Part 1: Servicing Small Duct/High Velocity Systems

Part 1: Servicing Small Duct/High Velocity Systems


Small duct, high velocity (SDHV) systems have been a niche product in this industry for many years, but are now growing in popularity, especially in the retrofit market. The most common application for these products is a tight-fitting equipment and duct application, where a conventionally ducted system would compromise the architecture of the home.

The difference in servicing SDHV and traditional HVAC split systems lies in how the components are engineered to work together. There is really nothing new here, they're simply put together in a unique manner to match the application. If you're new to SDHV service, this article will provide some basics you'll need to know before you can undertake a successful, high-quality servicing of a system.

Deep Coils As SDHV systems are used to many "tight fit" applications, the coils used are generally smaller to transfer the required Btus. This requires more fins per sq inch, rows and passes of the coil. Tighter coils require better filtration and/or better cleaning care of the coil.

TXVs SDHV products never used fixed-orifice metering devices. Therefore, knowledge of superheat and subcooling for proper charging of a line set (length and lift) is required. If you don't know how to service a refrigeration system, don't service the refrigeration cycle of SDHV.

Duct Pressure Adjustment of the static pressure may be required during or after servicing. Measuring static pressure both before and after service is one method to record the evidence of proper service performed. Static pressure in the main plenum (trunk duct) will range from 1.2-in. w.c. to more than 2-in., depending on length and number of supply ducts, the amp draw of the motor, the restrictor plate setting, and return static pressure.

Measure the static pressure at least 2 ft. from the blower with the insertion tube penetration equal to no deeper than the inside duct wall. If you're lucky, you'll have the record of the original settings at installation to use as a comparison. Regardless, changing any of the settings during service will allow you to balance the system to current service requirements.

High Velocity The air in the main trunk transfers to high velocity at the take-off. Measurement of airflow at the outlet will indicate if there's a restriction in the duct run. If the take-off becomes restricted, the cfm for that duct will be reduced. Look carefully for restrictions if other trades work near your duct runs. The insulated 2-in. duct makes a good-looking knee pad to everyone except the SDHV contractor!

Return Air Usually return location, in relation to the outlets, is not critical to the operation of the SDHV system, so you might find it in a location that is not "typical HVAC." The best location is as high as possible, because of the greater temperature differential across the coil. We would like to see as warm air as possible. Resist the temptation to blame return location for standard HVAC service problems discovered during service. Occasionally, the return duct will become restrictive over time if standard flex duct for return and elbows are not supported. If return static is out of OEM specifications during service, the best technique is to investigate the elbows, by feeling them during operation, for a collapse inside the insulation.

Air Outlet (Floor Location) SDHV uses a principle of induced airflow to circulate the air in the room and condition the air. Sometimes it is referred to as aspiration. The proper installation of the outlet allows for aspiration to work. The air out of the outlet will be traveling between 1,000 and 2,000 fpm to create proper induced airflow at the outlet.

The air traveling out of the outlet in this range will drag the air in the room with it, creating a vacuum or negative pressure differential around the outlet. The air in the room moves to and comes in contact with the conditioned air as long as the fan is running. The re-conditioned air then mixes in the room.

If the SDHV supply outlet is installed in the floor without a screen, objects can fall into it, thus blocking airflow and stopping aspiration. Testing the outlet with a turbo meter will indicate blockage. If blockage is indicated, detach the duct from the outlet and remove the golf ball, marbles, toys, or whatever else is small enough to fit in a 2-in. hole, then install the OEM screen to prevent this from happening again. And by the way, the system will blow out a ping pong ball, which is sort of fun to do, especially at a home show.

Temperature Differential SDHV systems are engineered to operate at 200 to 250 cfm per nominal ton. Standard HVAC systems are engineered to operate at 400 cfm per nominal ton. As a service test, the temperature differential (TD) will be close to twice a conventional system. Proper TD will be 30F at 200 cfm per nominal ton and 25F at 250 cfm per nominal ton.

The TD can be measured in either heating or cooling. Measure the cfm at each outlet on the system, using a turbo meter, and add the total for the system to ensure it's delivering at least 200 cfm per nominal ton. If you have proper airflow, check you subcooling on a DX system or the entering water temperature, water flow rate, and water TD on a hydronic system. This is science, not magic: one or the other must be repaired if you do not have proper TD.

Relative Humidity SDHV used for air conditioning will remove 30% more moisture than a conventional system. The coil is colder because the cfm is reduced. The system is engineered to remove all of the moisture possible at the temperature of the air delivered.

Use a hygro-thermometer at the outlet to measure delivered air temperature and RH percentage to ensure proper system operation. The delivered temperature should be 25F to 30F degrees colder than the return. The RH from the outlet of a properly sized, properly installed, matched system will be 99%. However, it will have at least 30% less grains of water and be twice as cold as the air delivered by a conventional system.

Amps The amp reading on a SDHV system is the "source" measurement. A proper amp reading will always give you an indication of any changes in system operation since the last service. If amps are lower the motor is working less, less airflow moves across the coil, less Btus are delivered, and subcooling is affected. More amps means the motor is working harder, more airflow moves into the duct work, and superheat is higher. Usually this indicates the beginning of duct leakage. I recommend you always start a service by taking an amp reading, on the motor only, and always end a service by recording the amps for the next service.

Restrictor Plate The sole purpose of the restrictor plate is to quiet an SDHV system down at the outlets. However, never restrict the airflow below the SDHV industry minimum of 200 cfm per nominal ton. Setting the restrictor plate lower than industry standard minimum will cause coil freezing. Adjusting the restrictor plate during service isn't required. Should you decide to try and quiet the system down, be sure not to go below the OEM's recommended amps, static pressure, and cfm.

Aspiration (Induced Air Flow) Making a visual inspection of every outlet and moving any obstruction that limits a free airflow of at least 3 ft. from the outlet is required for proper service. Sometimes the customer even blocks the outlet. SDHV systems use induced airflow or aspiration to create a negative pressure differential around the outlets and transfer the Btus in the first 3 ft. of the outlet. Be absolutely certain that there is at least 3 ft. of clearance during service.

This is probably one of the most commonly overlooked service-related checkpoints in the SDHV industry, and careful education of the customer might be required. The diameter of the airflow from a SDHV outlet is very narrow and directional. If the customer blocks the outlet because it's moving something or blowing on someone, either move the outlet or install a 15 or 25 degree offset outlet pointed away from the moving object or traffic patterns.

Condensation (P-Trap) SDHV systems remove 30% more moisture than conventional HVAC systems. If a proper full SDHV p-trap is not operational, it usually means 30% more water damage. There have been recent innovations in primary drain protection. Upgrading the primary drain protection and serviceability is a good excuse to get out those old SDHV customers and schedule a service inspection. If a proper SDHV p-trap is not on the system, the water will not leave the coil when the fan is running because the coils are always positioned on the "return side" of the system. This will lead to a flushing when the system shuts down. Holding the water on the coil because of improper ptrapping will affect the system's ability to condition the space in both sensible and latent load. If you are a frequent visitor to a SDHV home that just seems to run forever, check for a proper SDHV p-trap.

Anti-Frost Switch In temperate seasons (spring and fall) during normal operation any air conditioning system, including SDHV, will have a tendency to frost up the evaporator coil. Engineering for lower cfm per nominal ton needs to allow for this condition. Some manufacturers protect from freeze up with simple anti-frost switches located on the top and bottom of the coil. Service should include the cycling of these switches to ensure that they indicate and satisfy properly.

These servicing points include some common best practices for all SDHV products. However, not all SDHV systems are engineered or built the same. When in doubt, always check with the manufacturer for advice.

Next month: a step-by-step checklist for servicing a SDHV system

Ted Brown is director of contractor training for Unico Inc., St. Louis, MO. He can be reached at 800/527-1939 or [email protected]. For more on SDHV service, visit

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