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Minimum outside air on VAV

Minimum outside air on VAV

Minimum outside air on VAV

(OP)
Hello,

Does anyone have a genuine control strategy to control and maintain minimum outside air on a VAV system?

Thanks!

RE: Minimum outside air on VAV

Having a separate minimum outside air and variable air louver seems to have worked best for me, and is a lot easier on TAB contractors.

RE: Minimum outside air on VAV

Use a dedicated injection fan

RE: Minimum outside air on VAV

A dedicated injection fan.

RE: Minimum outside air on VAV

you can use OA reset with a CO2 sensor in return.
Even better would be to use an ERV for ventilation, that would equal the above injection fan.

RE: Minimum outside air on VAV

Consult the rooftop manufacturer, they have the best knowledge of their product and product control.

RE: Minimum outside air on VAV

Are you asking about ensuring the minimum OA gets into the air handler, or that the min OA reaches each individual space served by a VAV box?

RE: Minimum outside air on VAV

The ventilation calculations have a very good hold on this question. 62MZCalc, while a little difficult to initially understand, does a fabulous job in illustrating the amount of outside air at the design condition. And the International codes follow the same path, with differing equations to a point.

My main question would be, can you supply 30% of air to the space for heating (see ASHRAE 90.1) and still maintain the minimum outside air required?

RE: Minimum outside air on VAV

Have the VAV return fan track the VAV supply air fan with constant differential equal to the minimum OA CFM. You need airflow measuring station at the supply & at the return. Consider Air Monitor airflow measuring stations at each fan inlet.

RE: Minimum outside air on VAV

Calculation is more important and more complex than control strategy. You have to size VAV units to both provide cooling and fresh air in all conditions, including minimum load conditions.

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