# 30-ton A/C unit
**URL:** https://boards.straightdope.com/t/30-ton-a-c-unit/320804
**Category:** Factual Questions
**Created:** [September 9, 2005, 12:50pm UTC](https://boards.straightdope.com/t/30-ton-a-c-unit/320804 "2005-09-09T12:50:21Z")
**Posts on this page:** 10
**Page:** 1
### Author:  [@SlickRoenick](https://boards.straightdope.com/u/SlickRoenick)
#### Post date: [September 9, 2005, 12:50pm UTC](https://boards.straightdope.com/t/30-ton-a-c-unit/320804/1 "2005-09-09T12:50:21Z")
When I see these multi-ton units, what does the number mean? Obviously the machine doesn’t weigh that much.
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### Author:  [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)
#### Post date: [September 9, 2005, 1:18pm UTC](https://boards.straightdope.com/t/30-ton-a-c-unit/320804/2 "2005-09-09T13:18:12Z")
> [@SlickRoenick](#):
>
> When I see these multi-ton units, what does the number mean? Obviously the machine doesn’t weigh that much.
That’s the amount of ice the machine could make from 32F water in 24 hours. It’s an old specification. A 30 ton unit is 360000 Btu/hour. A pretty big unit.
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### Author:  [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)
#### Post date: [September 9, 2005, 1:27pm UTC](https://boards.straightdope.com/t/30-ton-a-c-unit/320804/3 "2005-09-09T13:27:03Z")
> [@David Simmons](#):
>
> A 30 ton unit is 360000 Btu/hour.
OK, 'nother question. Why would you rate A/C at Btu? I thought Btu was a measure of _heat_ production.
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### Author:  [@RandomLetters](https://boards.straightdope.com/u/RandomLetters)
#### Post date: [September 9, 2005, 1:32pm UTC](https://boards.straightdope.com/t/30-ton-a-c-unit/320804/4 "2005-09-09T13:32:26Z")
> [@CookingWithGas](#):
>
> OK, 'nother question. Why would you rate A/C at Btu? I thought Btu was a measure of _heat_ production.
Btu is a measure of heat - for A/C units, BTU/hour is how much heat it can move in an hour.
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### Author:  [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)
#### Post date: [September 9, 2005, 1:38pm UTC](https://boards.straightdope.com/t/30-ton-a-c-unit/320804/5 "2005-09-09T13:38:35Z")
> [@RandomLetters](#):
>
> Btu is a measure of heat - for A/C units, BTU/hour is how much heat it can move in an hour.
Right. Think of Btu as a quantity of heat like a gallon is a quantity of water. 1 Btu/hour for an AC is analagous (in the strictest sense in this case) to 1 gal/hour for a pump.
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### Author:  [@UncleRojelio](https://boards.straightdope.com/u/UncleRojelio)
#### Post date: [September 9, 2005, 1:58pm UTC](https://boards.straightdope.com/t/30-ton-a-c-unit/320804/6 "2005-09-09T13:58:33Z")
> [@CookingWithGas](#):
>
> Why would you rate A/C at Btu? I thought Btu was a measure of _heat_ production.
An A/C unit does produce heat. The more heat it produces outside, the more cool air it can produce inside. You can’t get sumpthen fer nuttin’.
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### Author:  [@danceswithcats](https://boards.straightdope.com/u/danceswithcats)
#### Post date: [September 9, 2005, 2:26pm UTC](https://boards.straightdope.com/t/30-ton-a-c-unit/320804/7 "2005-09-09T14:26:32Z")
> [@David Simmons](#):
>
> That’s the amount of ice the machine could make from 32F water in 24 hours. It’s an old specification. A 30 ton unit is 360000 Btu/hour. A pretty big unit.
Never heard it expressed that way. I’ve always considered 1 ton of cooling to be the amount of cooling produced by the melting of 1 ton of ice in a 24 hour period. Wouldn’t ice production be variable based upon machine efficiency, ambient temperatures, and so forth?
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### Author:  [@zagloba](https://boards.straightdope.com/u/zagloba)
#### Post date: [September 9, 2005, 2:45pm UTC](https://boards.straightdope.com/t/30-ton-a-c-unit/320804/8 "2005-09-09T14:45:07Z")
> [@http://www.answers.com/topic/air-conditioning](#):
>
> Air conditioner equipment power in the U.S. is often described in terms of “tons”. A “ton” is defined as the cooling power of one ton US (2000 pounds or 907 kilograms) of ice melting in a 24-hour period. This is equal to 12,000 BTU per hour, or 3510 watts ([http://physics.nist.gov/Pubs/SP811/appenB9.html](http://physics.nist.gov/Pubs/SP811/appenB9.html)). Residential central refrigeration cycle air conditioners can be 1 to 5 tons (3 to 20 kW).
This curious comes about because the first air conditioning systems actually did cool the air by melting ice.
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### Author:  [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)
#### Post date: [September 9, 2005, 2:58pm UTC](https://boards.straightdope.com/t/30-ton-a-c-unit/320804/9 "2005-09-09T14:58:46Z")
> [@danceswithcats](#):
>
> Never heard it expressed that way. I’ve always considered 1 ton of cooling to be the amount of cooling produced by the melting of 1 ton of ice in a 24 hour period. Wouldn’t ice production be variable based upon machine efficiency, ambient temperatures, and so forth?
It takes the same amount of heat to melt ice as it does to make ice. The specification is of the output capacity of the machine. The input power required to produce this output depends upon the efficiency.
I’m pretty sure the specification arose because of the imagery. The nontechnical can visualize the making, or melting, of ice and thus the cooling effect while taking Btu’s out of the building and moving them to the outside is mostly ncomprehesible.
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### Author:  [@zagloba](https://boards.straightdope.com/u/zagloba)
#### Post date: [September 9, 2005, 4:17pm UTC](https://boards.straightdope.com/t/30-ton-a-c-unit/320804/10 "2005-09-09T16:17:47Z")
Oops, I see I garbled the last sentence of my previous post.
I believe the spec is not just due to imagery, but rather that in the first indoor air cooling systems, melting ice actually provided the cooling, much as the ice box preceded the refrigerator.