1. Technician’s Guide and Workbook for Zoning Section 11: Long House 3 Zone Balancing

2. Flip The Long House
Let’s imagine the Long house example in Manual J has undergone a house flipping/energy savings update. The insulation was increased, the HVAC system and all of the ducts were installed in conditioned space, sealed and leak checked. The kitchen, entry hall, and first floor hall areas have been opened up to the living and dining room for an open floor plan on the first floor. The closets in the Master bedroom were rearranged to expand the original half-bath to a modest full bath. The basement was completely refinished with the addition of a half-bath and the two rooms that were not conditioned originally were conditioned.

3. Zone Plan 1st Floor

4. Zone Plan Basement

5. Load Calculations

6. Proportional Balancing CFM (1)
To calculate the actual balancing CFM the airflow measured in the field must be made proportionally equal to the Manual D design values. In some cases, the proportional values may be adjusted. For example, as the kitchen design CFM value could be adjusted slightly towards the cooling value and be slightly warmer than design year-round. This is an important concept for balancers to understand. As they balance, the individual diffusers will generally be slightly above or below the design value. It is important for balancers to keep in mind how leaving the diffuser slightly above or below the design value will change the operation of the area served in heating and cooling applications.

7. Proportional Balancing CFM (2)
The balancing CFM calculation is done in the field using the measured airflow across the heat exchanger value. The actual field value is then corrected so that it aligns proportionally with the design values.

8. Proportional Balancing CFM (2)
For this example, the Manual D values will be used for calculating the final proportional airflow values for balancing. Calculation 1 Balancing CFM: 672 ÷ 712 = ; for front hall 24 × = 23 (rounded); enter 23 into column 3 Front Hall. Calculation 2 Final Balance percent of design: (24 ÷ 23) × 100 = 104%

9. Proportional Balancing CFM (3)
Room/
Diffuser
Manual D
Design Value
Balancing Design for Field 672 CFM
As Found CFM
(all dampers open)
Final Balanced CFM
(all dampers set)
Final Balance % of design
Front Hall 24 23 40
104%
Front Hall Cl 7 8 6
89%
Great room/1 43 41 30
98%
Great room/2 38
102%
Great room/3 72
1 Bedroom/1 37 35 45 36
103%
1 Bedroom/2 57
97%
M Bath 22 18
105%
2 Bedroom/1 34 32 33
2 Bedroom/2 62
100%
3 Bedroom 60 27 56
Kitchen/1 42
Kitchen/2 44
Bath 1 14 13
108%
Rec Rm/1 58
Rec Rm/2
Utility 53 50 54 49
Exercise 46 26
95%
Total
712
672
670
99.7%

10. Zone Balancing (1)
Obviously, setting the individual dampers to the required airflow as described in the last section is only one step in the zone balancing process. That balance would normally be done with every zone calling for cooling. After it is completed, what happens when one or more is closed? We know the system is providing 670 CFM after the balance was completed. We need to check the equipment manufacturer’s expanded data to find out the minimum allowable airflow across the heat exchanger. Remember, if we go below that CFM value, the DX coil may ice up in cooling, and a fossil fuel heat exchanger may cut off on high temperature safety. For this example, lets imagine the minimum CFM is 500 CFM. That means that when we are done with all of our balancing, we must double check to make sure that 500 CFM or more are crossing the heat exchanger in every operating configuration.

11. Zone Balancing (2)
After we know our operating CFM of 670 CFM and our minimum allowable CFM of 500, we need to know the total CFM for each individual zone. The CFM design values and value after balancing are:
Zone 1 design = 249 CFM; Total after balancing = 235
Zone 2 design = 240 CFM; Total after balancing = 229
Zone 3 design = 223 CFM; Total after balancing = 206
Always double check numbers before starting to balance. In this case = 670 and that matches our final balance CFM total; = Thus, the total proportionally matches our Manual D design total.

12. Zone Balancing (3)
The zone that uses the smallest amount of airflow is 3, at 206 CFM. If zones 1 and 2 are closed zone 3 will have 206 CFM plus some amount of overblow. The system will also have a decreased airflow across the heat exchanger due to the blower curve related airflow reduction due to the higher system external static pressure (ESP). If the over blow is greater than or equal to 500 CFM the system will operate safely if the over blow total for zones 1 and 2 is 500 or more CFM each. This overblow design is possible when an ECM motor is used to control the minimum CFM that goes through the heat exchanger.

13. Zone Balancing (4)
The diffusers must be designed to operate at the higher CFM without excessive noise, and without creating drafts in the conditioned space. The operating cost of the blower motor may rise but, the system is easy to balance, and future system operational diagnostics are made simpler. A variation of this zone design type could be achieved by adding a minimum setting on the zone dampers. If the zones were balanced with a minimum airflow setting of 10% of their design value, rather than simply closing all the way, less air would need to go to the active zones as overblow. Similarly, a zone or room could be used as a dump zone.

14. Zone Balancing (5)
Draft testing in the space can be done with an anemometer. Ideally, there will be no measured airflows above 50 fpm in the occupied space where people may be located.

15. Main Balancing Points
There are two main points that must be addressed and documented on the balancing report when the balancing is completed:
The airflow across the heat exchanger must be equal to or greater than 500 CFM for all operating conditions.
The motor amperage must be within the acceptable design range for all operating conditions.

16. Fan Laws and Motor Amperage
Motor amperage is an important consideration, that has not been mentioned earlier. To explain why, lets take a quick look at the fan laws:
Fan Law 1 covers changes in RPM as the relate to CFM:
(CFMbeginning ÷ RPMbeginning) = (CFMfinal ÷ RPMfinal)
Fan Law 2 covers static Pressure’s relationship to CFM:
(CFMbeginning ÷ CFMfinal)2 = (SPbeginning ÷ SPfinal).
Fan Law 3 covers CFM’s relationship to motor break horsepower:
(CFMbeginning ÷ CFMfinal)3 = (BHPbeginning ÷ BHPfinal)

17. Motor Amperage
For balancing, you only need to make sure the system is operating within 10% of the motor nameplate rating.

18. Balancing Report
The Final balancing report should have the final CFM measurements for all zones in closed and open positions and for any minimum air settings, dump zone totals, and/or bypass totals, as well as the blower and motor information. In the case of hydronic systems, a hydronic balance report is also required. For sample fan and pump law calculations and more detailed information on balancing systems, and the relationship between CFM (or GPM), RPM, BHP, and SP (or ΔP) see Manual B or Technician’s Guide and Workbook for Quality Installations.

19. Field Notes
The homeowner’s complaint is the house I warm and the AC does not seem to be working. During the system check the technician found that the blower motor was operating at 9 Amps and 110 Volts. According to the motor nameplate, it was rated to operate at 8 amps and 115 volts. Since 9 amps is above 10% of the name plate value, the motor may need to be replaced. However, first the airflow needs to be evaluated. If it is too high, the airflow is adjusted and the motor voltage and amperage is tested again. In this case there was a large hole in the return plenum after the filter that needed to be repaired. After the plenum was repaired, the motor ran fine.