[ksguy]

Hey folks, yesterday I pulled out a section of subfloor during some bathroom work and found a supply duct running through a panned return. This supply runs to our spare bedroom that's never the same temp as the rest of the house...and this helps explains it. I should have realized this sooner since at one point I noticed a supply coming out of a panned joist bay, but I digress.

My question is how to best address this. I'd like to have this supply insulated somehow but a section of it is directly above the main return plenum above the blower. The run spends about 15 feet in the return, and the joist bay that it runs in is one of two that are panned to come from a big return vent in the hallway. There is one other single-bay return at the other end of the hall, and each bedroom also has a single-bay return. The vertical runs within the walls don't have ducting, they're just in the wall cavities.

The pic with the electrical box is where the supply leaves the return, the one with the plumbing is above the blower where it enters the return, and the pic with just the duct running right to left with panning behind it is a view under the bathroom floor.

 

[user_12345a]

I guess if most of the pipe isn't accessible, you have to open up the ceiling below to get in there an insulate.

 

[beenthere]

Don't insulate supply runs in a return joist Bay. It will reduce the return air flow.

 

[Yodaman]

Regarding temps in the spare room, it is unlikely the supply running through the cold air is the primary reason for the poor performance.

I would be looking at undersized supply duct or lack of return air, or placement of each. Balancing with dampers may help, but be cautious, because it can have negative effects on the overall system performance.

 

[ksguy]

The temp of the air leaving the duct in this room peaks at about 83 degrees. Thermostat is set to keep the house at 69.

There's a 6x14 return grill in this bedroom (and an identically sized one in the other two bedrooms). There's also a 6x28 and 6x14 in the hallway that leads to the bedrooms.

Forced air temp in the kitchen as a point of reference is 95, they're about the same distance from the furnace, but there's no return anywhere near the kitchen.

I don't want to reduce return air volume, but it wouldn't be too difficult to add another 6x14" return in the hallway, or I could add one to the basement family room, and there are no returns in the basement at all.

 

[user_12345a]

The temp drop of that run can be measured to see if there's a significant loss.

Air at trunk around the take-off vs air at vent and compare with other runs not going through the return.

 

[ksguy]

Good call, I'll check it out a little later today. I suspect I'll find I really should insulate all of the supply ducts.

 

[user_12345a]

They don't need to be insulated when they're in the conditioned space.

it's just the airflow in the return may be taking too much heat from the supply duct going through it.

beenthere may be right about insulating the run -> takes away free area for airflow in the return.

 

[ksguy]

Does an unfinished basement with no open supply ducts count as conditioned space? Most ductwork runs through the unfinished part of our basement.

 

[user_12345a]

only if you have insulation in the basement ceiling and it's isolated from the rest of the house is it unconditioned.

you should have a little heat going into the basement to stop condensation (absolute humidity level same as upstairs) on cold surfaces so i would not insulate most of the ducts down there.

 

[ksguy]

Makes perfect sense about condensation and humidity. It's definitely not insulated from the rest of the house, so I'll insulate the ductwork sparingly.