# Gas Furnace: How many btu's for 1500 sqf?



## beenthere (Oct 11, 2008)

Use  HVAC CALC to determine what size furnace and A/C you need.

Standard min size blower is 3 ton. Doesn't mean that you need a 3 ton A/C.


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## zootjeff (Jul 11, 2007)

lloydb said:


> Im thinking of replacing my mom's furnace. How many btu's do I need for a 1500 sqf house?
> I've heard 12,000 btu's per ton and it has a three ton, but 36,000 btu's doesn't sound right.


How cold do your winters get? How good is the insulation?

Without knowing the details a 50-70K BTU furnace should be about right.

A 36K BTU furnace would probably heat the house most of the year but it would run for long cycles and if you had a cold winter your house would likely get pretty cold as it wouldn't keep up.


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## Clutchcargo (Mar 31, 2007)

Does the current furnace keep up with the winters? If so just match up the output BTUs with the new unit.
You need the wall R values (including windows and doors) and air infiltration value to determine the output of a unit from scratch.
GL


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## Yoyizit (Jul 11, 2008)

This 
http://www.degreedays.net/
will give you the HDD for your area.

This 
http://www.homeenergy.org/consumerinfo/benchmarking-energy-usage.php
gives you a way to calculate your needs, but the tolerance on the answer is pretty wide.

Some say that 6 BTUs per SqFt per HDD is average but according to this link 52% of the homes use less than 10.0 BTUs/HDD/Sq.Ft. so they're saying ~10 is avg.

BTW, it should be BTUs/(HDD-sq.ft), not BTUs/HDD/Sq.Ft.; that is, as either HDD or sq. ft goes up, BTUs go up. 
The English sentence "BTUs per SqFt per HDD" could reduce to either formula but BTUs/(HDD-sq.ft) is correct. Mostly people get this wrong or write it in a way that is ambiguous.


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## beenthere (Oct 11, 2008)

Clutchcargo said:


> Does the current furnace keep up with the winters? If so just match up the output BTUs with the new unit.
> You need the wall R values (including windows and doors) and air infiltration value to determine the output of a unit from scratch.
> GL


Thats the one of the worst things you can do. Most furnaces are already oversized. So why stick in another oversized furnace?

Good chance his place only needs a 40,000 BTU. Or it could need a 50, or 60,000 BTU.

The idea is to get the right size. And have LONG run cycles when its near design temps.


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## reckless (Jul 25, 2010)

*Btu*

I live in NJ and 70,000 does good for me. At 80% efficiency usuable 56,000BTU


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## Marty S. (Oct 31, 2009)

lloydb said:


> Im thinking of replacing my mom's furnace. How many btu's do I need for a 1500 sqf house?
> I've heard 12,000 btu's per ton and it has a three ton, but 36,000 btu's doesn't sound right.


 That's for an AC. Takes more BTU's to heat a house to maintain 72 degrees when it's 20 below out then it does to cool a house to 74 when it's 95 out. 

No way to give a correct size without knowing location or how tight the house is. A modulating or multi stage furnace will give your mom the best comfort though.


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## Binkstir (Mar 6, 2009)

This is for Northeast Oklahoma. Most of the winter here is above freezing with some dips into the teens or single digits sometimes.


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## Clutchcargo (Mar 31, 2007)

beenthere said:


> Thats the one of the worst things you can do. Most furnaces are already oversized. So why stick in another oversized furnace?
> 
> Good chance his place only needs a 40,000 BTU. Or it could need a 50, or 60,000 BTU.
> 
> The idea is to get the right size. And have LONG run cycles when its near design temps.


I wouldn't say it's the worst thing you can do. Taking rules of thumb from the internet is the worst thing.
I would think my method is a better barometer for sizing the next unit than estimating the R-value of the wall and how drafty the windows are.
If it's doing it's job and doesn't run short cycles, why not put the same output unit in? You just have to keep an eye on the run time during the cold spells.

If you really want to nail down the size unit the textbook "Modern hydronic heating" goes into great detail.


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## zootjeff (Jul 11, 2007)

In any case and any sizing you should probably just weigh the factors that are important to you.

Are you considering a high efficiency furnace or an 80%? 

How annoyed will you be with a furnace that can't keep up on those one or two weeks, vs how much better you'll feel about having a lower loss furnace and saving that green energy?

A furnace that puts out less heat also puts out less waste heat, so that's why you want a long cycle and a smaller furnace to be the most efficient.

If you think you might have winter that dips into the negatives and you're worried about it to the point that it will keep you up at night because grandma might freeze to death... Then the physiological benefits of running an over sized furnace outweigh the costs.


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## beenthere (Oct 11, 2008)

Clutchcargo said:


> I wouldn't say it's the worst thing you can do. Taking rules of thumb from the internet is the worst thing.
> I would think my method is a better barometer for sizing the next unit than estimating the R-value of the wall and how drafty the windows are.
> If it's doing it's job and doesn't run short cycles, why not put the same output unit in? You just have to keep an eye on the run time during the cold spells.
> 
> If you really want to nail down the size unit the textbook "Modern hydronic heating" goes into great detail.


Doing a load calc, is much better.

Just putting in the same size. Leads to new furnaces being over hated. Since new furnaces need to move more air then the old one did. The heat exchangers aren't made as thick as they use to be. So the temp rise needs to be lower.

Lots of houses have 100,000 BTU furnaces in them. And the house only needs a 60,000 BTU.

Using the same efficiency furnace, but sized right. Saves a lot of money on the heating bill.


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## beenthere (Oct 11, 2008)

Clutchcargo said:


> I would think my method is a better barometer for sizing the next unit than estimating the R-value of the wall and how drafty the windows are.


Try doing a few load calcs. And you'll see how accurate they are.

And how oversized the furnaces you have been putting in are.


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