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cbendock 02-27-2011 12:41 PM

is my cold air return opening too small???
my hvac contractor installed a furnace (coleman Model # TG9S060B12MP11) rated at 1200 CFM with a 14"x20" cold air return opening. the grill whistles like mad when installed. recently i've read the rule of thumb is 2CFM per sq in. of cold air return opening. am i reading this correct that my opening is too small and the cause of the whistle noise?

a little background....

the hvac contractor flaked when it came time for him to do the trim work. since there's a 10% holdback in my contract with him for completing trim, and that 10% amounts to almost $900 i've given up after a couple unreturned calls and decided that installing vent screens and hooking up my water heater myself is worth the savings. it's been a year since i've seen hide or hair of him.

but if i don't hunt him down i worry that i am giving up the chance to have a significant problem fixed. plus the adjustable baffles he said he would install to adjust the airflow to the registers in the various rooms in the house. however if i do hunt him down, i will likely be billed for work that i did but he should have done

Grampa Bud 02-27-2011 12:58 PM

Lets see..... 14 x 20 = 280. 1200 divided by 2 = 600 Hmmmmmm...... The rule of thumb that has never gotten me into dutch is if I have a supply at the furnace that measures 24 x 24 then I had better have returns that add up to 576 sq. in. coming back. The air vanes that you mention... are they motor or solenoid controlled or just manual levers???? If they are all used or not they could cause some whistle depending on how you load or unload the house with the system running.

Poppyloppy 02-27-2011 03:37 PM

While it is true that the furnace blower in the furnace is designed to blow 400cfm per ton AT THE FURNACE, a 2-3 cfm per sq." air flow at the supply or return grille is correct and depends on duct size, shape, length, turns,etc., as well as how restrictive the grilles are (smaller spaced louvers = more restriction).

Important note: It's not really the size of the grille opening, but the opening behind it. Example: low wall grille that is 24"x10" (= 480CFM), but behind the grille, the wall space is only 3.5" deep; therefore 24" x 3.5 = 54x2 = 108cfm and we haven't considered the 1 or 2 vertcle 2" wall studs. Air flow is King.

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