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Increase in cooling setpoint -> substantial drop in reheat energy

asked 2017-03-03 12:14:36 -0500

kpp gravatar image

updated 2017-05-17 12:46:42 -0500

I am running the prototype building energy models for medium size office buildings in energyplus. In the parametric study, I am testing the change in energy consumption with respect to increase in cooling setpoint (72 to 77F) while keeping the heating setpoint constant 68F.

For an increase in cooling setpoint, I see the drop in cooling energy which is expected, however, most of the HVAC savings are coming from a drop in terminal electric heating energy. For example, for San Francisco, the increase in cooling setpoint from 72F to 77F is resulting in 68% drop in reheat coil energy. Am I missing something?

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isn't this the expected behavior? higher supply air temperature means less heating of air is needed for zones that are calling for heating.

mdahlhausen gravatar image mdahlhausen  ( 2017-03-03 16:06:19 -0500 )edit

Thanks Matthew. True, the return air temperature will be higher but, the central cooling coil will the cool the (recirculated + OA) supply air to 55F anyway. Therefore the high return air temperature should not have any impact on the reheat coil. Reheat coil should kick in only if the zone temperature falls below the heating setpoint 68F and that does not change in any of the cases I am running. Therefore I am trying to understand how the change in cooling setpoint affects the reheat coil operation if the supply temperature from central cooling coil is kept at 55F.

kpp gravatar image kpp  ( 2017-03-03 18:02:37 -0500 )edit

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answered 2017-03-05 08:02:19 -0500

Archmage gravatar image

The increase in cooling setpoint results in lower volume of air flow into the zone. That is where the cooling savings come from. But at the same time the lower volume of 55F supply air requires less reheating (to the heating setpoint).

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Yes, that makes sense. This matches with the results I am getting after exporting detailed output files from the energyplus simulation. The autosize input in these models is reducing the overall design supply airflow rates. That is reducing the minimum airflow through the terminal box during reheating mode (which is approx 25-35% of design airflow rates). In milder climates since most of the time the terminal box is operating at the min. VAV damper position the savings in reheating are higher in these climates.

kpp gravatar image kpp  ( 2017-03-13 16:15:35 -0500 )edit

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Asked: 2017-03-03 12:14:36 -0500

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Last updated: Mar 05 '17