Converting a cold air return to Hot air supply line

[crankshaftdan II]

Try to be informative as possible. Realitives house is story and one half-large bedroom upstairs with a 10x7' closet which is going to be converted into 3/4 bath(Shower-stool-sink) need heat duct in same. The only HD is located on the opposite wall from the BR area(18') cannot access to hook in for the BR.
There is a dead cold air return(Not hooked up in basement) four feet away which could be re=routed and installed for hot air return & hooked up in basement hot air plenium. I know this is not acceptable for code and will probably make the stairwell the cold air return for the upstairs(drafty under the door). Question would be that I would probably have to re=balance the air flow to the rest of the registers to even out the heat balance for the thermostat?
Anyone out there done this before? I would have to otherwise tear out a stud wall cavity on outer wall from basement to second floor to box in a run! I cannot either go over the second floor ceiling with a run from the existing one!
I don't want to put a electric heater in the BR due to moisture/shower/safety.
Suggestions/comments/advise appreciated.......Cranky

 

[DC73]

Don't know the codes and not sure I can give you a complete answer anyway but here's some food for thought until others chime in.

A few years ago, I researched HVAC before installing a system in my workshop. I read a few articles online describing how to calculate the minimum return air flow (and thus grille size). You'll have to search as I no longer have those sites bookmarked.

I recommend you start with that calculation to find out how much return air you need, then add up how much return air the house currently has so you can see the impact of taking that one particular return air duct out of service.

The return air can come from multiple sources. Using the stairwell as a return air path can be acceptable as long as enough air can flow through doors, walls etc.

Here are some options for improving return air flow through walls and doors: http://www.tamtech.com/return-air-pathways

Maybe you don't need the entire existing cold air duct for the new supply. Could you run a smaller supply duct inside the existing cold duct to preserve some of the return air capacity of the duct? Not sure it would be allowed by code but furnaces use concentric supply/exhaust vents so I don't think there is any technical reason it couldn't be done as long as the materials used are appropriate.

Good luck,

DC

 

[AA7483]

If I am reading this correctly the return duct you want to use is unused. You want to connect this return and use it a supply to heat the new bath? Is it a piped return or does it use the wall cavity? What does the rest of the bedroom do for a return now?

 

[6768rogues]

As I understand, you want to use an abandoned return as a supply for a new bathroom. We never put returns in bathrooms. Undercut the door 3/4” for return and do it. No one professionally balances typical residential systems. There is no code issue unless your new bathroom lacks an operable window OR an exhaust fan. I am only curious about why there is an abandoned return duct in the first place.

 

[crankshaftdan II]

The abandoned cold air return is located in a bearing wall in the center of the house-unattached completely in the basement above the heat plenium. It is rectangular in shape so a round supplemential hot air return would have to be like 2.5" dia. to be used in conjunction with the existing cold air return. I think the BR is now using the stairwell for a CAR to the second floor? The furnace uses out side air for combustion from a pvc pipe which is open in the basement and exhaust is piped outside with pvc?? I don't understand why the CAR was not hooked up as well? There will be a vent in the bottom of the new bathroom door when installed and a exhaust fan-the ceiling will be slanted for the out side wall due to the steep pitched roof, so the only window option will be for a fixed or opening sky-light type-no reg. vertical wall for a std bath window.

 

[mrobins297aaa]

Is the cold air return a metal duct in the stud space? or is it just the stud space with the exposed wood?.
You need to have a metal duct up thru the stud space to the second floor maybe 10 x 3 1/4, after you get in the basement but a booth to transform to 6" round. If at all possible try and tie the 6" right into the plenum.
As far as the return for upstairs just make sure the air has a way to get back down to the first floor where there are other returns.........forget about re-balancing the system nobody does that.
I'm not sure what you mean by the 2.5" dia duct but if your talking about heat that's way to small you'll never get any air out of it, 6" should be the min.

 

[AA7483]

What he said. You can't use a wall cavity for a supply without a duct inside. Make sure your bedroom has a path for return through an undercut door or a jumper duct in the attic.

 

[crankshaftdan II]

Is the cold air return a metal duct in the stud space? or is it just the stud space with the exposed wood?.
You need to have a metal duct up thru the stud space to the second floor maybe 10 x 3 1/4, after you get in the basement but a booth to transform to 6" round. If at all possible try and tie the 6" right into the plenum.
As far as the return for upstairs just make sure the air has a way to get back down to the first floor where there are other returns.........forget about re-balancing the system nobody does that.
I'm not sure what you mean by the 2.5" dia duct but if your talking about heat that's way to small you'll never get any air out of it, 6" should be the min.

What he said. You can't use a wall cavity for a supply without a duct inside. Make sure your bedroom has a path for return through an undercut door or a jumper duct in the attic.

I will have to check out the duct work as it looked to be a square plate screwed into the wall w/adapter from 6"? round to rectangle-maybe 10x3 1/4 into the cavity-will check to see if it is metal or just wood run(maybe that was what they used back in 1960's).
The 2.5" referenced was if I was to use existing CAR for both Hot & cold runway-which would probably be too much of a squeeze in a 10x3 1/4 duct way. I would place a grill in the bottom of the new BR door and also at the bottom of the stairs-so the cold air would slide down the existing first floor cold air returns. Should I be really concerned about the output from the existing furnace making it work harder/longer to put enough warm/hot air up stairs in the large BR and the new bathroom or am I just over imagining that there will be more btu's required? The area is the same-just another room with a door? If I would re-adjust the first floor HAR's and restrict their air flow should I not get more warm/hot air upstairs-I.E. longer the run less heat? Maybe it is more simple than I am over engineering the solution to the new bathroom? Thanks much for the comments and suggestions so far..........

 

[crankshaftdan II]

Don't know the codes and not sure I can give you a complete answer anyway but here's some food for thought until others chime in.

A few years ago, I researched HVAC before installing a system in my workshop. I read a few articles online describing how to calculate the minimum return air flow (and thus grille size). You'll have to search as I no longer have those sites bookmarked.

I recommend you start with that calculation to find out how much return air you need, then add up how much return air the house currently has so you can see the impact of taking that one particular return air duct out of service.

The return air can come from multiple sources. Using the stairwell as a return air path can be acceptable as long as enough air can flow through doors, walls etc.

Here are some options for improving return air flow through walls and doors: http://www.tamtech.com/return-air-pathways

Maybe you don't need the entire existing cold air duct for the new supply. Could you run a smaller supply duct inside the existing cold duct to preserve some of the return air capacity of the duct? Not sure it would be allowed by code but furnaces use concentric supply/exhaust vents so I don't think there is any technical reason it couldn't be done as long as the materials used are appropriate.

Good luck,

DC

If I am reading this correctly the return duct you want to use is unused. You want to connect this return and use it a supply to heat the new bath? Is it a piped return or does it use the wall cavity? What does the rest of the bedroom do for a return now?

As I understand, you want to use an abandoned return as a supply for a new bathroom. We never put returns in bathrooms. Undercut the door 3/4” for return and do it. No one professionally balances typical residential systems. There is no code issue unless your new bathroom lacks an operable window OR an exhaust fan. I am only curious about why there is an abandoned return duct in the first place.

I will have to check out the duct work as it looked to be a square plate screwed into the wall w/adapter from 6"? round to rectangle-maybe 10x3 1/4 into the cavity-will check to see if it is metal or just wood run(maybe that was what they used back in 1960's).
The 2.5" referenced was if I was to use existing CAR for both Hot & cold runway-which would probably be too much of a squeeze in a 10x3 1/4 duct way. I would place a grill in the bottom of the new BR door and also at the bottom of the stairs-so the cold air would slide down the existing first floor cold air returns. Should I be really concerned about the output from the existing furnace making it work harder/longer to put enough warm/hot air up stairs in the large BR and the new bathroom or am I just over imagining that there will be more btu's required? The area is the same-just another room with a door? If I would re-adjust the first floor HAR's and restrict their air flow should I not get more warm/hot air upstairs-I.E. longer the run less heat? Maybe it is more simple than I am over engineering the solution to the new bathroom? Thanks much for the comments and suggestions so far..........

RE-checked the 10x3 1/4 duct and it has NO metal from the bottom basement to the second floor-I would have to drop the heat run in the basement in an attempt to slide one up and don't know if there would be enough clearance to do so??

 

[mrobins297aaa]

the only way to find out is to try it with a pc of 10x3.25. There could also be some 2x4 blocking between the studs half way up, if you run into that situation best way to handle that is with a long pc of 3/4" gas pipe using it like a ram up into the stud space from the basement. you'd be surprised how well that works. Anyway you probably won't have that problem since it was at one time a RA.

Any chance of just cutting a new one from scratch? it's not easy but it can be done. even if you can find a wall below that comes up into the center of the room upstairs, if your replacing the carpet you can cut a section of the floor out and run the duct in the 2nd floor joist space to get it into the room you need.

you said the home was a 1 1/2 story sometimes these homes have a knee wall on the second floor with a small attic space behind it. Any way you may be able to get that new duct up from the basement into that attic space and cut a wall register in the knee wall.

I don't know what you have there but just some suggestions that may put you on to something that may work for you.

take a look at all the walls below and which direction the 2nd floor joists run and see if there may be something that may work.

there's also the possibility of running a new duct thru a closet on the first floor.

 

[brewchief]

You may have a better chance at sliding a piece of 6" or 7" oval pipe up instead of square wall stack, it's basically 6" or 7" round pipe but in a flatter version so it can be run in a stud space.

 

[bobbyjean]

sounds like a ***** of a job for sure...if you open a wall midspan you could get the oval pipe in...i've done it...it *****...any chance you could check for obstructions with a inspection snake camera? a wire running through would spoil your fun.
go with the vented skylite for sure...good luck with it

 

[LS6 Tommy]

If you use return air from the basement you need to route the combustion air outside, too...

Tommy

 

[crankshaftdan II]

Will check into the snake camera before attempting to shove any type of flex/fixed sheet metal up the opening. Better to know what you can see and getting frustrated as hell. I did check with a few of the long time neighbors on street to see if they had done any simular upgrades and most have-they just used the stairwell for the cold air return and seems to have worked out for them! This will turn into a major task for me-but we all like a good challenge!! Will advise as the progress unfolds.

 

[6768rogues]

I thought it had a metal duct when I commented. It cannot be an unlined stud bay.

 

[crankshaftdan II]

I thought it had a metal duct when I commented. It cannot be an unlined stud bay.

As far as I can determine it is a wood run(Cold air return)thus the question if it could be used as a hot air supply. Will check it out and advise.