2 Fieldpiece tools + 10 mins = 1 easy BTUH calculations.
I just wanted to make a post explaining how easy an actual output BTU calculation is using only two of Fieldpiece's tools. FP really has simplified some of the hardest HVAC calculations with the introduction of these tools.
What I used:
Fieldpiece SDP2 Dual In-duct psychrometer
Fieldpiece STA2 In-duct hot wire anemometer
Drill w/ 1/2" step bit.
Operate your system in cooling mode. Drill one 1/2" hole in the supply and one 1/2" hole in the return. Using the SDP2, insert the probe marked with red in the return and the unmarked probe in the supply. After the system has ran for a bit and settled down, record the enthalpy. The SDP2 will display supply enthalpy and return enthalpy. Subtract the supply from return.
Return Enthalpy: 26.04
Supply Enthalpy: 19.50
Enthalpy change (Δh): 6.54
Using the tape measure - measure your return duct. Using the STA2, input your square duct measurements or round duct diameter. Refer to the table in the STA2 manual; mark and drill 1/2" holes in appropriate traverse locations. Take timed measurements.
Feet per minute: 556
Cubic feet per minute: 988.4
CFM x 4.5 x Enthalpy change (Δh) = BTUH
988.4 x 4.5 x 6.54 = 29,088 BTUH.
I have a 2.5 ton (30,000) system. Not to bad. I would like to make a few adjustments and maybe get a little closer to the nominal tonnage. This whole process took no time at all. I stated 10 minutes and that is only due letting the system run and settle down. By performing this calculation on each system I service, I can tell exactly what changes need to be made produce more output BTU's.
Hope this helps!
Brent A. Ridley