[Master Brian]

I'm trying to finish off the ceiling in my basement, but I have a single trunk that is kind of in the way. It is 8"x14" and is currently supplying air to 4 spots with 6" round ducts. It also has a 4"x12"? register cut in for the basement, but in all honesty it normally stays closed because the basement stays pretty constant. There is also an approximate 6"x7" beam that runs parallel to this trunk. What I'd like to do is create a bulk head that looks even and in order to do that I need to split the current trunk into two separate trunks, each supplying 2 of the 6" ducts and possibly a small vent for the basement.

I'd like this as small as possible obviously, but can anyone tell me what I can get away with? I've tried looking at calculators and I believe I've asked before with mixed answers. Can I get away with 2) 6x14" trunks or can I go narrower yet? Does the basement figure much into this equation, because if so, the furnace sits right off this room and I could always try to figure another way to push air to the basement, unless it's fine to place a couple of registers in the new trunks for when I want to send a little conditioned air to the basement. I am for what it's worth going to add a return air to the basement, unless of course that is a bad idea.

 

[Master Brian]

Adding to this as I just priced the local supply place on this, what if I just bypass the trunk style setup and run 6" round duct all the way back to basically the furnace room and hook in where they branch off?

They want approximately $30 for two sides and two tops to make either a 6x12 or 6x14 duct and I have a bunch of 6" round pipe laying unused in the basement. Right now everything is mostly un-insulated, but I've been slowly trying to tape and insulate everything as I've been working on the finish job of the basement. Not sure if that matters or not.

Also, any reason I can't wrap this with the bubble foil style insulation? I got a heck of a deal on 2 large 4' tall rolls.

 

[beenthere]

It would take a 6x20 to be equal to a 8x14.

If you want to run round pipe instead of rectangular trunk line. You may need to go to 7" to get the same amount of ir flow from the longer runs.

You can use the bubble wrap, but it has a low insulation value.

 

[Master Brian]

It would take a 6x20 to be equal to a 8x14.

If you want to run round pipe instead of rectangular trunk line. You may need to go to 7" to get the same amount of ir flow from the longer runs.

You can use the bubble wrap, but it has a low insulation value.

Regarding the 6x20 or the 8x14, keep in mind....I'm wanting to split that into two runs. Each run handling two 6" round ducts, each to a separate room. Currently the 8x14 is handling four 6" round ducts.

The current setup is the furnace sets in the room right off the room I'm working on, so the trunk exits the furnace and within a foot it takes a 90* turn and goes about another foot then immediately enters the room I'm working on. The trunk runs about 15', after about 10' one 6" duct comes off and feeds the first room with a run of about 6'. After about 12' the next 6" duct comes off and feeds a 2nd room with a 6" round duct and a run of about 10-15'. At the very end of the 8x14 trunk two 6" round ducts come off and run about 6-8' each and feed my walk in pantry and around the corner in my kitchen.

Not sure if that changes much, but if so then please let me know. Being as the beam I'm trying to box out is made from 3 2x8's sandwiched together, using a 7" duct shouldn't be an issue.

Is it better to use the sleeve insulation that slides on around the rigid duct? I do have some flex duct, but my understanding is it doesn't flow as well. I've also thought about loosely blowing some cellulose insulation into the bulk head I create AFTER I wrap the pipes as I'll also have insulated water pipes in the same chase. Although being as it's in a "conditioned" space I also partly understand the insulation and sealing of the duct work isn't as large of an issue as it just helps heat/cool that space....hmmm!!!

Like I said I have a ton of that bubble insulation so even two times around wouldn't matter much to me, if that is better. I gave $50 for the two rolls that are 4' tall by about 2' diameter!

 

[beenthere]

Split into 2. It would be 2-6x10 ducts.

Ran as round, still would need 7". Else the other side of the house will heat and cool faster then the side you are redoing.

You can double wrap bubble wrap to get a higher R value.

 

[Master Brian]

In the opinion of the experts, which would be the best method 2) 6x10 trunks with two 6" feeds on each or going to individual 7" round duct for each of the four feeds?

Not sure the overall cost or implementation is much different.

 

[Bret86844]

If each duct is 100 CFM, and the basement vent is another 100 CFM, that's 500 CFM. Sizing the duct for 0.08" water column pressure loss per 100 feet, you need almost exactly an 8x14. These are all assumptions since I don't know the actual CFMs.

Let's delete the basement register and make it 400 CFM total. Two 6x10s would work, or two 4x16's. Also, two 8" round ducts would work.

If you want the basement register, you may want to tap off somewhere else, because if you try to make two ducts, one at 200 CFM and one at 300 CFM, your pressure balance would be off because you would be in-between duct sizes.

 

[user_12345a]

Do not compromise the system by using smaller equivalent ducts or removing registers. Also don't do anything that will increase the effective equivalent length of the trunks - certain fittings and elbows are equivalent to quite a bit of length, may need slightly lower friction rate to compensate.

The basement may be warm now with it closed, but if you seal the trunk line and it's semi-insulated by a bulkhead, drywall, you'll need the extra heat down there.

Basement is warm due to duct leakage and conductive heat loss.

Fix those problems and the basement will get a lot colder.

 

[Master Brian]

Do not compromise the system by using smaller equivalent ducts or removing registers. Also don't do anything that will increase the effective equivalent length of the trunks - certain fittings and elbows are equivalent to quite a bit of length, may need slightly lower friction rate to compensate.

The basement may be warm now with it closed, but if you seal the trunk line and it's semi-insulated by a bulkhead, drywall, you'll need the extra heat down there.

Basement is warm due to duct leakage and conductive heat loss.

Fix those problems and the basement will get a lot colder.

I'm sorry, but I'm a bit confused by this? Are you saying DON'T change the current trunk out for something else and split it into two? I do realize elbows, t's, etc reduce airflow. I wouldn't be putting any more 90's in place than are currently there now. I just want to isolate two ducts to one trunk and two ducts to another trunk.

I realize if I insulate the duct work I'll loose some residual heating/cooling to the basement, but I think at the same time, I'm somewhat combating that by better insulating the basement as a whole. When I started the basement was just block wall, but I've been finishing it off and now well over half of it has been very well insulated with rigid foam, covered by framed walls with at a minimum of R-13 batting in the framing. I've also been insulating duct work that will be untouched by any renovation to the basement. Over the years as I've been working on this, slow process, I've been monitoring the temps in the basement and what I've seen is where it once fluctuated A LOT, it now stays a pretty constant 65* down there. Also, IF I loose the residual "conditioned" air in the basement, wouldn't I be maximizing the efficiency of the HVAC system on the main living floor, thus making it somewhat ok to tap into the system to vent some of that air to the basement when/if needed?

Please correct me if my logic is incorrect. If I shouldn't insulate the ducts I don't have to, I just figured better putting that conditioned air into the room vs between the joists. Also, FWIW, I've been placing 6" thick batting between the floor joists to help stop noise from traveling between the upstairs and downstairs. I know there are better ways, but this is fairly cheap, easy and actually has been as effective as I need it to be. My daughters room is above an area I've done this and I used to hear every move she made up there and could talk in a normal voice if I needed something while in basement working, now I have to yell and she can barely hear me....unless that's just her growing up and ignoring her father!!:surprise:

 

[user_12345a]

Insulating/sealing the ducts is fine, just don't eliminate a vent feeding the basement at the same time.

You may need it.

Sealing is a good idea especially when you finish a basement - right now the air leakage isn't really wasted.

Splitting trunks is okay, just don't compromise the system - making the trunks smaller.

If u add elbows etc it may make sense to go with a slightly lower friction rate.

 

[Master Brian]

Yesterday I had found a site explaining how to calculate CFM's needed per room based on room size, air turnover, etc, but today I can't find it. WAIT....maybe I just found another one. It says to Calculate Volume of Room, then to multiply by required air changes and gives a table. For residential it says Bedrooms are 5-6, Basement 3-4, Kitchen 7-8, Family 6-8 and Bathrooms 6-7. This says then divide that number by 60minutes and you get the Required CFM.

I also looked again and two rooms actually have 7" round ducts feeding them, not 6" as I originally said. The kitchen and the pantry do have 6". So there are two 7" round ducts and two 6" round ducts feeding off the 8x14 trunk.

Here is what I know if it helps at all: (now based on the calculations I found)

1) Bedroom #1 (located on corner of house so two walls are exterior) is using 7" round duct and is 10x16x9, so it's 1440 cubic feet. Based on formula above that says I need 144cfm for this room. 30' from furnace to register with 3 90's, if you count flowing into side of register and where it exits the furnace. I believe 7" duct is 160cfm, so would that need stepped up or will running straight off furnace as 7" suffice? The 3 90's are off furnace, coming off trunk and going up to joist bay. It goes into the side of the 9x10 floor mounted register, no sure if that counts as a 90. This room has a return air grill.

2) Bedroom #2, center of house one wall is exterior) is using 7" round and is 10x11x9, so 990 cubic feet. Formula says I need 99cfm for this room. Duct is 19' from furnace to register, which is about a 12" square wall mounted register. (keep in mind old house) There are 3 90's off of this as well, again the furnace, off the trunk and then into the register box that feeds the wall register. It looks like that should be well under the 160cfm of a 7" duct, so can I stay with 7" on this one? Could I get away with stepping it down to 6"? This room tends to be one of the coolest in summer and hottest in winter. No return air.

3) Walk-In-Pantry just off kitchen with no door. 6" round duct is 7x6x8 or 336 cubic feet. Using formula for kitchen as it's connected. Says it needs 39.2cfm of air and the run length is 23' with about 3 90's. Obviously gets way more cfm's than required, but might help make up for kitchen!? as vent is right near door opening. Standard 4x10 floor register.

4) Kitchen 11x13x9 or 1287cubic feet. Calculation specifies 171.60cfm requirement. Obviously the 6" round duct supplying it is a little under sized but possibly offset by Pantry? The total run is 26' and has about 4 90's and a 4x10 floor register.

Being as the kitchen and pantry require 210cfm and they are being supplied with about 240cfm, do I stay with 6" duct or jump up to 7" for most of the run? I could downsize the pantry to 5" if that would be better and might be able to run 7" all the way to the kitchen.

Also, like was posted above I see that 14x8 trunk is 500cfm, that seems just about right for my current setup, which would appear to require 453.8cfm, if you don't adjust for friction loss, which I'm not certain how to figure or implement. I will also say at one point, it appears, there was a 2nd 7" line going to the kitchen but it was closed off. I don't notice the kitchen being hot or cold, so I'm guessing all is good there.

The basement area in question is 28x16x7 so 3136 cubic feet and thus should require about 209.07cfm. Looks like if I run an 8-9" duct off furnace I should be good for that room. If this is all correct, it'll help me check the rest of the house, especially since there is a trunk feeding the other side of the house.

Please let me know if I'm thinking too hard on this and over-complicating it. I'm also trying to figure out how the return air fits into all of this. Being as it's an old house there are things that have been changed over the years such as some registers were probably return air registers at one point and vice versa. Am I correct that I just want to match what comes out to what goes in?

Also if it matters at all the furnace is a Goodman GMP 100-3 installed in the early to mid 90's by my best guess as I didn't own the home at that time.

 

[Master Brian]

Insulating/sealing the ducts is fine, just don't eliminate a vent feeding the basement at the same time.

You may need it.

Sealing is a good idea especially when you finish a basement - right now the air leakage isn't really wasted.

Splitting trunks is okay, just don't compromise the system - making the trunks smaller.

If u add elbows etc it may make sense to go with a slightly lower friction rate.

I won't be adding any elbows per say....currently the two bedroom ducts are taken off the top of the trunk, so I'm guessing if I go with round pipe for each run adding a 90 in place won't hurt much. Otherwise all will be the same.

I'll add a register for the basement, but may do so directly off the furnace. I'm also trying to figure out the best way to get a return air down there as well. The easiest method for that would be just adding one in the room that is the laundry/craft room, which is also open to the furnace.

I understand there is friction loss, but how much does it come into play in my situation? Based on what I just posted above it would appear to me that even with some friction loss, I'm basically over sized the way it is...well, except for the kitchen(unless the pantry vent helps there).

I really appreciate all of this help. I tried to get an HVAC contractor(s) to quote me when they replaced an a compressor and again when my furnace was worked on, but I guess there isn't enough money to be made on running the duct or they felt it was well within my capabilities as none of them were interested.

 

[Bret86844]

The insulation you've added is one of the proper methods for sound-proofing. There are other things you can do but they get more expensive and harder to retrofit.

With sealing off the ducts, even if your temperature is good in the basement, it's still a good idea to have airflow so you don't have stagnant air and humidity problems.

I have not had a chance to look at the details of what you wrote, but the CFM's sound pretty reasonable. Based on the furnace model number, my best guess is it is putting out 1200 CFM total with a pressure of 0.5".

Best practice is have a return for every supply. Otherwise make sure doors are undercut so the air can escape to a central return.

Friction loss from just the ductwork and radiused elbows is probably very minimal. The bigger culprits would be supply and return grilles, duct plenums, and abrubt changes in direction.

 

[user_12345a]

Yesterday I had found a site explaining how to calculate CFM's needed per room based on room size, air turnover, etc, but today I can't find it. WAIT....maybe I just found another one. It says to Calculate Volume of Room, then to multiply by required air changes and gives a table. For residential it says Bedrooms are 5-6, Basement 3-4, Kitchen 7-8, Family 6-8 and Bathrooms 6-7. This says then divide that number by 60minutes and you get the Required CFM.

You're getting ahead of yourself.

You don't need to re-design the system, just make sure the new ducts are sized right.

If you count the registers and the sizes you can pull a friction rate out of thin air (0.8, 0.1) and make sure that the trunks are sized proportionally to the branch lines.

So if u know that the branches need 400 cfm at 0.8"/100 ft or whatever, you make sure the trunk can deliver that at the same or lower friction rate.

Good idea to have return in the basement for sure.

----------------------
The formulas, rules of thumb online are bogus and would only get you very poorly performing system.

CFM is based on the amount of heating and cooling the room needs rather than volume.

For ventilation there are other formulas which may include the volume but that's not a factor with resi equipment, only mostly commercial/industrial where the ventilation system is used to provide fresh air.

Just don't compromise the design; there are houses were I am where they cover vents right over with hardwood floor an drywall rather than re-locate. Lots of crappy work out there done by people with either not the best judgement/education or who just don't care.

 

[Master Brian]

You're getting ahead of yourself.

You don't need to re-design the system, just make sure the new ducts are sized right.

If you count the registers and the sizes you can pull a friction rate out of thin air (0.8, 0.1) and make sure that the trunks are sized proportionally to the branch lines.

So if u know that the branches need 400 cfm at 0.8"/100 ft or whatever, you make sure the trunk can deliver that at the same or lower friction rate.

Good idea to have return in the basement for sure.

----------------------
The formulas, rules of thumb online are bogus and would only get you very poorly performing system.

CFM is based on the amount of heating and cooling the room needs rather than volume.

For ventilation there are other formulas which may include the volume but that's not a factor with resi equipment, only mostly commercial/industrial where the ventilation system is used to provide fresh air.

Just don't compromise the design; there are houses were I am where they cover vents right over with hardwood floor an drywall rather than re-locate. Lots of crappy work out there done by people with either not the best judgement/education or who just don't care.

Thanks for the reply. I'm not trying to redesign the system, just trying to get this to make sense. I did run through those formulas to see and it actually seems like everything is right on. You say they are bogus, maybe they are....part of my concern is that being 100yrs old, I know lots has changed over the years and it is obvious that some registers/return air locations have been changed, etc. For the most part the house seems pretty balanced.

I'm still a little undecided how I plan to run the return air as well as add some venting in the basement. Individually each are easy enough, but combined it gets a little tricky because the joists run perpendicular to how I can easily run ducts.

I'm going to ask two other questions. Is it a bad idea to add a return air or even a supply duct right off the furnace, say a 1-3' distance or will that kind of cause an imbalance? Also, am I correct in thinking even if I make the basement into living space it doesn't need the supply nor the return it would need if it were above ground.

Thanks again!

I was digging around the extra parts in storage in my basement and it turns out I have about 4) 5' sections of 7" duct and a few 90's, so that'll keep some of my costs down!

 

[user_12345a]

Is it a bad idea to add a return air or even a supply duct right off the furnace, say a 1-3' distance or will that kind of cause an imbalance? Also, am I correct in thinking even if I make the basement into living space it doesn't need the supply nor the return it would need if it were above ground.

It's potentially dangerous to put return air right off of the plenum without sealed combustion appliances.

Puts the furnace room under negative pressure and may cause backdrafting of a water heater.

You can tap supply off a plenum, i'm sure it would have to be dampened down to not starve the rest of the house. It's like cutting a hole in a supply duct and leaving it.

 

[Master Brian]

Either would be in a different room from furnace, but maybe still to close based upon what you said.

 

[user_12345a]

I did run through those formulas to see and it actually seems like everything is right on. You say they are bogus, maybe they are....part of my concern is that being 100yrs old

They may have been a reasonable ballpark at a certain point for a typical room of a certain size, but really supply is always based on heat loss, not volume.

Heat loss is not proportional to volume or floor area -> double perimeter of square room and the area/volume quadruples. But you'll only need double the amount of heating.

It would also get you in trouble if you had some rooms with insulation and some without or a bedroom at the front with three out of four walls exposed vs one.

Better to not use rules of thumb unless it's to make a quick judgement call.

 

[beenthere]

Goodman GMP's are known for popping their crimp rings. So be fore you go any further. Check if it has popped them. If so, you need to replace the furnace. Best way to check if it did, is to pull the blower, and then look in where the blower was, and see if any rings are laying on the deck/plate.

If you increase the friction rate to any of those rooms, you increase the amount of air going to all other rooms.

Your house may need far more air to the rooms then that rule of thumb says.

 

[Master Brian]

I just had the blower replaced this last winter and all was good on the furnace.