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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1st- upgrade mechanicals(only kidding!)

2nd- go with your suggested progam, but get contractor price on new supply runs, which should be pretty costly you would think, unless you're doing it. vent fans are available, but tend to be unreliable

3rd- make the second story its own zone, with appropriate improvements: window air, space heaters, maybe split/ductless heat pump


Thanks for the input Tom.

Definitely going for an upgrade of the mechanicals (after I punch a big hole in the roof, and vent the water heater to the kitchen).

Thought about the 3rd option, but HO is def not going for that one.

Time to further investigate option #2.

Best Regards,


What are the airflows per register, wheres the duct leakage, how much, did you do a static pressure test, did you review or do a manual J or D? Ahh now you know if a new run is required &/or what on the existing system needs work

I've got my work cut out for me. I ran the Manual J, and the current system is about right for the house as is. By the time I make the envelope upgrades the system will be over-sized for the house. On to a Manual D calculation.

I will get the airflows per register.

I know that the majority of the duct leakage is in the basement.

I know I've heard much about the static pressure tests. But, I have yet to have to perform one. How is the test performed?

Thanks a lot Sean!

For static pressure (especially as I am not in the mood to do a bunch of typing) - 

Virginia right? The Manual J calcs should tell you how much air each room needs allowing a licensed HVAC tech to air seal & balance the system

Great, thanks Sean.

Before you go through the expense of upgrading the HVAC system consider adding a mini split for upstairs. Upstairs must be REALLY poor on insulation to be COLDER than the main floor, perhaps spray foaming would be an option?


As you probably encounter enough, folks don't always want to spend the real money required to improve their houses.

Having said this, spray foam in limited quantities is an option (such as flash and batt), and a mini split is out of the cards at this point.

I can't think of a better way to make a believer out of my client (with a roughly $4K budget) than to improve the envelope. You hear what I'm saying?


You need more information before you suggest what the appropriate answer to the customers problem would be.  Do loads on the house and each floor.  Loads tell you equipment sizing which tells you air flow requirements.  Now you have something to compare the existing duct to what you actually need.  Then you can get get an accurate estimate of options and costs.

No matter how much you reduce the loads on the second floor ( and I don't think you can do it by just air sealing) keep in mind that the thermostat is located on the main floor and that is the area it will pay attention to.  You need to increase the ductwork and balance it so the two areas act as one. (good luck with that)

Keep in mind you may need to increase the equipment size to deal with the increased load since the upstairs probably wasnt figured into the original design. You will probably have to damper the main floor runs to force the air upstairs otherwise it will blead off through the main floor runs. 

Seperate systems will allow for better control and comfort but can be a price and space issue.

There are damper sytems available that will help but duct sizing is critical on this option.  Get a qualified company to do this option. No matter what the pressure you can only get so much air through a single pipe.

As A "HVAC Specialist"   I can only say that depending upon the age of the system the single run was probably installed to get heat to the second floor and not A/C.  Age of the house and duct system will tell you that and someone may have just changed equipment without looking at the duct. Or heaven forbid the previous home owner just didn't want to spend the money to do it right in the first place

Last but not least - put his mother in law up there and now more than one problem is solved   LOL

You did not mention any HVAC system returns on the second floor. is there any?

No sir. We have already visited that topic, but he wants more supply (airflow) to the 2nd story before we rip wallboard down to add a return. The return pathway is currently the stairwell from the 1st to 2nd story (no party walls on 2nd floor).

In your original entry and in your replies to other's comments you said a number of things that rule out some options - but also rule out some potential issues. The homeowner's request to add another supply (by itself) would provide little, to no, improvement.

One thing you said was that you did a Manual J and that the existing system is adequate and will likely be over-sized once your recommendations are carried out. This is relatively good news in the situation in which you find yourself; there apparently is sufficient capacity available, it's just not being properly delivered or controlled. 

You wrote that a separate system - even a ductless - is out of the question. At this point you have really one option that will make an impact (after your prescriptions are carried out): Zoning. A decent zoning system (properly designed and installed) could provide significant improvements. It will provide temperature control in the space and direct airflow where needed - when needed. (Depending on sizing the single run that is there may need to come out or be changed.)


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