Reduced plenum design questions.

George*RLCC · 02-23-2012, 06:39 PM

Greetings good people,

I decided to convert my 2flat that’s currently running on steam heat to forced air. I love steam heat but boiler it’s on its last leg and its taking too much room in the basement. I have intentions of finishing basement and duplexing it with the first unit.
I'm planning on installing two 95% AFUE Goodman units. Each apartment it’s going to have its own unit
These are the units I’m going to use;

Model # GMH95 0704CXA
BTU 69000
2 Stage Upflow
http://www.goodmanmfg.com/5744/ResidentialProducts/GasFurnaces/tabid/1337/ResidentialProducts/GasFurnaces/GCH959/tabid/1343/Default.aspx

This is what I know;

I know that new system should be configured using Manual J and manual D. Problem is I don’t have an access to any of them. So I’m figuring this all out using common sense and previous experience I have working with HVAC contractors. I’m a general contractor.
I know that main trunk cannot exceed 20' feet and that in needs to be reduced afterwards. I also know that total length of the trunk cannot exceed 50’ and that there should be no takeoffs within 4’ from the reducer.
I know that main trunk needs to be connected 6” from the top of the plenum and that return needs to be equivalent to the supply.
I know how to size and test gas supply, combustion air intake, combustion clearance, exhaust, drain, dedicated circuit and all that but I DON’T know how to implement 50 % rule with reduced plenum.

So what do you guys think…is this going to work?

See attached drawing…

I appreciate all of your help, I really do.

Thank you.

George

qbert · 02-23-2012, 06:51 PM

Furnace size seems a bit large for two systems. The duct work should be sized to match blower drive and the runs should be sized to the room loss. without starting with the heatloss/heatgain calc ,it is all a guess. You can find calculators on line i am sure. Over sizing furnaces and under sizing ducts will cause headaches later

hvac5646 · 02-23-2012, 06:51 PM

Your in luck. There is a man j program for sale here for just $50.00

And man d is available of the net by just googling it.

George*RLCC · 02-23-2012, 09:37 PM

What are the CFM's for 6" insulated duct- 110-120?

REP · 02-23-2012, 09:46 PM

For flex duct its a lot more like 70 cfm s .
Thats if its installed properly.That is ;short a run as possible,stright as humanly possible and metal 90s or 45's for bends.No dips allowed.

**beenthere** · 02-24-2012, 05:38 AM

Yep, 70,000 sounds big for 1200 sq ft.
That build out ill be cold without any air blowing directly into it.
There is no 50 foot max limit on a trunk line.

bungalow_steve · 02-24-2012, 02:59 PM

Return is too small, should be slightly larger then the supply.

The plenum should be reduced probably after the sixth branch, I doubt the 300 square foot room will get much supply as is.

George*RLCC · 02-24-2012, 05:04 PM

Quote:

Originally Posted by beenthere

Yep, 70,000 sounds big for 1200 sq ft.
That build out ill be cold without any air blowing directly into it.
There is no 50 foot max limit on a trunk line.

Yeah i thought of that before beenthere and i think i'm just going to put in one of those hydronic baseboard heaters by the windows to compensate for the loss. I’m thinking 1000W should do it...
I've got original 8" maple crown around the perimeter of the both rooms and i hate to mess it up with soffits. If you go back to the attached drawing you’ll notice that all of the ductwork is routed thru the bedrooms and closets, thats mainly because there is no crown in those rooms.

In addition to the crown there is also a steel header in between sunroom (build out) and living room- i would have to drop my trunk bellow the header to get to the windows.

What would you recommend here..?

Thank you for your input.

George*RLCC · 02-24-2012, 05:24 PM

Quote:

Originally Posted by bungalow_steve

Return is too small, should be slightly larger then the supply.

The plenum should be reduced probably after the sixth branch, I doubt the 300 square foot room will get much supply as is.

Steve i changed my design and reduced it after the sixth branch.

However now first trunk its about 30 long ...and i believe it is not suppose to exceed 20 ' ?

http://www.rses.org/assets/serviceap...al/630-148.pdf

**beenthere** · 02-24-2012, 05:42 PM

Not much you can do with that room then, without sofitting.

Are you going to also have A/C?

The limitation on trunk length without a reducer. Is mostly due to low air flow caused at the supplies closest to the unit/blower.

One solution is to use dampers in all supplies and balance as needed. The other, is of course not to exceed 24 to 28 foot without a reducer.

George*RLCC · 02-24-2012, 11:13 PM

Quote:

Originally Posted by beenthere

Not much you can do with that room then, without sofitting.

Are you going to also have A/C?

The limitation on trunk length without a reducer. Is mostly due to low air flow caused at the supplies closest to the unit/blower.

One solution is to use dampers in all supplies and balance as needed. The other, is of course not to exceed 24 to 28 foot without a reducer.

Yes BT i was thinking of putting a 3 ton A/C unit.
Do you think 46000 95% AFUE would cut it here?

**beenthere** · 02-25-2012, 07:09 AM

Quote:

Originally Posted by George*RLCC

Yes BT i was thinking of putting a 3 ton A/C unit.
Do you think 46000 95% AFUE would cut it here?

That room won't cool real well without air directed toward the build out.

Might also want to consider increasing your supply trunk size. The evap coil adds resistance to air flow.

What are your summer temps like.

George*RLCC · 02-25-2012, 04:29 PM

Thank you BT i did not think of that...
right now , I’m using heart & cooley ductulator, i'm getting 717 FPM's for 24x8 rec. duct @ 40' and 1000cfm's...
for the remaining 200 cfm i'm just going to branch of 8" round from the plenum box.
How much resistance does the coil add ...percentage wise...???

George*RLCC · 02-25-2012, 04:36 PM

Quote:

Originally Posted by beenthere

That room won't cool real well without air directed toward the build out.

Might also want to consider increasing your supply trunk size. The evap coil adds resistance to air flow.

What are your summer temps like.

Summer temps here in Chicago ordinarily reach anywhere between 78°F and 92°F.

George*RLCC · 02-25-2012, 04:42 PM

What is the recommended return velocity .. 700FPM's?
Right now i'm getting 1303 fpms with 16x8 @ 35' ...I guess i need to bump it to 24x8 to match the supply right?