Separate Energy Consumption for space Heating and Domestic Hot Water (DHW)

It is often necessary to distinguish between the energy used for space heating and for domestic hot water (DHW). Accurate separation enables better reporting, benchmarking, and identification of efficiency improvements. This article explains two methods for separating the energy consumption of heating and DHW, with practical steps and important considerations for building energy simulations.

Methods for Separating Energy Consumption

1. Independent Plant Loops

The most straightforward method is to model separate plant loops for space heating and DHW. This approach allows each system’s energy use to be tracked directly via its own meters.

Benefits:

• Direct measurement of each system’s energy use.
• Clear and accurate reporting.

Tip: Use the Clone tool to duplicate existing plant loops, then modify each for its specific function (space heating or DHW).

2. Using Energy Meters in Summary Reports

If separate plant loops are not modeled, you can estimate the energy consumption for heating and DHW using EnergyPlus Summary Report Energy meters. This involves identifying the relevant meters for each end use and extracting their data from simulation outputs.

How to proceed:

• Locate the relevant demand meters for heating and DHW in your simulation output reports.
• Use these values to allocate total energy consumption between heating and DHW.

Example:

Suppose your simulation provides the following energy meter data:

Total heating delivered: Q_total = 185,179.94 kWh

Step 1

Use the Total Heating Electricity Consumption from the End Uses Table.

E_total = 52,886.65 kWh

Step 2

Calculate the electricity use proportionally to the load.

DHW: E_DHW = E_total x Q_DHW / Q_total = 52,886.65 x 146,392.24 / 185,179.94 = 41,809.04 kWh

Space Heating: E_heating = E_total x Q_heating / Q_total = 52,886.65 x 38,787.70 / 185,179.94 = 11,077.61 kWh

Important Considerations

• Separating systems in the model is an approximation. Splitting the system may result in small or significant differences in energy consumption, depending on the complexity of the original system and its controls. Actual energy use may differ due to system interactions, changes in loop temperatures, and flow rates.  
• Verify meter definitions in eplusout.mtd to confirm no shared components between heating and DHW systems.