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Decreased outdoor air flow results in lower heating energy but higher cooling energy?

asked 2017-05-26 20:00:22 -0500

rysit's avatar

updated 2017-08-06 07:36:04 -0500

I am using OpenStudio 1.14, and have created a baseline and proposed building, where the proposed building has a lower outdoor air flow rate by adjusting the following variable: OS:DesignSpecification:OutdoorAir > Outdoor Air Flow per Floor Area.

I ran a baseline and proposed building for all 16 California climate zones.

  • Heating energy decreased in all 16 climate zones
  • Cooling energy increased in all climate zones except 11, 13 and 14.

The HVAC system is modeled as a single zone package unit.

I would have expected both heating and cooling energy to decrease with a decrease in ventilation? Any explanation for this?

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answered 2017-05-30 02:26:47 -0500

updated 2017-05-30 02:35:43 -0500

There is too little information to pinpoint the exact cause, but I'd put my money on free-cooling (especially during nighttime).

The common intuitive conception is that whenever you bring outside air you need to heat it in the winter, and cool it in the summer. That's slightly inaccurate and a cause for misconceptions.

On a 32°F day, you will definitely need to heat up this air to bring it to the zone setpoint. On a 95°F day, you will definitely need to cool down this air to bring it to the zone setpoint. But what happens when it's 68°F out and sunny? Chances are that the sum of your envelope gains (conduction, solar radiation, etc) and internal gains (people, equipment, lighting) is positive, meaning your zone as is would tend to have a temperature that drift upwards, and likely exceeding your cooling setpoint. In this case, the 68°F outside air you bring in is actually helping!

I'm not that familiar with the 16 California zones, but I assume the 11, 13 and 14 are very hot (and probably dry climates).

I suggest you take a look at Climate Consultant from UCLA, which happens to ship with the 16 California Climate zones handy. It should provide some good, useful insights.

Here's an example of zones 5 and 14 on a psychrometric chart (click to enlarge). You will notice that the quantity of red dots (>75°F) is much greater (and they are more extreme temperatures too) in climate Zone 14, meaning you will likely get a cooling penalty. On the other hand in climate zone 5, only 4% of the hours are hotter than 75°F and very few are above 80°F, while you have 11% in the 68-75°F range, so you're likely to get cooling savings there.

California Climate Zone 5

California Climate Zone 14

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Thank you Julien! Very helpful. I appreciate the response.

rysit's avatar rysit  ( 2017-06-05 00:52:13 -0500 )edit

If that solved your problem please mark the answer as accepted so we know the thread is resolved. Otherwise, we'll need to wait for someone else to chime in (though maybe unlikely since it's a week old)

Julien Marrec's avatar Julien Marrec  ( 2017-06-06 03:04:55 -0500 )edit

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Asked: 2017-05-26 20:00:22 -0500

Seen: 112 times

Last updated: May 30 '17