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Discussion Starter · #1 ·
The furnace is located inside the garage, it's a two story house, the downstairs vents are coming up from the crawlspace and the upstairs vent comes down on the ceiling through the attic.

Here's what the furnace looks like, it sits on top of the evaporator coil
http://s33.postimg.org/drszw75cv/20160611_102111_resized.jpg

There are two 10" insulated galvanized pipe that runs through the garage ceiling and upstairs into the attic. One pipe goes into the furnace and the other goes down into the enclosure/stand that the furnace sits on.

The smaller 3" pipe is the vent that goes out the roof.

Based on the info given, do you know if I have an up or downflow furnace?

I'm also trying to figure out how air is traveling through my ductwork. The downstairs vent have very strong airflow while the upstairs air is weaker.
 

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There is also a good chance you don't need a furnace as big as that one as far as BTUs.
 

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Make sure a manual j is done for replacement, especially if your going to a condensing furnace. Your going to have a furnace that's way oversized, and will be problematic if you change it out for the same size.
 

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Discussion Starter · #6 ·
the one there is 115k btu is probably 70% AFUE,

I'm looking at Goodman GDH81005 100k btu 80afue

or pay double to get a 96% 80btu variable speed GCVC960804

problem is the VS unit only have a 1600 cfm blower while i need 2000 cfm blower. The 100k btu 96% is 2k cfm but I'll be oversized.

Based on previous quotes, difference between the two after installation is going to be $1000-$1500

We run our furnace between end of october to late feb/march. Not sure if it'll justify the higher upfront cost.
 

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Discussion Starter · #10 ·
It's based on what a few local contractors told me.

Told them I was going to get an 80btu 80afue unit (original thoughts) and he told me that my heater is going to be working real hard with that size. He recommended that I go at least 100k btu @ that AFUE based on his experience with my area, home size and age.
 

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Experience doesn't mean you know how to do it right, it just means you've been doing wrong for a long time.
"Working real hard" isn't a description or mathematical calculation. Doesn't sound like they did any sizing.
Bigger isn't better. I cannot stress that enough.
 

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It's based on what a few local contractors told me.

Told them I was going to get an 80btu 80afue unit (original thoughts) and he told me that my heater is going to be working real hard with that size. He recommended that I go at least 100k btu @ that AFUE based on his experience with my area, home size and age.
Total non-sense.

No such thing as working really hard, if it has enough heating capacity for the most extreme cold you experience, you're covered.

Have too much and the furnace won't run long enough to ever reach peak efficiency, will be noisy, may even cycle on high limit from not enough air flow.

It's not the continuous operation that hurts furnaces, it's the cycling on and off.

The manufacturers specially make 2-stage and modulating furnaces to maximize runtime for comfort and noise reduction.

I don't know how you arrived at 2000 cfm either; you only need that if the a/c is 5 tons. Hopefully it's not.

Also when you size furnaces, it's the output that matters.

A 60 to 70% afue unit could be 75 to 80% when actually running. The afue is the seasonal rating, not the operating BTU output.

Sizing furnace based on size of existing unit is flawed unless it was done right to begin with.
 

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I'm looking at Goodman GDH81005 100k btu 80afue

or pay double to get a 96% 80btu variable speed GCVC960804
Why jump to variable speed?

You can get a 96% single stage furnace.

Compare apples to apples.

If you have a real heating season - ie dropping down to freezing or lower for 4 weeks or more each winter, I would recommend not getting anything below 90%.

They're obsolete up here in canada. The only reason to get a mid efficiency furnace in a cold climate is if venting a condensing model would be impractical.
 

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It's based on what a few local contractors told me.

Told them I was going to get an 80btu 80afue unit (original thoughts) and he told me that my heater is going to be working real hard with that size. He recommended that I go at least 100k btu @ that AFUE based on his experience with my area, home size and age.
A properly sized furnace should run 27/7 on the coldest nights. Some contractors don't understand this. Others are afraid of getting complaints from customers that don't understand this.
 

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Unless you have an enormous old house, I suspect your previous furnace was grossly oversized. I see you live in the Bay Area which is a pretty mild climate. How big is your house? How old is it? How good is the insulation? Do you have AC?
 

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Discussion Starter · #19 ·
Unless you have an enormous old house, I suspect your previous furnace was grossly oversized. I see you live in the Bay Area which is a pretty mild climate. How big is your house? How old is it? How good is the insulation? Do you have AC?
the house was built in 1978, bad insulation, 2 stories 2300 sq ft and yes on the AC.
 

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I don't know what bad insulation means.

Do the walls have? are they 2x4 or 2x6?

Is there anything you can do within your budget to reduce heat loss?

A house of your size where I am would probably heat on a 60k 95% furnace with probably a colder climate, and you can get an 80k 2-stage for safe measure.

Now if you have ducts in a unheated crawl space, the heat loss may be significantly higher. In that case best thing to do is would be to fix up the crawl space.

Best thing u can do is get the load calc done.

If you have a heat loss 45 to 60k you don't go and put in a furnace with 80k output; that would be nutty.

The experience means nothing, nor do my sizing recommendations based on my own limited experience being in houses. Go and get a load calc done.
 
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