Economizers Open in Winter?
Spoke with my boss today about one of my stores. This particular store has 9rtu's, all with economizers. About a month ago, I noticed the economizer was about 25% open on the last unit toward the back of the building. Shortly after discovering this, I talked to the main guy at the hvac servicing company that takes care of our units. He said it was designed like that to keep a fresh air supply. It didn't make sense to me considering it was winter and this was causing the front door to stay cracked open.
As I was talking to my boss today about the store, I mentioned this unit to him. He also thought it was crazy to have this unit cracked open in the winter. He called the supervisor that I was referring to earlier. He said this was not only the way it was designed (keeping fresh air), but also it was a law that all units with economizers had to remain 10-15% open at all times. Is there any truth to this or is he blowing smoke up my butt? Thanks.
Depending on alot of factors, but most what is required BY LAW for fresh air requirements in a particular jurisdiction / application, this is not unusual at all.
Ashrae standards are frequently called for, but I won't quote any numbers to muddy the issue - find out what is called for in your particular application.
ALSO, don't look at a damper, say it is open 25 % and assume it is giving you 25 % fresh air - all dampers have a point, depending upon style, design etc. that will pass 100 % air before they are open 100 % - the only way to tell is to have an air balance / volume test done by someone who knows what they are doing. Don't guess on this - it is important for energy consumption / savings.
I can guarantee things aren't set up properly if the front door is being blown open - the building is too positive. By the way, if your boss doesn't know anything about fresh air in buildings - be careful - I will bite my tongue before I write something that will get me banned.
Good luck - - - -
Last edited by freonguy; 01-15-2008 at 08:48 PM.
Reason: add another tidbit
Superheat and subcooling tell it all !
I'll throw some numbers out there since you don't want to
IMC says that retail stores should have 0.3 cfm / sqft of outside air. If you have a 1500 sqft store that requires 450cfm of outside air. Now say you have a 5-ton RTU, that would be 22.5% outside air. It is very common to see the economizer / outside air damper shut completely, its not code, but it is done frequently.
As the other poster stated, the economizer doesn't have anything to do with the pressure issue in the space, that means the unit is improperly balanced.
With lighting and people loads, it is also not uncommon to see the economizer open past its minimum set point, after all that is the whole point behind it.
Call the HVAC Company back and explain the situation you see with the doors (not just the fact that your poking around in the units). Depending on the unit Manufacture model they may install barometric relief dampers and/or exhaust fans to relieve pressure. Personally I like to have the relief based on building pressure sensors, but they usually don't get that technical unless they are a considerable large size.