Stratification or Short-circuiting Airflow?

[twangmaster]

We've got an office, approx. 2400 sq. ft. w/ open structure ceiling at 14'. We're putting 3200 cfm w/ 200/250 mbh gas heat rooftop unit. The unit is tripping out on high limits, finally found out we were getting 88F return temp. When the unit is shut down, and the fan off, we waited about 30 min. started unit back up. return temp was 74F. Duct system is exposed with spiral duct, with diffusers angled at 45 degress toward the exterior walls from a 10' elevation. Return is at approximately 12'. Since i'm seeing a 74F return temp when the unit is first starting up, i'm thinking that it's not stratification, but rather short-circuiting from the supply diffusers to the return. Any ideas? Thanks!

 

[Accystan]

Hi there:

I'm not sure it matters whether it's stratification or short circuiting - either way you should look into one or both of two things:
1.Try to return from low level, and/or
2.Use double deflection grilles instead of the diffusers to get the air down. Try to size the grilles such that the air is reaching its terminal velocity (50 or 35 fmp)as it reaches the outer walls. The vertical deflection bars (preferably aerofoil) in the grilles can be adjusted to give the right throw. The horizontal bars can be adjusted to avoid drafts. Ideally you'd want to aim the air such that it covers windows.

Cheers.

If you can get the grilles right you shouldn't need to move the RA duct/grille