Quote:
Originally Posted by aerohead
The protocol is to leave the returns alone,and add adjustable and lockable dampers behind the diffusers on the supply side.Intake ducts can collapse with a restriction,not so for the pressure side.
Systems which are engineered will spec out the design flow volume which can be directly measured with traversing of the actual duct with a pitot-tube and manometer/magnehelic,or at the face of the diffusers with an average of velometer readings,based upon the manufacturers criteria.
You're just trying to choke down an overflow condition until the entire space is at a uniform temp.By having the dampers behind the diffusers,you won't be accelerating the flow beyond 1,200 feet/minute,where things begin to get noisy.
Once its adjusted you shouldn't ever have to touch it again.
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Winner chicken dinner.
Most $$ disappear with duct leaks (30% is norm). After that is ducting improperly-sized. Then, “tight turns”.
In short - with an AC system — the money is in the ducting both as to integrity, but mainly as to design where flow is emphasized. Any radius reduction or turns are a problem (which will have been “solved” as cheaply as construction permits allow).
It’s perilous to mess with it as Aerohead notes. Flow to each room needs balance with each other. Have a look in the GRAINGER catalog as to the types of tools sold to pros (will aid in understanding).
Flow measurement is the thing.
On the oldest houses with designed-in HVAC the far more efficient ceiling-central location was used for a circular diffuser. I far prefer these and would retrofit for them. The supply air flow is EVERYTHING (versus SEER ratings, etc).
Ducting for returns isn’t as critical. The basic measurements suffice so long as supply and return have a basic match in each room.
Were it possible (floorplan) a single chimney from the basement upwards with its own time-controlled fan would be worth looking into versus screwing with the HVAC system.
The guys using in-ground cooling loops might have insights.
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