3.2 Electrical Multimeters, Combustion Analyzers & Electronic Leak Detectors

Electrical Multimeters, Combustion Analyzers & Leak Detectors

Modern gas appliances rely on solid-state electronics, microprocessor controls, flame rectification sensors, and power draft inducers. Safe servicing requires competence not only with gas pressures, but also with digital multimeters (DMMs), electronic combustible gas detectors, approved chemical bubble solutions, and flue gas combustion analyzers. Under Ontario Regulation 215/01 and the CSA B149.1 Installation Code, an Ontario Gas Technician 3 (G3) must master these diagnostic tools to isolate electrical faults, confirm flame safety shutdown, verify piping gas-tightness, and evaluate combustion efficiency.

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Digital Multimeters: Meter Selection, CAT Ratings & Safety

Gas technicians service line voltage ($120\text{ VAC}$) and low-voltage control circuits ($24\text{ VAC}$, $5\text{ to }24\text{ VDC}$). Using an unrated meter presents serious shock and arc-flash hazards.

Under IEC/CSA 61010 standards, test instruments are classified by Overvoltage Categories:

• CAT I: Signal-level electronics and circuit boards isolated from mains.
• CAT II: Portable tools and single-phase plug-in branch receptacles.
• CAT III: Distribution wiring, feeder panels, and hardwired equipment including furnaces, boilers, and water heaters.
• CAT IV: Origin of utility electrical service, outdoor conductors, and electricity meters.

Always practice the Live-Dead-Live protocol: test the multimeter on a known live voltage source, verify zero voltage on the de-energized appliance circuit, and re-test on the known live source to confirm meter operation.

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Core Electrical Diagnostics on Gas Heating Equipment

A professional DMM performs four primary measurements in heating system diagnostics:

1. AC Voltage ($120\text{ VAC}$ and $24\text{ VAC}$)

• Line Voltage: Line ($L_1$) to Neutral ($N$) confirms nominal utility power ($115\text{ to }125\text{ VAC}$). Measuring $L_1$ to Ground should also read $120\text{ VAC}$.
• Polarity and Grounding: Neutral to Ground must register under $2.0\text{ VAC}$ (ideally $0.0\text{ VAC}$). A reading of $120\text{ VAC}$ indicates reversed polarity, which blinds flame safeguard controls and triggers safety lockouts.
• Low-Voltage Control: The step-down transformer secondary supplies $24\text{ VAC}$ between $R$ and $C$. A reading of $24\text{ VAC}$ across safety switch terminals indicates an open safety control.

2. Resistance and Continuity (Ohms, $\Omega$)

Resistance testing must only be performed on de-energized, isolated circuits:

• Gas Valve Coils: Dual solenoid shut-off valves contain coils typically measuring $20\ \Omega\text{ to }100\ \Omega$. An infinite reading ($\text{O.L.}$) indicates an open, failed coil.
• Hot Surface Igniters (HSI): Silicon carbide igniters measure $40\ \Omega\text{ to }100\ \Omega$ cold at room temperature; replace if above $100\ \Omega$. Silicon nitride igniters operate between $10\ \Omega\text{ and }20\ \Omega$.
• Transformer Windings: The primary winding typically measures $50\ \Omega\text{ to }150\ \Omega$; the secondary measures $1\ \Omega\text{ to }5\ \Omega$.

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Flame Rectification Testing (DC Microamperes)

Modern automatic gas burners verify flame presence through flame rectification. Ionized gas molecules in combustion flames conduct electrical current.

Because the grounded burner metal surface area is much larger than the slender flame rod (typically a $4:1$ ratio or greater), alternating current ($120\text{ VAC}$) applied to the sensor passes more readily in one direction. The flame acts as a half-wave rectifier, converting AC voltage into a tiny direct current measured in DC microamperes ($\mu\text{A}$):

$$1\ \mu\text{A} = 1 \times 10^{-6}\text{ A} = 0.000001\text{ A}$$

Diagnostic Procedure

To measure flame rectification signal:

1. Disconnect power to the appliance and remove the flame sensor lead wire.
2. Configure the DMM to DC Microamperes ($\text{DC }\mu\text{A}$).
3. Connect the meter test leads in series: one lead to the flame rod terminal and the other to the wire leading to the control board.
4. Restore power, initiate a heating call, and record the microampere reading once main flame establishes.

Low readings typically stem from non-conductive silica or carbon glaze on the rod, an oxidized burner ground wire, or flame lifting caused by excessive manifold pressure. Clean rods with fine steel wool or a fine abrasive pad as the appliance manufacturer directs; never use coarse sandpaper.

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Combustible Gas Leak Detectors (Sniffers)

Electronic combustible gas sniffers detect hydrocarbon fuel gases at concentrations down to roughly $20\text{ to }50\text{ ppm}$ (about $0.04\%\text{ to }0.1\%$ of the methane LEL).

Sensor Mechanisms & Protocols

• Catalytic Bead Sensors (Pellistors): Combustible gas oxidizes on a heated catalytic bead, raising temperature and resistance relative to an inactive reference bead in a Wheatstone bridge.
• Metal Oxide Semiconductor (MOS) Sensors: Fuel gas reacts with a heated tin dioxide ($\text{SnO}_2$) element, lowering electrical resistance across methane, propane, and butane.
• Clean-Air Calibration: Sniffers must always be turned on and warmed up ($30\text{ to }120\text{ seconds}$) in clean outdoor air. Starting inside a gas-filled room zeros out background levels, blinding the sensor.
• False Positives: Volatile organic compounds (pipe dope, PVC primer, spray lubricants) trigger false alarms or contaminate catalytic beads.
• Screening Role: Sniffers identify the general area of a leak quickly. CSA B149.1 Clause 6.22.3 accepts either a liquid solution or a leak-detection device; most technicians pinpoint a sniffer reading with solution before repairing the joint.

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Approved Liquid Leak Detection Solutions

CSA B149.1 Clause 6.22 covers pressure testing of new piping (air, inert gas, or carbon dioxide) and the leak test after gas is introduced, when every connection is checked with a liquid solution or a leak-detection device.

Purpose-made leak detection fluids are formulated to be:

• Chloride-Free: Chemically neutral formulation prevents stress corrosion cracking.
• Persistent Bubbling: High-surface-tension surfactants create durable foaming bubbles over pinhole leaks even under low gas pressures.
• Freeze-Protected: Winterized formulations are rated down to $-40^\circ\text{C}$ ($-40^\circ\text{F}$) for winter outdoor service in Ontario.

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Flue Gas Combustion Analysis & Carbon Monoxide Diagnostics

Electronic combustion analyzers draw flue gas through a stainless steel probe inserted into the venting system, utilizing electrochemical cells to measure oxygen ($\text{O}_2$) and carbon monoxide ($\text{CO}$), while a thermocouple measures net stack temperature. An internal processor calculates carbon dioxide ($\text{CO}_2$), excess air, and combustion efficiency.

Stoichiometric Benchmarks

To ensure complete combustion and prevent soot or carbon monoxide formation, appliances operate with controlled excess air:

Air-Free Carbon Monoxide Calculation

Excess combustion air dilutes flue gas, masking high carbon monoxide production. Combustion analyzers and appliance standards therefore express CO on an air-free basis ($\text{CO}_{\text{air-free}}$), normalizing readings to stoichiometric air:

$$\text{CO}_{\text{air-free}} = \text{CO}_{\text{measured}} \times \left(\frac{20.9}{20.9 - \%O_2}\right)$$

For example, an analyzer measuring $60\text{ ppm CO}$ in a flue sample with $8.9\%\text{ O}_2$ calculates:

$$\text{CO}_{\text{air-free}} = 60 \times \left(\frac{20.9}{20.9 - 8.9}\right) = 60 \times 1.7417 = 104.5\text{ ppm}$$

The often-quoted $400\text{ ppm}$ air-free figure is the laboratory certification limit used in ANSI Z21/CSA appliance standards (for example ANSI Z21.47/CSA 2.3 for central furnaces); it is not a CSA B149.1 field pass mark. In the field, compare readings with the manufacturer's instructions and investigate any CO well above the appliance's normal baseline. Ontario does set one hard number: the Gaseous Fuels Code Adoption Document (clause 7.1.13, Annex Q) requires G.1, G.2, and G.3 technicians servicing a residential natural-draft boiler of $300{,}000\text{ Btu/h}$ or less between October 15 and April 30 to measure CO upstream of the draft control device. Above $100\text{ ppm}$ the boiler is an immediate hazard: correct it, or shut off the fuel and give the notices required by O. Reg. 212/01 s. 13.