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Forced Air Ducting Changes - Advice Needed

JohnLaPlante Member Posts: 49
Hi all,

I am a proud owner of a Mouat Steam System that the uber-talented Gerry Gill will be working on this summer.  However, I also have a forced air AC system that I need to do some work on, and I thought I would get your opinions. 

A little about my house. Built in 1837, heated by steam for the most

part. AC unit installed about 7 years ago when we moved in. There are

no steam radiators in the kitchen area, so my contractor added a 10kw

heat coil about 4 years ago, which helps the kitchen stay warm.

However, there are not enough vents to cover the heat loss. Also, I

don't need the heat coil to provide heat to the entire house, just the

kitchen area. Also, we are remodeling our kitchen this summer, and

unfortunately we have to relocate a stairway...and the stairway is

going down right above my main HVAC duct. So there are a few ducting

changes I need to make. My air handler is a variable speed Trane 3 ton


Below is a picture of the way my current system is configured, and my idea for re-ducting the system. Please note that I am:

1. Adding a manual damper to separate the front and back of the house.

I don't need the heat in the front part of the house (steam radiators

cover this), but I need AC everywhere.

2. Re-piping the ductwork around the new staircase (black dotted square).

3. Adding a new vent in the kitchen area.

Thoughts? Any advice?


  • Ironman
    Ironman Member Posts: 7,376
    edited March 2011
    Some Advice

    Even though you have a variable speed blower, you can't force its total air flow through one half of your system. Recommendations:

    1. Leave the damper partially open to allow 25 - 30% air flow.

    2. Run the new register that you're showing on the left side directly off the trunk line. You can't piggy back off of another run out unless its capacity is increased back to the trunk line.

    3. The same applies to the new lines around the stair case: the initial portion of the run out needs to be sufficient size to carry both outlets. It takes an 8in. pipe to feed two 6in. pipes. Use "Y" fittings, not a "T" to split to two outlets. Notice a "Y", not a "y" fitting. The latter gives a 70/30% split.

    4. Minimze "L"'s. Each "L" is equvalent of 15' of pipe in friction (resistance). Air flow will take the path of least resistance.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • JohnLaPlante
    JohnLaPlante Member Posts: 49
    Bob, thanks!!

    Bob, thanks for the advice about the damper...will do.

    On the new register, I don't think I'm going to be able to do that.  The spacing in the basement of our house just won't allow for another tap to be run...we had to break away part of the foundation to fit this one in.  The run that is there now is 7" round, and the extra register will only be 4 feet from the "Y".  Do you think this will work?  I know it's not optimal, but I'm not sure I have a choice.

    On the staircase...my other option would be to bring 2 lines directly off the trunk...one on either side of the staircase opening.  I would need more 6" round ducting, but do you think this would be a better solution?


  • Ironman
    Ironman Member Posts: 7,376
    Can You...

    Enlarge the 7" to 8" between the trunk and the "Y"? And yes, two runs back to the trunk would be a little better.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • JohnLaPlante
    JohnLaPlante Member Posts: 49

    Yes, Bob, there is enough room for that...perfect idea!  That will work wonderfully.  Thanks for your help. 
This discussion has been closed.