Steve Groulx wrote:
Hi,



We had a new furnace installed last year and we're planning to change
the ducts. We have a few questions we're hoping you can answer.


Sorry not that easy. You need to do a manual D. I suggest you also
should do a manual J first. These are mathematical calculations based on
your home and HVAC equipment.



Furnace Info:

50K in, 46K out.

From the manual, at a static pressure of .5", CFM is 1093. (Fan Speed
Med-Hi - Heat Only)

Currently set to Low Fan Speed due to undersized ducts.



Existing Layout:

Plenum is approx. 22" x 20"

Main Heating Duct is 5" x 11" x 16' (Height and Width severely
undersized)
7 runs off Main Heating Duct. 4" diameter. Each run is approx. 14 ft.

3 runs off Plenum. 4" diameter. Each run is approx. 14 ft.

3 runs into Cold Air Return. 8" diameter.



Questions:

What size should the Main Duct be? 18" x 8", 18" x 10", smaller,
larger?
Should the Main Duct reduce in size farther from the furnace to
maintain pressure? If yes, when should it reduce? Existing Main Duct
is 16' long.
What diameter should the runs be? 5" was suggested?



Thanks for your help.

Steve.

--
Joseph Meehan

Dia duit