The values calculated by Design Master HVAC match the values calculated by other loads programs. Any differences in output is typically the result of the building being modeled differently in the two programs. The difference goes away as soon as the two models are identical.
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.
Area: 40 SF
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
|Design Master HVAC||2,300 btuh|
|Trane Trace||2,500 btuh|
|Carrier HAP||1,700 btuh|
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 HVAC reports for this load. It is based upon the proven CLTD method, and it matches two other industry standard programs, Loadsoft and Trane Trace.
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.
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))
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.
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