Maintenance & Reliability · SOP
Make Up Air Unit (MUA) Gas Burner Inspection & Combustion Analysis
Standard procedure for performing a thorough inspection and combustion-efficiency analysis of a make‑up air unit gas burner. Covers gas pressure verification, flue‑gas sampling, burner tuning adjustments, and leak‑tightness confirmation to ensure safe, code‑compliant operation.
Estimated Time
90 min
Difficulty
Advanced
Regulation
CSA B149.1 / ASHRAE 62.1
Version
1.0
Purpose
To define the mandatory steps for safely inspecting, testing, and tuning the gas burner and combustion system on a make‑up air unit. Regular combustion analysis ensures the burner operates at peak efficiency (O₂ 3–5%, CO < 50 ppm air‑free), minimises fuel waste, reduces emissions, and prevents the formation of carbon monoxide. This procedure also verifies gas train integrity, flame signal reliability, and compliance with CSA B149.1 (Natural Gas and Propane Installation Code) and applicable local mechanical codes.
Scope
This SOP applies to all roof‑mounted or indoor make‑up air units equipped with atmospheric or power‑burner natural gas / propane burners rated up to 5,000 MBH (1,465 kW). It covers the complete inspection sequence — visual burner checks, manometer pressure readings, flue‑gas analysis (O₂, CO₂, CO, NOx, stack temperature), air‑shutter adjustment, and final gas leak verification. Units with modulating or electronically staged burners require manufacturer‑specific setup data; refer to the applicable OEM manual for trim‑pot and PID gain values.
Safety Precautions
- Positive lockout/tagout of the main gas supply valve and the unit disconnect switch. Verify zero energy state with a gas pressure gauge and an electrical contact voltmeter before opening any access panels.
- All gas connections and pressure taps must be leak‑tested with approved Snoop or equivalent non‑corrosive leak detection fluid. Never use a flame or open‑air source to check for leaks.
- Combustion flue‑gas contains carbon monoxide. Never operate the burner without the flue stack properly connected and vented to atmosphere. Use a personal CO monitor if working in an enclosed mechanical room.
- CSA‑rated safety glasses, cut‑resistant gloves, and flame‑resistant clothing (FRC) required when working on live gas equipment. Hearing protection mandatory if the MUA is operating during inspection.
- Burner housing and flue stack surfaces can exceed 260°C (500°F). Allow a minimum 15‑minute cool‑down period after burner shutoff before accessing internal components. Verify surface temperature with an infrared thermometer.
Required Tools & Materials
- Combustion flue‑gas analyser (O₂, CO, CO₂, NOx, stack temperature) — calibrated within last 90 days
- Digital manometer (0–35 in. w.c. range, ±0.1 in. w.c. accuracy)
- Gas‑rated Teflon tape or pipe dope (CSA‑listed for fuel gas)
- 3/4‑in. open‑end wrench and 1/2‑in. crescent wrench
- Flat‑blade screwdriver (insulated), Allen key set (metric/SAE)
- Leak detection spray (Snoop or equivalent) — non‑corrosive, non‑staining
- Intrinsically safe flashlight (Class 1, Div 2 rated)
- Calibration record tags / PM log labels (weatherproof)
- LOTO kit: padlock (6‑pin, key‑retaining), hasp, danger tag, gas valve blank flange (if required by site policy)
- CSA‑approved safety glasses, cut‑resistant gloves, flame‑resistant clothing
- Personal CO monitor (0–1000 ppm, low‑alarm 35 ppm, high‑alarm 200 ppm)
Procedure: Step-by-Step
1
Perform Lockout/Tagout (LOTO). Isolate the main gas supply valve (mechanical lock + hasp). Lock the unit disconnect switch in the OFF position. Verify zero energy using a gas pressure gauge (0 psi at the upstream tap) and a contact voltmeter (0 VAC at the unit power terminals). Attach your personal padlock and danger tag.
2
Gain access and visually inspect the burner compartment. Remove all access panels. Inspect the burner tray, flame rod, igniter electrode, and gas orifice for debris, soot buildup, corrosion, or cracking. Verify that the orifice is clean and the gap between the igniter and ground rod is within the manufacturer’s spec (typically 3–5 mm). Document any irregularities on the PM log.
3
Measure incoming gas pressure. Connect the digital manometer to the upstream pressure tap (between the main shut-off valve and the first regulator). Record static pressure. For natural gas the acceptable range is 5–14 in. w.c., for propane 11–13 in. w.c. If outside range, stop and notify the gas utility or facility manager before proceeding.
4
Measure burner manifold gas pressure. Locate the pressure tap downstream of the combination gas valve (manifold side). Connect the manometer and record the reading while the burner is OFF. Then restore power and gas (per LOTO removal procedure), initiate a call for heat, and record the manifold pressure once the burner fires and stabilises. Adjust the gas valve pressure regulator to the nameplate spec (typically 3.5 in. w.c. for natural gas).
5
Allow the burner to reach steady‑state. Let the MUA run for a minimum of 10 continuous minutes after the first ignition. This ensures the heat exchanger has fully warmed and the combustion chamber has stabilised, giving accurate flue‑gas readings.
6
Perform combustion analysis. Insert the flue‑gas analyser probe into the exhaust stack sampling port. The probe tip must be at least 12 in. downstream of the last bend and centred in the stack. Wait for the O₂ and CO readings to stabilise (typically 2–3 minutes). Record the following values: O₂ (%), CO₂ (%), CO (ppm air‑free), NOx (ppm), stack temperature (°C or °F), and ambient air temperature.
7
Evaluate combustion efficiency. Use the analyser’s built‑in efficiency algorithm (or an ASHRAE efficiency chart) to calculate combustion efficiency. Acceptable targets for a natural‑gas MUA: O₂ = 3–5%, CO < 50 ppm air‑free (preferably < 25 ppm), stack temperature ≤ 204°C (400°F) above the ambient temperature. If any parameter is out of range, proceed to Step 8.
8
Adjust combustion and re‑sample. If O₂ is too high (>5%), close the air shutter slightly (turn the locking screw, rotate the shutter disc 5° at a time). If O₂ is too low (<3%), open the air shutter. If CO is elevated, verify that the manifold pressure is correct, then adjust the air‑fuel ratio. After each adjustment, wait 3 minutes and re‑run the flue‑gas analysis. Repeat until all parameters are within the acceptable range. Record the final air‑shutter position and manifold pressure on the PM log.
9
Perform a gas‑tightness leak check. With the burner operating, spray Snoop (or equivalent) on every gas connection: main shut‑off valve, union, combination gas valve inlet/outlet, gas pressure regulator adjustment cap, and all threaded fittings. Bubbles indicate a leak — immediately close the main gas supply valve, relieve pressure, and re‑make the connection with fresh gas‑rated Teflon tape or pipe dope. Re‑apply leak detection spray and verify zero bubbles.
10
Close up and run a full cycle verification. Reinstall all access panels and safety interlocks. Reset the LOTO. Initiate a full heat cycle and observe the ignition sequence: spark → flame sense → stable main burner fire. Allow the unit to run through one complete cycle (fire → modulate → shut down). Verify that the flame signal stays above the minimum threshold (typically > 1.5 µA for rectification systems). Log all readings, adjustments, and observations on a weatherproof PM tag affixed to the unit panel.