New furnace installation and ducting questions


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Old 11-18-15, 02:42 PM
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New furnace installation and ducting questions

Hi,
I'm having a new 90%+ heater installed and adding an AC unit. It's a 3 storey victorian twin in Philadelphia and each floor is about 850 square feet.

Aside from wanting a new, more efficient replacement for my 30+ year old furnace, I've been a little disappointed by how little heat gets to the 3rd floor. I've been told that my blower is a bit oversized for the heater ...and in fact, my system cycles a lot!

One of the contractors who came out for an estimate suggested that the way that the branch ducts are set up creates a lot of static air flow. For instance, a branch comes off the main duct work, and then the main duct work reduces about 2 feet further down. There are multiple examples of this... He thought that these areas act kind of like a 'buffer' for the air continuing down the duct and slows the air moving away from the heater (thus making it harder for the blower to move the air to the upper floors).
He says he's an ex-union duct folder and has been in business for a few years on his own installing systems. He gets outstanding reviews on Yelp, so I trust that he knows something about this (I myself am an electrician on Yelp with outstanding reviews, and we seem to share much of the same attitude about our work, so I'm inclined to trust him).
However, I freely admit that I don't know jack about this.

What say you?

(And thanks in advance!)
 
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Old 11-18-15, 03:44 PM
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Your contractor may be an excellent "tin bender" but he doesn't understand duct design. Your duct SHOULD taper down the farther it gets from the furnace because there is less air the farther you get from the furnace because of the air that goes to the side ducts and registers. Since there is now a lesser amount of air being moved the duct needs to be smaller to maintain the proper velocity through the duct to allow it to get to the end.
 
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Old 11-18-15, 04:34 PM
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Thanks!
...But I don't think I made this clear:
So he agrees that the duct should taper down, but his thought is that it should *immediately* taper down, instead of going (about) 2 feet before doing so.
His feeling is that the part of the trunk that remains the same size after a branch creates a 'cushion' of dead air in the section that doesn't taper down before it tapers down 2 feet later at another branch.

In other words, should the duct taper down immediately after the first branch, or is it OK if it continues after the first branch and then tapers down?

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Old 11-18-15, 04:57 PM
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The taper should be some distance after the take off. The reason is that having it immediately after the take off (as shown on the right-hand drawing) will cause turbulence in the air flow and that will disrupt the airflow. It should be a smooth transition to the smaller duct size to reduce the chance of turbulence.
 
  #5  
Old 11-19-15, 05:56 AM
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I agree, at least a foot would be better than nothing.
 
 

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