new zone in basement

[icepilot]

Hey all. I'm installing a third zone in a basement. House already has two zones. It's about 600 sq/ft. I'm installing a 12x8 supply with a 24volt damper to feed three 8inch diffusers. Everything is pretty straight forward although I'm thinking about using a static pressure regulating damper, although I've never used one and not quite sure how to go about it.
Also, the homeowner is installing a drop ceiling and I'm going to use the ceiling as the return plenum. Just adding a couple of return grills in the ceiling. Has anyone used this method before. I come from a commercial background and thats pretty common. thanks for any advice.

[beenthere]

Are you sure there aren't any air leaks from the out side after the drop ceiling is installed.

No romex allowed in a ceiling plenum, all cables must be rated for ceiling plenum use.

The barometric bypass damper should be sized to bypass the difference between the smallest zone, and the units required cfm.

Are you taking out the old zone panel, and installing a new 3 zone panel.

[firecontrol]

If you live in a area that has extended winter heating periods the returns in the ceiling are not going to do a thing to return the cold air on the floor from the basement zone back to your heating equipment. Warm air in from the ceiling, right back out via the ceiling.
Put your returns near the floor.

[icepilot]

Yes, I am replacing with 3 zone panel. This system was originally installed without a bypass. Why would I need one now? I thought the only time I would need one is if there would be a cicumstance where the fan would be on without a zone open. The t-stat calls the fan on and zone open/closed at the same time, so I didn't think I would need a bypass. thanks for the info

[beenthere]

If the smallest zone is 60% or greater then the blowers cfm setting then you don't need one if the duct work is sized 125% of that zones cfm requirement.


In other words, if the blower is a 3 ton, set for 1200 cfm, then your smallest zone should use atleast 720 cfm, and the duct is sized for 892 cfm, then you don't need a bypass, or dump zone.



Hope that helps.

[icepilot]

thanks for all the info. One more for you. How do you figure out the return size. I'm using 8 * 14 for the supply, with (3) 8" flez duct diffusers. this is the 3rd zone in the house. In the basement.
thanks again.

[beenthere]

Can't put any more air in then your taking out.

If your figuring your 8" flex at 200 cfm a piece, then you need to return 600 cfm.

16 x 8

How did you come up with 14 x 8

[icepilot]

14 * 8, the room is 600 sq/ft. I'm judging in on 1 cfm per foot. 14 * 8 gives me 600cfm

[beenthere]

Not knowing insulation values for the basement, or glazing, its doubtfull you'll need 600 cfm, but its easier to damper down, then to increase duct size later.

Are they/you going to use the basement full time, summer and winter.

Remmember, it doesn't take alot to cool a basement, but controling humidity is another story.

[icepilot]

Yes, they are using it full time. I figured 600 was probably high also, but my problem is I'm really limited on room coming out of the AHU. My only option might be to come out with a 12 inch damper into a 12" insulated flex until I get up into the drop ceiling. Then hard pipe the supply. I know that the flex will probably cut down on the cfm a bit, but I'm oversized a little with the 600 cfm, so It could work out. Trust me, I think using flex when everything else is pipe is going to look like a hack, but trust me when I tell you, I'm very limited on space. Plus, the buddget isn't there to custom fabricate duct. I'll switch from 12 flex to 8 * 16 until the first take-off, then reduce to 8 * 12 for the second, then reduce to 8*6 for the last. that should give me 200 cfm to work with at each diffuser. thanks for the feedback.

[beenthere]

I understand.
I just meant you have air to spare, and could cool the basement too fast.