Are AHU 100% OA Dampers Necessary? 1

[tfraz]

I have numerous air handlers used for constant volume, 100% outside air(OA) ventilation that contain control dampers at the OA intakes. Virtually all the dampers are seized (in various positions) from lack of maintenance, and need to be replaced. The question has been raised as to whether or not they're really needed. Our primary concern is heat loss through convection or building pressurization when the systems deactivate. Other then sealing the buildings, do they serve another purpose? Has anyone in a temporate climate (I'm located in San Jose, California) dealt with this issue or done any studies to determine the effectiveness of these dampers, or the potential energy loss if they are removed? Thanks.

 

[235zzzo]

Hi Tfraz!

1.First you can be attempted to implement the "easy solution": - just remove all the dampers. but you have to wait and see all along the year for some complains. I'm sure some will arise.

And the engineer's approach:

2.And why? (related to Point 1)? If you remove the dampers, the building will have several open entrances, for the OA "take several walks by night" (or when the system is off), driven by natural convections or by the differences of the pressure fields, outside and inside of the building.

3.The resulting natural flows can be significant, for instance, it can drive in the air pollution besides that, it can interfere negativelly with the building thermal inertia.

4. Other point you must ask to yourself, why the project engineer has decided to consider those dampers, I think he had some good reasons to do that, (or not?, are you able, to come out to that kind of conclusions concerning the project?. Are you able, qualified to audit the project?).

5. Tying to balance the situation: You can skip a few dampers, maybe it is possible! But you have to know which dampers, based on some factors, like as:

- Pressure fields (relative pressure), outside and inside the building;
- The specific and the importance of each damper, concerning its particular sub-system, "what is on the table";
- The cost, depending on the dimensions if they are of the same type;
- Very important, you have to be sure if some of those dampers are part of against-fire enhancement systems or not!
- Finally, you better to have a friendly talk with the project-engineer. That is the most clever approach!

That's all for the moment. Good luck
zzzo

 

[hosec]

In a temporate climate the intake dampers are often used as part of the frost protection system, and shut when the ahu is inoperative.

There are usually separate FP systems in place to protect against freezing coils, but in the event of failure of these, a low temperature detector downstream of the coils will automatically shut off the plant and initiate closure of the dampers if the detected temperature drops below about 4 degrees C.

 

[stevenw]

Last time i checked the Calif. code required units capable of 100% economizer, so they are required by code.

 

[timr]

If the areas being served by the air handlers have unoccupied hours, i.e., the areas are vacated at night, more likely than not the dampers are required by code. Typically, outside air dampers serving an air handler are required by code to fully close during unoccupied hours. You may want to check local building codes to see if that is the case. Additionally, the previous points made regarding pressurization, control, contamination, the intent of the original designer, etc. are valid and should be considered.
timr

 

[MPE]

What is the reason for the 100% outside air? If it is due to the processes within the building, there might be a need for the dampers to adjust/regulate the airflow provided.

localized Building/space over pressurizing where medium to high efficiency filters are changed out is a concern if there are lab hoods.