Terrible Design - help needed to correct

Old 10-02-19, 05:18 PM
Thread Starter
Join Date: Sep 2001
Location: Philadelphia, PA
Posts: 62
Upvotes: 0
Received 0 Upvotes on 0 Posts
Terrible Design - help needed to correct

I live in a 22 year old, 3-story townhouse in Philly. The entire house and all of its systems were constructed by unskilled labor. I have repaired the hvac several times but never replaced it. My system is a split gas unit. The furnace is in the basement in a heater closet that is adjacent to my garage. It's time to replace the entire system as it is as old as the home. If you look at my basic drawing, you will see that air exits the top of the plenum which is 18x18 at that point and immediately has to make a 90 turn that then is forced into an 18x8 90 that feeds the 1800 square feet of the top 3 floors. Keeping the unit from being directly under the trunk line by about 20" off the wall is a soil line and ducts into the slab that feed floor registers in the basement. Questions:

1. How badly do the 2 90 degree bends compromise the efficiency of the system?
2. How undersized is my 18x8 duct that feeds the upper 3 floors?
3. I don't want to move soil line but if I may be able to figure something out with the 2 ducts that go into the slab. If I only have a 6" offset, how much better is that than what I have now?
4. I could potentially install a horizontal unit but that would require 1-90 degree bend. Any better than what I have?
5. Any other suggestions?


Attached Images  
Old 10-06-19, 06:59 PM
user 10's Avatar
Join Date: Mar 2004
Location: NA
Posts: 1,829
Received 57 Upvotes on 50 Posts
Sharp 90s are bad especially when the inner part of the duct fitting isn't a smooth curve.

18x8 is pretty small - i don't know if it's undersized, depends on the airflow requirements of your system. if the trunk is too small, your branch lines are probably also too small and there's not much you could do short of tearing apart the house and re-doing the entire duct system.

Townhouses don't need much heat, it may be okay if you have a small furnace and a/c.
Old 10-07-19, 12:11 AM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Jersey
Posts: 65,451
Received 4,051 Upvotes on 3,634 Posts
You're only showing the supply ducts in your diagram. The return ducts are important too.

Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: