CFM question.

Icarus

Admitted amateur here on this subject.

Condition, small, ~1100 sq ft house pretty well insulated. Currently heated with a 2 stage 40 k btu goodman 2 stage forced air furnace. Currently all heating duct terminations are equal sized off a main trunk in the basement 6” round with floor/base grills. With open floor plan, the heat is very even without undue hot or cold spots. When it is generally running it runs on low stage nearly all the time except when bringing up rooms from overnight reset. Low fan on heat is 731 cfm. High is 990. There are currently 9 warm air outlets and one return air, all located downstairs.

Plan, to add a goodman heat pump to provide some heat when conditions are right, but to also provide A/C. Planning on using 2 ton HP unit. Moderate cooling climate, coastal Maine. The goodman air handler blows 403 cfm on low cool, nearly 600 on high.

Problem, up stairs attic style bedroom, ~200 sq ft with sloped ceilings, quite well insulated but subject to heat gain from the roof. (The room keeps warm without any ducting simply from natural convection as long as the stair top door stays open).

I can easily run a ~6“ Duct to the upstairs through a downstairs closet. I could even run two, one on each end of the room. I would probably control this duct (or duct) with thermostatic controlled dampers so as not to cool or heat the room if not needed.

Question then...is on 6” duct likely to be enough to cool this room? Is two? My intuition is that one would be minimal, two probably enough.

Any advice appreciated.

Icarus

ChrisJ

There's variables here like what static pressure everything else is running but to give you an idea I'm cooling two of my second floor bedrooms each 200sqft with a single 9" drop each and those runs are only 5' long or so.

I have a third bedroom that's 120sqft and that has a single 8" run.

Every room has its own return also.

Generally speaking, I don't see a single 6" run cooling 400sqft. Maybe a 10".

Can you fit something bigger if it's rectangular?

Icarus

I can actually run virtually anything. The trunk in the basement is ~8x12 at the end I would have to tap to. The ideal add on duct would come off the end of the trunk, and end up in the floor in middle of the room, just under the window. A second duct would be near the opposite end of the room, but also in the middle. I would prefer to have RA in the room, but that is going to be difficult. Additionally, this is a room that is seldom used (especially in the winter) so choking off RA in an closed unheated room might be a problem. This house is a vacation rental fro most of the summer, longer term rental/owner occupied in the late fall, winter and spring.

Icarus

PLEASE NOTE THAT THE ORIGINAL POST SAYS 400SQ FT. THE ACTUAL AREA IS 200 SQ FT!

EBEBRATT-Ed

@Icarus
a two ton unit needs more like 800-900 cfm (400-450 cfm/ton is normal) 1 6" round will not do much especially on a long run to the second floor.

I would guess you need 200 cfm to the second floor which would be an 8" round or equal in square or rectangle. But on the second floor something larger would be needed due to the long run.

You should consider a return up their as well. Pulling the hot air out is just as important as pushing the cold air in.

Are you using a new air handler or putting a heat pump coil in the existing furnace??

Icarus

Using the existing air handler, Goodman furnace. The fan on low speed cool is 450 cfm, 600 on high. (I can configure it a lot higher, but I have it set now for heating comfort. Adding the extra duct might allow (require) a slightly higher fan speed. In reality, 2 ton is going to be over cooling. I need to check to see if there is a smaller hp/coil combination out there.

EBEBRATT-Ed

@Icarus you can get 1.5 ton which would be 600-700 cfm might be a better fit if the cooling is adequate. Suggest a heat loss/heat gain calculation to size equipment and duct

motoguy128

I’m a cool mild humid coastal climate, 700-750. 1.5 ton 525-600. Not sure why people on here want to run more than 400cfm per ton if it’s not a dry climate. You never get anY humidity removal especially when the oversized indoor coils these days.

Some brands like carrier and Rheem run 340/ton as nominal, but rates output is lower than nominal.

B_Sloane

Icarus said:

I can actually run virtually anything. The trunk in the basement is ~8x12 at the end I would have to tap to.

never come off the end of a Duct

Static Regain !

mattmia2

You need to get a return up there as well. if the air coming in has no place to go, it won't move.

Jolly Bodger

Put in a two head mini-split HP. This would provide zoning and modulate to meet the needs and provide the best efficiency.

mikeg2015

1.5 Tons will be plenty and longer run times will make it more balanced. A single 6” or wall stack to get to that room will be fine. Shouldn’t need a dedicated return if there’s descent door undercut or open stairway.

1.5 ton HP only need 600CFM, Carrier and Rheem only 525, especially matched with a 2 ton coil. Not sure why most pros on here wan more than 400cfm/ton. SOme MFG nominally only want 350 like carrier and Rheem.

ChrisJ

mikeg2015 said:

1.5 Tons will be plenty and longer run times will make it more balanced. A single 6” or wall stack to get to that room will be fine. Shouldn’t need a dedicated return if there’s descent door undercut or open stairway.

1.5 ton HP only need 600CFM, Carrier and Rheem only 525, especially matched with a 2 ton coil. Not sure why most pros on here wan more than 400cfm/ton. SOme MFG nominally only want 350 like carrier and Rheem.

You feel a 6" duct on a home HVAC system of any length is good for 600 CFM ?

I come up with 85 CFM for 6" hard pipe.

Since the room size changed to 200 sqft, if It was mine I'd want an 8" minimum if using hard pipe being it's on the second floor. A 10"x6" could work as well.

Hot air will tend to stay up there and not want to be "pushed down". Cold air will tend to sink and fall down the stairs on it's own.

If this is done without a dedicated return it will always be the room that's too hot and will likely end up with a window air conditioner.


One of the reasons my system at home works so well is the main return is on the second floor right above the stairs where all of the hot air naturally goes. It cannot be reversed, those systems always work terrible in the summer.

Jolly Bodger

@ChrisJ you understand! Most think nothing of return. With A/C, return is more important than supply.

Icarus

I’m still here reading all the thoughtful comments. My bugaboo is that local HVAC contractors want so much money to do a pretty simple install. Debating between the fundamental efficiencies of mini-splits vs the simple install of a coil and compressor in the existing air handler.

Having been in the trades my whole life, I under stand profit and overhead, but I also know how long it takes to do things. For example,I just did a two zone minisplit, fairly simple install, one unit on the outside wall upstairs, the other into an interior wall into a crawl. Including pulling the circuit, hanging the units, running the line sets and coves I had about 10 hours into it. I had a friend do the purge and evac and paid him well for his time. (This is not where the house in the OP is located...there i can’t find any one to do the evac only).

The quotes I am getting is ~$ for a coil and Goodman heat pump that I can buy for under $. As I said, I expect it to take only about one day for a journeyman to do the install, it would seem that ~$ is pretty steep for one days labor, including P/O.

The quotes for a 3 zone minisplit has been bid at ~$. Once again, an install that would take me alone perhaps 3 days. Hardware costs ~$.

Icarus