Consider the Duct Portion of the Building Load

Dec. 7, 2011
Over the past year NCI has collected a substantial volume of field data on the duct portion of building load from NCI contractors in the field using advanced diagnostic testing methods.

Over the past year NCI has collected a substantial volume of field data on the duct portion of building load from NCI contractors in the field using advanced diagnostic testing methods. How would it change your business if you were able to show homeowners that one-third to one-half of their home’s heat loss or heat gain was coming from a poorly operating duct system?

As building performance and HVAC contracting are being moved closer together, savvy HVAC contractors are finding realistic ways to control the building load by better executing their profession. Replacing windows and light bulbs doesn’t seem very appealing to most HVAC contractors, but reducing the building load up to 50% by improving the duct system and proving it to homeowners, enables us to remain the kingpin in the building performance game.

Find Duct System Losses

Without diagnostic testing, typical inputs into load software normally assign a BTU loss of about 15%. Under summer and winter weather conditions this can miss the target by more than 300%.

To identify actual operating conditions, field diagnostic tests can be taken on a live system to reveal defects that can then be corrected. Without identifying and correcting these operating system defects there’s no way to control the load of a building or deliver quality HVAC system performance.

Duct Leakage

Duct leakage is a real threat to the load of a building, and can be reduced in almost every situation. BTUs that are lost or gained through leaky duct systems directly add to the load of a building. So by tightening up a duct system correctly, the load of a building may be reduced.

But you can’t just pookey up the ducts and expect energy savings as is done in most of today’s energy savings programs. With the average US residential .50-in. w.c. fan currently operating at over .80-in. w.c. of total external static pressure, additional duct capacity must be added to the system before duct sealing or the result will be substantially increased static pressure, which will result in lower airflow and reduced system capacity. How efficient is a 20 SEER system at 250 CFM per ton? This is the typical result of many duct sealing jobs.

Before duct sealing, measure total external static pressure first. You will find it’s typical to need additional supply and return duct capacity before duct sealing can begin to assure adequate airflow. Then pookey the duct system to assure airflow and BTUs will not be lost from the system on the supply side or pulled into the system on the return side. This will have a substantial and immediate measurable impact on the load of the building.

How to measure duct leakage? Measure it live. Measure the fan airflow, then measure the airflow at the supply registers and then at the return grilles. Subtract the total of the supply registers from the fan CFM to find live supply duct leakage. Do the same on the return side.

Example: The system fan is measured at 1000 CFM. The air through the supply registers is measured at 800 CFM; the difference is the supply duct leakage of 200 CFM. On the other side of the fan the air at the return grilles measures 700 CFM. 1000 Fan CFM minus 700 CFM at the return grilles reveals 300 CFM of return duct leakage. Duct leakage of 200 CFM on the supply side and 300 CFM on the return side total 500 CFM of live duct leakage.

Seal the duct? Sure, usually after adding more duct capacity.

If you've been using duct pressurization methods to test duct leakage, you will find a substantial difference between the two duct leakage numbers. While duct pressurization testing is more widely accepted, the duct leakage number it provides has never claimed to represent a live duct leakage CFM number. Compare the two methods for yourself.

Duct Temperature Loss

Under live operating conditions, ducts lose some of the BTUs generated by the equipment. The question is how much? The answer is to measure the actual duct losses.

On the supply side, measure the temperature as the air leaves the equipment. Next measure the average air temperature leaving the supply registers. Subtract the two temperatures to find the temperature change through the duct system. Do the same on the return side.

Temperature is lost (or gained) through the ducts due to duct leakage and also from thermal loss due to inadequate duct insulation. Both of these losses can be effectively measured in the field.

Example: The air entering the return grille is 70F. The return duct passes through an attic at 40F. The temperature entering the equipment is 60F. Subtract 60F from 70F to find 10 degrees are lost through the return duct system from duct leakage and thermal loss.

To find the percent of BTU loss, divide the degrees of duct temperature loss into the temperature rise of the equipment.

Example: If the equipment temperature rise is 40F and the duct temperature loss is 20 degrees then 20F divided by 40F equals 50% of the equipment generated BTUs lost through the duct system. If the equipment output is 60,000 BTU, multiply by 50% to find 30,000 BTU of duct loss.

Does this turn 90 AFUE equipment into a 45 AFUE system? Let your competitor explain that to your customer.

One challenge of measuring live system BTU losses and gains is that the losses will vary under current operating conditions. To adjust for these changes and to annualize the duct losses or gains, system performance software such as CommonCents™ must be used.

Load Calculation Software Questions

For decades a building’s load has been determined by various load calculation software. While these programs are the industry standard and are required documentation, estimating the heat resistance value and performance of each individual building component and modeling what should happen may not reveal the realities of how each individual building operates under actual conditions.

Measuring the operating performance of an HVAC system under actual live conditions provides hard data that modeling can’t begin to consider using generic inputs and assumptions.

Consider the idea that has been mentioned in “Doc” articles for the past 5 years that the typical residential HVAC system operates at 57% of equipment rated capacity. If that’s true, how is that accounted for in building load-modeling software?

In order for an HVAC contractor to select properly sized equipment from a load calculation that works, wouldn’t the heat loss or gain of the building have to be overstated in order for the math to work? Or what happens when a quality contractor installs and verifies a system operates near 100% of capacity based on the modeled load of a home and the standard modeling inputs were used? Won’t the equipment on well performing system be oversized?

It’s clear that new opportunities exist in our quest to deliver better and better performing HVAC systems. It’s also becoming more and more evident that those that can educate their customers actually reduce building load and offer better performing, smaller and more efficient systems. These contractors will have a substantial advantage in every sales situation.

Rob “Doc” Falke serves the industry as president of National Comfort Institute an HVAC based training company with technical and membership organization. If you're an HVAC contractor or technician interested in a free report on how to measure live system BTU loss contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at for free information, articles and downloads.