1. Technician’s Guide and Workbook for Zoning Section 10: System Diagnostics

2. Temperature and Comfort Complaints
Temperature and comfort related complaints may be directly related to HVAC equipment. However, they may also be related to the systems design or how the home was constructed. Once again, before deciding a zoning system will fix the problem, a full home evaluation might be in order.

3. Common Sense Required
Technician’s should use “common sense” and make sure the home is functioning properly before adding a zone system. For many home comfort issues, servicing the equipment to bring it back to proper operating condition, sealing the ducts, and balancing the system’s airflow will solve most of the comfort problems.

4. Thermostatic Control Issues
Thermostat location is important for all systems. The basics of placing the thermostat on an interior wall, where it will not be affected by direct sunlight at about 54 inches above the floor, or 48 inches above the floor to be Americans with Disabilities Act (ADA) compliant, holds true for individual zone thermostats too.

5. Verify Thermostat’s Operation
To test a zone for proper operation the thermostat must be set so the zone control is calling for a temperature change. The zone airflow, or hydronic flow is then measured and compared to the design requirement (auxiliary equipment should come on). Then the thermostat is adjusted so the zone controller is in off position and the air or hydronic flow is again tested (auxiliary equipment should be off).

6. Damper Control Issues
Automated dampers have motors that are controlled by a central zone control module. Dampers are generally designed to be normally open (NO), or normally closed (NC) when power is applied. A third option is the damper stays in the last set position until a control signal is sent to open or close it.

7. Zone Control
The zone control can be wireless or provide wired signals in ac-volts, dc-volts, mili-volts or mili-amps through a floating type of signal. For these systems the damper motor then interprets the control signal and applies it to move the motor and control the damper’s position. This type of system may fail-to-open, fail-to-closed or stay in the last position the damper was in, when the signal was lost.

8. Electrical Diagnostics
Today's HVAC systems are more complex than ever. The HVAC technician must not only have refrigeration skills, but also have superior electrical troubleshooting skill. The ability to read and understand equipment wiring diagrams and measure voltage, current and resistance in electrical circuits is a must. Correctly troubleshooting and diagnosing control circuits and their components is a critical skill.

9. Moisture and Condensation Issues
For the optimal comfort, homes should operate at a relative humidity (RH) of 50%. Operating at 60% RH is still reasonably comfortable in very humid areas where maintaining 50% RH is difficult and expensive.

10. Moisture Failures
Mold and mildew blooms may grow wherever moisture is present. Generally, they appear in locations where the RH is above 60%. Sheetrock and other common home surfaces make excellent areas for mold growth.

11. Minimizing Grains of Moisture (1)
Moisture gains can be minimized by implementing the following:
Minimize infiltration from the outside.
Seal all Ducts. Duct in conditions space can condense moisture if there are small air leaks.
Repair and seal foundation leaks and make sure moisture does not sit against exterior walls.

12. Minimizing Grains of Moisture (2)
Items like dryers, gas appliances and fireplaces that cause a negative pressure in the home need to have a conditioned pathway for makeup air to enter the home.
Humidity control incorporated into engineered ventilation design.
Size cooling equipment correctly.
Cycle blower fan so it does not pick up moisture from the drain pan when the coil is not in the cooling function.

13. Minimizing Grains of Moisture (3)
Close off cooling only ducts in unconditioned space that can condense moisture in the winter.
Install a dehumidifier for humidity control.
Make sure refrigerant charge is correct.
Make sure cooling coils are clean, and filters are changed frequently.
Condensate drain must work properly.

14. Retrofit Damper Location
The fact that many contractors simply install a zone control, add zone thermostats and zone dampers to an existing system and skip all of the design work is problematic on many levels:
They do not know how the blower performance will change.
They do not know what the current airflow across the heat exchanger is.
They do not know how much air is getting into and out of the zones.

15. Airflow Across The Heat Exchanger

16. Retrofit Damper Evaluation (1)
At a minimum before attempting to add dampers to an existing system the following must be done or known:
Drawing of the complete duct system with size and existing measured SP on trunks.
Knowledge of the minimum and maximum airflow required by the system for heating and cooling.
The blower motor name plate FLA and voltage.
The actual airflow across the heat exchanger and a corresponding SP on the main trunk.

17. Retrofit Damper Evaluation (2)
A Manual J load calculation for each room and a corresponding CFM value for each room (and diffuser if more than one in a room).
The as found CFM from each diffuser.
The existing equipment must be clean, properly charged and operating properly.
If it is determined that the existing system is not operating correctly, the system deficiencies must be addressed, prior to designing/installing the zone system.

18. Retrofit Damper Evaluation (3)
Once it is determined that dampers are going to be installed, and an excess airflow management plan is in place, the dampers, control system, and thermostats may be installed. Fan Law 2 can be used to quickly calculate the CFM when a single zone is opened. The basic formula in a calculation format for evaluating the CFM after the adjustments is:
CFMfinal= CFMbeginning ÷ (SPbeginning ÷ SPfinal)1/2

19. Example Calculation CFMfinal= 1,327 ÷ (0.30 ÷ 0.12)1/2
For an example say we had a CFM of 1,327 measured when the supply trunk measured 0.30 iwc SP and we added dampers and closed down all but the smallest zone. The SP on the main trunk is now found to be 0.12 so we use the formula to quickly calculate the operating CFM. CFMfinal= CFMbeginning ÷ (SPbeginning ÷ SPfinal)1/2
CFMfinal= 1,327 ÷ (0.30 ÷ 0.12)1/2
CFMfinal= 1,327 ÷ (2.5)1/2
(Note the ½ power equals the square root)
CFMfinal= 1,327 ÷ 1.58
CFMfinal= 839.9

20. Sample Problem For Class
What if we measured a CFM of 1,400 and the supply trunk measured 0.20 iwc SP and we added dampers and closed down all but the smallest zone. The SP on the main trunk is now found to be If we use the formula to quickly calculate the operating CFM. CFMfinal= CFMbeginning ÷ (SPbeginning ÷ SPfinal)1/2
CFMfinal= 1,400 ÷ (0.20 ÷ 0.14)1/2
CFMfinal= 1,400 ÷ (1.43)1/2
(Note the ½ power equals the square root)
CFMfinal= 1,400 ÷ 1.2
CFMfinal= 1,167

21. Bypass Damper Issues (1)
Many older zone systems have large bypass ducts that allow air to go directly from the supply trunk back into the return trunk. Many variations of length and duct sizing have been used to try and make sure the bypass trunk does not return too much air to the return duct. When a technician sees this design in the field there are three basic configurations that should stand out for having probable design flaws.
A bypass duct with a barometric damper but missing a balancing damper.
A bypass duct with an open/close electronic control damper, and no balancing damper.
A bypass duct that is too large without any dampers at all.

22. Bypass Damper Issues (2)
For all bypass systems not designed and installed in accordance with Appendix B directions in this Guide and Workbook, the worst-case bypass airflow should be measured and evaluated. The location of the equipment and the equipment manufacturers entering air temperature limits for heating and cooling must be met. In some instances, the above types of bypass system may work correctly. However, in the vast majority, there will be a bypass airflow volume control problem. Some may never have been set up correctly, others were not designed properly.

23. System Diagnostics Summary
Technicians must verify that the systems they install and maintain are working as designed. Too many zone systems fall into a state of disrepair because basic zone and thermostat testing is considered too hard and takes too much time. In reality, a complete zone function test is easy, and the only way to verify that a zone system is functioning correctly.

24. Field Notes
The homeowner’s complaint is the new system worked fine for a couple of years, but lately the bedrooms have not been comfortable. The Technician had copies of the original duct SP measurements on a startup sheet. After measuring the current SP values and the system’s an ESP, the technician knew the problem was a dirty coil, a dirty filter, or dirty blower blades and recommended the equipment be cleaned.