Thanks Jeff. Allison reached
Thanks Jeff. Allison reached out to me, but I do think I’m still too early in the preliminary stages so I’ll have to get some things firmed up first.

Debbie,
Debbie,

I didn’t have a stairway issue, I had an orientation issue. We are using I-beam trusses: running ducts perpendicular to them is problematic so I needed ducts to fit within the bays between them as much as possible. But it raised the same question — I wanted to know about duct sizes early on.

The first step in the design is to calculate the heating and cooling loads (“Manual J”). Not until the loads are known can zones be determined, equipment selected, CFM of flow to each room be determined — and then the delivery system (aka ductwork) be designed.

The good news is that the Manual J depends on things you probably already know like room size, envelope construction, glazing, and so forth. (Maybe also stairwell location; I’m not sure.) Once you know e.g. how many CFM will be required, and basically where ducts might go, you can do some guessing as to what the max size of the duct might be.

One happy discovery in my case: Like you, I was “sure” that I’d need two zones. However, the Manual J “adequate exposure diversity” (AED) calculation showed that I could get by just fine with one zone.

My suggestion: talk to Allison and see if it’s reasonable in your case to do your Manual J first, and then an A-B (or A-B-C) comparison between duct alternatives to help you decide on a layout. Of course, this presumes that the details of your room sizes, volumes, glazing, building orientation, etc. are known.

All the best,
Jeff

Great article, Thanks Allison

I did contact Allison early
I did contact Allison early on but he said he couldn’t do anything until I had the design completed. 🙁 All I asked was for an estimate as to how much room to leave for the main trunk. Maybe that was the wrong question. I did tell him I absolutely want him to do the design, but I guess I can understand his not wanting to give out that advice when all I had was the square footage and the fact that two systems will be needed. It would have been nice to know though because I had 8 feet and since I didn’t know if that will be enough, I redesigned the stairway and some adjoining rooms to get more area. The redesign isn’t as nice as the original. I’m not the architect so once I get it all figured out in my mind, I will turn it over to the architect. I really can’t go to Allison before that because the architect might make huge changes. I should probably negotiate a redraw for issues like this.

Did you have an issue with where to put the stairway?

You are right on with this
You are right on with this comment, my concern for building more efficiently is that it is possible to build energy efficiency into a structure and do so affordably, however the market today tells us that we have to have this or that to green build a home when we do not have to have such things, everything we need is already within the local marketplace of construction materials. My concern when looking at the HVAC system is to first know my building losses and then consider the air volume and the latent heat capacity. With an energy efficient design I always calculate the losses for the lowest winter temperature and my average summer time temperature, I want to know the base load in terms of Btu and Kilowatts since I might be adding renewable energy to the design also.

Debbie, I was in your
Debbie, I was in your position not all that long ago. I, too, had chicken-and-egg framing/HVAC decisions to make. Energy Vanguard just recently finished my design, and I think it came out rather well and takes all of my considerations and tradeoffs into account. One piece of advice I would offer you: get them involved sooner rather than later, even if you have not nailed down your exact framing layout. Allison offered me that advice early on and I wish I had taken it sooner than I did. Nevertheless, I was pleased at how well Andy and Allison worked with me to get me what I needed.