Field Readings Tutorial

TPD™ System Reading Entry

1. Nom. Tons (Nominal Tonnage):

Enter the system tonnage found on the condenser name plate.
(Nom. Tons is also used in electric heat… see below.)

2. CFM Over-ride:

CFM Over-ride was revised in early 2013 to auto calculate low indoor air flow.  By allowing the program to recalculate low air flow events, TPD can more accurately estimate (and communicate to clients) the actual capacity and SEER losses  associated with reduced indoor air flow.

Normally left blank. The CFM Over-ride is designed to be used whenever the program shows calculated BTUs exceeding the total system BTU capacity (as this is obviously an impossible situation). The system’s actual CFM can be determined by using the CFM Over-ride to decrease the Actual CFM displayed on to report. To display a more understandable and accurate report, lower the CFM air flow until BTUs are within 93% of rated capacity. The alternative is explaining to a client why a system producing 110% of rated capacity is a “bad thing”.
NOTE
: see “The Perfect Storm” below.

3. Indoor Unit Voltage:

Enter the volt meter voltage reading obtained from the indoor unit.

4. Static Pressure:

Normally left blank.

This reading may be necessary to identify duct problems. Use standard static pressure procedures to calculate this number. The program does not calculate this number.

5. Auto Return Temp:

Temperature reading should be taken with the thermostat fan setting set to AUTO. This reading is used to determine actual heating CFM.

6. Auto Supply Temp:

Temperature reading should be taken with the thermostat fan setting set to AUTO. This reading is used to determine actual heating CFM.

7. On Return Temp:

Temperature reading should be taken with the thermostat fan setting set to ON. This reading is used to determine actual cooling CFM.
Note: You must first verify that Fan On and cooling auto use the same fan speed.

8. On Supply Temp:

Temperature reading should be taken with the thermostat fan setting set to ON. This reading is used to determine actual cooling CFM.
Note: You must first verify that Fan On and cooling auto use the same fan speed.

9. Element Amp Draw:

Enter Amp probe reading for total amp draw of all electric heat elements.
Note: When electric heat is used without air conditioning entering the number of 5kw heat strips in the Nom. Tons column will correctly calculate needed CFM.

10. Input BTU Rating:

Enter the BTU input as listed on the furnace name plate.

11. Output BTU Rating:

Enter the BTU output as listed on the furnace name plate.

12. Rated Temp Rise:

Enter the high and low temperature ratings as listed on the furnace name plate

13. Stack Temp:

Enter the temperature of the flue gas as it exits the heat exchanger.
Note: Be certain you take the temperature of the flue gas and not the pipe. This may require removing a screw or drilling an access port in the flue pipe.

14. Combustion Air Temp:

Enter the room temperature, or other air source temperature, that supplies combustion air to the furnace.

15. CO Reading:

Enter the CO level as recorded from a reliable CO meter.

16. Return Wet Bulb:

Enter the Wet Bulb reading taken as close as possible to the fan intake.  All WB readings should be to the nearest tenth (56.8) of a degree)
NOTE: When WB temperature readings are taken at the filter grill:

1) If cooling BTUs are within range there are probably no significant air leaks in the return air side of the system

2) If cooling BTUs are low, hot air entering the return air at a point after the WB reading should be eliminated as a possibility.

17. Supply Wet Bulb:

Enter the Wet Bulb reading taken as close as possible to the air leaving the evaporator coil.  All WB readings should be to the nearest tenth (56.8) of a degree)
Note: When the WB reading is taken at a supply grill it should be taken at the grill closest to the air handler in order to eliminate the effects of “duct gain”.

18. Outdoor Unit Voltage:

Enter the volt meter reading as taken at the outdoor unit.

19. Outdoor Fan Amps:

Enter the Amp probe reading as taken for the outdoor fan.

20. Compressor Amps:

Enter the Amp probe reading as taken for the compressor.

21. Indoor Fan Amps:

Enter the Amp probe reading as taken for the indoor fan.

22. Ambient Temp:

Enter the outdoor ambient air temperature.
Note: Always take ambient air temperatures in the shade… not in direct sun light.

(Nom. Tons is also used in electric heat)

Most heat pump applications require a minimum of 5Kw of electric heat per ton in order to avoid ‚”cold blow‚” during defrost. Cold climates may require more than 5 Kw per ton.

In cases where electric heat is used without air conditioning, the number of 5Kw heat strips or the total heat strips should be entered in the Nom. Tons space.

The “perfect storm”

Since TPD™ initially uses an assumed air flow to calculate cooling BTU production it is possible… but extremely unlikely… that a system could have a total enthalpy heat loss that perfectly matches an increase or decrease in air flow, resulting in a calculation that would exactly match a systems expected BTU production. We refer to this situation as the “perfect storm” and though possible it is extremely unlikely.

Other considerations

The CFM Over-ride function has been added to determine a more accurate CFM range when the assumed (company preference adjustable) CFM per ton, causes BTU calculations to exceed a systems rated BTU capacity. See above.

TPD™ does however provide a method of calculating actual air flow using the “fan-on” setting in the system heating mode. Just verify that fan-on and cooling auto are set to the same fan speed.

DO NOT EXPECT TPD™ CALCULATIONS TO MATCH OTHER STANDARD AIR OR PSYCHROMETRIC CALCULATORS THAT DO NOT COMPENSATE FOR OUTDOOR AMBIENT TEMPERATURE OR ALTITUDE .

All CFM related calculations are altitude adjusted using a modified ‚”standard air‚” model. TPD™ also further temperature adjusts Cooling BTU and other readings based on outdoor ambient conditions.