Coil/Furnace width, CFM 17.5" vs 24.5"

[Chevsky]

EDIT: I just noticed there's a GM9C800804BN -- 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" -- 1200 1600 CFM or 2000 CFM.

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"/1200cfm1600cfm or 24.5"/2000cfm, Goodman, both two-stage burner, 5-spd blowers, rated for 80Kbtu, 80%afue. The old year-2000 furnace it's replacing is 23"-wide/20"-deep, 1000Kbtu, and single-stage (and I feel it's oversized). Why Goodman? B'cuz it's two-stage. If you think "only using gas when heat-pump is not delivering, then no need for two stage", then I could go MrCool on furnace, rather than Goodman.

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 total heat load of the whole house 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.

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 12001600 cfm	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 12001600 cfm too small? ...or ok size?

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

Anyone think I should find a different brand/series matched-AHRI-certified pair at 21" & 1600 cfm?

Thanks for reading this far!

 

[Fitter30]

Supply is one out of two return is just important. Return air for 3 ton 240 sq. In. Then have to figure in the grill or grills stamped metal 80% at best of free air. When ever the coil doesn't match furnace it's a crap shoot. Neither coil will give u what ever the design load the hp or ac manufacturer is giving. The smaller A coil will have a smaller opening than the furnace causing hot shops in the heat exchanger. The larger in ac or hp mode the air flow won't be even pick your poison. Will either work will capacity suffer yes. Wood manufacturers don't care they won't spend big $$ to partner with a ac company because it would cost to much. Best system would be a air handler for wood and a furnace for hp or ac with a damper system like a check valve between the two ahu's.

 

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[Chevsky]

@fitter -- the choice is between 17.5" and 24.5" on BOTH coil and furnace. If I get a 17.5" coil, then I get a same-widrth 17.5" furnace. If I get the 24.5" coil, then I get the 24.5" furnace. The CFM rating is on the blower, and the ducts I measured for what CFM they can handle. The question is about whether a 17.5" coil AND 17.5" furnace is too little at 1200 (oops, 1600) CFM. and if a 24.5" coil + 24.5" furnace is too much @ 2000 CFM (I know it's not too much, I'm just saying that I would prefer the 17.5" setup for he reasons I stated, and I'd go with that size if the consensus is "ok")
Your response reads as if I was choosing between just two coil sizes, and not matching a new furnace to the same size as the new coil. That's not what I'm describing. Yes, returns are important too. I measured them, not a problem. What I'm asking about is:
1) Heat load of home being X = ~34K btu/hr
2) CFM of furnace being Y = 1200 (oops,1600 CFM!)
3) Duct size supporting Z = 1800 CFM
This is NOT about mismatching the coil and furnace widths -- it's about whether 1200 (oops, 1600) max CFM from the furnace's blower will be ok given my house size & ducts.

 

[Fitter30]

What is the complete model number of furnace that gmvc 80 comes in more that one hp blower?

 

[Chevsky]

GM9C800803BN is the 17.5"
GM9C800805DX is the 24.5"

BTW, Returns are (2) 12x6" stamped grates on wall, and (1) 24x7" floor grate -- the floor grate is thin/tall grid, so seems efficient for air flow.

 

[Fitter30]

The 803 BN with the 1/2 hp motor with .5" normal external static across.furnace and another .2" across coil would give u 1300 cfm. Should be ok with the 17.5 coil.

 

[Chevsky]

EDIT: I just noticed there's a GM9C800804BN -- which is a 1600 CFM version of the same 80 Kbtu, 80% afue, 17.5" wide cabinet!! So I guess my problem is solved!

---original comment---
Fitter, thank you for looking at these options for me. I appreciate your help.
Let me share one other thought...

I did a room-by room estimate for all the spaces on this furnace+coil, but since the house is "open concept"-- the kitchen, living room, and foyer are all three connected by wide passages or no wall at all. I first took them by recommended ACM (kitchen, living room, bedroom, bath, etc), but then did another calculation but used the kitchen 8-ACM for each of these rooms. The first calc gives me just under 1200cfm for the whole 1st floor, but the second--with 8-ACM--gives me 1500cfm. (The dining room, tho it has two registers, only one is connected--duct runs back to the trunk but then isn't tied in (lazy installer?) I had planned on connecting it, but maybe now I wont.)

Now, the 17.5" Goodman 803BN will do 1200-1300 cfm MAX, So do you still think it's ok? I mean, if the t-stat delta is low, say just 2 'F, then wouldn't the blower be coasting at a medium speed? It doesn't know how much cfm each room is getting--just whether the temp at the t-stat is close to the set point, and the temp outside. So if yes, it runs at a medium speed, then the overall home cfm would be too low.


(As far as cooking, tho I don't currently have a vent-to-outside, I am planning on installing a new vent routed outside, which would help lower the ACM for the open-concept areas (if I actually use it--which I should, since the range is propane)).

Anyway, I guess I have to decide if I want to live with a system that's sized to only just make it, or to go with a marginally oversized unit (oversized today isn't as bad as yester-year's single-stage furnace short-cycling, right?). I'd want to avoid winding up in a situation where the heat pump struggles to heat/cool because of having no margin of error on blower capacity. Not sure if I'm making a good case for going with the 4-5ton HP and 24.5" furnace & coil, or just falling into the oversize trap. It's really a shame that MrCool doesn't sell a 21"/1600cfm coil (in the Universal series AHRI-matched, for the rebate)--maybe they feel there's no need for that middle ground.

 

[Fitter30]

Wouldn't want to oversize more than 1/2 ton but it probably isn't necessary with two systems. Need more cooling when companies over turn on the second system. Furnace has a 9 speed ecm motor so it will give enough flexibility to adjust fan speed to a 20° temp drop across evap coil. Be sure to check with all the utility companies, state and fed for rebates or tax breaks.

 

[Chevsky]

Fitter, So are you saying that with the 2-3ton condenser setup as 3ton, I only need 400cfm x 3 = 1200cfm anyway? So no need to be able to do 1600cfm? In that line of thinking, I was worried that with 803BN I'm sizing the furnace's blower at 100% of it's capability -- no room to tweak up if I need to. With 804BN I can speed the blower below it's max, and have room to adjust.

The two furnaces are only $40 diff in price, and I don't see any specs different than blower size & hp (and amps/weight)

 

[Fitter30]

Fitter, So are you saying that with the 2-3ton condenser setup as 3ton, I only need 400cfm x 3 = 1200cfm anyway? So no need to be able to do 1600cfm? In that line of thinking, I was worried that with 803BN I'm sizing the furnace's blower at 100% of it's capability -- no room to tweak up if I need to. With 804BN I can speed the blower below it's max, and have room to adjust.

The two furnaces are only $40 diff in price, and I don't see any specs different than blower size & hp (and amps/weight)

Go with the next size up with a ecm motor theres just more speeds than just a psc motor 4 vrs 9