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mattorrell 02-18-2013 12:46 PM

Duct work sizing for 800 CFM unit
I am renovating my 700 sq ft apartment. It has a 2 ton Skymark HVAC unit that produces 750 CFM for heat and 800 CFM for A/C. The unit is located in a bedroom closet.

There are 2 supplies coming off the HVAC unit supply plenum:

- 16 x 6 rectangular sheet metal duct feeding the bedroom
- 12 x 6 rectangular duct feeding the living room (and remainder of apt), fed by 9" flex duct elbow from the HVAC supply plenum.

If my calculations are correct, the current supply set-up carries ~ 750 CFM.

The bedroom is hot when the heater is on and cold when the a/c is on, so I would like to correct this. Based on the sq. ft. of the bedroom I only need ~160 CFM. I have the following questions:

Could I change the 16 x 6 rectangular bedroom supply to an 8" round flexible duct supply with a damper? This would reduce total apt. supply to 525 with damper open. (According to the Silent Flex II friction loss chart 8" should carry 225 CFM at .1 friction loss)

Will a total CFM supply less than the rated capacity (800 CFM) of the unit damage the HVAC unit / cause the coil to freeze when a/c is running, (especially if I close the damper to achieve ~160 CFM supply in bedroom)?

Unfortunately, the other supply (12 x 6 feeding the living room) is in the ceiling and cannot be changed. The only component of the living room supply that is accessible is the 9" flex duct elbow. Is it correct to assume that increasing the diameter (or to metal) of the elbow will not help because ultimately the supply is limited by the smallest component (in this case, the 12 x 6 rectangular duct)?

Any ideas (I've searched other threads but can't find anything that specifically helps here)?

Thanks is advance for your help.

beenthere 02-18-2013 02:30 PM

Probably freeze up.

Is the closet near a wall at one of the other rooms?

mattorrell 02-18-2013 02:47 PM

1 Attachment(s)
No it isn't. I attached a sketch. Are there any basic balancing approaches that could be used?



beenthere 02-18-2013 02:52 PM

Not really. And since the only return is in the bedroom, Your kind of stuck. I'd think about running another supply across the bedroom ceiling to the living area.

mattorrell 02-18-2013 04:20 PM

Unfortunately, getting access to the ceiling to run another supply isn't possible.

What is your opinion of running a by-pass from the supply plenum to the return plenum based on the following set-up:
Living room supply: 300 CFM (12 x 6 duct)
Bedroom supply: 225 CFM (8" flex)
By-pass: 325 CFM (9" flex)

I thought this might keep the supply at 800 cfm output.

beenthere 02-18-2013 04:24 PM

While it will keep your CFM up, it could still freeze the coil as the return air temp will be much lower now. Unless you not going to use the A/C when the apartment/condo is below 74.

How high is the bedroom ceiling that you can't run an exposed duct across it.

mattorrell 02-18-2013 10:29 PM

I usually set the a/c thermostat 70 - 75 degrees. Would this cause coil freezing problem if a by-pass is utilized?

Ceiling height is ~ 7.5 feet. Starts to become a low ceiling if I enclose duct on the ceiling, but something I am still considering.

The duct run is only 10.5', but it is attached to z-channel about 3' into run which would make it difficult to remove/replace duct. The biggest duct I can fit in the cavity is 20 x 6, which gets me close on CFM (600 cfm for the 20 x 6 for living room + 160 cfm for 8" damper in bedroom).

What is your opinion re: use of a duct booster fan like this:

beenthere 02-19-2013 05:03 AM

On a short run a duct booster may be a bit too loud for comfort. Fantech has some quiet boosters. Might have to turn up the TV or sound system to hear it.

Increasing to 6X20 for the living room will help the most. Bypassing 325 CFM out of 800 would require a freeze stat to shut down the compressor if the line set temp drops too low, which it probably most of the time when the outdoor temp is under 76 and the indoor temp is under 75.

Zoned systems that bypass a lot of air use a DAT(discharge air temp sensor) to shut off the compressor when the air temp drops too low.

mattorrell 03-24-2013 05:01 PM

What is the correct formula (rule of thumb) to determine filter grill size?

I have read 200 square inches per ton as a rule of thumb.

For my 2 ton unit (moving 800 cfm), this would mean 400 sq. inches of filter area...or a filter grill size of 30 x 14 (420 sq inches)? Should it be bigger?

This is going in a bedroom, and I know best practice is to oversize in order to keep air flow noise to a minimum, but I also want to keep size reasonable.

Thanks in advance for your advice!

beenthere 03-25-2013 04:20 AM

CFM divided by 300(FPM) equals sq in. This would be text book air flow. You can use 400 as your FPM, it should still be fairly quiet.

mattorrell 03-25-2013 08:34 AM

Sorry, but I'm not following the math.

800 CFM / 300 fpm = 2.67. Not sure what to do with the 2.67

Thanks for the help!

Marty S. 03-25-2013 02:01 PM

Multiply it by 144 to convert to square

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