ReplyFree cooling during colder times when there is an air conditioning load... To meet fresh air requirement in relation to occupancy and indoor air quality.
Professional Member
Hello all,
any one can explaine the porpuse of motorized fresh air damper on roof tops since it is a code's requirement instead of manual fresh air damper.
Thanks
I'm learning
Professional Member*
Free cooling during colder times when there is an air conditioning load... To meet fresh air requirement in relation to occupancy and indoor air quality.
Professional Member
My question is about fresh air not economizer.
I'm learning
Professional Member*
Does the unit have an economizer?? Is the f.a. damper going to the return of the unit?? Motorized v.s. manual keeps the cold or hot out when in off cycle.
Professional Member
I work for a large university, and many of our dedicated fresh air dampers are motorized because they are controlled through the Building automation system off of a return CO2 sensor or CO2 sensor located in the space. If the CO2 level rises then the fresh air damper opens to bring in more fresh air and vice versa. It saves energy by only bringing in the amount of fresh air required to satisfy what is needed by the occupants and not having to condition a specific amount of outside air constantly that you would with a manual damper. I believe me try to keep the CO2 level below 800ppm.
Professional Member
I see that a lot, and have had to program the same sequence, but have a fundamental issue with it. Essentially you are forcing the co2 level up to a point just short of making people sick. I know, gotta save energy....Originally Posted by kayHVAC
Sorry, rant mode off.
Oh, and here they usually make me measure osa co2 and control 400 ppm above that rather than a fixed setpoint
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Professional Member
Bingo. Especially if the bldg. static pressure is much different than ambient you can have significant infiltration/exfiltration when the unit is not running.
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Sponsor & Professional Member*
People will not get sick from CO2 levels above 1,000 ppm.
CO2 is used as marker of people occupancy indicating the need for fresh to purge the indoor pollutants. The CO2 is not a dangerous pollutant.
Regards TB
Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
Use +Merv 10 air filter. -Don't forget the "Golden Rule"
Professional Member
It apears that I did not explain my question properly. Let me clear it up.
With manual dampers, you can adjust the percentage of fresh air (outdoor Air) from zero and up., normaly 15% ( code Requires ??) for unit over 5 tons is recomended to presurize the building. Equipment brings the outdoor air regardless of low or high OA. there is no sensor. you get warm air in the summer, cold air in winter as long as the blower fan runs. There is some aluminum filters to prevent insects and rain to get in the retun side.
Now they changed the code and want motorized damper to open blades to let OA get into the retun air. Also there is minimum setting from 15% and up. as soon as blower fan starts, actuator opens the blade to preset position. The damper motor opens the blades as long as blower fan motor runs. And it spring closes the blade when there are no power to damper motor. There is no sensor no thermostat, nothing, no control but blower fan motor low voltage. Now my question is that what is the purpose of this Motorized damper which is inspector asking for it? I just Like to know what's the logic behind it. The manual damper wouldn't do the same?
I'm learning
Professional Member*
I'm no code expert......but I never heard of a code for a certain percentage for an outdoor damper. There is codes stating how much outdoor air(volume) and where ever your dampers are they are.
As far as motorized dampers......I would assume it's for smoke/fire control. They need to fail closed.
Professional Member
I think the other guy that replied about closing the damper in the 'off' cycle answered your question.
Regular Guest
10% fresh air mandated in most commercial bldgs by code
Professional Member*
In our area any hvac equipment over 2000 cfm requires a duct smoke detector which will provide a safety shut down of fans and OA. Fixed position damper will not provide closure.
ckartson
I didn't write the book I just read it!
Professional Member
That is your OAT intake, your economizer and the dampers are set to maintain MIN of proper fresh air intake per code.. I assume this is a new install the inspector is checking only what is required...Like you said some years ago all units over 5tons need to have auto dampers no fixed. I will assume you will have all this balanced. Once everyone is gone and if you have a control system you should be able to set the min to what you want...In deep summer, high temps-high Humidity i shut mine to 0%. and depending on the size you may need spill dampers to maintain static. This is basic HVAC
Just a note about CO2...so many people worry about indoor CO2 but very few have sensors outside... I have seen where the CO levels outside are worse than inside and the dampers still open because many forget to measure outdoor and indoor CO2. If your going to install CO2 sensors do it right.
Professional Member
I've seen some codes that wanted a motorized damper for smoke/fire shut down instead a RTU mounted chimney with barometric relief and fixed % intake.
Professional Member*
Steam that was the "standard" or "rule of thumb" before we had 62.1. and the code requirement for ventilation. There is no "10%" standard anymore. The IMC 2006 calls for different ventilation rates based on the type of building and rooms such as restrooms, classrooms, office areas, etc.
Typically you are talking about 15% or more in most cases. LEED points for air quality requires a lot more. The 2012 code does require motorized dampers for outside air. We don't have that code yet in GA but will have it Jan 1, 2014. Don't know to much about that yet other that in some cases you are allowed to have less OA than in the IMC 2006. Thank you very much
Last edited by glennac; 02-27-2013 at 09:18 AM.
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