Return Ducts and Building Load
Let’s focus first on the return duct system and understand how this part of the HVAC system can drastically affect the load of the building.
Heat loss of a building in winter is often looked at as the heat that escapes a building envelope through a leaky window, poorly insulated ceilings, or oversized plumbing penetrations. While these are true, each of these lacks a major driving force to move the heat out of the house – the fan pressure found in a duct system.
On the other hand, a return duct leak from outside of the building envelope pulls cold outside air with a force 50-times greater than nature would impose on the home. Near the equipment, the return duct often sees a pressure of .40-in. w.c. or greater, that pulls cold air into the building.
Follow the BTUs
Let’s walk through the process of how to calculate the BTUs that are pulled into the building from outside. Say its 30F outside and 70F inside. This gives you a temperature difference between the outdoor air and the indoor air of 40 degrees. You can easily measure this number.
Measure the fan total external static pressure at .83-in. w.c (the national residential average) and verify the fan speed is set at high. Plot airflow on the manufacture’s fan tables at 1480 CFM.
Next, measure the return grilles with a balancing hood and find the total return air at the grilles is 1080 CFM. Take the fan CFM of 1480 minus the return grille CFM of 1080, to find the measured return duct leakage of 400 CFM.
With this information, you can calculate the BTU of outside air being pulled into the home. This is added building load.
The BTU formula is CFM of leakage x temperature difference x 1.08. (1.08 is the default BTU multiplier). So, 400 CFM x the 40 degree temperature difference between the outside and inside air temperature x 1.08 = 17, 280 BTU loss per hour. This is a measured and hidden increase in the load of the building.
Pretty scary, isn’t it? What’s even scarier is that this part of the building load is invisible, unless it’s measured. Say a Manual J load calculation shows the building’s heat loss at 52,000 BTU per hour. The 17,280 equals an increase in the building load of 33%.
Imagine the amount of calking, weather stripping, low flow shower heads, electrical outlet gaskets, compact florescent lights and Energy Star appliances it takes to equal the energy savings an everyday HVAC guy gets done in a two-hour duct repair job?
What’s more interesting is that homeowners like this repair and get immediate comfort benefits from it effortlessly. No hype, no sacrifice, no suffering or inconvenience; just comfort.
The repair consists of tightening the duct system with a little pookey, and then exceeds code by blanketing the duct where the temperature loss is measured with 48-in. wide R-19 insulation. Then, measure airflow and temperature change to verify that the repair eliminated the cause of the excessive building load culprit.
In summer months the testing is similar, although the BTU formula is for total heat. Email me for more information at [email protected].
Supply Duct and Building Load
Often the supply duct impact on the building load is less because the only losses are heat loss through the ducts for duct leakage and thermal loss due to poor insulation. Unlike return ducts, supply ducts do not pull in the cold or hot outside air.
Here’s the measurement process. As before, plot the fan airflow — stick with the 1480 CFM. Then measure each supply register with a balancing hood and add them together to find the supply CFM into the building of 1180 CFM. 1480 CFM minus 1180 CFM = 300 CFM of supply duct leakage.
To determine duct temperature change, measure the temperature leaving the equipment (downstream of the equipment at least 5 duct diameters) let’s say it’s 110F. Measure the average air temperature leaving the supply registers of 100F. 110 F - 100 F degrees = a 10 degree duct loss.
Apply the sensible BTU formula: 10 degrees of supply duct temperature loss x the 300 CFM loss x 1.08 = 3,240 additional BTUs of build load that’s lost through the supply ducts.
Add together the 17,280 BTUs of return duct loss and the 3,240 BTU of supply duct loss to find an increase in the building load of 20,520 BTU. Divide the 20,520 BTU by the 52,000 BTU per hour Manual J estimated heat loss, to discover nearly a 40% increase in building load that can be easily reduced to the default Manual J duct loss by tightening the ducts and adding some duct insulation.
Is this testing perfect to the exact CFM? No, neither is building science testing. But the improvement you make in performance of the HVAC system is accurately measureable and when you begin real HVAC system testing, you will discover a new dimension of success never before enjoyed by you or your customers.
Caution: Do not use duct pressurization testing CFM numbers to calculate duct BTU losses. There’s no correlation between operating duct CFM loss and the theoretical CFM loss calculated by duct pressurization testing.
Rob “Doc” Falke serves the industry as president of National Comfort Institute an HVAC based training company and membership organization. If you're an HVAC contractor or technician interested in a free procedure to calculate duct loss BTU, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles and downloads.