Sizing Return and Supply

[skidoox800]

Hey all,

Well I had a guy come out and look at putting in a new system for a rehab house. Its 700 sq feet single floor like 34x20. He said he will be putting in a 1.5 ton Trane R-410A A/C 13 seer and a 40,000 BTU Furnace. I asked him the sizes on the ducts also for supply and return.

He said he is doing a down flow furnace, supply under the house and return in the ceiling.

He said he is doing a 25x10 return duct, with 5 8" return duct branches going to the 25x10 return duct.

1 in each bedroom, 2 in living room and one in dining room.


The supply he is making the coil drop down into a 20"x8" supply 5 feet long, then reducing to a 14"x8" that's 8 feet then dropping it to a 10"x8" that's 10 feet long.

6 supplies in the house, 6" round running to the Supply Vents.

1 supply in each bedroom (2 bedroom) 2 supplies in living room, 1 supply in kitchen, and 1 supply in bathroom.

It's a rectangle house. 4 windows facing south 2 facing east, 2 facing west, and 3 facing north. R-19 in exterior walls. New windows and R-19. Can someone tell me if the sizes for returns and supplies are close. And if the amount of returns and supplies are good. House in Northern Illinois.

Please help just want to make sure this sounds close.

[rob_riley]

sounds like he is giving you a big duct system. 25x10 is more like 5 ton but at least he is giving you multiple returns. system will have low pressure and velocity but will certainly be very quiet. I don't think you have enough throw at the supply outlet. Ductwork is too big for 1.5 ton, unless he plans on moving 3-3.5 tons of air (silly) And Since it is a downflow, (assuming a crawlspace) i woould probably be doing some low return as well.

[skidoox800]

Thank you! What do you recommend for the return size? Also supply size? I would like it to be quiet but want airflow. Doesn't there have to be a certain amount of airflow to the furnace? So what should the proper size be and how many returns should there be to make sure furnace gets enough air.

[rob_riley]

depends on so many factors. did your contractor do a manual J, Manual D? i doubt he did. i cant give you that info here b/c this is not a DIY website. you should talk to another contractor or 2. most guys give free estimates for installations> if you are a professional, you should apply for pro membership and will be able to get alot of help here.

[REP]

He could install 16x8 down to 10x8 on supply depending on where the 6" take offs are.
He could give you a 20x8 return and you would still have a quiet system.

[skidoox800]

ok cool. I have done some HVAC work in the past just never done a load calc before. He didn't do a load calc so thats why i'm asking. Thats all. Anyone else?

[rob_riley]

sounds like a DIY. You will not get someone here to layout your duct system......and if you doo..it will get flagged.............GOOD LUCK!!!
Call a professional.

[02powerstroke]

to me it looks like the return is sized for a 2.5 ton system and the supply is sized for a 1.5ton system... the sizing is inconsistent from supply to return

[rob_riley]

25x10 for a return for 500-700 cfm??? maybe if its like 300 ft long. way oversized

[skidoox800]

not a DIY sorry man... I wish i was doing this myself, save money but i don't know this side of it.

[skidoox800]

I thought the same thing a really big return. I've flipped a house before of the same size and the ducts were alot smaller. Thats why i'm wondering. Its a different place now doing the work.

[beenthere]

The return size is fine. As far as the drop size.

The 6-6" supplies is border line on being enough.

The 20X8 starting trunk is also border line if its duct board.

[skidoox800]

What is duct board? He didn't use that word... Also that is enough supplies or not enough?

[seatonheating]

What is duct board? He didn't use that word... Also that is enough supplies or not enough?


Shouldn't you ask him these questions?

[Integrity Aire]

There are 2 primary materials (simplified) used in residential system design ducts, duct board which is cut, assembled, sealed and installed and round flex duct that comes in many diameters that are used as branches off the main trunkline (often made of duct board).

[beenthere]

Duct board is a fiberglass duct. It is more restrictive to air flow then sheet metal. there for. It has to be sized differently.

The number of 6" supplies may not be enough. Can't say, since the length is unknown.

But. Think of it this way. Your bathroom, doesn't need nearly as much air as your bedrooms do. So your not going to be getting/putting 100 CFM into the bathroom.