Flue gas temperature: how to measure it and read it on a boiler

What flue gas temperature did you write down last year on the boiler you’re servicing tomorrow? If the honest answer is “no idea” or “it was in range,” tomorrow’s number won’t tell you much. A single stack reading is like a blood test with no reference range: you get a number, not a diagnosis.

Yet it is the most talkative reading in a combustion analysis. O2 tells you how the burner is tuned; flue gas temperature tells you everything that happens between the flame and the chimney — the condition of the heat transfer surfaces, the water temperature, the real firing rate. This guide covers how to take a reading you can actually compare, and how to make it talk.

What does flue gas temperature actually tell you?

Short answer: flue gas temperature shows how much heat is leaving the boiler without reaching the water. Read it as net stack temperature (flue gas minus combustion air), at the same firing rate, against a baseline taken when the boiler was clean. A rise of about 40°F (22°C) over that baseline costs roughly one point of efficiency.

That rule of thumb comes from the U.S. Department of Energy: the effect of dirty heat transfer surfaces can be estimated from the rise in stack temperature over a clean-operation baseline, at about 1% efficiency per 40°F. The key word is baseline. Without one, the rule has nothing to work on.

There is a floor, too. Natural gas flue products have a dew point of roughly 131°F (55°C). On a condensing boiler, dropping below it is the whole point; on a non-condensing unit, it means corrosion in the heat exchanger and the chimney. The dew point also moves with excess air: the higher the residual O2, the lower it sits, and the colder the return water has to be before anything condenses.

Tools you’ll need

• A calibrated combustion analyzer (Testo, Bacharach or equivalent) with a flue probe and a combustion air temperature probe.
• A 1/4-in. bit and metal plugs to close the test port.
• A thermometer or BMS readout for supply and return water temperatures.
• The commissioning report or last service report — without it, today’s number has nothing to stand next to.

⚠ Safety — The probe and the vent are hot: wear gloves when pulling the probe. A badly sealed test port leaks flue gas into the room, especially on a positive-pressure vent. Close every hole with a metal plug, never with tape.

Six moves for a reading that holds up

1) Drill where you can find it again

The test port goes upstream of any draft hood, draft diverter or barometric damper, as close to the appliance outlet as you can get. Farther along, the air pulled in to balance the draft cools and dilutes the flue gas. On a boiler with a bell-shaped diverter on top, Bacharach’s guidance is to drill the vent connector directly below it. Mark the hole and note its position in the report so next season’s reading comes from the exact same spot — the same discipline that applies when measuring draft.

2) Measure combustion air

Put the air probe at the burner inlet, not at the boiler room door. The analyzer subtracts that value to give net stack temperature, the figure that feeds the efficiency calculation. In a basement boiler room that runs at 60°F in September and 95°F in January, the gross reading can climb 35°F without a single tube getting dirtier.

3) Lock in a known firing rate

A boiler that just lit hasn’t warmed its own mass yet, so its flue temperature keeps climbing for several minutes. Force high fire, wait until the reading stops rising, then record it. On a modulating burner, repeat at low fire. Comparing this year’s high fire to last year’s low fire is how you invent a trend that isn’t there.

4) Find the core of the flue

Flue gas isn’t uniform across the vent: the edges run cooler than the middle. Slide the probe in slowly until you hit the highest reading. Testo’s training material treats that hot spot as the repeatable measuring location. Once you find it, set the probe stop so the depth stays the same.

5) Read it next to its neighbours

On its own, net stack temperature can’t settle anything. Log O2, net stack temperature, return water temperature and firing rate on one line. That combination is what separates fouling from a tuning problem.

6) Check the baseline — and the vent rating

Compare to the clean baseline, then check the vent material’s limit. In Canada, ULC S636 rates plastic venting by temperature: up to 65°C (149°F) for Class IIA (PVC), 90°C (194°F) for IIB (CPVC) and 110°C (230°F) for IIC (polypropylene). A condensing boiler vented in PVC whose flue gas is creeping toward 149°F hasn’t just lost efficiency: it is running at the edge of what its vent is rated for. The appliance manufacturer’s venting table and CSA B149.1, enforced in Québec by the RBQ, have the final word.

What is a rising or falling reading telling you?

That last row deserves a word: on a non-condensing boiler, a very low stack temperature is not an efficiency win. It is often a short-cycling boiler running on cold water, with flue gas dropping below its dew point inside the chimney.

Terrebonne: 83°F recovered from a tube bundle

A school in Terrebonne, heated by two non-condensing firetube boilers. The commissioning report, kept in the boiler room, gave a net stack temperature baseline at high fire. Three heating seasons later, at the first fall service, the same reading from the same marked port came in 83°F (46°C) higher. O2 hadn’t moved, and gas input matched the nameplate.

With steady O2 and the right firing rate, that left heat transfer. Opening the smoke boxes showed an even layer of soot in the tubes, left over from a stretch of poor tuning that had been corrected the previous winter without a cleaning afterwards. After a full tube brushing, net stack temperature came back to within a few degrees of baseline. By the DOE rule of thumb, the gap had been worth about two points of efficiency, on two boilers, all winter.

The most useful fix wasn’t the brushing, though. The new clean reading went into the file as the baseline, with the firing rate and port location, so the next drift shows up at the very next visit.

Before your next visit: find your baseline

Pull the last commissioning or service report for each boiler and look for a net stack temperature with its firing rate. If there isn’t one, the next visit after a cleaning becomes your baseline. Record it with O2, return water temperature and port location, then compare every season on the same line.

A stack reading you can compare from year to year turns a calendar-based cleaning into a justified one, and a drifting gas bill into a known cause. That’s the kind of tracked reading Montréal Combustion keeps in its clients’ preventive maintenance programs: for the building manager, it is the difference between paying for a tube brushing out of habit and ordering one because the chimney has started costing efficiency points.