Servicing Small Duct/ High Velocity Systems

Servicing Small Duct/ High Velocity Systems

By Ted Brown

Figure 1. Airflow is critical to a SDHV system, so carefully check duct runs and take-offs for signs of leakage.

Figure 2. Always check the amp reading of the blower motor.

Small Duct High Velocity (SDHV) central heating and cooling systems have been installed in the U.S. since the 1940s. Originally just heating systems, SDHV systems today can provide heating, central air conditioning, air filtration, humidification and outside air to the conditioned space. The SDHV blowers, coils (both heating and cooling) and duct system form the heart of the comfort provided in any application.

Other manufactured industry products, such as boilers, geothermal, and tankless water heaters are the heat source for the SDHV system heating coils. Major manufacturers' condensing units, heat pumps, chillers and geothermal systems provide the cooling source for the SDHV system cooling coils.

Additionally, all items that can be installed on the return side of a traditional HVAC system, including air filtration (electronic, media, and UV), humidification (power and steam), heat recovery and energy recovery ventilators (HRVs and ERVs), and even simple outside air systems can be installed on the return side of an SDHV system.

It's critical that you know exactly what comfort the SDHV is providing and what the source of each layer of comfort is. You may be servicing a system that has a blower module, a hot water heating module connected to a condensing boiler, a chilled water module connected to a reverse cycle 5-ton chiller, a steam humidifier for humidification, a UV filter for air filtration, and an outside make-up air system, all coordinated through a central printed circuit board brain. You need to know about chillers, SDHV systems, condensing boilers, UV lights, steam humidifiers, make-up air systems, and printed circuit boards in order to service the system professionally.

The following is a step-by-step process to effectively and professionally cleaning a SDHV central heating and air conditioning system.

Step 1. Identify all of the sources of heating and cooling.
Locate the thermostat for the SDHV system. Then, locate the return air location and the filtration method being used. Locate any boilers, chillers, condensers, HRVs, ERVs humidifiers, or air cleaners connected to the system. Identify if the entire system is being coordinated through a printed circuit board, and ascertain its location. Make certain that the filter is clean. If it's not, clean it or replace it. Small duct systems are all about air flow. If the return filtering system is restricting air flow in any way, your cleaning process will be compromised.

Step 2. Turn the blower of the SDHV system to the fan "on" mode, with the system selector switch of the thermostat in the "off" position.

Step 3. Visually inspect all accessible main trunk line and small duct. With the blower on, inspect the ducting for any tears, exposed uninsulated duct, or loose connections at the main trunk line (see Figure 1). Check the 2-in. ducting for a loose or blown off end cap, loose or leaking take-off at the blower to the main trunk line, or any other place where air may be leaking. Make any necessary repairs to the leaking or uninsulated ducting.

If the system has a single return that uses an SDHV return duct (typically connected to the top of the return filter grill in the ceiling of a hallway, approximately 10-ft. in length and adapted to the return side of the fan and coils in the attic), inspect it internally up through the filter grill. Look for a build-up of dirt on the inner lining. If the SDHV is an older system with a large build-up, recommend replace ment. This entire step is critical before pro ceeding to measuring the airflow in Step 5.

Step 4. Locate every outlet on the system.
Count the number of total outlets. The standard small duct outlet is round, with a 2-in. opening in the middle. Some small duct manufacturers make a slotted diffuser approximately 8-in. long, by 13/4-in. tall, with a 1/2-in. high slot in the middle for air distribution. These also count as outlets.

Step 5. Calculate and measure the airflow of the system.
Using an airflow reading tool called a Turbometer, calculate the cfm of air flow of each outlet. Set the tool to the "knots" position. Place the round opening of the Turbometer over the 2-in. outlet, and record the knots reading. For all round outlets, multiply the knots by two to obtain cfm. For the 90-degree slotted outlets, multiply the knots reading by four and then add four to get the total. For a straight slotted outlet, multiply the knots reading by six to get cfm. Add up the cfm to get the total cfm of the system.

Step 6. Take an amp reading with your amp meter at the blower motor.
Place your probe around the wires leading to the blower motor inside the SDHV blower module. Record the amp draw (see Figure 2). Check the setting on the restrictor plate on the front of the blower module. The cfm reading calculated in Step 4, the amp draw of the blower, and the setting of the restrictor plate give you the information necessary to check with the manufacturer's air flow chart to determine if all three are in alignment.

Note: An SDHV system installation at sea level can be calculated at between 200 and 250 cfm per nominal ton of cooling. On a 3-ton system, an acceptable range of cfm as calculated in Step 4 would be between 600 and 750 total cfm. If your application is at high altitude (over 2,500 feet above sea level) you must have at least 250 cfm per nominal ton of cooling. Therefore, a 3-ton system at high altitude would have at least 750 cfm of total airflow out of the outlets.

Step 7. Inspect the blower motor and blower wheel inside of the small duct blower module.
Remove the side doors of the blower module for a visual inspection of the blower motor and wheel. If you find that the wheel is dirty, disconnect and remove the blower and wheel. Clean the wheel thoroughly and make certain that it's completely dry before reinstalling. If your amp draw reading in Step 6 didn't line up with the manufacturer's chart, retake the reading with the clean wheel and recheck the chart. Reattach the door and tighten the screws holding it firmly in place.

Step 8. Inspect the heating and cooling coils.
Remove the side doors of both the cooling coil (DX or chilled water), and the heating coil (hot water). Note that in chilled water/hot water applications, there may only be one coil that is used for both heating and cooling. As with a traditional coil, you're looking for signs of dirt restricting the heat transfer capabilities of the coil. Should the coil appear dirty, clean it in place using a spray bottle and coil cleaner. Leave the access doors off until you complete Step 9.

Step 9. Inspect and check the drain lines for all coil modules with drain lines.
Many SDHV systems are installed in the attic spaces of homes, or above the ceiling in commercial applications. Proper drainage of each module that holds or creates water is important. If the SDHV unit being serviced is in a horizontal configuration, the drain line from the unit must have 1/4-in. slope for every foot of drain line from the drain line opening of the unit to the exit point of the drain line to the outside.

Visually inspect the drain line completely for any cracks, denigrations, deteriorations, dips, or compromises. If you find any of these, repair them immediately. Once you have determined that the drain line has the proper slope and is in good shape, pour 2 gal. of water down each drain line. This will ensure that the drain is open and that condensate will flow appropriately. Every SDHV system must have a P-trap in the drain line as well. If one does not exist, install at this time.

Once you've determined the primary drain line is clear and working, test any and all secondary drain lines in the same manner. If a secondary drain pan doesn't exist in an application in an attic or above a ceiling, now is the time to recommend this to the customer. If there is a secondary drain pan, make sure it's drained to the outside, and that the float switch in the secondary pan will break the contacts to the condensing unit. If there's a float switch in the primary drain pan, remove it and immediately recommend and install a secondary drain pan. Nothing will shorten the life of the compressor portion of a SDHV system like a float switch in the primary drain pan (you'll see this primarily in older applications).

Once you have determined that all drain lines are working appropriately, reattach all service doors to all modules, tightening the screws so that each is firmly in place.

Step 10. Clean and check any condensers, chillers, heat pumps, boilers, humidification products, ERVs, and HRVs.
These cleanings should always be done according to the individual manufacturer's specifications and cleaning procedures. Check with these companies for their clean and check procedures.

Step 11. Check operation of the entire system, making certain that all are coordinated properly and working seamlessly in conjunction with each other and with the brain of the SDHV system, the advance printed circuit board.

Step 12. Check the refrigerant charge of the system.

Step 13. Fill out your service ticket.
Make certain that you fill out all of your work and all recommendations to the customer on the ticket. The ticket is the reference guide for what happened and what you checked. It's an important document.

Step 14. Get Paid.
You've just performed a very thorough, professional cleaning of a customer's SDHV system. You and your company deserve to get paid. Simply ask, "Will that be MasterCard, Visa, Discover, your personal check or cash for today's service call?" For service agreement customers, total the ticket as if they were paying. Then draw a line through the dollar total and write next to it, "No charge, covered under your service agreement." Customers wants to know that they're getting value from their service agreements. You're reinforcing behavior by presenting it in dollar format and then congratulating them on having a service agreement with your company.

Step 15. Congratulate yourself on a job well done!

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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