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Gas Furnace for 950-1200 sqft home in Orange County

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qtchic86

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Discussion starter · #1 ·
Hi all,

We're looking to replace an old furnace with a new Bryant 315A two-stage 80%, and we're also replacing all of the ductwork (asbestos removal), but we're having some difficulty figuring out the correct sizing. The house is currently 950 sqft, but we're planning to expand to a 1200 sqft in the next 5-10 years, and really don't want to replace the furnace all over again.

I know there are a lot of technical aspects that usually go into sizing a furnace, but in this case we're kind of winging it. Our HVAC guy, who we've known for a few years and trust, is trying to get the best of both worlds for us. But I'm not sure if the furnace is being sized correctly. He says it will be 45000 BTU, but it's a 3-ton and goes up to 1800 sqft. Does this sound right?

Thanks for your help!
 
#2 ·
No, none of that sounds right at all. There is no "winging it" unless you want to hack things together.
Manual j, d, and s.
Size the furnace to the existing house. Add separate equipment for the future addition.
 
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#3 ·
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Discussion starter · #4 ·
Are you saying it's impossible to size up a bit, temporarily, to handle the square footage of our future addition? We were under the impression that a two-stage would run almost entirely in the lower stage for our current sized home, and when it is expanded, it would then switch between the two stages as needed. Does it not work that way?
 
#5 ·
Multistage runs based on setpoint vs space temp.
Without the addition being in place, what would you do with all of the extra air that would otherwise be going in that specific area? You need to move around 350-400 CFM/ton.
Do you have final approved plans of the addition? Plans that can be used to do an accurate manual j and manual d?
Temporary to me is a couple weeks. Not 5-10 years........that's half the life of the equipment. Maybe the whole life if the compressor dies from lack of airflow and overcycling.
 
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Discussion starter · #6 ·
Yeesh, ok sounds like we'll need to rethink this.

No, we haven't even started drafting the plans. It's just something we plan on doing in the near future. The house is pretty much 70's original, except for new windows, and we're adding R38 insulation in the attic. We figure when we expand, we'll improve more of the insulating factors (probably ending up with a similar, or only slightly higher, Manual J calc).

But, I have a little more clarification from our HVAC guy. Here's the unit he was recommending:
furnaces.homeowl.com/l/321/Bryant-315AAV-036070-upflow

The specs show 54000 BTU, two-stage variable speed fan, and it's supposedly the smallest unit we can get with these features. Our house is tiny at 950 sqft, but even with the addition, it will be around 1200 sqft, still within this unit's specs. He says there are no issues with too much power, because of the dual-stage and the variable speed fan. We're also installing an Ecobee smart thermostat to fine-tune and control the furnace's variable features and comfort level.

I should also mention that we have considerably moderate winters here, so a gentle fan heat is ideal - most of the time. It gets down to maybe 40-50 degrees at night, very rarely 30 on a cold winter, then we'll need a little more blast of heat. But in the summer, the extra fan power might be a lifesaver when it's 100+ degrees.

Does this plan sound any better, or is it still way off base?
 
#7 ·
How is sq ft relevant to a units "specs"?
A unit doesn't come from the factory rated for xxxx sq ft. The ONLY way to size is via manual j, manual d, and manual s. Anything else is guessing.......with your money.
We get at least a few people on here a month, complaining that their new high efficiency unit is too noisy. This is a result of undersized duct, and usually ends with the blower motor dying a very early death and the homeowner is stuck with a repair bill in the 4 digit range. That's 4 digits with a comma.
Ideally you should never "feel" the airflow. You don't want it to be 100* outside with air blasting out of the vents moving curtains and such. That's too much duct pressure, and not a comfortable feeling for the occupants once your core temperature reduces.
 
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#8 ·
I'll wing it.
You won't need 20,000 btu's with the addition.
 
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#9 ·
it's only a 200+/_ ft addition. my 1100 sft house has a 40000 requirement for heating. At the time, Lennox had a 40k two stage, but I got a 60k trane furnace. It does run 1st stage most of the time, but then, the 2 ton ht pump runs 1st stage for a/c about 90% of the time as well.
 
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