What size duct for make-up air?

Question

I have an industrial exhaust fan venting heat from the top of a stairwell. It is a CentriMaster ModelPRN PRN110 with specification sheet here: http://www.acmefan.com/InformationFiles/ModelInformation/OLDLiterature/ModelPRN.PDF

I'm installing a vent at the bottom of the stairs for make-up air, and have a question about the duct size.

1. From the 'dimension data' (p. 6), the fan ceiling cut is 16 x 16" (A), but it says "Duct size 10". Does this mean I should use a 10" duct for the make-up air, or a 16 x 16" square?

2. From the 'performance data' (p. 5), the PRN110 has RPMs 2477 through 4464 with CFM 600 through 1090 at .000", or CFM 578 at 1.000".

   1. What are the inch rows in that table (i.e. .000", .125", etc.)?
   2. How do I determine the RPM (what row of the table do I read)?
   3. How do I determine the CFM (what column of the table do I read)?
   4. Once I know the CFM when the fan is on, how do I know what size passive duct to install for the make-up air? It seems like just using an equal size duct (10" or 16x16") should be sufficient.

I'll be using an electric-open default-close damper tied to the same 120V thermostat that drives this fan.

Answer 1

1. You'd use a 10" duct that comes off of a plenum/metal box which is 16x16 to match the hole. You will find it... tricky... to try and attach a duct directly to an irregular surface like a ceiling, floor, wall, etc. (in other words, don't do that).

2. These are best answered by an HVAC technician running Manual S, T and D calculations after inspecting your system. The inch rows you ask about are static pressure values based on your actual duct; you have to measure it with a manometer to get the location and value of static pressure, you will see these in inch values or inch WC values, typically.

   Regarding which rows you read, I assume the PRN110 lets you choose which RPM setting to use, from 860 up to 1550 (the values you assigned to RPM in your question are actually the tip speed of the fan blades). Once you know what RPM you want to use, you know which row to read. Conversely, if you have an HVAC technician calculate the values on the right side of the table first, that will tell you what RPM you need to choose on the blower.