Trunk Re-sizing - HVAC - DIY Chatroom Home Improvement Forum
Advertisement


Go Back   DIY Chatroom Home Improvement Forum > Home Improvement > HVAC

CLICK HERE AND JOIN OUR COMMUNITY TODAY...IT'S FREE!

Like Tree1Likes
  • 1 Post By roughneck
Reply
 
Thread Tools Search this Thread
Display Modes
Old 04-21-2020, 08:35 PM   #1
Newbie
 
Join Date: Aug 2018
Posts: 13
Rewards Points: 8
Default

Trunk re-sizing


I recently purchased a new furnace due to exchanger crack in old.

Looking for advice/tips and tricks

My current return trunk measures 13x20 and the new furnace needs 8x20 here's the twist the return was already reduced from 16x20 to 13. I have no room to cut the already reduced part off and start back with the 16. What's the best way to reduce down to what I need? Lots of cutting?

The hot side trunk is 20x20 needs reduced to 16x20 seems easy enough as this truck hasn't been chopped up already.

Currently just replacing trunks isn't an option I must build from what I have the return has an 8 maybe 10 inch lead coming into the what would be 8inch side.
Would duct board be an option?
Where do you even find duct board? As far as I can tell major hardware stores (home Depot ext.) Do not carry it and sadly I only have 2 supply stores near me one mainly plumbing and other contractors only.

Thanks in advance.

Sent from my SM-A205U using Tapatalk
rebelboiis2211 is offline   Reply With Quote
Sponsored Links
Advertisement
 
Old 04-21-2020, 08:43 PM   #2
Member
 
user_12345a's Avatar
 
Join Date: Nov 2014
Location: Canada
Posts: 12,449
Rewards Points: 56
Default

Re: Trunk re-sizing


normally the trunk lines aren't resized, the plenum is just adapted to fit the new furnace with a transition piece, if not replaced.

the trunk lines only need to be replaced when they're the wrong size for required airflow, and when they are, usually the branches are also wrong.

pictures may help us get clear on what you're trying to do.

furnace replacement you may want to leave for a pro, there's lots to screw up.
__________________
I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.

Last edited by user_12345a; 04-21-2020 at 08:46 PM.
user_12345a is online now   Reply With Quote
Sponsored Links
Advertisement
 
Old 04-21-2020, 09:24 PM   #3
Member
 
Join Date: Nov 2014
Posts: 5,842
Rewards Points: 732
Default

Re: Trunk re-sizing


You’ll have to make transitions out of metal.
The entire trunk isn’t replaced.
Hopefully you knew what size furnace to buy. Many are way oversized from the original contractor and can be downsized.
There’s quite a bit of commissioning to be done as well. Hopefully you have the proper tools.
roughneck is online now   Reply With Quote
Sponsored Links
Advertisement
 
Old 04-22-2020, 05:00 AM   #4
An old Tradesmen
 
Join Date: Oct 2008
Location: Somewhere
Posts: 39,259
Rewards Points: 86
Default

Re: Trunk re-sizing


Pics of the duct your trying to reduce would help, along with the brand and model number of the furnace.
__________________
When posting in certain forums, knowing your location will help others give better feedback/advice/solutions to your questions.
beenthere is offline   Reply With Quote
Old 04-22-2020, 06:59 AM   #5
Newbie
 
Join Date: Aug 2018
Posts: 13
Rewards Points: 8
Default

Re: Trunk re-sizing


For anyone who may worry, I discussed replacement with a pro, he encouraged me to try myself BUT NOT to turn on fuel or power. He 1 will come finish if I'm too stuck to continue or 2 will inspect and do first fire up. He said "I charge 3500 to 4000 (as all the contractors do) to install or you install for 1000-2000 and pay me a service call to inspect and fire or whatever the balance on installation is if needed.

Old furnace: Williamson Temp-o-matic oil convert to Nat gas. Hot and return out top.

I do not have photos currently but will upload a quick drawling until actual photos are possible.

New Furnace: Goodman gmes960803bn. Hot on top, return out side.

The problem isn't going to be getting the hot hooked up.

The biggest issue *I think* I'm going to have is getting the return air approx 40 inches down to the return boot and making it fit.

I'm pretty sure I know where I'm tangled up at now that I've pondered it awhile and thats the supply house gave me 20x8 duct. If I where to get (2) 20 and (2) 16 pieces instead of the two you slip together and it made. I should be able to size the original down to what I need?

*My thoughts. Cut the already chopped up return back to 20x16 and use s clips to attach 20 and 16 pieces and cut the bottom to fit at the furnace.



Sent from my SM-A205U using Tapatalk
rebelboiis2211 is offline   Reply With Quote
Old 04-22-2020, 07:01 AM   #6
Newbie
 
Join Date: Aug 2018
Posts: 13
Rewards Points: 8
Default

Re: Trunk re-sizing


Circles are pipes headed to the registers. The "trunks" are actually just boxes that close off at the top.



Sent from my SM-A205U using Tapatalk
rebelboiis2211 is offline   Reply With Quote
Old 04-22-2020, 07:06 AM   #7
Member
 
Join Date: Nov 2014
Posts: 5,842
Rewards Points: 732
Default

Re: Trunk re-sizing


Was this an old gravity system at some point?
Pictures would help.
Are you certain you need 80k BTU? That’s a big furnace.
roughneck is online now   Reply With Quote
Old 04-22-2020, 07:17 AM   #8
Newbie
 
Join Date: Aug 2018
Posts: 13
Rewards Points: 8
Default

Re: Trunk re-sizing


Quote:
Originally Posted by roughneck View Post
Was this an old gravity system at some point?
Pictures would help.
Are you certain you need 80k BTU? Thatís a big furnace.
I will get real pics as soon as possible I do have that drawling for you to get a feel of what I'm talking about.

It does have a blower in it. No plates for model number if I recall my aunt said it was installed by my grandfather when she was 10 and shes 45ish now .

1940 built 1400 sqaure foot house in NW pa very poor insulation (maybe none?) and windows are original it's a cold cold house and I think 80k is slightly large but not unreasonably. By my math It comes out to about 73k btu needed. The supply house came to a slightly larger btu needed both slightly under 80k the only furnace they had that wasnt smaller was this 80k.

Sent from my SM-A205U using Tapatalk
rebelboiis2211 is offline   Reply With Quote
Old 04-22-2020, 07:27 AM   #9
Member
 
Join Date: Nov 2014
Posts: 5,842
Rewards Points: 732
Default

Re: Trunk re-sizing


How was the furnace size calculated?
roughneck is online now   Reply With Quote
Old 04-22-2020, 07:49 AM   #10
Newbie
 
Join Date: Aug 2018
Posts: 13
Rewards Points: 8
Default

Re: Trunk re-sizing


Quote:
Originally Posted by roughneck View Post
How was the furnace size calculated?
How they did it I'm not sure.
I did what was suggested to me square footage times climate zone number of 50-55.
I'm more north and I sit in a snow belt with weird weather. I see temps of -15 over night at times "the norm" is more likely to be low 20's

Sent from my SM-A205U using Tapatalk
rebelboiis2211 is offline   Reply With Quote
Old 04-22-2020, 08:52 AM   #11
Member
 
Join Date: Nov 2014
Posts: 5,842
Rewards Points: 732
Default

Re: Trunk re-sizing


That’s not how equipment is sized.
An AHRI approved manual J is how equipment size is determined.
This is a free version-

www.loadcalc.net
user_12345a likes this.
roughneck is online now   Reply With Quote
Old 04-22-2020, 11:35 AM   #12
Member
 
user_12345a's Avatar
 
Join Date: Nov 2014
Location: Canada
Posts: 12,449
Rewards Points: 56
Default

Re: Trunk re-sizing


It's very possible a house of that size with zero insulation needs 80k, but in that case it's better to insulate and air seal as much as you can and downsize the furnace to 40 to 60k.

What can be done depends on the budget and construction of the house - framed walls can have dense pack cellulose blown in. Double brick without framing - too expensive to insulate the walls unless a major renovation is being done anyhow.

The air ducts needs to be assessed to see if they're large enough for the furnace - the old one guaranteed has lower airflow requirements.

There's a lot of theory that goes into doing this type of job properly.
__________________
I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.
user_12345a is online now   Reply With Quote
Old 04-24-2020, 05:50 PM   #13
Member
 
Join Date: Jun 2018
Location: Colorado
Posts: 524
Rewards Points: 904
Default

Re: Trunk re-sizing


You have a "Low Boy" furnace that you want to replace. Since you have already purchased the furnace you might as well use it. First, do not use 1 - 20x8 duct opening on the furnace, too small, you will regret it. Take a look at the attached sketch and see if you have the room to achieve something like this. It's easy enough to measure once you have the furnace and filter box in place. Have a local sheet metal shop make what you need, don't scab it in with duct purchased from HD or Lowe's.


Filter Box:

http://www.flowrightbox.com/furnace-filter-cabinets/


Trunk re-sizing-low-boy.png
57TinkerMan is online now   Reply With Quote
Old 04-24-2020, 06:40 PM   #14
Member
 
Join Date: Jun 2018
Location: Colorado
Posts: 524
Rewards Points: 904
Default

Re: Trunk re-sizing


Think along these lines, disregard cooling coil on top of furnace.



Trunk re-sizing-low-boy-replace.jpg
57TinkerMan is online now   Reply With Quote
Sponsored Links
Advertisement
 
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off





Top of Page | View New Posts