Exhaust Fan when Heat Pump is in Econo

[Dymalica]

I have noticed that all manufactures of equipment (Trane, Carrier, York) have no power exhaust on side discharge units. I have a 3 unit heat pump system with 15 tons each (2 units will be running at a time, 1 backup). The system max cfm is 12000 cfm (6000cfm/unit), existing exhaust fan for bathrooms, closets, etc is 2000 cfm. If one Heat pump is running i sized one fan for 4000 cfm (6000cfm -2000cfm), and another at 10,000 cfm (12,000cfm-2000). Should I size exhaust fans smaller and let a slight build up in pressure? Is there a rule of thumb for power exhaust units? I noticed that many manufactures use about 1/3 to 1/2 inlet amount to size power exhausts. i.e. 8000 cfm inlet, 4000 cfm power exhaust. Are there any exhaust fans that have 2 speed, that can be controlled by control unit without VAV drive?

 

[DrRTU]

Check the installation instruction and you should see a note that states you can use the standard factory field installed power exhaust units but exhaust fan is installed on the return duct. You will have to extend the power / control wiring harness with field wiring and sealtight. We build a short joint of duct with ductmate and add frames around the tap due to the cantilever of the fan box hanging off the duct.

 

[lilliput1]

Power exhaust would be useful only to allow use of outdoor air economizers in buildings with mostly interior spaces requiring cooling year round. The return air is exhausted and the outdoor air is brought. I think your units are residential type and do not fall in the above application. I recommend forgetting the economizer feature. The heat pumps would be heating during winter.
DrRTU is correct. Power exhaust units can be installed in the return air ductwork. Usually they are multiple fans and are sequentially turned on as the OA damper open to attain the design economizer mixed air temperature (which should be reset up as it gets cooler outdoors). Ideally there should be about 0.05 CFM/SF floor area not returned for pressurization (to avoid infiltration).

 

[DrRTU]

Page 42 of the York technical guide

http://www.yorkupg.com/PDFFiles/259337-YTG-B-0606.pdf

shows the horizontal application. York units are an on/off type of design. A switch is installed with a cam on the economizer damper shaft. When the oa damper reaches 30% to 40% open the fan is turned on. You select the fan speed to match your flow rate. Review the cataloged tables and estimate a duct static. The problem with on / off is you have too much fan when the economizer is 50% and you are short at 100%. Carrier offers on / off and full modulation on selected RTUs

http://www.xpedio.carrier.com/idc/groups/public/documents/techlit/48_50a,hg-2si.pdf

I add up my total exhaust subtract it from the total supply and select the PE based upon duct static loss. Most of the time you need high speed and you will be ok for the small cabinets in a line i.e. 7 ½ ton but short for a 12 ½ ton.