# Explain, please, 'Tons' in re HVAC. **URL:** \ **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:** 20\ **Page:** 1
**Author:** ![handsomeharry](https://avatars.discourse-cdn.com/v4/letter/h/e9c0ed/32.png) [@handsomeharry](https://boards.straightdope.com/u/handsomeharry)\ **Post date:** [August 30, 2014, 4:27am UTC](https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940/1 "2014-08-30T04:27:59Z")
My A/C is out, and I’m a hurtin’! At any rate, I called an A/C guy, and he said that, when I pay him money, he will replace my central heat and air with a 4-ton unit. So, the last time I had to buy any kind of CHA system was in the mid-80s, for a small 700 sq. ft. house, and, IIRC, on the bill was listed “40,000 BTU’s”. (I could be way off on that, but, I’m just putting that number there for filler.) There was no mention of tonnage. I told the AC guy to put it in, and he said he’ll be here next week. I was outside with him while he was inspecting it, and I marvelled at how small the heating/cooling apparatus proper was. I liked the price he quoted, but, I rather staggered when he told me I would be getting a 4-ton unit. I envisioned myself wearing a parka in the middle of summer just to avoid frostbite, since the old unit couldn’t have weighed even 1/2 ton, so, the new one would be 8 times heavier, ergo, at least 5 times cooler (yeah, right). Alright, I’m pretty certain that a 4 ton unit doesn’t mean how much the whole thing weighs. He would need some monster truck just to get it to my house, let alone install it. Therefore, my question is: What does a ‘4-ton unit’ mean, in relationship to home central heat and air? Thanks, hh ---
**Author:** ![PastTense](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pasttense/32/14550_2.png) [@PastTense](https://boards.straightdope.com/u/PastTense)\ **Post date:** [August 30, 2014, 4:44am UTC](https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940/2 "2014-08-30T04:44:04Z")
> **[Why Is Air Conditioner Capacity Measured in Tons? - Energy Vanguard](https://www.energyvanguard.com/blog/Why-Is-Air-Conditioner-Capacity-Measured-in-Tons)** > > A few years ago, a student of mine told a funny story in a home energy rater class. He was an HVAC contractor and said he was installing a new air conditioner for an elderly woman. As he was explaining… ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [August 30, 2014, 4:46am UTC](https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940/3 "2014-08-30T04:46:25Z")
A ton of ice melting over 24 hours provides a ton of refrigeration. This is equivalent to 12000 BTU/hr ---
**Author:** ![handsomeharry](https://avatars.discourse-cdn.com/v4/letter/h/e9c0ed/32.png) [@handsomeharry](https://boards.straightdope.com/u/handsomeharry)\ **Post date:** [August 30, 2014, 12:08pm UTC](https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940/4 "2014-08-30T12:08:02Z")
Thanks for the answers, guys! ---
**Author:** ![The\_Second\_Stone](https://avatars.discourse-cdn.com/v4/letter/t/bbe5ce/32.png) [@The\_Second\_Stone](https://boards.straightdope.com/u/The_Second_Stone)\ **Post date:** [August 30, 2014, 6:05pm UTC](https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940/5 "2014-08-30T18:05:36Z")
You want the right size unit for your premises. Too big will waste energy as well as too small. The right size hits a sweet spot for turning on and off. Get a good brand (Trane or Lennox were what I found when I did research) and have them check it at least once a year. ---
**Author:** ![RickJay](https://avatars.discourse-cdn.com/v4/letter/r/bb73d2/32.png) [@RickJay](https://boards.straightdope.com/u/RickJay)\ **Post date:** [August 31, 2014, 6:15am UTC](https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940/6 "2014-08-31T06:15:20Z")
A four ton AC is big. You should have a big house to merit four tons of AC. ---
**Author:** ![handsomeharry](https://avatars.discourse-cdn.com/v4/letter/h/e9c0ed/32.png) [@handsomeharry](https://boards.straightdope.com/u/handsomeharry)\ **Post date:** [August 31, 2014, 6:21am UTC](https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940/7 "2014-08-31T06:21:08Z")
My house is about 1700-2000 ft. How is 4-tons, size-wise? ---
**Author:** ![RickJay](https://avatars.discourse-cdn.com/v4/letter/r/bb73d2/32.png) [@RickJay](https://boards.straightdope.com/u/RickJay)\ **Post date:** [August 31, 2014, 6:35am UTC](https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940/8 "2014-08-31T06:35:31Z")
Assuming you’re in the South, which if I recall correctly you are, that is a bit on the large size. Depends on your home and how well is resists heat, of course (is it shaded?) ---
**Author:** ![don\_t\_ask](https://avatars.discourse-cdn.com/v4/letter/d/e68b1a/32.png) [@don\_t\_ask](https://boards.straightdope.com/u/don_t_ask)\ **Post date:** [August 31, 2014, 8:40am UTC](https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940/9 "2014-08-31T08:40:47Z")
Here is a company in Arizona [explaining sizing](http://www.aricoplumbing.com/airconditioning/air-conditioner-system-sizing.aspx) > [@](#): > > The Rule of Thumb > > Each home in Arizona has different insulation properties. But the general rule of thumb is to have 1 cfm per square foot of living space. So a 1600 square foot home would have 1600 CFM of cooling capacity, or a 4 ton unit. Said another way, each ton of air conditioning will keep cool 400 square feet of living space. Southern Arizona is very hot, so 400 cfm per ton is probably the smallest number you will find in the country. Numbers may go as high as 800 cfm per ton in cooler climates. Also very well insulated homes may also experience higher numbers. ---
**Author:** ![handsomeharry](https://avatars.discourse-cdn.com/v4/letter/h/e9c0ed/32.png) [@handsomeharry](https://boards.straightdope.com/u/handsomeharry)\ **Post date:** [August 31, 2014, 10:45am UTC](https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940/10 "2014-08-31T10:45:06Z")
> [@RickJay](#): > > Assuming you’re in the South, which if I recall correctly you are, that is a bit on the large size. Depends on your home and how well is resists heat, of course (is it shaded?) Oklahoma City, and, not shaded. ---
**Author:** ![rbroome](https://avatars.discourse-cdn.com/v4/letter/r/838e76/32.png) [@rbroome](https://boards.straightdope.com/u/rbroome)\ **Post date:** [August 31, 2014, 2:34pm UTC](https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940/11 "2014-08-31T14:34:54Z")
> [@The\_Second\_Stone](#): > > You want the right size unit for your premises. Too big will waste energy as well as too small. The right size hits a sweet spot for turning on and off. Get a good brand (Trane or Lennox were what I found when I did research) and have them check it at least once a year. Too large is worse than energy inefficient. An A/C does two critical things: cools and dehumidifies. 78deg and 100% humidity isn’t comfortable at all… If your A/C is too large, it won’t circulate enough air to remove the water. It is always a balancing act. Smaller is better-as long as it is just big enough for the hottest days. In general, it is better to be uncomfortably hot a few afternoons a year than uncomfortable for most of the year because the A/C isn’t drying the air. And while it depends on the house, I would think 4 tons for a 2000 sft house is too large. 3 tons sounds more like it. But it depends. Getting a good A/C guy who can make the right recommendation is the hard part. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [August 31, 2014, 3:20pm UTC](https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940/12 "2014-08-31T15:20:11Z")
I think a 4 ton unit might even be a bit small for 2000 square feet but it depends on a lot of other factors. Personally I would trust the professional on site rather than random opinions on a message board. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [August 31, 2014, 3:40pm UTC](https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940/13 "2014-08-31T15:40:45Z")
FWIW, our 2,400 sq. ft. house (in the blazing-hot Sonora desert) has a 5-ton unit. ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [August 31, 2014, 5:43pm UTC](https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940/14 "2014-08-31T17:43:46Z")
This thread is a great example of why the US needs to use metric. 🙂 ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [August 31, 2014, 5:48pm UTC](https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940/15 "2014-08-31T17:48:37Z")
> [@don\_t\_ask](#): > > Here is a company in Arizona [explaining sizing](http://www.aricoplumbing.com/airconditioning/air-conditioner-system-sizing.aspx) The link article explans that one “ton” of cooling capacity is equivalent to 400 cubic feet per minute (CFM) of air change. Unless this is somehow standardized, I don’t see that that necessarily follows. It might be standard in the particular machines they sell, but these seem to be so many other factors. Shouldn’t temperature change be mentioned in there somewhere? I can easily imagine moving 800 CFM of air, and cooling it half as many degrees. ---
**Author:** ![crowmanyclouds](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crowmanyclouds/32/19884_2.png) [@crowmanyclouds](https://boards.straightdope.com/u/crowmanyclouds)\ **Post date:** [August 31, 2014, 6:37pm UTC](https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940/16 "2014-08-31T18:37:06Z")
> [@Sunspace](#): > > This thread is a great example of why the US needs to use metric. 🙂 “Explain, please, ‘[Tonne](http://en.wikipedia.org/wiki/Tonne)’ in re HVAC.” :dubious: 😉 CMC fnord! ---
**Author:** ![Chefguy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chefguy/32/138_2.png) [@Chefguy](https://boards.straightdope.com/u/Chefguy)\ **Post date:** [August 31, 2014, 6:52pm UTC](https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940/17 "2014-08-31T18:52:47Z")
> [@Sunspace](#): > > This thread is a great example of why the US needs to use metric. 🙂 You mean like British Thermal Units? 😃 ---
**Author:** ![GusNSpot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gusnspot/32/436_2.png) [@GusNSpot](https://boards.straightdope.com/u/GusNSpot)\ **Post date:** [August 31, 2014, 11:07pm UTC](https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940/18 "2014-08-31T23:07:38Z")
Hummmmmm, I just might start a thread on the best way and the correct things to do to make the most efficient use of what ever A/C you already have on a daily basis. Not adding stuff, just how to leave doors, set thermostats, etc. … That would help manny peoples … ::::::::::::::::::::::::::::::::::::: flee :::::::::::::::::::::::::::::::::::::: ---
**Author:** ![Lightlystarched](https://avatars.discourse-cdn.com/v4/letter/l/eb8c5e/32.png) [@Lightlystarched](https://boards.straightdope.com/u/Lightlystarched)\ **Post date:** [September 1, 2014, 11:43pm UTC](https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940/19 "2014-09-01T23:43:44Z")
I have a 1300 square foot house with 8 foot ceilings. Built in 1952 with block construction. The A/C is a 3 ton. We do tend to keep the thermostat at 80 in the summer and with a ceiling fan in every room it is comfortable (as long as you are dressed for it - shorts and tees). I think the usual rule is 1 ton per 400 square feet. If you have higher ceilings or less insulated walls/roof you might want to get a bit more. I know some of the longer (more sprawling?) ranch houses around here have 2 a/c’s - one for the living areas and one for the bedrooms. Then you can adjust the temp in each area as you need it. Each A/C would then be a lower tonnage. ---
**Author:** ![usedtobe](https://avatars.discourse-cdn.com/v4/letter/u/ec9cab/32.png) [@usedtobe](https://boards.straightdope.com/u/usedtobe)\ **Post date:** [September 2, 2014, 5:41am UTC](https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940/20 "2014-09-02T05:41:45Z")
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. [Next page](https://boards.straightdope.com/t/explain-please-tons-in-re-hvac/696940.md?page=2)