Furnace Repair & Combustion Diagnostics
A furnace is a machine that controls fire. Whether it is doing that safely is a question with numbers behind it — draft pressure, flue carbon monoxide, manifold pressure, temperature rise. We bring instruments and read them.
The Problem Is Rarely the Part You Expected
When a combustion safety program reviewed the results of a thousand residential inspections, the most common finding was not a failed component inside the furnace. It was open return air ducts — found in more than one in five homes.
An open return depressurizes the area around the furnace and competes for the air combustion needs. The furnace itself may be in perfect condition and still be unable to burn cleanly or exhaust properly.
This is why we do not diagnose a furnace in isolation. You do not work on a furnace; you work on the whole system it lives inside.
Findings across 1,000 inspections
Share of homes where each condition was present
Depressurizes the space around the furnace and starves combustion
Overheating from restricted airflow
Conditioned air dumped into attics and crawlspaces
Immediate fire and explosion hazard
Incomplete combustion reaching measurable levels
Combustion products entering the living space
Our Combustion Safety Standard
Every one of these is a measurement with a pass threshold, taken after the furnace has run at least five minutes so the draft has stabilized and we are seeing how it truly behaves.
Flue Carbon Monoxide
Sampled at each flue before dilution air enters. Four burners means at least four readings. The accepted ceiling is 100 ppm, but we treat anything over 25 ppm as a defect to correct — field experience shows most of those units can be fixed.
Draft & Spillage
Draft is the vent's ability to pull exhaust out of your home, and the minimum it must produce changes with outdoor temperature. We verify it with a gauge and confirm with a smoke test that nothing is spilling back out of the draft diverter.
Temperature Rise
The difference between return and supply air must land inside the range printed on your furnace's rating plate — typically 30°F to 60°F. We tune blower speed to hit the middle of it. Too high and the heat exchanger cooks; too low and it condenses.
Why Draft Standards Change With the Weather
Warm air rises, so a vent naturally pulls harder when it is cold outside. That means a furnace can pass a draft test in January and fail it in September. A system tested only in cold weather has not really been proven — if draft is weak when it is cold, it will be weaker when it is warm.
| Outdoor Temperature | Minimum Draft | Where This Applies |
|---|---|---|
| Above 80°F | 0.005 in. W.C. | The typical Los Angeles service call |
| 32°F to 80°F | 0.01 in. W.C. | Overnight and winter mornings |
| Below 32°F | 0.02 in. W.C. | High desert and mountain communities |
What Your Furnace Is Telling You
A loud "boom" on startup
Delayed ignition. Burners light at one end and the flame must travel across to the rest. When that crossover is obstructed, gas builds up first and then lights all at once.
We check: crossover tube alignment, plugged orifices, air leaks around the furnace base — and spider nests, which build inside orifices and crossover tubes and are a real cause here.
Scorching or rust in front of the burners
Flame roll-out — flames escaping forward out of the burner compartment instead of drawing into the heat exchanger. This is a serious combustion fault, not cosmetic wear.
We check: blackened or rusted areas ahead of the burners, burnt wiring, carbon deposits, combustion air supply, exhaust restriction, and manifold pressure.
Heat that quits after a few minutes
The high limit switch is cutting gas to stop the heat exchanger overheating. Low airflow is the usual cause, and it is often self-inflicted.
We check: external static pressure, filter condition and size, closed or blocked supply registers, and whether the furnace is over-firing.
Clicking, then nothing
The control board attempts ignition three times, fails to confirm a flame, and locks out for an hour. The igniter is often blamed when the real fault is the furnace failing to see a flame that is actually burning.
We check: flame sensor current, which must read at least 1 microamp and typically runs 2–5. A microscopically thin carbon film on the rod is enough to break sensing.
Only works with the closet door open
A textbook combustion air problem. With the door shut, the furnace cannot get the air it needs — or worse, it is pulling exhaust back into the return.
We check: leaks around and behind the furnace base that let combustion air migrate into the return duct, plus combustion air openings sized to the furnace's input rating.
Trips when it is windy — or when the fireplace is lit
Both point at venting. A lit fireplace pulls air up the chimney and puts a tight house into negative pressure, and the furnace can no longer exhaust against it.
We check: the vent for downdraft vulnerability, whether a storm cap at the termination is warranted, and exhaust fans and duct leakage as competing pressure sources.
Three Things We Will Not Do
Shortcuts that get offered on heating calls, and why we decline them.
Bypass a safety switch to get you through the night
A tripped safety means a fault was detected. Even if we correctly identified and fixed that fault, a second one may exist that the same switch would have caught. There is no version of this that is worth the risk to you.
Plug an orifice or drop gas pressure to derate an oversized furnace
Both reduce exhaust gas temperature. Cool exhaust condenses inside a vent never designed to stay wet, and that water runs back down and rusts the furnace out from the inside. The comfort complaint gets solved; the equipment pays for it.
Fit a cheaper non-factory replacement part
Furnaces are certified as complete assemblies, tested with the components the manufacturer specified. Substituting a part moves responsibility for how that machine behaves onto whoever installed it — permanently.
Repair It, or Replace the Whole Problem?
Most furnaces are worth repairing, and we will tell you plainly when yours is. But if the heat exchanger is compromised or the unit is near end of life, a heat pump removes combustion from the equation entirely — no flue, no gas valve, no carbon monoxide to test for. California rebates and federal tax credits can offset a significant share of the cost.
Compare heat pumps →Understanding the fault
If your furnace keeps stopping, it is usually protecting itself from something. These explain what.
Why Does My Furnace Keep Shutting Off After a Few Minutes?
A furnace that starts then quits after a few minutes is usually protecting itself from something.
Read the article →How to Tell If Your Furnace Is Producing Carbon Monoxide
The warning signs of a carbon monoxide leak, and which ones mean leave the house now.
Read the article →What Size Furnace Do I Need? Why Bigger Is Never Better
An oversized furnace costs more, wears out faster and heats less evenly. Here is why.
Read the article →Furnace Repair FAQs
How do you know if my furnace is producing carbon monoxide?
We measure it directly. A sample is taken from each flue before dilution air mixes in, and a furnace with four burners gets at least four separate readings. The working standard is under 100 parts per million in the flue, but our field threshold is tighter: anything above 25 ppm is treated as a correctable defect, not an acceptable result.
What does a healthy furnace flame actually look like?
A properly tuned burner shows a dark blue interior, a lighter blue exterior, and at most a light orange tip. The flame should be steady and straight, never wavering or fluttering. White tips, ghosting, weak flames, uneven patterns across burners, or flames visibly touching the heat exchanger all indicate a combustion problem.
My furnace makes a loud boom when it starts. Is that dangerous?
Yes, and it should be diagnosed immediately. That sound is delayed ignition: gas accumulates before the flame crosses from burner to burner, then lights all at once. Common causes are misaligned crossover tubes, a plugged burner orifice, or an air leak. In Southern California, spiders nesting inside orifices and crossover tubes are a genuine and frequent cause.
Why does my furnace shut itself off after a few minutes?
That is usually the high limit switch protecting the heat exchanger from overheating, and low airflow is the normal cause. We measure external static pressure, inspect the filter, and check that supply registers are open. Closing off registers in unused rooms is a common homeowner energy-saving attempt that directly causes this fault.
Do you check for a cracked heat exchanger?
Yes, by inspecting the exchanger directly and watching the flames for interference when the blower starts. We do test for it, but we test draft, spillage, and carbon monoxide first. Those conditions are more common and more immediately hazardous, and a cracked exchanger is frequently blamed for symptoms that a draft or airflow problem actually caused.
What gas pressure should my furnace be running at?
Incoming line pressure should measure between 6.5 and 14 inches water gauge, and manifold pressure at the burners is nominally 3.5 inches water column. Some manufacturers permit a narrow adjustment between 3.2 and 3.8 to account for fuel heat content. Pressure outside that window produces incomplete combustion and wasted fuel.
Can you just derate my oversized furnace instead of replacing it?
We will not plug an orifice or drop gas pressure below spec to do it. Both lower exhaust gas temperature enough to let water condense inside the vent, and that condensate runs back down and rusts out the furnace. It trades a comfort complaint for equipment failure and a liability we would own.
No Heat? Let's Find Out Why.
We arrive with combustion analyzers, draft gauges, and manometers — and we leave you with the readings, not just an invoice.