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dual supply runs for zones?

7K views 47 replies 4 participants last post by  beenthere 
#1 ·
OK, I need some expert advice on how this should be done...

I want to zone my basement from the rest of my house... basically two zones, upstairs (2 floors) and basement... I originally was thinking put circular zone dampers on each of the runs to a register... but that came out at 25 dampers *aaaaaaggh!* and ontop of that, I assume they would have to be "accessable" since they are hard wired 24v lines and we had plans to drywall our basement ceiling. This would of hid all them basically and made them inaccessable.

since 25 dampers would be $,$$$ just for the dampers... I was thinking maybe it is cheaper to have two rectangle dampers put on the supply...

right now we have one 20x8 supply running the length of the house, and everything branches off that... would it be better to have two supply's one larger one for the upstairs and one smaller one for the basement and have rectangle dampers on them in the furnance room where it will be unfinished basically forever? they would be accessable... and would be a LOT less money in the end...

what is your opinnions? Our house is 2,500 sq ft above ground and about 1,600 below (conditioned space) and right now all that is being fed by a single 20x8 supply with manual dampers on the branch lines (6" insulated R4.2 flex duct)
 
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#47 ·
Oh yeah, and I'd of never thought of that if it wasn't for this thread and your input... I just don't know what type of filter box to put in... I still have that horrible one that is there now and undersized return... oh well, summer is the time for that fix finally
 
#2 ·
Yes, you could do it that way also. I don't know how much cheaper it would be to build 2 new trunk lines and have them installed. Thats the way I have my zone system and it works well and the dampers are accesible all the time.

Either way you go don't forget to install a by-pass for the extra air and the needed sensors.

Sizing the ductwork properly is critical to make any zone system work right. Make sure you plan before you install.
 
#5 · (Edited)
Yes, you could do it that way also. I don't know how much cheaper it would be to build 2 new trunk lines and have them installed. Thats the way I have my zone system and it works well and the dampers are accesible all the time.

Either way you go don't forget to install a by-pass for the extra air and the needed sensors.

Sizing the ductwork properly is critical to make any zone system work right. Make sure you plan before you install.
You mention needed sensors, what other sensors are in a zoned system besides the thermostat per zone and the outside temp sensor and/or supply temp sensor? The system I am looking at is basically this one http://www.alpinehomeair.com/viewproduct.cfm?productID=453059062
 
#4 · (Edited)
Yep.

And you system will appreciate it also.

Don't for get. You will need a bypass for when only the basement is calling.
When you say bypass, what exactly do you mean?

Do you mean bypass that zones supply line back to the return line?
 
#7 · (Edited)
Is there any guides out there or website calculators that can help calculate demand on a furnance for heating spaces? I want to check to see if the furnance I have is too big or not for the space that is conditioned... they put in a 125,000 btu furnance and it seems like it's just not performing like it should...

anywhere the furnance would list how many CFM it puts out in air flow, or anything else I would need to figure this out? Would like to look at it myself before I call in the pro's if I would need them

I calculated it out to 1,875 cfm going by its a condensing ufrnance at 125,000 btw... but how can I check the duct is sized properly?
 
#9 ·
HVAC Calc its a load calc program.
Worth its 49 dollar fee.

If your supply or return duct is rectangular, and has only 240 sq in cross section less or less. Then its probably under sized.

Yes. Bypasses air from the supply plenum back into the return.

That is a good zoning kit you posted to.
 
#10 · (Edited)
HVAC Calc its a load calc program.
Worth its 49 dollar fee.

If your supply or return duct is rectangular, and has only 240 sq in cross section less or less. Then its probably under sized.

Yes. Bypasses air from the supply plenum back into the return.

That is a good zoning kit you posted to.

well the main supply duct that runs the house length is 24x9 (or 8) I think and the return is smaller in size.... so thats what 192 sq inch at the smallest size? (i need to measure it again, i cant remember if its 20 or 24 but its arond that size)

How is the bypass sized? Is it what ever is left over in CFM that has to be returned? say I have 1,875 CFM coming out and I am using 1,200 CFM in the 1st and 2nd floor... I need a bypass duct for that zone that can return the last 675 CFM back to the return?

I also have a humidifier on the supply with a bypass to the return that has a 6" diamater flex tube insulated... how would that factor into the calculations? its only open in the winter

this whole calculating CFM for a house intrigues me :)
 
#11 ·
The bypass must be able to bypass all the air the the smallest unit doesn't use when only it is calling.

So if your furnace/air conditioner needs to move 1875 CFM. And your smallest zone by itself only needs/moves 400CFM. Then the bypass must allow 1275CFM to flow through it.

Humidifier doesn't get counted. Since it should be closed in the cooling mode.
 
#12 ·
The bypass must be able to bypass all the air the the smallest unit doesn't use when only it is calling.

So if your furnace/air conditioner needs to move 1875 CFM. And your smallest zone by itself only needs/moves 400CFM. Then the bypass must allow 1275CFM to flow through it.

Humidifier doesn't get counted. Since it should be closed in the cooling mode.
Awsome, thanks! You guys are a big help!
 
#13 ·
Well with all this sizing talk, I starting wondering, is my filter box the right size?... right now this furance when they put it in used the old return boot... well that has a filter box for a 16x24x1 filter and right now it has a 3m pleated filter in it.. http://www.amazon.com/gp/product/B000HE5E7I/ref=oss_product this one to be exact... is this hurting my furnance blower or furnance in anyway or add to the list a new return boot and filter box?
 
#14 ·
Use a general purpose pleated air filter.
One that isn't made by 3M. They are TOO restrictive.

It si making your system run hotter in heat mode. How much? Can't tell from here. Also causing low air flow in cooling mode, which could cause liquid flood back to your compressor.
 
#15 ·
Use a general purpose pleated air filter.
One that isn't made by 3M. They are TOO restrictive.

It si making your system run hotter in heat mode. How much? Can't tell from here. Also causing low air flow in cooling mode, which could cause liquid flood back to your compressor.
Alrighty, thanks for all the info... was trying to figure out why my nat gas bill was outragiously high the past couple months (this is the first year in this house) and its all starting to make sense now... you've been a REALLY big help! :thumbup: any filters out there you would recommend?
 
#16 ·
I don't frequent box stores or air filters. So not sure what brands they carry.

You want a pleated filter. that the pleats are rounded like a roller coaster. Not one with sharp pleats like a saw blade.

General, flanders, etc make them
 
#17 ·
alrighty, last night took out the 16x24 3M filter and put in a fiberglass mesh one temporarly just to see if there is a difference in air flow, and boy is there ever... I do have a problem though... my furnance wants a filter box that is 19x24... right now its 16x24... and its not even a filter box... someone just cut a hole in the top of the return boot and put a flap on the bottom to make a holder and they called that a filter box... it is HORRIBLE you cant even pull the filter out because its so uneven and bent up... where can you buy real premade filter slots / racks so I can have a real one that has a removable cover plate? thanks
 
#19 · (Edited)
been playing with the HVAC calc tool and its nice, just not sure on the results though... it's claiming for my house that I need a 30x14 duct :huh: for each floor... I had to do something wrong... that's huge... the current duct for the entire house is 24x8 split into left and right sections so its really two ducts branching off the center point (the furnance)... I'd love to be able to run three 12x8's one for each floor... divided into two sides so really six ducts... plus the return... but either I'm doing something wrong or my house is just too oddly done...

entered everything from the floor to the insulation values and got some high numbers for CFM for some rooms....


also the trunk plenum, shouldn't branches be at least 18" from the end caps? the person who did this put branch lines all the way to the end and there is one littleary 1" from the end
 
#20 ·
18" helps prevent turbulence.

What size furnace and A/C does it say you need. And how many CFM for for each(total).

What FPM did you enter.
 
#21 ·
18" helps prevent turbulence.

What size furnace and A/C does it say you need. And how many CFM for for each(total).

What FPM did you enter.

I'm still trying to learn this program, where do you get it to tell you what size Heating / Cooling system you need? I seem to be asked how many CFM my current one provides
 
#22 ·
Right now it is calculating the floors with the following CFM

Second Floor - 699 CFM Total
Main Floor 903 CFM Total
Basement 566 CFM Total


this is from a 1875 CFM system...

and it is telling me for the second floor I need (at 900 for the air speed)

18x8 or 12x12

Main Floor (900 speed)

18x8 or 12x12

Basement (900 speed)
16x8 or 12x10

that is if it all came off a single branch... right now we have a T from the furnance for two sides of the house... So I was thinking more like this

Second Floor (900 air speed)
- Far half - 10x5 (279 cfm) near side 12x8 (557 cfm)
Main Floor
- Far half - 8x8 (381 cfm) near side 16x8 (708 cfm)
Basement
- Far half 8x8 (~200 cfm) near side 12x8 (412 cfm)

So I think I figured out some of the reasons why it was giving me such large sizes... had some numbers wrong... but for a basement with top down heating, what is a good air speed? also is 900 pretty much standard in 1st and 2nd floors?
 
#23 ·
800 to 1000 FPM is a good FPM for residential trunk lines.
Trunk line FPM isn't important for throw from the register/grille. The register/grille throw is depedant on how many CFM your moving through it.

After you enter all of your data for the walls floors, windows, doors, etc. You can get a load report.
That wil tell you the BTU of the equipment you need.

HVAC Calcs duct sizing module leaves a bit to be desired. And actually tends to undersize the duct, since it doesn't have an allowance for length.
 
#24 ·
well after entering al the room specs... it gives me a heat loss / gain report

Sensible Gain - 27,737
Latent Gain - 3,478
Total Heat Gain - 31,215
Total Heat Loss - 72,621

I dont know if these numbers are low but the last HVAC company put in a 125,000 BTUH furnance but we are looking to expand the house someday another 1,000 sq ft so it might be sized right?
 
#25 ·
Big furnace for that load.

To zone the basement. Most of the air the furnace needs to move would have to be bypassed. Or blown out a dump zone.

A 1,000 sq ft addition may add a total of another 40,000 BTUs to your load. But will have almost the same problem for zoning. Best to put in a unit just for it. If you build it.
 
#26 ·
Big furnace for that load.

To zone the basement. Most of the air the furnace needs to move would have to be bypassed. Or blown out a dump zone.

A 1,000 sq ft addition may add a total of another 40,000 BTUs to your load. But will have almost the same problem for zoning. Best to put in a unit just for it. If you build it.

Yeah when talking to the place that put the furnance in a two years ago (they are a big HVAC company here) they basically said "you need one that big for your house"... tried to explain I think its oversized and they just kept saying you need it trust us... then proceeded to say my calculations were way off... but thats what the program said so now I'm just left with how to handle it... right now that load is going down a 24x8 duct that gets smaller as it goes... it has two legs though left and right of the furnance... so I'd think it could handle the load? not sure... all I got from them was "its fine, you dont need to zone your house" well the basement doesnt need cooled in the summer and it does need heat in the winter... changing manual dampers yeah fine and all but gets to be a pain when you have three stories (if you include the basement as one) and you can zone the 2nd floor off to keep it from doing something up there that you dont need on the other floors... zoning makes the most sense to me...

starting to think I need to get someone to come in to do the new plenum, because this is starting to get to be a lot of work... not sure how to run three branches off my furnance as it is now... the main plenum comes off a big 24x24 plenum box ontop of the ac coil on the furnance... which isnt wide enough for 3 12x8 runs...
 
#28 ·
Post pics.


Once they told you that you need that size furnace. They told you they weren't the right company.
I'll get some pics this week...

yeah, I'd of asked them more questions, but I didn't put the furnance in, it was the previous home owners before they sold the house.... so they probably didn't care much about it...
 
#29 ·
Some pics of the current setup...

the center has a 24x8 run going off to the front and back of the furnance from the 24x24 ish box that is ontop of the AC coil box...

I tried to mark some of the plenum with C or S to show where it is a cold air return plenum or supply
 

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#30 ·
You have a 14X8 supply coming off the front and back. But only a 24X8 return drop. Hmmm, a bit undersized.

I'd increase that.

Move the 2 flex supplies from the right side of the plenum, and tap them into the supply trunk.
Then install a new trunk where those flex lines where. And use that for the basement. Then you only need 3 dampers.
 
#31 ·
You have a 14X8 supply coming off the front and back. But only a 24X8 return drop. Hmmm, a bit undersized.

I'd increase that.

Move the 2 flex supplies from the right side of the plenum, and tap them into the supply trunk.
Then install a new trunk where those flex lines where. And use that for the basement. Then you only need 3 dampers.
The supply coming off the front is 24x8 when I measured, it's also 24x8 going out the back.. the two flex ducts coming off the side I actually do want to move because they arn't where they should be when you look where they actually go. which one do you think is undersized, the return or the supply?

Also, where do you guys generally buy plenum? I found a couple places online that sell it prefabed.. but was just wondering where you get it at.. thanks!
 
#33 ·
I meant to type 24X8 but did a typo.

The return is undersized.

I get them from a sheet metal shop. Actually cheaper then online.

That's what I thought when I first saw the return boot on it.. because the boot is 24x16 and the furnance wants 24x16 or 18 I think
 
#34 ·
Its not uncommon to have a smaller drop then the furnace opening. The furnace opening is for about 1750CFM at a low velocity for an air filter. And to allow the air to have an easier path to the other side of the blower cabinet. So both sides of the blower can draw in air.

24X10 or 24X12 would be better.
unless you have a 5 ton A/C. Then the install instructions will want a return on both sides of your furnace.
 
#35 ·
Its not uncommon to have a smaller drop then the furnace opening. The furnace opening is for about 1750CFM at a low velocity for an air filter. And to allow the air to have an easier path to the other side of the blower cabinet. So both sides of the blower can draw in air.

24X10 or 24X12 would be better.
unless you have a 5 ton A/C. Then the install instructions will want a return on both sides of your furnace.

I have no clue what my AC is right now, it's been too cold and we are under 2ft of snow... so hard to look at it's info panel at the moment... thanks for pointing that out though
 
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