Calculated peak loads don't match up with ReadHVAC output

I’m trying to understand peak loads by trying to calculate them myself, but the result isn’t matching up with what the ReadHVAC component is outputting.

Here is a very simple model:

peak (47.2 KB)

The ReadHVAC component is telling me that the cooling peak is 1.37kW and the heating peak is 1.54kW. First of all, why don’t these values match up with the “comp_values” (1.76kW and 2.20kW)? I’m using IdealAirLoads and a simulation time step of 1/h.

I’m running a yearly simulation, so I would expect to be able to replicate the peak loads by finding the maximum values in the “cooling” and “heating” outputs from the RoomEnergyResult component. However, this is not the case, and in fact the values are flipped in that one calculation method tells me the result is cooling dominated while the other is heating dominated:

Am I making a mistake in my calculation? I would like to know where this discrepancy comes from.

Hi @MaxMarschall,

The zone sizing is calculated in a different way then the energy use. The difference you noticed can be caused by the following differences.

  1. With the peakload calculation for heating load, internal heat load (electric light, people, equipement) is not taken into account. That is probably the reason why the peak load for heating are higher then the maximum enegy use.

  2. The sizing is done based on a ddy climate file. This is a location specific weather data file. The energy simulation is done based on the EPW file. This file differs from year to year. With a not to cold winter your maximum energy use will be lower than the calculated peak loads in the sizing simulation.

  3. Energy plus does not take heat recovery into account during the sizing calculation. This can have a hudge effect on the results.

For more info:


Thanks @Erikbeeren, that was really helpful

Erik’s explanation is a good one. For your particular model, Max, reason #2 that Erik listed is mostly to blame. The zone peak loads that the “Read HVAC Sizing” component outputs are the maximum load that has to be added/removed from the room on the heating/cooling design day within the .ddy file. Typically, engineers will apply a “safety factor” on top of this zone peak load to size HVAC equipment and this helps cover some of the gap that you see between the annual peak and the design day peak (the factor is typically somewhere between 1.15 and 1.25)

This “safety factor” is also why the zone peak heating/cooling is different than the HVAC component sizes. EnergyPlus will automatically multiply the zone peaks by a factor in order to size the equipment for the annual simulation. You can change the factors that EnergyPlus uses with the “HB Sizing Parameter” component.