A Philadelphia homeowner was told to stop using a Goodman furnace and pay $4,900 for a replacement after technicians detected carbon monoxide near 100 parts per million inside the flue.

However, because technicians did not identify visible heat-exchanger damage, many commenters said the situation sounded like one that warranted more testing before jumping to a full replacement.

Here's what to know

According to the homeowner's post on Reddit, the unit in question was a Goodman GMSS920803BNAA made around 2016. The technicians said several 45-degree bends made the vent configuration too restrictive for that furnace, and after finding about 100 ppm of CO in the PVC exhaust pipe, they proposed replacing the furnace and correcting the venting for $4,900.

In an Oct. 7 update, the original poster said the contractor later sent over a Testo 310 analyzer photo showing 98 ppm CO and 5.1% O₂ and said the reading kept climbing until it reached 105 ppm. The homeowner wrote, "They haven't visually seen any heat-exchanger damage, even after opening the furnace up."

If a replacement really is needed, upgrading heating and cooling equipment can be one of the best ways to cut utility bills and protect against rising energy prices. Palmetto can help homeowners understand their HVAC options and reduce energy bills with newer, more efficient HVACs and heat pumps, while its Comfort Plan network can connect households with vetted installers and efficient heating and cooling solutions.

A number of commenters said the homeowner should pay for an independent diagnostic visit before agreeing to a replacement. One commenter wrote, "I'd want to see a full combustion analysis, not just a CO check," adding that gas pressure, venting, a clogged secondary heat exchanger, or a cracked primary heat exchanger could all affect the fuel-air ratio.

More background

A big part of the debate was how much the available test data actually proved. The homeowner said they were still trying to determine "whether it stabilized, or whether it was raw or air-free CO," and they did not yet have indoor or supply-air CO results.

For many commenters, that missing information was central to the issue. One commenter, citing more than 30 years of Goodman dealer experience at their employer, wrote, "100 co ppm is on the upper end of what I'd let pass on a maintenance," while another commenter said some brand-new furnaces can read in that range.

The poster also said they are the second owners and believe the original Goodman warranty does not transfer. Other commenters said it would still be worth checking for parts coverage or a heat-exchanger warranty, especially because a problem involving gas pressure, dirty burners, fan speed, or vent termination could be much cheaper to fix than replacing the whole furnace.

What can be done?

The homeowner said they shut the furnace down, asked for the analyzer results, and started seeking another opinion on whether the problem is a heat exchanger issue or a repairable venting or combustion problem.

Commenters also laid out several specific checks they would want done next. Those included checking gas pressure against the furnace specifications, clocking the gas meter, inspecting the intake for flue-gas cross-contamination, calculating whether the vent run exceeds the allowed equivalent length, and performing a full combustion analysis instead of relying on a single CO number.

For homeowners who do wind up replacing aging equipment, Palmetto's Comfort Plan includes $0-down options that can lower heating and cooling costs by up to 50% and 12 years of free maintenance. Homeowners can also pair solar panels with electric appliances, including efficient HVACs, to drive utility costs even lower. EnergySage makes it easy to compare solar options and find the best solar system and installer for a home and budget, and it can help homeowners save up to $10,000 on solar installations.

The homeowner is still waiting for a clearer diagnosis. As they put it: "I'm willing to replace it if that's the appropriate fix. I'd just like to understand whether correcting the venting and professionally retesting the existing furnace is a reasonable option."

Where can I learn more?

These stories get at why so many homeowners seek a second opinion before agreeing to major HVAC work. They cover disputed furnace and AC diagnoses, carbon monoxide concerns, and contractor sales tactics.

• In Pennsylvania, regulators said a company used fear-based furnace warnings to pressure homeowners into replacements.

• A homeowner was shocked after a $30,000 quote followed a push to replace a working furnace.

• Homeowners said some contractors relied on trust-me sales tactics to steer them toward expensive systems.

• After routine AC service, one homeowner said a suspicious $20,000 quote followed basic maintenance.

• A worried homeowner learned 180 ppm is too high after noticing gas smells near a new furnace.

Get TCD's free newsletters for easy tips, smart advice, and a chance to earn $5,000 toward home upgrades. To see more stories like this one, change your Google preferences here.