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Old 08-29-2011, 08:23 PM   #1
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Duct Work Too Small?

We have a new modular home (in Minnesota) with a 3 ton heat pump & 60K BTU Propane Furnace. The house is very well insulated, good double pane windows, storm & insulated exterior doors, R50 attic, 2x6 walls, etc. The furnace is a downflow, 95% eff but it is real noisey and runs forever in heat or cool mode. The home is 30 wide X48 long, with the furnace at one end of the home. In the basement there are two main air ducts from the furnace.. both are 5 x 12 in size. One of the main supply ducts routes the air to the rooms closest to the furnace. The other 5 x 12 duct supplys air to the other half of the house. There is only one return air... in the wall right next to the furnace. No return airs from the far end of the house.
I think the supply ducts are too small and there should be numerous return air ducts to provide some sort of air flow from the end of the house opposite of the furnace.
Per the design specs that we got with the home & with the furnace.. the furnace has a 1200 cfm capability but the duct work was designed around a 652 cfm design capability. Specs are below
External static pressure 0.60 in H2O 0.60 in H2O
Pressure losses 0.30 in H2O 0.30 in H2O
Available static pressure 0.30 in H2O 0.30 in H2O
Supply / return available pressure 0.26 / 0.04 in H2O 0.26 / 0.04 in H2O
Lowest friction rate 0.069 in/100ft 0.069 in/100ft
Actual air flow 652 cfm 652 cfm

Total effective length (TEL) 434 ft

I'd like some comments on the duct work size and the number & size of the returns... Thanks,


Last edited by Dakotamade; 08-30-2011 at 07:38 PM.
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Old 08-30-2011, 04:03 PM   #2
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Sounds like you are right - Ductwork is not up to par with furnace size.
As per Thermal enviromental comfort association(TECA) (canada) specs and without actually seeing ductwork I can only provide following info on duct sizing:

When trunk duct serves second floor runs or any runs with a maximum of 6 fittings (ie. Plenum Take-off - pipe take-off - floor boot or duct turns (90 degree) all equal 1 fitting.

CFM Trunk size
601-685 20x8 (as you said 652CFM - depending on how it's split)
189-225 8x8
226-290 10x8
291-370 12x8
371-455 14x8

Almost sure that your Return Air is under sized too as it should be equal in equal out. 1200 cfm (furnace output) for 1 return grill the duct should be 28x8 or 16" round

if it's a wall grill the maximum CFM per trunk depends on size of wall and size of return hole - using standard RA grille to account for K factor here is a basic guideline.
2x4 wall -- 14X6 grille = 170 CFM
2x6 wall -- 14x8 grille = 220 CFM
2x6 wall -- 24x8 grille = 390 CFM
2x4 wall -- 30x6 grille = 370 CFM

Hopefully this helps.
Tin2Win (Journeyman sheet metal worker)


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Old 09-02-2011, 12:19 PM   #3
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Is there a formula to calculate duct size/cfm?
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Old 09-02-2011, 12:34 PM   #4
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check this chart
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**Always kill the power ** Hot air rises, but heat will always move from higher to lower temperatures. ** Real man shoot in manual. ** If it ain't grounded, it ain't dead.
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