How We Made a Heating System Really Work

The team of elves assembles: Henry Sterling, Jim Davis, Rob "Doc" Falke, Dominick Guarino (l-r).

NCI's Al D'Ambola played a key role in the project's success by replacing the home's original, uninsulated return duct system.

CO expert Jim Davis has a word or two with Betty's furnace before he works his magic

Several contractors had been to Betty's home and assured her that the system was just fine, even though she was always cold and her heating bills were astronomical. Unfortunately, as an industry we have digressed to the point of believing that if the equipment is working, then the system must automatically be working as well. Focusing only on the box has become the standard practice.

HSER and system performance rating of installed systems is available for progressive contractors who are committed to leading this industry forward. As the truth is revealed about a system's performance, the contractor can then offer extremely high-margin repairs. The best part is that the value of the repairs to the customer exceeds the high-margin sales price. It's a win for everyone.

Full HSER diagnostic testing takes a trained technician or salesperson about 45 minutes on a typical residential system. The service version of system performance testing takes about 10 minutes per system on a service call or during a startup. With Betty's system at 31%, we were concerned if we would be able to get the system above 90% which is the acceptable NCI standard. However, we were there to serve her, not to impress anyone. So we got to work.

Btu Output
The equipment was rated at a Btu output of 64,000. Ideally, all the heat created by the equipment should be delivered into the home, but equipment defects and duct system losses exceed 30% in most systems when weather is extreme. The real test of a duct system is the loss it suffers on the hottest or coldest days — when the heating or cooling is needed the most.

This system was only delivering 19,671 Btu. With the initial delivered Btu established, it was time to evaluate our test numbers, interpret what was wrong with the system, and determine the repairs we needed to make to increase the delivered Btu of Betty's system.

Airflow
Airflow is the lifeblood of an HVAC system and is the fluid that moves the heat throughout the building and the system. Live measurement of air properties in an operational HVAC system is the only accurate way to measure performance and diagnose it. Anything less is highly inconclusive.

To evaluate airflow at Betty's house, we carefully measured total external static pressure at 0.93-in. We plotted airflow on the manufacturer's fan performance tables and determined the fan was moving 870 cfm. Supply airflow was measured at each supply register, and the airflow totaled 706. The difference between fan airflow and supply register airflow identified 164 cfm of supply duct leakage. Therefore, the supply ducts needed to be tightened up. The return grille measured 692 cfm. This, subtracted from the 840 cfm that the fan was providing, revealed 178 cfm of return duct leakage.

So, live duct leakage testing proved supply airflow was just under 200 cfm short, but true measured duct airflow loss was 432 cfm — which is actually a little better than most systems that NCI contractors test around the country.

While airflow is a key test, it is only the beginning. Several of the supply registers were only delivering a small percent of the required airflow. NCI CEO Dominick Guarino and I removed five 6-in. ducts and replaced them with 8-in. ducts with new takeoffs and balancing dampers.

While doing this, we inspected the duct joints and found several poorly installed or damaged joints that needed to be replaced or repaired. We found these even though the materials that had been used in the original construction were typically better than we find in most new construction. The rectangular metal trunk duct and sheet metal branch ducts were all wrapped with high quality foil-backed insulation, and taped with top-rated insulation tape. The last few feet of each run was flexible duct. All of the ducts were oozing with mastic. It appeared to meet all code requirements but earned the grade of "F" for performance. Can codes and quality installation practices deliver an "A" duct system? Not without live performance testing.

Static Pressure
In addition to total external static pressure measurement, we use pressure as a key diagnostic test to evaluate the duct sizes and rate system components. Leaning to take and interpret static pressure readings requires a rich knowledge of how static pressure works, and a commitment to field practice. We measured static pressure on both sides of the filter and identified a pressure drop over the filter of 0.25-in.

One of NCI's standards used to evaluate filter pressure drop is that it should not exceed 20% of rated fan static pressure. Since Betty's fan moved 400 cfm per ton in cooling mode at 0.70-in. of total external static pressure, we multiplied that by 20% to identify that the filter should have a pressure drop of no more than 0.14-in. So the electrostatic filter had to go, and we added an additional return air grille and installed some new, low pressure drop filters that the fan easily could afford.

The next thing we learned from static pressure is the initial suction pressure from the return side of the fan measured 0.55-in. This indicated the return duct was severely undersized. There were several return grilles at the far end of the house with no airflow. We removed a grille to find a box with two jumper ducts into the bedrooms. These may sound like a good idea, but they seldom accomplish any measured results.

Contractor Henry Sterling dug in and built the sheet metal for a new return duct system for the far end of the house. NCI's Senior Trainer, Al D'Ambola demolished an old single return duct, and ran a new larger one in a more direct route with four fewer elbows. Henry also built a new return air plenum at the furnace to replace the oval hole that had been cut in the side to receive the old single return duct.

With these changes, pressure on the return side of the fan fell from 0.55-in. to 0.16-in. This, along with the other duct repairs and changes we made, caused the airflow to increase to more than 900 cfm at the supply registers, without a fan speed change.

Remember, airflow is a multiplier in the Btu formula. Increasing airflow by 100 cfm will increase Btus by 3,500 to 7,500 per hour, if the temperature rise is maintained.

Temperature
Temperature is also a multiplier in every Btu delivery formula. A 1F temperature change will make a 1,000 Btuh difference in Betty's system.

Are you beginning to see the steps to increasing system efficiency? You already understand each of the principles. It's likely that all you're missing is the basic testing and how-to procedures to enable you to pull them all together. You're closer than you think to being able to offer top-performing systems to your customers, and being handsomely paid for the results.

First, we used temperature to diagnose what was happening to the system. One of my favorite tests is to measure the temperature difference between a plenum and the farthest grille. Temperature testing confirmed that there was a 14F loss through the return duct. At the same time, the temperature change through the furnace was only 41F. Simple math of 14 divided by 41 proved that 34% of the equipment's heat was being lost through the return duct.

A visual inspection confirmed that the original return duct had no insulation. It was just a flexible duct made of foil. Al solved the insulation problem when he replaced the duct and sealed the new plenum and ducts at every new joint.

Combustion and Venting
A 41F temperature change through the system was also a concern. So we called in Jim Davis, one of the nation's leading authorities on combustion efficiency.

Jim's initial temperature testing confirmed only a 41F rise through the furnace. He also identified falling oxygen content in the flue gasses, and CO levels ranging from 21 to over 85 ppm before he shut down the equipment.

"The inducer fan sounds funny." mumbled Jim, "Take off that flue pipe and see what we've got." As Henry removed it, a couple cups of rust and soot spilled out. Jim issued an order to replace the flue and clean out the inducer fan.

Meanwhile Henry and I balanced the airflow into each room. The duct modification had done what we'd expected. We took final readings and completed most of the reports so they would be ready for the final numbers. The system was looking good, but we wondered if we could get it above 90%. The performance was so poor when we started, but we had completed all the repairs that the tests indicated the system needed.

With a new flue and a clean inducer fan, the system was retested, and the gas pressure was adjusted to maximize burner performance. The oxygen in the flue gasses stabilized at 6%. The CO landed and stayed steady at 76 ppm, well below the 99 ppm maximum standard. The temperature rise through the system soared to 60.4F.

With combustion maximized, ducts renovated, insulation problems solved, and the system balanced, we all assembled in the front room to do the calculations and enter the final numbers on the report. We were tired, sore and really dirty as we each expressed our admiration for the installers and service techs who get this valuable work done day after day.

The moment came as I calculated the system's final HSER value. The last stroke of my calculator revealed a 99.07%! As I announced the final score, there were widening grins and then cheers. We were thrilled with the results of our tests and happy for Betty. We cleaned up and went back to the office and enjoyed a Christmas party where we reveled in the discovery that our old bodies could still make it through an attic.

Betty came back from her Christmas vacation a few days later and was amazed at how warm and cozy her home was. She was planning to move her sewing into the sun room that she had been unable to use during the winter since the home was built. The local newspaper came over and she posed comfortably for the photo shoot. At one point she realized the furnace wasn't running, and commented how nice it was not to have to mentally count the heating cost as it ran constantly like it used to.

To completely describe every test and process we used to diagnose and repair this system could fill a book (as a matter of fact, it fills several of them). More than 400 tests were taken, more than 25 test procedures were used, and dozens of calculations and repair procedures were completed to achieve these results.

It sounds nearly impossible when you look at it from that perspective. But these tests, calculations, and procedures are reproduced and used to diagnose, renovate and test more than 20,000 systems each month around the country.

There's an army out there of trained and certified contractors that have learned to apply time proven principles of HVAC system performance measurement in their companies and jobs every day.

We feel great about the work we did for Betty. With training and dedication, you can feel great about your systems, too - and your bottom line.

Rob "Doc" Falke serves the industry as president of National Comfort Institute, a training company specializing in measuring, rating, improving and verifying HVAC system performance. If you're an HVAC contractor or technician and would like a no-cost duct renovation procedure, contact Doc at [email protected] or call him at 800/633-7058. NCI's website is www.nationalcomfortinstitute.com.

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