Current floor vent location is on right, with a dedicated duct to it from trunk line in basement. The fancier grate is propped into a former vent location, and there’s a capped duct that terminates just in front of it, running under floor from the duct that was in the open space on the opposite wall.
In tearing things out for the bath rehab, I uncovered some rotted 10″ x 3-1/4 duct work under the floor (currently under the OSB above)…that goes to nowhere. A patch in the bathroom floor (where the fancy grate is sitting above) indicates there used to be a boot on it and a floor vent. Now, the heat for this room is served off a separate duct that goes only to this room (see the grate to the right).
The dead-end duct is a 90° off the run that feeds a floor vent in what will be the guest bathroom, and did (and will again) continue up to a single boot/vent on the third floor.
Vent location in “new” bath, off stack that comes up inside wall on first floor.
The run up to the third floor was slapped on the outside of the wall in what will be my shower.
So, having already torn off most of the plaster, I plan to run new (non-rotted) 10″ x 3-1/4″ duct in the wall up to the third floor, which will necessitate building out that currently 1-3/4 deep wall a bit before putting up Wonder Board. I have to build it out anyway to fit the plumbing.
What’s left of the ductwork to the third floor. It will be pushed into the wall, which I’ll then build out 2-1/2″ to fully enclose the ductwork and the plumbing.
I doubt there’s any issue with replacing the straight run and the boot in the “new” bathroom and on the third floor.
But here’s my question: Is it acceptable to replace that (currently dead-end) 90° run to the old bathroom, attach a boot and move the floor vent back to where it clearly was before, thereby having three boots/vents on that one run? (Any issue from an HVAC standpoint, I mean…I do realize the vent location in front of the walk-in shower is not ideal, but it would solve another problem.)
The problem it would solve is to get rid of the ductwork currently serving the old bathroom. That run was slapped on the outside of a wall in my pantry, and when I redo the kitchen (If I don’t give up on this crazy house project before then, that is), I’d dearly love to push into what is currently a full bath on the first floor (off the dining room, which is weird) to make what is now the pantry large enough to be a 1/2 bath (which is to say I can’t just bury the ductwork in that wall, because it would still be in the way of making the room larger).
Quite annoying placement for this ductwork in the pantry below the bath – it stymies my future kitchen rehab planning. (This really was a decent if too-small pantry…until I had to rip out all the shelving to access the plumbing chase. And now that whole wall has to come down to remove the black mold on the backside of the drywall – yuck.)
That full bath on the first floor? It was once a butler’s pantry. And if I can figure out the HVAC problem, it will be again. Someday. Not soon.
Oh – and in case it helps, here’s the view in the basement of both of the runs in question.
The one on the left is the one that runs in the wall up to the second floor – the one I would like to use to serve three vents. The one on the right, which runs through the pantry and serves only the old bath, would go. Note that they are both on the same trunk line.
Rude Mechanicals Press Blog 
I was in hvac way back in my 20s and we did tons of this kind of remolding work . When you have a 3.25 x 10 duct like that only one outlet should be used per duct ,not 3 as yours shows and is fed by a 6″ round off of the main. Is there any way you can run new ducts from the main to the ,lower floors and leave this one for the top floor? If not an you have to run it this way then make sure you use registers that has a damper in it so you can control air flow where needed. Good luck
Paul E.
Well bummer! But thank you for your response. I think, then, I’ll combine the two on the second floor (I can see no other way to get heat to my bathroom and remove the in-the-way run on the pantry wall). Two is better than three, I suppose. And abandon the duct to the third floor, but run a dedicated 110 up for a small wall heater. The room in question is a closet; no need for it to be comfortably warm, really (and it wasn’t, even with the duct work in place).
I’d agree the existing venting is undersized, the uncomfortably cold room is proof. In my reno days the conversation with my hack guy would have gone like this.
Me. – If you cut out the upper and lower wall plates could you fit a larger vent using the stud space?
Hvac guy. – That’s easier said than done.
Also remember return air! You can’t force air into a closed space. If the airflow is restricted the space will not heat properly no matter how big the hot air vent is.
My experience so far is having to replace ALL of my duct work. My home was built in ’72 so it was much easier than… From the sound of your post you seem frustrated and that’s to be expected. Step back, have a bourbon and toast yourself thus far. Only one though as you’re not done yet.
Happy New Year, Fitz! We’re all here to cheer you on.
Megan:
John with HeatRegisters.com here. Got your catalog request and will send one out to you. To elaborate on what fairwoodworking says here, you may want to consider a register faceplate design that has a larger percentage of open area than usual. That way you could get more air in the cold room without enlarging the size of the duct opening, which may not be feasible. Also, I concur that you should make sure you have adequate open area on the return air grille(s). As a side note, many registers have open area of about 50%, especially on air returns. we have a lot of designs that are more in the 60% to 66% range or even more. A 66% faceplate has 32% more open area than a 50% so you can see where that would be helpful.
This may be one demolition too far, but you could put another 3-1/4 x 10 duct into the stud bay next to the existing 3-1/4 x 10 duct.
If my understanding of your domestic geography is correct, that would be straight up from the trunk where the offending round duct feeding your existing bathroom starts. Earlier photos of your hallway shower-to-be show open joist bays, so you could run from a new wall duct across the shower room and under the wall to your existing bathroom.
This assumes that there are no pipes or wiring in the stud bay. I’m not clear on just where this wall falls on the first floor. Despite the many as-bought photos of your house, I never got the kitchen/pantry/bathroom layout straight in my head and that was before you started unremuddling.
You might find it worthwhile to have someone do heat loss and required airflow calculations to make sure any proposed changes leave you with adequate heat and AC.
I envy you your project. Good luck.
Andrew Adams
Yeah – my first thought was to put a new run in the next stud bay, but unfortunately, there’s already stuff going up therein to the first floor. And breaking into the kitchen walls right now to move that might send me (farther) ’round the bend! So I’ve decided to combine just the two on the second floor, and run dedicated electric up to the third floor room above so that I can install a wall or baseboard heater if I find I need it (doubtful – I’m never in that room for more than a few minutes at a time)…or if I ever want to sell the house and need to call that third-floor room a bedroom.
As far as my bathroom being chilly, well, I’m installing a good combo fan/light/heat lamp.