#### Chevsky

##### Member

**EDIT**: I just noticed there's a GM9C800

**804BN**-- which is a 1600 CFM version of the same 80 Kbtu, 80% afue, 17.5" wide cabinet!! So I guess my problem is solved! (I changed 1200->1600 below)

---original post---

Looking for advice on selection of a width for Furnace+Coil pair -- 17.5" or 24.5" --

Is there any problem of having too low CFM maximum furnace/blower capacity? I mean, aren't modern systems running lower speeds when t-stat delta-T is small?

**Edit**: Calculated ~1200 cfm for the combined rooms based on prescribed ACMs for each room type; or 1500cfm if I treat the open-concept kitchen+living+dining+foyer rooms all as 8-ACM (e.g. all open to the heat load of the kitchen)

No 21" option avail for the brand/series I'm looking at -- I prefer the aesthetics of the 17.5", because outside there will be two heat pump condensers (two systems), so they'll be matched physical size. 17.5" coil gets matched to a 2-3ton HP. If 24.5" then matched to 4-5ton HP which is tall/double-stacked. (I know, silly criteria). I'm just a little worried that The 17.5" furnace CFM may be too low.

This is Dual-Fuel -- heat pump with w/ propane furnace backup. Options are 17.5" coil / 2-3ton (hp set as 3ton), or 24.5" coil / 4-5ton (hp set as 4ton). Paired furnace is either 17.5"/

My ducts support ~1800 CFM, based on dimensions of three main trunks. They're fiber board, and transition to round flex before each register. The house is an open concept, and I'm installing two systems -- one dual-fuel for the main house (~2000sqft) and the other HP-only for 2nd floor, partially above the garage (700sqft).

My dilemma is just for the larger space.

The

*is 28K-34K BTU/hr (before scale factor, and with keeping the upstairs cold when not in use). This is based on propane use (samples during cold months, different years) calculated with local degree days, design temp of 5'F. By manual-J block method =42Kbtu/hr (maybe because of high 9' ceilings? But house is tight), and PTSC calc =32Kbtu/hr heating with 30'F balance-point.*

__total heat load of the whole house__HP+coil is MrCool / Universal. Only 17.5" and 24.5" sizes have matched AHRI certs, required for Mass Save rebate.

Again, if the smaller, then I'll have the same model outside for both spaces (but set at diff capacity), side by side, same physical size, looking nice, running one in 2-ton mode, and the other in 3-ton mode. This is 5-tons/60Kbtu total for the whole house (that's just on heat pumps -- the larger space gets 64Kbtu (80K * 80%) when propane kicks in). I know I need to consider each space separately, which I why I'm asking.

My thinking is, because the observed/measured heat load of 34Kbtu is

*for the whole house*, I probably can get away with only 36Kbtu for the main house, considering possible pro's & con's below...

Main house gets size: | 17.5" cabinet / 3-ton / max | 24.5" cabinet / 4-ton / max 2000 cfm |

Pro's | • Not oversized, no short-cycling, satisfies heat calculations (but cfm?). • Only being used down to 30'F, then furnace takes over (dual-fuel). (maybe try to push as low as possible to save $, but COP-cost balance-point is prob 30'F) • Matched HP model/size units outside side-by-side is aesthetically pleasing. | • Lot's of cfm and 48Kbtu headroom -- no struggle to move air, cool, or heat. |

Con's | • Maybe too small to support full 1800 cfm, runs constantly when cold outside. • Heat wafts up to the 2nd floor via open staircase. | • Larger than necessary size(?). • Large double-stacked-size HP outside -- looks funny sitting next to other shorter HP |

I'm anticipating everyone saying "Go with 24" @ 2000 CFM. Duh."

Anyone think 36Kbtu

Anyone think 2000 cfm is too big? ...or ok size?

Thanks for reading this far!

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