David Richardson, NCI Trainer

Carbon Monoxide: What Do You Know? Four Package Unit Inspections That Expose CO Safety Issues

July 29, 2021

Most HVAC technicians don’t think about CO (carbon monoxide) while they’re working on a package unit in 90°F heat. They know that getting the cooling back on is the top priority. However, as a professional technician, don’t forget that health and safety are also top priorities. Here are some quick checks you can do with your summer service procedures that could expose potential CO safety issues. Let’s look at four inspections that may reveal hidden problems.

David Richardson, NCI Trainer
David Richardson, NCI Trainer

Inspect the Heat Exchanger for Cracks

First, check the heat exchanger. You likely already have the blower panel off have easy access to this component. In some cases, you may need to remove an additional panel on those units where the heat exchanger is separate from the blower compartment.

Look over the heat exchanger and inspect it for signs of overheating, metal stress, or cracks. It would be best if you addressed any opening, no matter how small. Cracks only get worse over time–they don’t improve.

When you identify a heat exchanger issue, don’t just offer a replacement. Instead, find out why the original failed and address the cause at the same time. Otherwise, the new heat exchanger will prematurely fail.

Low airflow is a common source of heat exchanger failure. It is also the reason you might be on your current cooling call. Some frequent causes of low airflow include:

  • Dirty or restrictive evaporator coils
  • Dirty blower wheel
  • Slipping or worn belts
  • Incorrect replacement blower motor
  • Dirty or restrictive filters
  • Closed fire dampers
  • Poorly designed and installed ductwork.

Some of these issues are easier to find than others. If you suspect low airflow is why the heat exchanger failed and nothing obvious stands out, grab your manometer and measure the equipment’s TESP (total external static pressure). Chances are high that airflow is low if your measured TESP is greater than the maximum rated TESP found on the unit’s nameplate.

Look for Rust in the Burner Compartment
Rusting of heat exchanger surfaces is another frequent source of failure. Rust weakens the metal to a point where it eventually fails. Because of the evaporator position on most package units, some condensation occurs inside the heat exchanger during cooling operation. This isn’t the primary rusting culprit as so many believe it is. Instead, this rust is often due to flue gas recirculation during heating operation.

CO2 (carbon dioxide) makes up a large part of flue gas. It is heavier than air and falls back into the burner compartment on some equipment designs instead of rising away. Flue gas also contains a lot of acidic moisture that attacks any metal components in the burner compartment. You have seen what I’m talking about if you ever had difficulty removing a rusted spark ignitor or burners from a unit.

Replacement inducer motors are another common cause of recirculation. Any gaps in the replacement inducer connections or misalignment will cause flue gas to short circuit and spill into the burner compartment. Look closely at the installation and make sure there aren’t any unintentional gaps.

Another source of recirculation has to do with equipment location. Rooftop equipment often sits inside parapet walls. The heavier flue gas stays near the equipment and recirculates into the burner compartment if there is no low opening near the equipment. Ground-level package units can also have this problem if there are privacy fences around the equipment.

Check for Soot in the Inducer Outlet
As you continue your inspection, look in the induced draft blower outlet. What does it look like? You may see a black, sticky substance inside. It is soot. Soot is carbon in its pure form and is not acceptable in any fuel-burning appliance because it is a visual clue of unsafe equipment operation.

If you see it, the equipment has produced high CO at some point. Soot is a sign of unburned fuel and the result of overfired or underfired equipment. Do not leave equipment with this condition in heating operation. Disable the heat until you identify the problem’s source and correct it.

Examine the Economizer Location
Once you finish the equipment inspection, step back and look at the economizer installation. Ask yourself, “Where is the outside air coming from that’s going into this unit?” Look for additional CO sources that could enter the equipment from the economizer or a leaking blower access panel.

Look for nearby water heater flues or the exhaust from other package units if you’re on a roof. If you’re working on a ground-level unit, check the parking locations and other outdoor activities near the unit, such as cooking on a charcoal grill.

Privacy fences around ground-level units can also cause flue gas recirculation into the economizer. Once inside the air handling system, these poisonous gases spread throughout the building for occupants to breathe.

Take Five
Many of these inspections tie to what you already do in your cooling service. You’re just looking at the equipment through a fresh set of lenses. I would encourage you to keep your eyes open – you may not revisit the same jobsite.

It only takes about 5-minutes to do these four inspections along with your other work. Don’t trust your eyes if you see something suspicious. Go the extra step and do some combustion testing to validate your concerns. No visual inspection is 100% reliable.

Your thoroughness will plant seeds for additional work that leads to future opportunities to help your company during the slower times.

David Richardson serves the HVAC industry as a curriculum developer and trainer at the National Comfort Institute, Inc. (NCI). NCI specializes in training focused on improving, measuring, and verifying HVAC and Building Performance.

If you’re an HVAC contractor or technician interested in learning more about adding carbon monoxide testing to your services, contact David at davidr@ncihvac.com. NCI’s website www.nationalcomfortinstitute.com is full of free technical articles and downloads to help you improve your professionalism and strengthen your company.