Furnaces in our climate get an easier life than almost anywhere else in the country. We run heat for maybe 30 to 60 days a year. The rest of the time the unit just sits there. So why do so many Houston-area furnaces fail at 12 to 15 years when they were built for 20 to 25? Down here they don't wear out from running. Humidity and disuse get them first.
Here is what kills them and what to do about each one.
Killer 1: Sitting in a humid attic for nine months
Most Pearland-area furnaces live in the attic. For most of the year it is hot and humid up there. The heat exchanger and burners sweat and cool off every day for nine months, along with the gas valve and the steel cabinet. Surface rust forms. Aluminum oxidizes. Capacitors degrade. Then in November you fire it up and put load on parts that have been corroding all summer.
What helps. Run the blower fan now and then in the off-season. Plenty of thermostats have a "fan circulation" setting that runs it 10 minutes an hour. That pulls drier air from the house through the furnace cabinet so it sweats less. A fall tune-up before the first hard freeze catches corrosion before it turns into a failure.
Killer 2: Cracked heat exchanger from short cycling
The heat exchanger is the most expensive part to replace, and the one your safety depends on. Heating up and cooling down over and over is what cracks it. Short cycling makes that much worse: the furnace runs 2 minutes, hits temperature, shuts off, and 5 minutes later starts again.
The causes vary. The furnace may be oversized or the return air undersized. A dirty filter can choke the airflow. A miswired thermostat or a wrong cycle anticipator setting will do it too.
What helps. If your furnace runs in cycles shorter than 5 minutes on a mildly cold day, have the system checked. The fix is usually in the ductwork. Sometimes it is just the filter or the thermostat, and sometimes the unit was oversized from the start and never had a chance.
Killer 3: Chronic dirty filter
A 1-inch fiberglass filter you forgot about for six months slowly suffocates the furnace. With airflow restricted, the heat exchanger can't shed heat fast enough and the limit switch trips. Trip after trip wears the limit switch out. Then the heat exchanger overheats past the trip point, and now we are looking at warping and cracks.
What helps. Change fiberglass filters every 30 days and pleated MERV 8 to 11 filters every 90. Put a reminder in your phone. A filter costs a few dollars. The heat exchanger is the most expensive part in the furnace.
Killer 4: Wrong thermostat or wrong settings
Smart thermostats are mostly great, but some settings fight your furnace. Aggressive recovery from setback is one. An overly tight comfort band is another, and so is the equipment "type" set wrong. We have seen Nest and Ecobee thermostats commissioned with single-stage settings on two-stage furnaces. The thermostat then short cycles the furnace by calling for full heat over and over.
What helps. When a thermostat gets installed or replaced, ask the installer to verify the HVAC type and equipment stages. Have them check the cycle anticipator and minimum runtime settings too. It takes about five minutes.
Killer 5: Unaddressed flue and combustion problems
A furnace burning at incomplete combustion runs hotter, leaves more soot, and corrodes its own heat exchanger from the inside. We have pulled flue caps off Pearland furnaces with bird nests, wasp nests, accumulated leaves, and once a deflated mylar balloon. Each of those obstructions partially blocks the flue and pushes the burn toward incomplete.
Dirty burners cause the same trouble, and so does gas pressure out of spec. So do oversized orifices and an undersized gas line. Every one of them means hotter combustion with more corrosive byproducts, and a heat exchanger that dies younger.
What helps. Annual combustion analysis. We put a manometer on the gas valve and a probe in the flue, then read inlet and manifold pressure, CO in ppm, O2, CO2 and stack temperature. We do this on every fall tune-up.
Killer 6: Condensate problems on high-efficiency furnaces
If you have a 90+ AFUE furnace, it makes condensate when it runs. The condensate is mildly acidic and corrosive. If the condensate trap clogs or the drain line backs up, that acid sits in the secondary heat exchanger and eats it from inside. We have replaced 6-year-old high-efficiency heat exchangers because the homeowner never cleaned the condensate trap.
What helps. If you have a high-efficiency furnace, the condensate trap and drain line need annual maintenance. We do it during the tune-up. If you have an 80 percent AFUE furnace (no condensate), this one doesn't apply.
Killer 7: No annual maintenance
Yes, we are biased here. Still, in our experience furnaces that get a tune-up every fall outlast the ones that don't by years, and we see it from Houston out to the coast. Twenty-five years of $99 fall tune-ups comes to less than one furnace replacement.
What helps. Annual fall tune-up. Combustion safety inspection. Filter check. Heat exchanger inspection. Drain trap clean (high-efficiency only). Burner clean. Igniter check. Flue inspection. Capacitor reading on the inducer and blower. A written record of every reading.
Stuff that doesn't help much
Things people obsess about that don't actually move the needle on furnace life:
- Brand. Once you are above bottom-tier, the brand differences in life are smaller than the install quality differences.
- Setting the thermostat lower. Saves energy, doesn't extend life.
- Single-stage vs two-stage. Two-stage is more comfortable and slightly more efficient, but single-stage furnaces installed correctly last just as long.
- Aftermarket "life-extender" sprays or treatments. Save your money.
Signs your furnace is in the back half of its life
You are aiming for 20 to 25 years and you are seeing:
- Surface rust on the cabinet or burners (worth a cleaning and an inspection, but not the end yet)
- Yellow or orange flame where it should be crisp blue (combustion is wrong, fix the cause)
- Soot accumulation around the burners (combustion is wrong, possibly heat exchanger leak)
- Thermostat says heating but air feels lukewarm (usually the gas valve or ignition, sometimes the limit switch)
- Cycling on/off in under 4-minute cycles (short cycling, address it)
- CO alarm chirping or activating (stop using until inspected)
What we do during a maintenance visit
Just so you know what good looks like:
- Visual inspection around the furnace: cabinet, gas line, flue
- Filter check, replace if needed (we charge what the filter costs us)
- Pull and inspect burners, brush clean if dust is present
- Heat exchanger inspection with borescope
- Igniter test and resistance reading
- Inducer motor amp draw and capacitor reading
- Blower motor amp draw and capacitor reading
- Combustion analysis at the flue with electronic analyzer
- Gas pressure check, inlet and manifold
- Limit switch test and high-limit verification
- Condensate trap clean (high-efficiency models)
- Thermostat operation test
- Written report with all measurements
Fall tune-ups are how 13-year furnaces turn into 25-year furnaces. Book yours, combustion safety inspection included: 281-992-7866 or book online.