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Old 11-02-2011, 03:27 PM   #16
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16 seer, 90k btu vs 15 seer 70k btu


Also if your current furnace is 80K but 80% efficient you have only 64K output. So the new 70k high efficiency unit would be more than you have now.

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Old 11-02-2011, 03:47 PM   #17
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16 seer, 90k btu vs 15 seer 70k btu


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Originally Posted by cypherx View Post
Ok these homes were built with 80,000 BTU furnaces, so the engineering firm that designed these homes determined that number, and must of determined it correct because there are literally thousands of these same floor plans.

Nope. They just made sure it was big nough to keep the house warm when its -20F outside. Reading hasn't been that cold ever.

This bi-level style was very popular in the late 70's because of the amount of living space obtained by a smaller foundation thanks to the upper and lower levels right on top of each other.

Usually put a 60,000 BTU input 95% in those houses.

Problem is, if 80,000 btu is the size... Goodman doesn't make an 80,000 btu (nor do some of the other guys). We have 70,000 btu or 90,000 btu. Would 70,000 be too little or 90,000 be too large? We need 80,000 btu, so what's a guy to do if that is not available.

You don't need 80,000 BTUs, your furnace is only 60,000 BTU output. Presuming its a standard 75% efficient oil furnace typical of the 70s.

Other data...
Both have a 1050 RPM Blower, but the 70,000 btu unit is a 3/4 HP motor and 10" x 10" circulator blower size, while the 90,000 btu unit is a 1 HP motor and a 11" x 10" circulator blower size. The speed is variable.

Both units: 95% AFUE
Temperature rise rate: 30 - 60 (F)

GMVC95 0704CX
Vent Diameter: 2"
No. of burners: 3
Disposable Filter (sq in): 384
Min Circuit Amps: 14.1
AC combination nets 15 SEER

Should use the communicating thermostat with this furnace.

GMVC95 0905DX
Vent Diameter: 3"
No. of burners: 4
Disposable Filter (sq in): 480
Min Circuit Amps: 14.4
Looks like approximately 200 CFM more than the other model
$200 more, and the AC combination nets 16 SEER

Too big of a furnace. Won't run in second stage except for coming out of set back, or if you have a power outage that last for a few hours. In first stage it won't be 95% efficient. It will be slightly less, and use more gas then the 70,000 would.

Also what if 90,000 btu IS too large (by 10k at least). Wouldn't it run in single stage and save money on the natural gas consumption? I had a Lennox dealer tell me that actually with a Dave Lennox Signature unit (98% AFUE). It was much more costly than this one though.

Nope, it will cost more to use.

Only other useful data I see is that it appears to me that I get a $500 tax credit for the 16 SEER and 95% AFUE AC/Heating combination, where as the 15 SEER drops the tax credit to $200. So it seems like it pays for itself, just do I want the money up front, or wait until tax time next year (if this change is really WORTH the $200 now).
A 60,000 BTU 95% efficiency or higher is all you really need. So a 70,000 BTU Goodman is ok, the 90,000 is too big, and will cost more to operate. plus, your duct work wasn't sized to move the higher air flow a 90,000 BTU 95% furnace needs to move. gas furnaces need to move more air then oil furnaces, as thir heat exchanges aren't made as thick, and will burn through if not kept cool enough.
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Old 11-02-2011, 04:09 PM   #18
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16 seer, 90k btu vs 15 seer 70k btu


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A 60,000 BTU 95% efficiency or higher is all you really need. So a 70,000 BTU Goodman is ok, the 90,000 is too big, and will cost more to operate. plus, your duct work wasn't sized to move the higher air flow a 90,000 BTU 95% furnace needs to move. gas furnaces need to move more air then oil furnaces, as thir heat exchanges aren't made as thick, and will burn through if not kept cool enough.
Thanks for your information.

That makes sense because when I look at the spec sheets and calculate 95% of the input btu's, I get the output btu's. I didn't think of that loss when regarding the current unit.

The communicating thermostat.... is that a proprietary protocol from Goodman? Curious as to what if you use something like a Honeywell IAC or 8000 and wire it dual stage heat, single stage cool. Isn't the Honeywell smart enough to engage the proper stage?

You could go wild and get that Nest (designed by ex-Apple employees - google it) but I want something tried and true (like Honeywell).
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Old 11-02-2011, 04:25 PM   #19
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16 seer, 90k btu vs 15 seer 70k btu


While an IAQ or 8000 series would work on it. The communicating will work better, and give you more control. Haven't taken the class/seminar on it yet. But the communicating will help to keep the furnace from going to second stage before it should, an d you won't have that clicking noise like the 8000 series does.
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Old 11-02-2011, 05:46 PM   #20
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16 seer, 90k btu vs 15 seer 70k btu


Ok beenthere...

I just got home and double checked the existing furnace. It's actually these numbers printed on the inside (bolded numbers stamped on the plate):

Burning Rate .70 GAL PER HR Oil No 2 C.S Draft .02 INCHES W.C.
Input BTU/hr 98000
Bonnet cap btu/hr 78400
Rise from 50 F to 70 F .50 IN WC External St
Electrical rating 8.65 Amps 115 volt 60 hertz
Maximum Fuse Size: 15 AMP Minimum circuit ampacity 9 AMP

Just wanted to throw that out there because I was a little off on the original size.

Last edited by cypherx; 11-02-2011 at 05:49 PM.
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Old 11-02-2011, 06:07 PM   #21
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16 seer, 90k btu vs 15 seer 70k btu


Ok, last year when we had that -6 weather, did it have to run non stop to keep your house warm?
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Old 11-02-2011, 06:30 PM   #22
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16 seer, 90k btu vs 15 seer 70k btu


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Ok, last year when we had that -6 weather, did it have to run non stop to keep your house warm?
Hmm, I wish I could remember.

I try to keep it at 67 and the wife always turns it up to like 72 and it's always a fight back and forth because Oil is so expensive.

Then when we are not home I have it set back to 62.

I honestly don't remember it running all the time. Let's look at my oil fill ups...
10/1/2010 - 115 gallons filled (first fill up since moving in the house)
11/30/2010 - 112.1 gallons filled (60 days since previous fill up)
1/6/2011 - 118.1 gallons filled (37 days since previous fill up)
2/2/2011 - 107.6 gallons filled (27 days since previous fill up)
3/3/2011 - 90.9 gallons filled (29 days since previous fill up)

Right now my tank is a hair above 1/2 way mark, which it usually takes about 115 ish gallons to fill from there.
I also have an oil water heater from 2003. Bock model 32E
32 gallons, recovery: 112 GPH.

New Low-E windows in the upstairs front of the house (Where the afternoon sun is). The Rear and lower level is still the anderson wood frame windows (though double pane). Still original sliding door downstairs and upstairs.

House built in 1978. 1650 SqFt.
1st floor (Family room (walk out sliding door under deck), 3/4 bath, Office, Laundry Room, Unconditioned garage) - It's usually much cooler. Propane fireplace in Family room because of that.
2nd floor (Living room/dining room with sliding door to deck, kitchen, 2 front bedrooms, master bedroom and full bath). - Thermostat in hall way.

Last edited by cypherx; 11-02-2011 at 06:35 PM.
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Old 11-02-2011, 07:13 PM   #23
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16 seer, 90k btu vs 15 seer 70k btu


Roughly 563 gallons for heat and hot water. I'd say a 60,000 BTU input 95% will easily cover your heating needs. So the Goodman 70,000 would also be ok, but the 90,000 way too big. it would need to move 1320 CFM to keep its temp rise at 60F.
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Old 11-02-2011, 07:37 PM   #24
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16 seer, 90k btu vs 15 seer 70k btu


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Roughly 563 gallons for heat and hot water. I'd say a 60,000 BTU input 95% will easily cover your heating needs. So the Goodman 70,000 would also be ok, but the 90,000 way too big. it would need to move 1320 CFM to keep its temp rise at 60F.
Thanks beenthere.

Maybe because it moves less air, the SEER rating goes from a 16 to a 15. But some online calculators only show that being about $20 different per cooling SEASON.

Yeah the tax credits are lower though but factoring in the slightly less up front cost, it's only really $100 difference.

I would like to keep my temperature at 70 (and stop fighting with the wife) rather than 67... so I think a more efficient heating portion trumps 1 SEER in the cooling season.
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Old 11-02-2011, 10:50 PM   #25
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16 seer, 90k btu vs 15 seer 70k btu


Might not even save 20 bucks a season on 1 SEER difference when your comparing a 15 to 16 SEER.

The right sized furnace will save you a lot m ore then that.
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Old 11-14-2011, 01:14 PM   #26
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16 seer, 90k btu vs 15 seer 70k btu


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Originally Posted by beenthere
Might not even save 20 bucks a season on 1 SEER difference when your comparing a 15 to 16 SEER.

The right sized furnace will save you a lot m ore then that.
Thanks for your input. Got the 70k btu unit installed and I am very comfortable. Much more so than the original furnace. So nice to have a quiet, energy efficient, clean burning natural gas system than the old smelly, noisy old oil burner!

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