Amateur attempting J/D Calculations/Duct/Plenum Questions

[skeer]

Stumbled across this Reddit post and the guy goes into a very detailed and in depth explanation, being a non-pro, about the calculations on a new furnace install and duct re-design. So I started following his steps and thought I could perhaps talk about it here, get some experienced advice even.

Existing details:
Inter-therm model G5RA096C-16 LP furnace 96k IN/78k OUT circa 1990s(?) I do not know the CFM of the blower motor
Existing plenum has two take-offs, 8" round.
Trunk is full 8x16 with 6 8" round take-offs, fittings of 1l, 8b, 1h, 2j, 8a. TEL for trunk is 308'
Longest runout has TEL of 361'

Single return, 24"x24", duct is a combination of 8x16 and spanned joists. Run includes 4 hard 90*. No vanes anywhere.
Return's TEL 457'


So far I realize that whomever installed this original was likely an idiot.. or the previous homeowner or both. I have most likely a very inefficient setup.. there are two runouts right off the plenum. And 2 more at the end cap, with another two flanking the cap within 6" of it.
The trunk itself has a flat el that connects to the main run of 8x16 in a mitered 90*.

Anyway.. since we are not going to be here forever I'm not investing in a whole new system or paying a local Hvac place to design a new duct layout. I will be doing what I can to make things better.. step one is turning vanes everywhere I can in the trunk and return.

Waiting on Nordyne to see if they can produce OEM cfm figures. I have relocated one of the end cap run outs to the side.. and the two sides are now at least 24" from the end cap. And I've read where spanned joists might not be a desired thing these days.. Not sure if that'd warrant ripping of that sheet metal and replacing it with two separate rect ducts or not.

The current setup warms the main part of the downstairs just fine, upstairs and the sun room (which is a walled in previous back porch slab) stays rather cold in the winter. Less static pressure should help that I'd think? But also perhaps if possible, adding one of two additional, smaller return grates?

 

[danski0224]

Anyway.. since we are not going to be here forever I'm not investing in a whole new system or paying a local Hvac place to design a new duct layout.

And this is the bane of residential HVAC work.

Good luck.

 

[Boogerman]

Spanned joists for a return are fine, as long as it doesn't touch an outside wall to pull in unwanted air from outside.

There's not enough information in your post to give any advice. Too much jargon and no meaningful information about what duct goes where, where the problems are related to what ducts, etc.

If you don't want to fix it right, the only meaningful approach is to look at each space that's deficient, and fix the duct or return to that room specifically. For instance, if you have 6" flex to a cold room, replace with 6" steel. Or, if you have a sealed room with no return, add a return. You may be able to increase airflow to some spaces by adding deflector vanes at the takeoff point of the duct to that space.

I had a professionally designed system that left one portion of my house 3 degrees colder than the rest. Rather than have the pros come in and do another guess job, I did some calc's, figured out I needed to increase airflow by 25%, and replaced the flex ducts with a hard pipe that stepped down sequentially from 14" to 6" through it's length, with wye's off to the registers at the stepdowns. I was lucky to have a crawlspace to work in. I guessed pretty well; now the entire house stays within 1/2 degree in winter, not quite as good in summer because of stratification of cooling air makes the upstairs run a couple degrees hotter. I had about $300 in the entire ductwork materials.

 

[brewchief]

That furnace came with a 1/2 hp blower rated at 1600 cfm at .5wc on high.

 

[skeer]

That furnace came with a 1/2 hp blower rated at 1600 cfm at .5wc on high.

Ooo thank you for that!

 

[skeer]

And this is the bane of residential HVAC work.

Good luck.

Oh I will readily recognize that.

 

[skeer]

Spanned joists for a return are fine, as long as it doesn't touch an outside wall to pull in unwanted air from outside.

There's not enough information in your post to give any advice. Too much jargon and no meaningful information about what duct goes where, where the problems are related to what ducts, etc.

If you don't want to fix it right, the only meaningful approach is to look at each space that's deficient, and fix the duct or return to that room specifically. For instance, if you have 6" flex to a cold room, replace with 6" steel. Or, if you have a sealed room with no return, add a return. You may be able to increase airflow to some spaces by adding deflector vanes at the takeoff point of the duct to that space.

The Manual D spreadsheet mentioned panned joists not being.. "Panning is not recommended by Manual D. Panned joist and stud space airway leakage increases duct loads, may cause space pressure problems and may cause indoor air quality problems."
It also mentioned that in a double joist space the max CFM is 800, which is likely fine on low speed.

I really have no way to add returns in places where it could help.

But I agree with you about them being fine, I do need to get in there to make sure there's no obstructions and maybe run some lengths of hvac tape.

I have no flex duct of any kind, it's all steel. I also have an incorrectly plumbed wood furnace tie in that will be remedied as well. At least 3 places for vanes, and two runouts to relocate. Last year I added two inline duct fans to help with the two longest runs, which is better but still not great.

 

[Boogerman]

I agree that panned joists aren't ideal, I'd avoid them at all times if I had a choice. But, if you have them, work with them if they're not causing problems that you can identify.

I can see you working with what you have instead of buying a new system, if the house isn't a long term proposition for you. With a bit of remedial work, you will have a system that works at 90% or more of what a new system would, for a fraction of the investment. When and if you sell, the selling price will most likely not be in any way affected by the ductwork as long as it doesn't look homemade. If you had paid $5 to $10K to reduct and redo the system, the house value wouldn't change.

The real answer would have been to have invested in a correct duct system when it was built, when the incremental cost was low; maybe 25% more cost to do a premium system over a hacked in one.

A lot of deficient systems are installed by the same broad group of professionals that have members complaining about homeowners hacking in work. This is driven by owners and contractors keeping the price down, of course. My house, for instance, was installed by professionals that insisted on running flex in a hard to work in crawl space in an existing part of the house. I absolutely made them run hard pipe in the closed in spaces of new construction where I knew I couldn't fix it in the future, and I made sure the sizes were right and there were adequate returns and registers. I gave in about the flex because I knew I was going to work on that crawl space later, and I could reduct it if needed. They assured me the flex would be adequate, and because the space was difficult, gave me a go away price to do it right. It might have worked, if they had installed it right.

In reality, they didn't stretch the flex to full extension, so it ran about 50% of the airflow it should have. They also didn't hang it correctly, so it sagged and pinched. In one spot, their connection to the boot tore loose and leaked. Unfortunately, I see that a lot with flex. If installed correctly, it works. But, the labor, especially in hard to work in spaces, makes it difficult to get installed right, so the techs just hack it in.

In my case, I demo'd out a bunch of stuff in the crawl space, removed rocks and junk and dead cats and dirt, changed floor supports from rocks and boards to concrete block neatly installed, laid down plastic tarps and then heavy duty polyethylene sheet on the dirt and generally cleaned it up and made it easier to work. Then, I ran correctly sized hard duct, insulated the ducts, sealed and insulated the perimeter foundation, insulated the floor, and did a finished installation on the entire space to make it clean, neat and orderly. Not a lot of dollars in the ductwork, but a hell of a lot of effort to make the space neat, clean and workable. And, re-arranged the floor supports so running hard duct was possible. The flex had let them work around the obstructions easily.

 

[danski0224]

The real answer would have been to have invested in a correct duct system when it was built, when the incremental cost was low; maybe 25% more cost to do a premium system over a hacked in one.

Sure.

But the builder dictates the level of work/quality based on the accepted bid, which is typically the lowest bid.

Then the contractor, usually cutting corners to get the job at the market price, then more corners are cut to "make money on the job".

More corners are cut with incentive based pay... or get it done in less than 8 hours, but take 8 hours pay, banking hours and other stuff.

These are completely separate issues from the skill or lack of skill of the people actually doing the work.

The builders that want (and more importantly PAY FOR) quality workmanship under the drywall are not common at all.

Homeowners that give a **** or even 2 shits about what's under the drywall are even more rare.

 

[fitter30]

400 cfm per ton of cooling is standard

 

[skeer]

Sure.

But the builder dictates the level of work/quality based on the accepted bid, which is typically the lowest bid.

Then the contractor, usually cutting corners to get the job at the market price, then more corners are cut to "make money on the job".

More corners are cut with incentive based pay... or get it done in less than 8 hours, but take 8 hours pay, banking hours and other stuff.

These are completely separate issues from the skill or lack of skill of the people actually doing the work.

The builders that want (and more importantly PAY FOR) quality workmanship under the drywall are not common at all.

Homeowners that give a **** or even 2 shits about what's under the drywall are even more rare.

In this area, there is a solid, measurable chance the original owner built this place himself. This is nowhere, Michigan. If he didn't then judging by the main panel wiring alone he paid a cross-eyed field hand to do it.

 

[skeer]

I agree that panned joists aren't ideal, I'd avoid them at all times if I had a choice. But, if you have them, work with them if they're not causing problems that you can identify.

I can see you working with what you have instead of buying a new system, if the house isn't a long term proposition for you. With a bit of remedial work, you will have a system that works at 90% or more of what a new system would, for a fraction of the investment. When and if you sell, the selling price will most likely not be in any way affected by the ductwork as long as it doesn't look homemade. If you had paid $5 to $10K to reduct and redo the system, the house value wouldn't change.

Yeah I guess the panned joists aren't actually causing problems, I just wondered about the blower cfm at the requested speed versus the cfm amount in the space.
And yeah, I accept that I bought a house without paying for an 'official' inspection and have therefor inherited some inefficiencies, bottlenecks and potential issues. But I will try to solve or remediate those to the best of my abilities, assisted by the good folks of the internet.

 

[gamp945]

The OP doesn't actually include any questions that I can see. What exactly are your questions?

I think you have questions about reducing static pressure, but I'm not sure. If you post specific questions, you will probably have better luck getting help.

 

[skeer]

The OP doesn't actually include any questions that I can see. What exactly are your questions?

I think you have questions about reducing static pressure, but I'm not sure. If you post specific questions, you will probably have better luck getting help.

Totally fair. I guess I'm seeking really anything. Am I being a *******? Am I going about this all the wrong way? Did someone here do this themselves and could maybe advise a bit?