#2780 Primary Secondary Pumping

[Felicia Lee]

I am modeling a central plant with primary secondary loops. I am getting two warnings, see below and attached for the file.

I have found and read some discussion online regarding the first warning and I see that this is documented in the CBECC-Com documentation. However, what is the work-around? This project has a pretty big pump penalty, should the system be modeled as primary only pumps?
Warning: Pump 'CHWP 1' is the primary pump for a chiller or boiler component included as part of a primary/secondary pumping system with more than one primary component. This configuration results in all primary pumps operating whenever there is any demand on the loop, regardless of whether the corresponding primary equipment is needed.

I’m unsure why I am getting the following error. CHWP 3/4 is defined on the secondary side, not the primary side. Is this setup correctly?
Warning: Pump 'CHWP 3/4' is 'VariableSpeed' and is defined on the primary side of a primary/secondary chilled water loop. This is currently not supported. The pump SpeedControl will be set to 'ConstantSpeed' for the simulation

[John Arent]

The second warning message you are seeing is related to the pumping configuration requirements for the primary and secondary chilled water pumps. I believe the warning message is a little ambiguous. But the system is being modeled correctly in your model (the proposed case) with constant volume primary pumps and variable volume secondary pumps.

The first warning message is related to a known issue in EnergyPlus, that turns on the second branch pump even when the load doesn’t dictate that It’s needed. Unfortunately as it’s an EnergyPlus issue (as of the current EnergyPlus version used with CBECC-Com, v8.5), there isn’t an easy fix for this. If the extra pumping energy is causing a compliance issue, you can use the following approach:

1) Document an Exceptional Condition in CBECC-Com to note that an adjusted calculation is used for the pump bhp or W/gpm.
2) Run the simulation with the as-designed primary pumps.
3) From the hourly results file, determine the fraction of the pump operating hours with the corresponding chiller on to the total pump operating hours
4) Multiply the as-designed pump power by this fraction from step 3 to get an equivalent operating pump energy use for the constant speed pump
5) Repeat for the other primary pump
6) Scale the motor nameplate horsepower (HP) as well. This is needed only because CBECC has a check that compares the motor bhp against the next smaller standard motor size, and adjusts the bhp upwards in the event that the bhp < 95% x HPi-1 where HPi is the actual motor horsepower and HPi-1 is the motor horsepower from the next smaller standard motor size. (This rule is in place to prevent the user from having an arbitrarily low bhp relative to the motor nameplate HP, which is often easier to check.

This is probably the best approach to address the issue and have a reasonable pump energy prediction.