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Seven steps to a clean-running high-efficiency furnace

Nicholas Woodworth analyzes a propane furnace at the Porter and Chester Institute.

Checking these system components will ensure your client is getting the most out of their investment in high-performance heating equipment.

By Joe Bousquin


Key takeaways:

  • Why seasonal maintenance is critical for high-efficiency furnaces.
  • How to undertake a combustion analysis.
  • The difference in gas pressure between propane and natural gas systems.

It's that time of year again. With fall approaching, contractors are gearing up to perform annual furnace checkups for their clients. But with more and more high-efficiency furnaces installed in homes nationwide, this yearly rite of passage for your service techs has become a little more complex.

Today's high-efficiency furnaces — generally those with 90 percent or higher annual fuel utilization efficiency (AFUE) — need more precise, sophisticated trade equipment to make sure they're operating at maximum efficiency. At the same time, since this more complex machinery is more sensitive, doing the annual clean and tune right is more important than ever.

"It's absolutely paramount to conduct regular, seasonal maintenance for high-efficiency systems," says Mike Agugliaro, co-owner of East Brunswick, New Jersey–based HVAC, electrical, and plumbing contractor Gold Medal Service. "Not only should you perform your normal gas pressure check, safety switches check, and blower wheel and motor clean, but it's imperative that clients understand how important it is to maintain a clean system and change filters on a regular basis."

It's beneficial for pros at any step of the construction, remodeling, and maintenance fields to know how to keep furnaces running their best. Here are seven steps to make sure you get it right on high-efficiency systems.

1. Do a combustion analysis

For techs going out into the field, one of the most important pieces of equipment is a combustion analyzer, which measures the composition of the outgoing flue exhaust to ensure the system is burning clean.

"Using a combustion analyzer is really the only way to know for sure how efficiently a high-efficiency system is running, though it's not always in the tech's standard tool bag," says Brian Schutt, owner of Indianapolis-based Homesense Heating and Cooling. "You've just got to carry it on the truck."

Before you enter the home, check for any blockage over the exhaust flue and intake pipe on the exterior. Calibrate your combustion analyzer outdoors to ensure you're not picking up any exhaust leaks inside the house, which could mask your readings.  

Once inside, since condensing systems are sealed, you'll need to drill into the PVC flue that carries exhaust out of the house to insert the combustion analyzer's probe. Each manufacturer will have its specific recommendations for clean exhaust on a high-efficiency unit; acceptable results can range from a CO level of 100 to 400 ppm air-free, though lower is usually better. O2 levels should be between 6 and 9 percent for optimal efficiency and may need adjusting from factory presets. Of course, don't forget to plug your inspection hole when you're done.

2. Check the heat exchanger and burners

While an accurate combustion analysis is the only way to truly measure how a high-efficiency system is operating, Berkley Little, education supervisor at the Porter & Chester Institute in Rocky Hill, Connecticut, says it's also imperative to cover the basics on these units. "The other steps you want to follow are the same as with most heating systems," Little says. That means checking the heat exchanger for excessive rust, cracks, or holes and ridding the burners of dirt or other particles that can create improper combustion.

3. Check the igniter and flame sensor

Use your ohmmeter to check for specified resistance at the igniter, and inspect the flame sensor for wear. Tom Wangler, Jr., owner of Batavia, Illinois–based Confident Aire, likes to polish the flame sensor with steel wool or a wire brush to remove any soot or dirt buildup. Use your multimeter to check specified amperage to the flame sensor and wire.  

4. Measure the temperature rise

Most high-efficiency systems will increase in temperature during heating by 40 to 70 degrees when operating properly. "We recommend a temperature rise somewhere right in the middle of that range," Schutt says. "At the lower end of the spectrum, the system is at risk to produce more condensation that can rust your system. At the higher end of the temperature range, it can put your heat exchanger at risk of cracking." 

5. Check the gas pressure

"The biggest difference with propane furnaces is that it operates at different pressures than a natural gas system," Schutt says. Measure the pressure on both sides of the valve to ensure each is within the manufacturer's specifications. Schutt likes pressure-sensitive safety valves in propane systems. "We always recommend installing a low pressure cut-off, which will shut the furnace down if the gas pressure is low," Schutt says. "This will reduce the amount of excess soot in the heat exchangers, which would reduce efficiency."

6. Check for gas leaks

Some pros recommend using a simple soap-and-water solution to check for leaks versus electronic leak detectors, which can result in false positives.

7. Check and clear the drain

"One of the most commonly neglected service aspects unique to condensing furnaces is the drain trap for the condensate," Wangler says. "But the trap collects dirt over time, and can back up with water. That can shut the unit down when it's running the hardest." Use pressure to blow out the line and ensure the drain is clear.

By approaching high-efficiency heating systems with the right tools, and in a methodical way, you can ensure your clients' equipment is running at optimal efficiency, to produce savings — and warmth — all winter long.

 

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