HVAC Load Calculation Comparison with Carrier HAP, Trane Trace, and Loadsoft

Wednesday, February 21, 2007

Before using our HVAC load calculation software, most of our customers do their due diligence and compare it to their existing loads program. When a difference is found between the Design Master load versus the other program's load, it is nearly always the result of the inputs actually being different. The difference goes away as soon as the inputs are matched.

There is one instance where Design Master is different from an existing load calculation program. Some glass loads calculated by Design Master and Carrier HAP are different. With the help of some of our customers, we ran a controlled experiment to compare the Design Master output with Carrier HAP, Trane Trace, and Loadsoft.

Load Calculation Input

Orientation: South
Area: 40 SF
U-Value: 0.8
SHGC: 0.87
Latitude: 32 N
Outside Air Dry Bulb High Temperature (July): 97 F
Daily Range: 18 F
Inside Temperature: 70 F
Peak Time: July, 3:00 pm

Calculated Glass Load

Design Master HVAC: 2,300 btuh
Trane Trace: 2,500 btuh
Loadsoft: 2,400 btuh
Carrier HAP: 1,700 btuh

Conclusion

Design Master HVAC, Trane Trace, and Loadsoft all calculate a glass load that is approximately the same. Carrier HAP calculates a glass load the is significantly lower than the other three.

Based upon these results, we are confident in the numbers that Design Master reports for this load: it is based upon the proven CLTD method and it matches two other industry standard programs, Loadsoft and Trane Trace.

Calculation Methods

Design Master HVAC is calculated using the Cooling Load Temperature Difference (CLTD) method. Loadsoft uses the new Radiant Time Series (RTS) method. Carrier HAP uses Transfer Function Method (TFM). It was unclear from the documentation what Trane Trace uses, but our best guess is that they use TFM.

Verification by Hand

One benefit of the CLTD method is that it is easy to verify the output by hand. Here is how the Design Master value is calculated:

Glass Load = Glass Conduction + Glass Solar

Glass Conduction = U * A * (Conduction(time) + CLTD Correction Factor)
Conduction(3pm) = 14
CLTD Correction Factor
        = (78 - Inside Temperature) + (Outside Temperature - 85 - (Daily Range / 2))
        = (78 - 70) + (97 - 85 - (18 / 2))
        = 11
Glass Conduction = 0.8 * 40 * (14 + 11) = 800

Glass Solar = A * (SHGC / 0.87) * CLTD
CLTD = 38
Glass Solar = 40 * (0.87 / 0.87) * 38 = 1520

Glass Load = 2320

The conduction function is based upon table 34 in 1997 ASHRAE Handbook - Fundamentals

The CLTD is based upon a table generated by software included with Cooling and Heating Load Calculation Manual - Second edition, McQuiston and Spitler, 1992.