Need Economizer Help

[markmast]

My company recently installed a rooftop unit at a newly constructed variety store, Its an Allied commercial rooftop unit with an economizer on it, the building is getting a positive pressure and holding the front door open an inch or so from the pressure only while the unit is running, we seemed to think it was the economizer but it seems to be set up properly as per the manufacturers specs. The barometric relief dampers are opening and closing when needed but still getting a positive pressure inside the building im not sure what is going on with the economizer or maybe it is the blower on too high of a speed?? any help would be greatly appreciated I am somewhat of a newcomer 2 and a half years as an apprentice and am working on this myself, Thank you in advance for any help!!!
:confused:

[maxster]

is there an exhaust fan on the dampers to pull the return up and discharge it out.if you don'yt ave a unit installed power exhaust the supply fan is BALLOONING the space thats why the doors are being pushed open.

[cxagent]

What does the TAB report say the air flows are? How do those airflows compare with the design? What is the pressure that is holding the door open (some one needs a micromanometer to measure this pressure)?

[John Culpepper]

Maybe a VFD on blower moter? Had similar problem on rooftop package unit couldn't resolve problem until unit got changed out with Trane Voyager with VFD and hot gas bypass.

[markmast]

Whats a VFD ???

My company does not have a micromanometer....

The economizer did not come with an exhaust fan for the dampers, ive never seen that before, anyway my boss expects me to fix this problem without adding any equipment, not possible??

[John Culpepper]

It is a veriable frequenquincy drive. It controlles how many hertz your motor runz at and controls speed.:)

[BaldLoonie]

From what I've seen, the relief dampers don't have the capacity to relieve the positive pressure from what the economizer can bring in. Is there a way to tell the economizer damper not to open fully to reduce the incoming air?

What size is the unit?

[du mech eng]

sounds like you need a power exhauster. is there a lot of return ductwork in the system? barometric relief is best suited for systems with minimal return duct. your supply fan is pressurizing the space enough to push through all of the return ductwork and out the relief damper. the more return duct you have, the higher your space pressure will be. undersized return duct will make this problem even worse.

[numbawunfela]

I'm gonna second the previous post. A power exhauster seems to be required. A power exhauster can be a kit that comes with a unit that is equipped with an economizer. You will likely see installation instructions for it as an option in the paperwork if it is all there in the unit.
When the economizer opens to get free cooling, it is dumping outside air into the return airstream of the space, presurizing it. A power exhauster is installed in the return and kicks on whenever the economizer opens. then it takes the air in the return duct and throws it outside, so that none of it mixes with the air the economizer brings in. this way you don't pressureize the space, since the proper cfm of air is being provided, and not the proper amount plus a bunch of outside air.

To know for sure, the front door should stay closed when the econ is closed, and the front door should open when the econ opens.

Bosses always want miracles, especially when they were the ones who poorly laid out the installation. It may be that the only way to keep the front door closed is to disable the economizer, but you would want the customer involved with that decision, of course.

[markmast]

Heres the situation now, my boss came with me to the site, we closed the economizer fully and the door was till opening therefore the blower is the problem, supplying too much air then its taking out!?!?? There is not much return duct it is a drop ceiling return! So my boss says that we need to slow down the blower, which doesnt make sense to me because if u slow it down ur supplying less but arent you returning less also>?????????????? ok so he wants me to change the pulley on the motor to slow down the motor and he thinks this will fix the problem..... any input is appreciated!!! it is an 8.5 Ton rooftop alliance, 3 phase, belted blower motor

[psmurf]

IF you closed the econ fully and door then econ is not problem
SO now you are recirculating inside air,Right? HOW can that force open
a door? Somehow you are getting air into the space from outside.
did you put all panels back on unit?Is there another unit for space
that could be the trouble? Is RTU placed on roof curb correctely?
That auhta keep you busy for a while!
Good Luck

[John Culpepper]

If you slow down the blower you may not get enough CFM across the evap coil while the compressors are running.

[numbawunfela]

Not to sound like a butt,

BUT ;) does the front door close of it's own accord at all? Are you sure that it's failure to close is an indicator of a problem with the AC and not just a bad door closer, hinges etc.

Positive pressure often causes front doors not to close, but not always.

Also You may wish to look for infiltration from the outside through non-ventilation sources. An example that won't help here, but illustrates the point: I know of a large McDonalds that had a really hard time maintaining air flow and cooling. it was on the ground floor of a 50+ story office building. The stairway door was propped open to allow for easy access to drop off garbage, and this 50 story tall staircase had a window on every floor. the solar gain heated up the air in the stair gradually, creating a stack effect, sucking better than 5000 cfm up the staircase, negatively pressurizing the space (opposite your apparent problem) and creating issues with make-up air, and exhaust.

In short - look outside the box

[jibroni69]

This is an interesting conundrum. If economizer is closed there is no way your rooftop can be responsible for the opening door, unless like the previous person pointed out that the curb may be leaking or a panel was off. Door closes when unit stops? Maybe an old fresh air vent tied into return air? look around perimeter of building for a fresh air hood? weird . . .

[John Culpepper]

How big is this store and what other kind of ventalation does it have? Exhaust fans for restrooms, make up air units?

[cxagent]

This is an interesting conundrum. If economizer is closed there is no way your rooftop can be responsible for the opening door, unless like the previous person pointed out that the curb may be leaking or a panel was off. Door closes when unit stops? Maybe an old fresh air vent tied into return air? look around perimeter of building for a fresh air hood? weird . . .

This is a common problem around here. If the ceiling plenum is used for a return, that plenum SHOULD be sealed as tight as a duct. Most buildings leak air badly between the roof and the wall. Those leaks are about the same as the economizer damper being open.

But the first thing I would do is what numbawunfela said. Make sure the door will close properly. If you don't measure what is causing the problem, you are just guessing. Measure the building pressure before looking for a building pressure problem.

[numbawunfela]

Hey Markmast - How did things turn out?

BTW - since it is probably too late anyway, this is how you can tell if you have a lot of infiltration. :)

There are two ways which you will likely not be able to do. .
1 Perform a blower door test. this is a door, with fans on it that pressurizes the space and the pressure that is reached tells you if there is a lot of infiltration. Rather impractical, time consuming etc.
2 Use a digital menometer (micro menometer). You already said you don't got one.
3 In the imperfect cenario we find ourselves in, the next best thing is to wait till it is significantly warmer or colder outside compared to the indoor space, and measure the change in return air temperature/humidity. A change if 3 or 4 degrees is significant, and humidity should not change at all. The temp change can also be from a lack of insulation on the duct. Realize that the low pressure in the duct can suck outside air into it, or in a plenum ceiling scenario like cxagent's it will draw in from the outside.

consider - if you get, say, a 20 degree drop across the evap. and a 4 degree rise in return air temp AFTER IT LEAVES THE SPACE (return duct - ceiling) then 20% of your cooling is dedicated to what is happening outside of the space you are trying to cool.