Is having a branch off the end of main heating HVAC duct bad and if so, why

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Question: Is having a branch off the end of main heating HVAC duct bad and if so, why?

Background:
A heating and cooling contractor sales representative came to my home today. He saw the situation pictured below and said that having a branch off of the end of the main heating duct is bad because it does not allow the main branch to maintain pressure.

I presently fail to see how a branch off the end compared to the same sized branch off the sides (recommended by the sales representative) of the HVAC would make any difference in the performance of the system. I would like to understand the physics behind why this is bad before I pay to have it corrected.

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Best Answer

IMHO:

I don't believe the position makes any difference. The pressure in the main duct, for all practical purposes, is uniform. There is a slight loss at each exit duct along the main duct, but the original main duct should have been designed to handle this. However, if you added the duct to the end of an existing system, and the duct is comparatively too large-you can effect the air flow of the entire system. Velocity changes with the duct size.

Related Solutions

It is reasonable to expect a contractor to replace the last foot of HVAC duct work, the connection between the insulated duct and the register

I believe it's called a "boot", and yes, they're generally pre-fabricated.

However, if the existing ones are in good shape, they shouldn't need replacing. If they're damaged, then the reasonableness depends on what your contract says.

Ceiling – reroute this air duct

I don't see a problem with that, other than the existing duct doesn't look wide enough to fit both. You'll have to get/build a reducer that goes from something slightly wider (that can support both circular ducts coming out of it) to the existing square duct size. Most metal shops can build this sort of thing for you - I had a couple 45-degree elbows built to re-route some ducts in my basement, it cost me around $50 I think.

 | |   | |
 | |   | |    <--  existing two round ducts (one moved)
 | |   | |
+---------+
|        /
|       /    <-- new reducer
+------+
|      |
|      |     <-- existing square ducts
|      |

The main thing is to keep the number of elbows to a minimum. In fact, in your case it may work better as it looks like you'll reduce the number of elbows by one.

Another option would be to extend the main duct, and split the two round ducts off at the end. Not sure how long those are now, but the box stores carry a few regular square duct sizes you can buy and assemble/install yourself.

Best Answer

Related Solutions

It is reasonable to expect a contractor to replace the last foot of HVAC duct work, the connection between the insulated duct and the register

Ceiling – reroute this air duct

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