Airflow velocity in a residential HVAC market duct system is an important design consideration.
Although designing for proper velocity applies to all ducted HVAC construction systems — heat pumps, air conditioners, gas furnaces, etc. — geothermal HVAC market systems benefit from a slightly different design. Because of the location of the refrigerant-to-air heat exchanger on a self-contained geothermal unit, these systems lend themselves to a lesson in improved velocity for residential comfort systems.
Geothermal heating and cooling continues to grow in popularity and HVAC sales. According to geothermal HVAC market manufacturer WaterFurnace, more than 1 million geothermal (or ground-source) heat pumps are operating in U.S. residential, commercial and government buildings. In fact, green HVAC geothermal heat pumps have become so common that most contractors do not give them any special consideration in terms of ductwork fabrication. As a result, duct for geothermal systems typically resemble the ductwork of any other home comfort system. But looking at these systems a bit closer reveals a difference.
It’s all about velocity
Appreciating this difference requires an understanding of air velocity, probably the most important concept in residential green HVAC duct performance. When designing a residential duct system, high velocities in ducts that are too small can cause problems. This means using larger ducts wherever possible. Any volume of air transported through a duct at a velocity of 700 feet per minute will travel at a lower speed when it is transported in a larger duct. Although this is a basic concept with respect to airflow, some HVAC sales technicians and even some HVAC construction designers do not appreciate the value of slow-moving air.
In some cases, slow-moving air is a good thing, while in other cases it is not. Adequate face velocity at a supply register is normally a positive, unless, of course, the register is located directly above a booth at a restaurant, causing napkins to move across the table. In this case, velocity is not so good. But homeowners typically like to know that each register is performing properly, verified by feeling the airflow at each register. Actually, it’s the volume of airflow that is the most important test parameter in energy-efficient homes, but using the more common, nonscientific hand test, we expect to feel air pressure against our skin. And that’s velocity.
Within a duct system, higher velocities increase resistance to airflow, and the increase is exponential. So, an increase in velocity from 700 to 800 fpm might produce an additional 15 units of resistance, but the move from 800 to 900 fpm will likely increase resistance another 18 units. And if the velocity increases to 1,000 fpm, resistance may increase another 20 units.
Most HVAC market contractors agree that resistance to airflow is bad. It causes the fan to work harder, creates extra noise, results in poor air distribution and balance, lowers efficiencies and even shortens the life of an HVAC system. What some HVAC sales contractors do not realize is how much duct velocity contributes to airflow resistance. For example, many in the HVAC construction business think a trunk reduction is needed to boost velocity back to a higher level. There are legitimate reasons to use trunk reductions, among them cost and space. But purposely trying to increase velocity is not a good goal, because this contributes to increased airflow resistance. If cost or space limitations favor trunk reductions, try to keep residential trunk velocity between 400 and 700 fpm by using appropriately sized ducts.
To better understand the negative consequences of velocity, imagine the Indianapolis 500 racetrack with 90-degree, square inside corners. If cars traveled just 15 mph, they could handle those poorly designed turns. However, higher-velocity racing requires long, sweeping inside radiuses on those turns. Car speeds of 200 mph with poorly designed 90-degree inside corners can cause significant turbulence in the outside of the first turn.
Geothermal and velocity
Understanding the benefits of lower velocity is a prerequisite to understanding why geothermal heat pumps have a built-in (factory-mandated, if you will) advantage in terms of good duct design.
A self-contained geothermal system is an all-in-one package that includes a compressor, fan, auxiliary heat and refrigerant-to-air heat exchanger coil. Because there is no room for the coil inside the cabinet, it is located on the exterior. Nor is there sufficient space to use a slant, “A” or “N” shape coil, so the coil is a large, flat panel. Those who do not appreciate lower velocities will see this as a burden, since it now requires a larger fitting and a larger air filter. But it is not a burden.
Consider what happens when the airflow of a 4-ton, 1,600-cubic feet per minute system, expands from a normal duct to the larger duct size that matches up to the coil on the side of a geothermal unit. In the case of the WaterFurnace 5 series 4-ton geothermal heat pump, the coil measures 28 inches by 30 inches, which is 840 square inches or 5.8 square feet. And 1,600 cfm of air running across that opening will result in an approximate air speed of 275 fpm, not considering cabinet framing. The return-air plenum size for a typical 4-ton system might be 14 inches by 26 inches. In this location, the same 1,600 cfm will have a velocity of just over 640 fpm. Slowing the air down to less than 300 feet per minute at the filter and coil is advantageous, especially if a 90-degree fitting is used with an inside radius and/or turning vanes to spread the air uniformly across the filter and coil. At this reduced velocity, both the filter and the coil produce a much lower pressure drop, noise is reduced and the fan does not work as hard.
In a conventional, non-geothermal 4-ton furnace or air handler, the 14- by-26-inch return drop turns into the 20-by-25-inch filter cabinet, where 1,600 cfm yields 460 fpm velocity. A 5-ton, 2,000-cfm system on the same 20-by-25 air cleaner produces 575 fpm. Review the specifications on the air cleaners to see the significant increase in the pressure drop, comparing 300 to 600 fpm velocity. Geothermal systems lend themselves to a very acceptable, low velocity where it is most important.
In the case of variable-speed compressor systems, like the WaterFurnace 7 Series, contractors typically design the duct system for the maximum size/speed of the compressor. So, a 4-ton 7 Series geothermal unit may have the same properly sized duct system as a 4-ton, two-speed 5 Series unit. But variable-speed compressor systems run at a lower-than-maximum capacity and fan output throughout most of the heating and cooling season. This means sometimes, 400 cfm may be put into a 1,600 cfm duct system, which results in very low velocities.
If an HVAC market designer is steadfast on the need to maintain a high velocity, then he or she may be reluctant to trust a duct system that is larger than it needs to be. But remember, low velocity reduces airflow resistance, so at part-load run times, the ducts and fittings have a much lower resistance or pressure loss. Also remember it’s the air volume delivery with which we are most concerned. So when 400 cfm of conditioned air is inserted into a low-resistance duct system, 400 cfm will come out, somewhere. Ensuring air volume is allocated as designed is the ultimate challenge for every HVAC market duct design. To simplify it: first, minimize resistance for the longest airflow paths; then, make short paths more restrictive or damper; finally, test and balance.
Every home and every duct system performs differently, but generally speaking, the larger the ducts, the lower the velocity, which results in lower airflow resistance. It makes sense to pay special attention to duct and fitting sizes to reduce velocity. This is especially valuable at the air handler or furnace, where the total airflow of the system is being pulled and pushed through a pretzel maze of ducts and several restrictive devices. Geothermal heat pumps will, by design, help familiarize contractors with larger ducts, lower velocities and less noise. But these lessons apply to all residential ducted comfort systems.
Dan Welkin is a comfort consultant and marketing manager for Precision Comfort Systems in Westfield, Indiana, and also conducts training on the science of indoor comfort.
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