EduKate me on this setup

[DonTx]

Today's home was 4038 sf 2 story, split zones with a thermostat for each zone and 1 five ton York unit.

This HVAC company installs York units for the national builder. On their set up, they run a large duct from the return plenum back around to the supply plenum.

Why would they do this?

What are the pro's and con's of this setup?

[Neal Lewis]

Donald,

It is a bypass damper. I can't explain exactly what it is for, but I see them on zoned scorched air systems. This link might explain it better.You can click on edukamate me on the left side of the page.

http://content.honeywell.com/yourhome/z ... _sec_3.htm

[Bill Kibbel]

First and most important, I believe the word is "edgumakate". Use your spell check please.

The by-pass duct is there to relieve static pressure when less than all zones are operating. There should be a pressure regulating damper with a counter balanced weighted arm installed in the by-pass.

[Bain]

And you guys tease us Southerners about OUR pronunciation?

I got thrown by this a couple of years ago, too, Donald. Luckily I phoned my HVAC-bail-out friend---who tolerates my insipid questions most patiently---who explained it to me before I made myself look like an arse by writing it up as a defect. It certainly does look strange the first time one sees it, huh?

OBTW--Don't you just hate guys like Kibbel--> [:-graduat who're bright, funny, and always know the answers to the questions?

John

[DonTx]

Thanks everyone. I've seen the counterbalance arm/damper on other systems, but not this one.

[Terence McCann]

Hi Don:

A little late here.

Some of the bypass dampers are operated off a static pressure sensing device or a amperage sensor from the blower motor. As Bill mentioned it allows a bypass when zones start shutting down.

The last kicker with these is that when zones start shutting down and the bypass opens it allows the return air temperature to really drop in the cooling mode which means that the supply air temp plummets. Most manufactures incorporate a temperature low limit to knock off the cooling, this prevents the evaporator from becoming a block of ice.