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CFM question.

IcarusIcarus Posts: 89Member
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 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.



  • ChrisJChrisJ Posts: 10,374Member
    edited December 2019
    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?
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • IcarusIcarus Posts: 89Member
    edited December 2019
    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.


  • EBEBRATT-EdEBEBRATT-Ed Posts: 6,307Member
    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??
  • IcarusIcarus Posts: 89Member
    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-EdEBEBRATT-Ed Posts: 6,307Member
    @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
  • motoguy128motoguy128 Posts: 48Member
    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_SloaneB_Sloane Posts: 36Member
    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 !
  • mattmia2mattmia2 Posts: 609Member
    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 BodgerJolly Bodger Posts: 203Member
    Put in a two head mini-split HP. This would provide zoning and modulate to meet the needs and provide the best efficiency.
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