What "warmup" and "cooldown" loads mean

What is a warmup or cooldown load?

In the peak loads tab you may come across values reported as "warmup" or "cooldown" loads. This article explains how we calculate these loads and where they come from.

 

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Calculation of the Space Loads from the EnergyPlus outputs

EnergyPlus provides a breakdown of peak space loads in the sizing report (you can download this report from Sefaira).

 

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For each zone it provides a summary report that looks something like this:

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Peak Cooling Load for a zone

Sefaira publishes the "Peak Design Sensible Load" in the UI (this is the value in the 3rd box from the bottom) (see the UI below for P02)

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Detailed Breakdown

To provide a bit of guidance, and to help make good decisions, Sefaira shows the top 3 loads. To get the 3 highest loads, we use the breakdown table below. In most cases the breakdown table Total / Grand Total (excluding latent load) does not equal the "Peak Sensible Design Load" that EnergyPlus publishes in its summary.

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For example, above the Grand Total for Sensible load is 625.68 + 475.18 = 1,100.86 W. The "Peak Cooling Load" in the summary was 1,259.51W

The difference (in this case 1,100.86 - 1,259.51 = -159) is what we call "warmup" or "cooldown". It basically reflects the extra load that's necessary to get a zone to the right condition. If the difference is negative, it's cooldown load)

This difference only typically occurs during the first hour of the day. You'll see high cooldown loads if:

  • You have a high setback temperature 
  • You have internal loads on after hours
  • You are turning on your building substantially after solar load has begun to affect a space (typical for east-facing zones with lots of glass)

In the above instances, the zone might be starting the hour substantially warmer than the desired space temperature. So not only does the cooling load need to include the loads if the space was already at the desired temperature, it also needs to include a "cooldown" load to get the space down from its starting temperature to the cooling setpoint.

"Warmup" is basically the same. The only difference is that it's caused when there are:

  • Low setback temperatures
  • Poor insulation of the facade
  • Cold overnight conditions

 

To continue learning about how to control the loads in your building see here.

To see more info about timesteps and thermal comfort and how they can affect your warmup loads or how we have updated the warmup over time

For further queries please contact support@sefaira.com

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