Here is a new area for me: The wisdom (or lack thereof) of adding supply duct runs in order to improve airflow to rooms that are uncomfortable.

I have been going back and forth with a client about improving air distribution to the 2nd story of his house. He lives in a converted 2-story house in northern Virginia. His 2nd story (previously an unconditioned attic) is hot in the summer and cold in the winter. The windows throughout the house are new, and show few signs of leakage. 1,900 square feet of conditioned floor area, un-insulated exterior walls, 2,900 CFM50 air infiltration rate (BAS near 1,400 CFM50).

He has only two registers for the 2nd story, and they are both connected to the same supply run (go figure). There is almost no airflow to the bedroom on the 2nd floor when the register to the bathroom on the 2nd floor is open (the register in the bedroom is further down the supply line).

I have told him, and continue to tell him, that he may be able to solve the comfort issues in the 2nd story by sealing up the ducts in the basement, adding insulation to the slopes, kneewalls and short attic space (all of which are under-insulated, and in some cases completely un-insulated), and sealing up leaks in the 2nd story. He now knows that such measures will improve overall comfort and efficiency, but is still pretty ardent about having new supply runs added to the 2nd story.

So, I am going to push for all the recommended envelope improvements, and I need guidance on whether adding a couple of supply runs to the 2nd story (through the 2nd floor) off of the main supply trunk makes sense. Or, am I totally missing the point.

Every time I turn to an HVAC "specialist" in my area, I hear a great deal about it being time for folks to upgrade and up-size their mechanicals. After all, it was most likely an HVAC specialist that ran a single supply line for the entire 2nd story.

I really need y'all on this one.

Thanks in advance.


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I like where this is going. Let me sit down, do some research, and I'll get back to you. Thanks for your input!

As a licensed HVAC contractor the key point to remember is its all about airflow. Does the airflow meet the comfort requirements of the area. If the area is not sealed and under insulated then the air flow from two registers is insufficient. Sealing duct work and insulating will help but the airflow may still be under comfort requirements. If supply runs are added make sure you add a return. Most systems are undersized for return air. The furnace no matter how efficienct cannot supply more air than it takes in. Zoning may be a solution if done right.

Thank you Bruce. I have a lot to read up on about zoning before I proceed with this client.

I performed a static pressure test on the system, and the results are as follows:

TESP (total external static pressure) = 150 Pa

Max pressure (read from nameplate data) = 120 Pa

Result: The system is not getting the recommended level of airflow.

So, does this mean that some thorough duct sealing is called, for or am I missing the bigger picture?





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