HVAC Question: Rigid metal vs. flex duct

[clemsonfor]

Tig if your only talking about 8' of flex and it will be easier i would just leave it!!

I have all flex i beleive in my home for the vents off the main trunct line. I dont notice any noise or lack of cooling. I think my AC is even a half ton undersized based on my lack of insulation and sq footage. But i have no trouble cooling my home even on those days where it was 104F. It ran a bit more but still could easily get 74ish which is on the cool side for us, we have it defaulted to 76 which is cooler than our first married days of 78!! But i could still bump it to 72. Some other folks i work with said 72 was impossible on those days? One guy at work keeps his air on 81 and bumped it to like 83 the hot days as they said they were freezing up! He lives in like a 6 yr old house as well.

 

[xtn]

Hey XTN,
You said that you designed systems and only put flexible at the last 3 or 4 feet before the register........is that to keep things more quiet??????

MFWD

No, although that is a very slight side benefit. It's because flex has a lot higher friction and pressure loss.

Your fan coil unit has some tonnage capacity that is available at its discharge opening, and the more friction and pressure losses you put between it and the air's final destination, the more you waste away that capacity with inefficiencies.

The real reason we use any at all is for ease of connection to the registers. The slop allowed by flex means you can field locate the collar wherever it is convenient. Also to isolate the vibrations that might be present in the unit and rigid duct from the wall, ceiling or floor that the register is installed into. In fact for commercial installations, in my area at least, code limits the use of flex to the last six feet of each terminal connection. We try to keep it down to three or four feet wherever possible.