My furnace was serviced last year. Do I still need a combustion test?

They answer different questions. A service is about whether the appliance runs well. A combustion safety test is about whether the exhaust leaves the building under the conditions your house actually creates — every fan running, doors shut, on a cold day. A furnace can be freshly serviced and still backdraft in a house that has been tightened up.

Can a combustion test find a cracked heat exchanger?

It can find strong evidence of one. The signature is a reading that shifts the moment the circulating blower starts, because blower pressure is moving air across a crack that should not be there. Confirming it means a heating contractor opening the appliance up, usually with a camera. The test gives you a reason to call one, with numbers attached.

Why will you not repair what you find?

Heating work in Michigan requires a mechanical contractor license and we do not hold one. It has a useful consequence: we have nothing to sell on the back of a bad reading, so the reading is just the reading.

Can air sealing my house make the furnace unsafe?

It can change whether a marginal flue still drafts, which is why combustion testing belongs alongside that work rather than instead of it. Reducing leakage is the point of air sealing; the appliances simply need checking once the house is tighter.

Should the water heater be tested at the same time?

Yes, where both are gas and especially if they share a chimney. Appliances on one flue influence each other, and testing one alone can pass a pair that fails together. Doing both is barely more than doing either.

A Serviced Furnace Can Still Be Unsafe

Two different questions

A furnace service answers one question: is this appliance running well? Filters, burners, ignition, the blower, the flame. A good heating contractor does that properly and your furnace runs better afterward.

A combustion safety test answers a different one: is the exhaust actually leaving the building, under the conditions this house creates? Those are not the same question, and an appliance can pass the first and fail the second.

That is not a criticism of the service. It is a description of where one job ends and another begins.

What the test measures

Carbon monoxide in the undiluted flue gases, once the appliance has run long enough for the reading to settle rather than catching it on the way up. Whether draft establishes from a cold start and holds. And what happens to all of that when the house is pulled into the worst state it realistically reaches — every exhaust fan on, the dryer running, interior doors positioned to make it hardest.

There is one more, and it is the one people have usually heard about.

The reading that moves when the blower starts

A furnace burns fuel on one side of a steel heat exchanger and blows house air across the other side. The two are never supposed to meet: exhaust up the flue, warm air to the rooms.

When that steel cracks, the blower pressure starts pushing house air into the combustion side, or pulling combustion gases out of it. What that looks like on a meter is a reading that shifts the moment the circulating fan comes on — steady while the burner runs alone, then changing as soon as air starts moving.

That signature is why the test is run across the blower cycle rather than as a single number. It is strong evidence. It is not proof: confirming a cracked exchanger means a heating contractor opening the appliance up, usually with a camera. What the test gives you is a reason to call one, with numbers attached, instead of a vague worry.

None of this shows itself any other way. A furnace with a cracked exchanger heats the house perfectly well and gives no sign at the vents.

Why a tighter house changes the answer

Air sealing, insulation and new windows all reduce the accidental leakage that used to supply replacement air to the building. That is the point of them and they are worth doing. It also means a flue that drafted adequately five years ago may not draft adequately now, with nothing about the furnace having changed at all.

The furnace that fails a worst-case test is frequently in the house that was improved last year, which is exactly why rebate programs require combustion testing alongside the work.

We do not fix furnaces, and that is the useful part

Heating work in Michigan requires a mechanical contractor license. We do not hold one, so we do not service, repair or replace furnaces. What is offered here is the measurement and a written report.

The side effect is worth stating plainly: we have nothing to sell you on the back of a bad reading. A free safety inspection from a company that sells furnaces is a perfectly normal thing to accept, and it is also an inspection performed by somebody whose next sentence may be a quote. When the person measuring cannot quote the repair, the reading is just the reading.

A failed test goes to a mechanical contractor with our numbers in hand, and you are told plainly what they mean and what to ask for.

Before you book anything

Carbon monoxide alarms on every level of the house, none of them past the replacement date printed on the back. Nothing in this article matters more than that sentence.
Look at the flue pipe where it leaves the furnace — rust, soot staining, or a joint that has pulled apart.
Watch the burner flame if you can see it. Steady and blue is right. Yellow, lazy or lifting off the burner is worth mentioning when you call.
Notice the pattern of any symptoms. Headaches that clear when people leave the house and return when they come back are the classic shape, and they are a reason to act today rather than book a test.
Check whether the furnace shares a chimney with a gas water heater. If it does, the two should be tested together, because appliances on one flue influence each other.

If you suspect carbon monoxide right now

Do not book a test and do not wait for one. Leave the building and call 911 or Consumers Energy from outside — they run a 24-hour line and they come out free. A scheduled test is for finding a problem you do not yet have symptoms of.

Where this stands today

Our BPI Building Analyst certification is in progress through the Consumers Energy program at Jackson College and completes in November 2026. It is not held yet and is not claimed anywhere on this site as held.

When it is: furnace combustion safety testing across Greater Lansing, best done alongside the water heater where both are gas.