My HVAC has an undersized air intake duct 14" round. Can/Should I GRM it by building a ~20" square duct from plywood?
Something that at least matches the inlet area of the furnace....
Do I need to maintain a ratio between the area of the output ducts to the inlet?
My fan speed is adjustable. (4? speeds fixed)
You don't generally see ducts made from plywood. Sheet metal is best. Many residential contractors build entire duct systems out of "ductboard" which is basically rigid insulation held together with duct tape. If you go with plywood, be sure it meets your local building code. Also be sure to observe minimum distance to combustibles in the area of the furnace.
Could you fab it out of light gauge galvanized sheet?
Duct sizing is a function of airflow (CFM). You should shoot for fairly low velocity in a return duct to keep noise and friction losses to a minimum. Say around 500 FPM. That would indicate one square foot of inside duct area for every 500 CFM delivered by the system.
The duct connection size of a furnace is not a great indicator of the CFM it can provide. Do you have any info of the model number? A 14" round is good for somewhere between 900cfm and 1000cfm.
Vintage Rheem 80+
RCLC07EC1GS
75k BTU
It shows some pressure ratings but no CFM
There is a 16X20 plenum on top of the unit that is fed by the 14" round duct
Rheem does not give data for vintage machines on their website.
Find a sheet metal guy for side work. Less hassles if you need to sell someday.
Why do you think it's undersized? Presumably Rheem did some engineering when they designed the thing and determined that the 14" opening was big enough for the furnace's airflow requirements.
If you do decide to do it, I'll add my voice to those saying don't use plywood. I don't know if that's a code issue, but it wouldn't surprise me. Besides, you can probably get bigger steel ducting at every Lowes Depot around.
An HVAC guy could check the return air w/ a manometer to see if it's ample.
It's been years since I did installs but our rule of thumb for checking return air was cracking the cold air door open 2-3", a little drawing in of the door was OK but if it slammed it shut the return air was short. That's assuming a clean filter too.
Pics of the unit and return would be nice.
I agree it may not be undersized. The system should pull in the air it needs from the return. It would be rare for a residential system to have a long enough to return duct for the size to affect actual unit operation. Typically just means the velocity will be higher (and maybe a bit noisier).
Sometimes, they will increase the duct size just before the filter in order to reduce the air speed through the filter. Or they will use the smaller duct in order to maintain a certain air speed through the filter and increasing the inlet duct might screw that up.
In other words - I agree with the question: Why do you think the duct is undersized?
While wood cavity plenum returns are not uncommon (my house uses one from the return grill to get to the return duct to the unit), I agree using plywood for actual duct would not be a good idea. Things like to grow on plywood when fed with a nice, constant - and sometimes humid - supply of air (ask me how I know...).
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