Boiler Combustion Test: What a Lowell Noritz Case Showed

The homeowner's description was specific: an orange-looking flame, fuses that kept blowing, and symptoms that got worse when several hot-water taps ran at once. The Noritz combi boiler in this Lowell, MA home had already had a replacement burner assembly and a new control board. The lesson from the visit is that a new part and a unit that lights aren't proof it's running right. A boiler combustion test and a separate electrical diagnosis come before any adjustment or verdict.

Safety first

If you smell gas, a carbon monoxide alarm sounds, anyone has symptoms of CO exposure, or you see flame outside the burner area, smoke or fire, get everyone outside without using switches, phones or appliances on the way. Call 911 from outside and stay out until officials say it's safe. Carbon monoxide has no smell, so an alarm matters even when nothing seems wrong.

Without any of those signs, repeated blown fuses, an abnormal flame, a leaking cabinet or recurring shutdowns still mean stopping normal use and arranging a diagnosis. Don't open the cabinet, adjust the gas valve, put in a bigger fuse or keep resetting the unit until it runs.

The Lowell case

The seven-year-old Noritz combi had a hard service history. About a year before our visit, a crack had reportedly been found in the burner assembly. The homeowner said a gas-valve adjustment kept the unit running while parts were ordered, and then a replacement burner assembly and a new control board went in. Reportedly, no combustion test followed that work, and the earlier adjustment and any readings weren't available to check.

Our inspection also found a dripping condensate trap and seepage at a union under the unit. Which line that union serves, and whether moisture had reached anything electrical, still needed confirming. This write-up covers the diagnosis, not a completed repair.

Why the combustion test comes first

A boiler combustion test uses a calibrated analyzer to compare how the unit burns with the manufacturer's procedure, at each firing rate and in each mode the manual specifies. Depending on the model and the work done, the commissioning around it can include:

• intake, exhaust, clearances, the condensate path, wiring and visible leaks
• the operating sequence for heating and hot-water calls: ignition, flame proving, fan, pumps, valves and shutdown
• combustion readings such as oxygen or carbon dioxide, carbon monoxide and flue temperature
• gas pressure checks under the prescribed procedure
• gas, water, condensate and safety-control checks

There's no universal target number worth publishing for homeowners. The model, fuel, firing condition and test method all change what the right result looks like.

Flame color can't stand in for those readings. Dust, viewing angle, burner condition, air supply and the point in the ignition sequence all change how a flame looks, so an orange flame is a clue, not a diagnosis or a reason to turn a gas valve. We don't adjust anything until the model allows it, the instrument is set up, the unit is at the specified firing condition and the starting readings are recorded. Testing first avoids moving an adjustment the wrong way or hiding a vent, fan, fuel or control problem.

Blown fuses need their own diagnosis

A fuse opens because its circuit drew more current than it should, but it doesn't name the failed part. A fan motor, pump, valve, transformer, harness, connector, control board, moisture path or short to ground could be involved. One theory in this case was that rich firing made the blower work harder and helped blow the fuses. That stays a theory until current measurements, fan data, the wiring diagram and the combustion readings line up.

The electrical check means documenting which fuse opens, confirming the correct rating, looking for moisture and heat damage, and comparing the loads with the schematic. Never install a bigger fuse or bypass the protection. That can damage the equipment and create a fire or shock hazard.

Two loads, two sets of tests

A combi heats the house and makes hot water from one cabinet, and each job creates different demand, flow and firing. Several hot-water taps open at once may push the unit harder, but its output and rated temperature rise still limit what it can deliver. Testing has to cover the modes the work affected, not only prove that one faucet gets warm. The heating system's pumps, zones, pressure and air removal can affect operation too, and they're separate from combustion.

Repair or replace?

Seven years old doesn't decide it. Repair can make sense when testing finds an isolated, fixable fault, the part is available, the heat exchanger and surrounding equipment are sound, and the unit passes its combustion test. Replacement deserves a look when a major failure can't be serviced, safety or venting can't reasonably be corrected, parts are unavailable, faults keep repeating or the unit no longer fits the home. Our combi boiler repair-or-replace guide walks through that decision.

Two more points belong in any comparison. Noritz's 25-year heat-exchanger warranty applies to its qualifying residential tankless water heaters. It isn't automatically the term for a combi, and it doesn't cover a board, burner, labor or installation work. And as of August 2026, Mass Save's boilers and furnaces page said broad rebates, incentives and financing were no longer available for natural gas, oil or propane equipment, with incentives remaining for income-eligible customers. Program rules change, so we recheck before any incentive appears in a written comparison.

Getting a measured diagnosis

Jeneral Plumbing is owner-operated by a Licensed Master Plumber (MA #9628831-PL-M) with 20+ years in the trade, including seven with a gas utility, and factory trained by Noritz. We're based in Westford, MA, and serve homes in Lowell, Acton and other towns within 20 miles across Middlesex & Worcester County. Our gas appliance work is Massachusetts-only, and we confirm your address and the work when you call. During regular hours, a visit carries an $80 dispatch fee — the cost of the visit, waived when you hire us for the work. We price repair and installation work at a flat rate, with the total and any fees shown for approval before work begins. Call (978) 392-7789 once any gas, CO or fire emergency has been cleared, or see our boiler repair service.