Have a max 1200 CFM vent hood in a new house. Considering the entire central air blower unit is 1450 cfm, how can I best do the make up air, if at all?
I would add an intake hood on the roof with a motorized intake damper, run a duct down to a grille in the ceiling in front of the exhaust hood. Interlock the motorized damper with the hood. Paint the intake hood to match the roof.
That's what I was thinking but then I was also thinking I could do the same thing with the central A/C coil. I could take that Make-Up damper and bring it into the return of the coil. I could also activate the e-static filter at the same time. Whatchya think?
It is a general practice with industrial air handling systems to provide fresh air in the return duct (after main return air damper and before the coil). The advantage is with one damper control (i.e return air) we can control fresh air also. You can have the advantage of redundant filtration too.
Based on hood requirements a 1200 cfm exhaust hood would be adequate for a considerable number or size of heat producing appliances. If your HVAC system only produces 1450 cfm (3.6 tons based on 400 cfm/ton)and you are exhausting 3 tons of conditioned air I would first question either the amount of the exhaust air and/or the capacity of the a/c system. Best case you can bring in 80% (960 cfm) make up air and introduce it through a supply plenum in front of the hood and then bring in 20% (240 cfm) through the R/A of the a/c system. Depending on what part of the country you are in this 240 cfm of O/A can be a considerable load to an arleady stretched a/c system.
This is not allowing for any additional infiltration air which must be accounted for. You stand a good chance of running an extremely negative pressure in the house which,
will bring even more O/A. You can also encounter a serious humidity problem.
