Issue overview

The attached archive includes three very similar models. They are a simple single-zone models served by an AirLoopHVAC:UnitarySystem which includes a single speed DX cooling coil and a hydronic heating coil. The heating coil is on the demand side of a simple plant loop. Hot water is supplied by a boiler. Changing the Flow Mode input along with pump inputs completely changes the results.

• BlrNotMod_IntPump_MinFlow0
  The boiler Flow Mode is set to NotModulated, the Pump:VariableSpeed's Control Type is set to Intermittent and its Design Minimum Flow Rate Fraction is 0.

Here are the annual energy use results

Simulation outputs show that the demand on the plant loop is the same as the hydronic heating coil heating rate which is expected (orange line is on top of the blue one). The pump flow rate is variable (not shown on the chart below).

• BlrCstFlow_IntPump_MinFlow0
  The boiler Flow Mode is set to ConstantFlow, the Pump:VariableSpeed's Control Type is set to Intermittent and its Design Minimum Flow Rate Fraction is 0.

Here are the annual energy use results. The gas energy use is much larger than the previous case.

Simulation outputs show that the demand on the plant loop is much larger than the hydronic coil heating rate. Since the coils is the only demand, this is unexpected but seems to explain why the gas energy use is so large. Pump flow rate is constant.

• BlrCstFlow_ContPump_MinFlow1
  The boiler Flow Mode is set to ConstantFlow, the Pump:VariableSpeed's Control Type is set to Continuous and its Design Minimum Flow Rate Fraction is 1.

Here are the annual energy use results. The gas energy use is much less than both previous cases

Simulation outputs show that there's no demand on the loop (blue line is at 0) but somehow the hydronic heating coil is still providing heating without using any energy! Pump flow rate is 0 (not shown on the chart).

Note that all three configuration appears to meet the loads (no unmet hours) and that there is no major differences between the .err files.

The results for the first case seem reasonable, but the two other seem wrong.

Operating System (Multiple choices)

Windows

Operating System Version

11

Version of EnergyPlus

25.1

Unmethours link or helpdesk ticket number

N/A

Defect file

plantenergybalanceissue.zip