Thermo Products OPB (24, 36)- 80, 30 service manual XI. APPENDIX B CALCULATIONS Example

All installations and services must be performed by qualified service personnel.

XI. APPENDIX B: CALCULATIONS

Example 1:

Problem: It is desired to install a warm-air furnace w/ an air conditioning evaporator coil at a residence. The net output of the furnace is 100,000 BTU/hr and the evaporator will provide 4 tons of cooling to the space. Estimate the minimum required airflow for the system to operate satisfactorily.

Solution: From the heat output of furnace, the heating airflow requirements can be estimated:

100,000 BTU/hr x 14 CFM/1000 BTU/hr = 1400 CFM.

From the required cooling performance, the cooling airflow requirements can be estimated:

4 tons x 400 CFM/ton = 1600 CFM.

The larger of the two airflow figures should be selected, i.e. 1600 CFM cooling requirement.

If only 3 tons (36,000 BTU/hr) was installed, the required cooling airflow is:

3 tons x 400 CFM/ton = 1200 CFM.

Then, the duct system should be sized for the 1400 CFM heating requirement.

Example 2:

Problem: A plan view of a residence with 1600 sq. ft. living area is shown in Figure 15. The supply registers and the OPB unit have been positioned. Ductwork interconnecting the registers is also shown. What is the required air flowrate at each register? What is the required duct size for each segment of the duct system?

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