furnace, one stop rule triggered

FURNACE START-UP SHEET        job 2611  2-stage 96%
Plate input      100,000 hi / 65,000 lo  nat gas
Plate rise       35 to 65 F   max ESP 0.50 in wc
Plate inlet      min 4.5  max 10.5 in wc
Plate manifold   3.5 hi / 1.7 lo in wc
Vent             PVC 2 in, slope back, 3 hangers
Terminations     all clearances measured OK
Inlet static     7.0 in wc   under load 6.6
Manifold set     3.5 hi / 1.7 lo
Clock 1          2 cu ft dial 72 s = 103,000  FAIL
Stop rule        manifold 3.5 -> 3.3, in manual
Clock 2          2 cu ft dial 75 s = 98,880   PASS
Rise             hi 50 F (68 in, 118 out)  PASS
TESP             0.18 + 0.28 = 0.46 in wc  PASS
Combustion       O2 8.2  CO2 7.1 pct  CO 18 ppm af PASS
Ambient CO       0 ppm furnace and hall    PASS
Limit / rollout / pressure switch  PASS PASS PASS
Restore          WH relit, 2 cycles clean  PASS
Tech: J. Ortiz   Lead review: approved 10/14

Clock 1. One revolution of the 2 cu ft dial took 72 seconds: 3,600 divided by 72 is 50 revolutions an hour, times 2 cu ft is 100 cu ft per hour, times the utility's 1,030 Btu per cu ft is 103,000 Btu/h. That is 3,000 over the 100,000 plate, 3 percent, so step 5's acceptance failed and its stop rule triggered.

The fix. Manifold was already at the plate's 3.5, so the over-fire came from gas richer than the plate assumes, not from a wrong setting. The manual permits trimming manifold to bring input to rating, and 3.3 sat inside its stated band. Flow goes roughly with the square root of manifold pressure, and the square root of 3.3 over 3.5 is about 0.97, so 103,000 should fall to about 100,000.

Clock 2. 75 seconds: 3,600 divided by 75 is 48 revolutions an hour, times 2 is 96 cu ft per hour, times 1,030 is 98,880 Btu/h. That is 1,120 under plate, 1.1 percent, inside the 5 percent band. The sheet records both clocks and the new manifold figure.

The rest. Rise 118 minus 68 is 50 F, inside 35 to 65. Static 0.18 plus 0.28 is 0.46 in wc, under the 0.50 maximum. Low fire was clocked and its rise read the same way on the sheet's back side. The lead reviewed the sheet against the plate, checked that the stop rule recorded what was changed and why, and approved.

References

• See related: Gas Pressure Set-Up Numbers Field Card
• See related: Gas Furnace Combustion Analysis and Tuning
• See related: How to Clock a Gas Meter to Check Furnace Input
• See related: Inducer and Pressure Switch Diagnosis
• Furnace manufacturer's installation manual and rating plate for every acceptance value