Minimum Return air duct size for forced hot air

[abyrne]

Hey all,

I have a pretty beginner question, I hope I have enough info. This is my situation.

I am buying a two family house, I am going to live on one floor. When we got our inspection we realized that the house has to gas forced air furnaces (one for each apt./floor). They are both four years old.

The problem is that they both had the air returns on the side of the units, which were in teh basement. So they were pulling in the air from the unfinished basement and pumping into the house. I was made a ware that this is not efficient and possibly illegal.

The seller had return air ducts installed this past sunday. The duct on the side of the unit is 16"x20". The new return air ducts are 8" round.

Is this OK? Are the ducts large enough? The apartments are not large at all. Maybe 800 sq.ft. each. Is this enough info?

Thanks guys.

[dash]

Post the model numbers of ALL equipment,but I can tell you I don't think anyone makes a furnace small enough for an 8" duct to be the only return duct.


In general the return is equal to or larger than the supply,in a properly designed ,safe ,system.

[casturbo]

BTU size of the furnaces are necessary to correctly figure the return and supply duct sizes. But for sake of discussion, I'd guess at least three 8" round ducts for EACH furnace return to be the minimum amount. This is assuming the furnaces are about the smallest in size there is.

[Toolpusher]

Unbleieveable what some will do,

How hard is it to do something right ?

If your hands are on it, then do it right the first time.
No wander some people are questioning what we say.

[abyrne]

What would you recommend I do? The guy already ran the duct. The seller is doing it to comply, but noone is sure with what.

Supposedly the town said it was OK to have the returns in the basement. Is that possible?

Is she breaking a law by not having big enough returns? Or is it just innefficient?

I spoke to the HVAC guy who did the work and he said that the 8" duct provided 350 cfm and that was enough for the unit.

[Jultzya]

Can we get some pictures? :D

[Jultzya]

Originally posted by abyrne
What would you recommend I do?

Post model # of said equipment... as you have been asked already.


Supposedly the town said it was OK to have the returns in the basement. Is that possible?

Again, what equipment? It's against code to pull return air from with in the equipment (utility) room.


I spoke to the HVAC guy who did the work and he said that the 8" duct provided 350 cfm and that was enough for the unit.

LOL, 350 CFM that's all? per unit?

What BTU are these units?

Sounds like the (so called) HVAC guy was on crack that day! :(

[abyrne]

I will try to get the model number. Hopefully I will be able to post that tomorrow.

[retired btc]

IAW IMC 918.2 minimum return duct size shall not be less then 2 square inches per 1000 btu output rating capacity of the furnace or not less then specified in the manufacture installation instructions

[millerman]

8" hard pipe round duct gives less than 250CFM maybe 230CFM with proper static :rolleyes:

I am sure that .5 ton system is just new that none of us have seen yet. Get a second opinion from another HVAC company, Ther are alot of parameters we won't know and they will be able to see, get that contingancy paperwork ready;)