# Explain, please, 'Tons' in re HVAC.

**URL:** <https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940>\
**Category:** Factual Questions\
**Created:** [August 30, 2014, 4:27am UTC](https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940 "2014-08-30T04:27:59Z")\
**Posts on this page:** 4\
**Page:** 2

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**Author:** ![CurtC](https://avatars.discourse-cdn.com/v4/letter/c/ce73a5/32.png) [@CurtC](https://boards.straightdope.com/u/CurtC)\
**Post date:** [September 2, 2014, 3:29pm UTC](https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940/21 "2014-09-02T15:29:06Z")

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> [@RickJay](#):
>
> A four ton AC is big. You should have a big house to merit four tons of AC.

Maybe in Ontario.

My house has three units: two four-ton and one two-ton.

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**Author:** ![fiddlesticks](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fiddlesticks/32/18129_2.png) [@fiddlesticks](https://boards.straightdope.com/u/fiddlesticks)\
**Post date:** [September 2, 2014, 5:19pm UTC](https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940/22 "2014-09-02T17:19:16Z")

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> [@usedtobe](#):
>
> Done properly, there is a complex formula (forget the name) the HVAC guy is supposed to calculate - it takes in house size, # of stories, directional orientation (N, S, E, W), shading. Then we get into room size and does it have a window? How large? Which way is it pointing.
> 
> But they just look at the plate on the old machine and swap it for the same size.

Manual J Load Calculation - [Manual J® Residential Load Calculation - ACCA](http://www.acca.org/technical-manual/manual-j/)

This is a a major purchase… stay away from guys who cite rules of thumb or “experience” and find one to do the actual calculation work for the house.

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**Author:** ![carnivorousplant](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnivorousplant/32/3563_2.png) [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)\
**Post date:** [September 2, 2014, 5:58pm UTC](https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940/23 "2014-09-02T17:58:08Z")

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I took a course in solar heating in undergraduate school.  
We calculated the heat loss for each room of a house based on dimensions, insulation and openings. We used the heating degree days for the city in there somewhere. The amount of air for each room was calculated. Is that what manual J does?

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**Author:** ![Mr.Natural](https://avatars.discourse-cdn.com/v4/letter/m/a9adbd/32.png) [@Mr.Natural](https://boards.straightdope.com/u/Mr.Natural)\
**Post date:** [September 2, 2014, 8:53pm UTC](https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940/24 "2014-09-02T20:53:03Z")

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Back in the day we estimated 500-600 square feet of heated/cooled space (living space) per ton. These days most hvac contractors will calculate the requirements with what they call a Manual J, which is required for new homes, at least in Georgia, where I have built since 1982.

edit: I read only the first few posts . I see that a post a couple above mine already posted the answer.

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