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IdKid 01-28-2013 10:58 PM

Residential HVAC system upgrades
1. Why aren't Horizontal Slab Coils used in residential A/c Systems if room permits? About the same cost but less restrictive in flows.
2. If I understand the 'Equal Friction Duct Sizing' correctly, all trunks are sized to the same friction factor with the branches adjusted to achieve the same friction loss of the longest branch.
I'm still a little hazy in how to figure the trunks and adjusted branches for a duct upgrade.

beenthere 01-29-2013 04:48 AM

They are used in residential. They're common in some areas, and not in others.

Your correct on friction rate. Trunk lines are all sized to the same friction rate, and as air is remove by supplies, when the velocity drops by 200 to 300 FPM, you reduce down to a size that gets it back up, but is still at or just under the friction rate you need/want. Supplies are ran out at what ever the friction rate came out to for the TEL for the CFM the room/area needs.

IdKid 01-31-2013 07:00 PM

Thanks, this has helped in the duct design math. To check, so the rest of my branches have the same pressure loss, the combined losses (of the flow path) of the trunk and a branch, should equal the combined losses of the longest branch.?

beenthere 02-01-2013 04:18 AM

Only the longest supply will have the loss of the TEL. The other ones will have the same per 100 foot of TEL, but will have a lower TEL. Balancing dampers are used to lower the air flow in the shorter branches that get too much air.

Not uncommon for a duct system to have a TEL of 300 foot plus or minus on the longest branch, and have a short branch with only 70 foot of TEL. But both are sized the to the same friction rate.

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