Replies

1. 
   mr_reference_Hugh | Oct 29, 2022 05:06pm | #1

   Kermit49, the 2 foot rule is really well explained on this web page.

   https://hvactoday.com/0719-two-foot-rule/
   Attached photo is taken from this url.

   Basically you always want to have the main trunk extend 2’ beyond the last takeoff.

   Your questions

   1. Can the final 8″ register be attached directly to end of 8″ duct?

   Response: it should not. If you want to maintain an appropriate static pressure to have enough air flow in your other takeoffs, don’t put a
   - boot (vent) at the very end of the main trunk;
   And/or
   - a takeoff right at the the end of the main trunk.

   You need to leave approx 2’ run at the end of the main trunk after the last takeoff (that ends in a boot).

   If you have a boot (vent) at the end of a pipe, that pipe must be a takeoff from the main trunk AND if this takeoff is the last one off the main trunk, the main trunk must 2 feet beyond that last takeoff.

   If you have a 28’ run of 8” duct that is an extension of the main trunk AND you end that run with a boot (register), then what you have is not a takeoff, it’s called a mistake (if can be a bit silly.)

   You can have a 30’ run of 8” pipe with a takeoff at the 28th foot. That takeoff would be perpendicular (approx 90 degrees) to that 8” trunk. That takeoff doesn’t need to be very long but you need a takeoff to feet that last boot)register.

   Your takeoff to the last boot (register) should be smaller than 8”. An 8” duct is really big and not sure how big of a room you will be heating or cooling.

   
1. 
   kermit49 | Oct 29, 2022 05:53pm | #2

   Thanks for the advice Mr Reference.

   I read Puryear's 2 foot article couple days ago while researching and have applied that rule, appreciate you mentioning it and including link for others to see.

   Your point on not maintaining proper trunk static is well taken. I see the trunk as a pressurized vessel with service outlets. You are right, a boot at end would be a big leak accompanied with pressure losses. The first two take-offs would suffer as well as the final one. Trunk extension it is. Two foot of course.

   The last room serviced requires 120 cfm. I can reduce room branch diameter but did not want to have higher that 400 cfm especially being our bedroom. In fact 400 cfm is the target velocity throughout this ducted mini-split system.

   Am I being unrealistic trying to maintain system flows more or less around 400 cfm mark? System noise is not something we look forward to. Also I only have 0.36 iwc to work with.

   Mr Reference, appreciate your advice.

   Kermit49

   EDIT: I should mention that all ductwork will be within conditioned space. Only thing going to the attic is lots of insulation and bugs if they can stand the cellulose chemical treatment.

2. Expert Member
   Akos | Oct 29, 2022 06:18pm | #3

   I wouldn't worry too much about it. I would guess the 8" run is your largest pressure drop, most likely you have to adjust the dampers on the 6" takeoffs to get the correct airflow there anyways.

   For ducted mini splits, I find home running each duct the simplest. I make a box plenum out of 36" of 8x16 duct with endcaps, unit connects on one of the 8"x36" side, the takeoffs on the remaining sides and endcaps. As long as the duct velocity is low, you can ignore most of the HVAC rules, size each run to about the same pressure drop at design flow and add balancing dampers at the takeoffs.

3. 
   kermit49 | Oct 29, 2022 07:14pm | #4

   Hi Akos,

   Is home running when you have all branches originate at plenum and travel directly to the register boot? Similar to how Zehnder distributes their 3" flex duct?

   What me worry? YES.

   Never designed a home before and aside from being fascinated with designing our retirement home, having a super comfortable indoor environment as we age is important. I only have on chance to get it right.

   Actually it will be alright as for some reason I have faith in myself. Additionally, I have tons of material at GBA, Energy Vanguard and Fine Homebuilding to research through. Plus great people willing to share and help.

   For an inexperienced old guy like me, subject to miscalculations, low velocity is my best friend.

   Thanks Akos.

4. 
   mr_reference_Hugh | Oct 29, 2022 09:31pm | #5

   I don’t see any issues with what you are doing. My sense is that you are doing a fine job.

1. 
   kermit49 | Oct 29, 2022 09:59pm | #6

   Well thank you Mr Reference, your confidence in me is appreciated. Thanks again for helping me out.