air return: cylindrical duct vs joist space?

[2cool2hear]

Is it safe to replace building cavities with metal cylindrical duct for cleaner, tighter air control? Most of the ductwork runs through the garage (unconditioned area).

Somebody mentioned that modifying the ductwork may throw HVAC balance out of control and possibly fry the system.

Suggestions/thoughts?

 

[pseudorealityx]

You need to better explain what you have now, and what you're proposing.

 

[bgarrett]

I've used the joist space for return air on two houses for 40 years. no problem

 

[mygarageone]

You can use any joist space but not if there's wire or gas line running through it.
But if it's in the garage , you'll have to use fire rated panning .

 

[rkevins]

you must make sure that the duct work will carry the same amount of air, if the duct is not large enough you will starve the system of return air

 

[mygarageone]

Rule of thumb for return air 2 sq inches per btu of heat, so for a 80 ,000 btu . You would need 160 sq inches total of all return air runs combined.

 

[James-W]

Rule of thumb for return air 2 sq inches per btu of heat, so for a 80 ,000 btu . You would need 160 sq inches total of all return air runs combined.

If you need 2 square inches per BTU then you would 160,000 square inches.

 

[mygarageone]

If you need 2 square inches per BTU then you would 160,000 square inches.

2x80 sorry

 

[pseudorealityx]

That is a ridiculous rule of thumb. Any duct size should be based on the airflow, not a heating value. I can buy a 2 ton or a 5 ton with a 80k btu furnace. They certainly need different return ducts.

 

[mygarageone]

That is a ridiculous rule of thumb. Any duct size should be based on the airflow, not a heating value. I can buy a 2 ton or a 5 ton with a 80k btu furnace. They certainly need different return ducts.

It was never meant to be an exact measurement only a guideline , have you never heard the term .

 

[pseudorealityx]

It was never meant to be an exact measurement only a guideline , have you never heard the term .

Sure, but the guideline should 'sound' more like....

~80 sq inches per ton of cooling (nominally 400 cfm/ton)

THAT makes sense and is consistent.

 

[mygarageone]

I'm not in disagreement at all , but some of these new homers , it's becoming harder al the time , especially a log home. We have had to become very creative .