Rooftop Barometric Relief

[humpy]

We have a constant volume hvac system utilizing package rooftop equipment with a supply fan, no return fan, and an economizer. When the unit was installed the barometric relief was missing(not installed). The unit is set up to supply a minimum amount of outside air for ventilation. The space is over pressurized when the unit is running. No provision was made to relieve pressure. Will adding the barometric relief dampers to the units relieve pressure from the building on a "non-economizer" day or does the relief only operate when the unit is on 100% OA (economizer mode)? Thank you very much for any reply.

 

[KiwiMace]

The barometric damper only spills air when there is an imbalance between the supply and return airflows. With the economiser closed there is still a slight surplus of supply to the space due to minimum ventilation requirement, and this will pressurise the space and to an extent, exfiltrate. A barometric damper will probably not operate under this condition depending on the minimum air and building tightness.

As the economiser opens, the space would pressurise to a higher equilibrium, so a barometric damper will be forced open to compensate. This may open gradually in proportion to the outdoor air, or flap open with a on/off economiser.

 

[stanlsimon]

You may not get much relief air out of your roof top unit. I'm not sure how your unit is set up but some of them had a little 6" flap that was supposed to let out 2000 cfm. The problem on less expensive units without a relief fan is that when the outside air duct is 100 percent open, then the return duct is shutoff by the damper.
So now your relief air can't get out. With this kind of unit you have to duct a relief duct off of the return, put in a relief damper, and duct to a separate opening thru the roof.

 

[AbbyNormal]

Been a while since I looked at a cheap one, the return damper is down stream of the relief on a good unit

 

[DrRTU]

If the pressure drop across the return grille, return duct and relief opening is greater than the force to close the doors on the building then they will stay open. Invest in power exhaust in place of a relief damper.

 

[bigwilly104]

I work with large VAV air Handlers frequently. All of the machines I work with use powered exaust. This can be controlled by a diferential presure switch. I have trane machines that use the percentage that the economizer is open. I did have a building that was getting over pressured even with the powered exaust fans. This was a carrier 90 ton with a 40HP Evap. fan but the powered exaust fans wre just two 3HP moters. This was a tech company with more heat sources inside the building thaan most. this made for cooling demand in the mornings all the time. with the economizer at 100% and the exaust fans on the doors wre being held open by the positive pressure inside the building. This was a relitevely tight building as far as security because of goverment contracts and having the security doors stay oprn was bad news. we ended up putting a aditional 10hp exaust fan in the building to change the relative building pressure. I kind of wrote a book here but your equipment manufacturer will be able to sell you a powered economizer for your machine if the unit is not to old. Stanlsimon's idea about a fan linked to your RA duct might be the best answer if the unit is to old for a factory economizer. Good luck

 

[humpy]

Thanks for the insight. We will look into power exhaust and /or gravity relief. I will try to post our results at a later date.