Modeling Equipment Curves

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Anyone know how to create an unloading curve for a residential split system? I was recently asked to do a life cycle cost analysis for an apartment complex owned by a unversity. They wanted me to model three different options. 15 SEER AC with 90% gas furnance, 15 SEER Heat Pump with backup 90% gas furnance, and 19 SEER Heat Pump with backup 95% gas furnance. The 19 SEER unit has the exact same EER number as the 15 SEER unit. The 19 SEER unit has two compressors to get to the higher SEER number. I think the only way to model this is to make a new unloading curve. I called the CDS help line and they understood the problem but couldn't help.

hvac-engr's picture
Joined: 2010-12-20
Reputation: 4

Yeah, the problem starts with the generic nature of SEER, since there are no details in the number itself. However, the answer is to probably make an unloading curve. It is important to make both an ambient relief and power consumed curve depending on the reason behind SEER (I'm guessing it operates better at lower temperatures)


In any case, you will need to have the following data points: KW consumed versus tonnage at design temperature (power consumed curve) and KW consumed versus temperature at Constant tonnage (ambient relief curve).


Once you get that data (from the manual most likely), you can create the unloading curves, which is pretty simple in TRACE. The hardest part here is getting the accurate data. If you can get the data points, then CDS could help you implement it.

Joe's picture
Joined: 1969-12-31
Reputation: 2200

I have not done this, but in theory, you could create an ambient relief curve. The major AC manufacturers have good detailed ratings for various outdoor temperatures and indoor conditions. You can use the rating table to create an ambient relief curve.

Split systems are essentially on-off systems...they do not unload...with the exception of two compressor systems and VRV systems.

The EER value reflects kW at peak (95 ambient 80/67 indoor coil entering and 400 cfm/ton).

SEER is an average of operating conditions under various ambients. I think you could find the equation in an ARI manual...but...if you can find the data to develop a curve, SEER becomes unnecessary. The ambient relief curve applied against outdoor temperature would take care of it.

Hope this helps...

Anonymous's picture