# Central A/C ? **URL:** \ **Category:** Factual Questions\ **Created:** [May 7, 2003, 11:49am UTC](https://boards.straightdope.com/t/central-a-c/173636 "2003-05-07T11:49:41Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![KnK](https://avatars.discourse-cdn.com/v4/letter/k/edb3f5/32.png) [@KnK](https://boards.straightdope.com/u/KnK)\ **Post date:** [May 7, 2003, 11:49am UTC](https://boards.straightdope.com/t/central-a-c/173636/1 "2003-05-07T11:49:41Z")
I have a 2400 sq ft two story home that is one year old located in the midwest US. 2x6 exterior construction with R-19 insulation. The furnace is a Bryant 90% plus efficiency. I would like to add central air conditioning to my home. My questions: 😕 \*How do I calculate the “ton” and the BTU requirements for the system. 😕 \*How important is the SEER rating. 😕 \*How much should I expect to spend (USD) on the complete system to install it myself. How much more for professional installation. Thanks! KnK ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [May 7, 2003, 12:27pm UTC](https://boards.straightdope.com/t/central-a-c/173636/2 "2003-05-07T12:27:27Z")
I don’t know how to answer your first two questions, but I have installed two central AC systems – one in our previous house and one in our current house. I didn’t do it completely alone; I worked jointly with a friend who does HVAC on the side. I would _highly_ recommend this arrangement if you want to “do it yourself.” In both cases we divided the work tasks: Friend’s Tasks Determine tonnage rating of unit Procure all hardware (compressor unit, A-coil, tubing, etc.) Custom sheet metal fabrication (if necessary) Charge system w/ refrigerant My Tasks Install ducts (if necessary) Install registers (if necessary) All wiring Trench digging ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [May 7, 2003, 12:32pm UTC](https://boards.straightdope.com/t/central-a-c/173636/3 "2003-05-07T12:32:06Z")
My suggestion would get some estimates, they will calculate the specs you need for you for free. They know your area and know what you should be using. It’s a sucky think to do but it’s better then installing the incorrect one. ALso it is important to get the right one as too small and you will be too hot and too large and you won’t dehumidify properly. Make sure your heating ducts have the capacity to handle the extra volumn of air needed for central A/C. Mine couldn’t but I could quickly add a section which made a big difference. ---
**Author:** ![li\_l\_Dickie\_Dirtz](https://avatars.discourse-cdn.com/v4/letter/l/2bfe46/32.png) [@li\_l\_Dickie\_Dirtz](https://boards.straightdope.com/u/li_l_Dickie_Dirtz)\ **Post date:** [May 7, 2003, 12:38pm UTC](https://boards.straightdope.com/t/central-a-c/173636/4 "2003-05-07T12:38:10Z")
As an estimate, you can figure roughly 500 square feet per ton of A/C. So for 2400 sq. ft. of living space (I’m assuming), you’ll need 4.8 tons of A/C, or a 5 ton unit. Being a two-story, you may want to get two smaller units, one for each floor, using the same ratio to figure how big each unit should be (if one floor is much bigger than the other). If your house is well insulated (high-tech windows, etc.) you may be able to go to 600 sq. ft. per ton, but that’s not usual. A well qualified, honest installer can help you size the unit. ---
**Author:** ![Patty\_O\_Furniture](https://avatars.discourse-cdn.com/v4/letter/p/96bed5/32.png) [@Patty\_O\_Furniture](https://boards.straightdope.com/u/Patty_O_Furniture)\ **Post date:** [May 7, 2003, 12:54pm UTC](https://boards.straightdope.com/t/central-a-c/173636/5 "2003-05-07T12:54:32Z")
Here’s an HVAC message board that might have your answers. > **[HVAC-Talk: Heating, Air & Refrigeration Discussion](https://hvac-talk.com/vbb/)** > > The #1 HVACR Community As I read their forum rules, they don’t want to see “do-it-yourselfers” and no trolling for estimates or cost/value opinions. So you’ll need to rephrase or drop your your 3rd question if you want to post over there. Most contractors will visit you and give free estimates anyway. Get three estimates and pick the middle one. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [May 7, 2003, 1:00pm UTC](https://boards.straightdope.com/t/central-a-c/173636/6 "2003-05-07T13:00:43Z")
> [@](#): > > \*Originally posted by kanicbird \* > **too large and you won’t dehumidify properly.** Good advice. My dad had central AC put in his house a few years ago. The installer recommended a 2.5 ton unit. Given his penchant to over-design things, my dad told him to install a 3.5 ton unit. You know, “more is better,” right? Wrong. The unit is too powerful. It kicks on, runs for 2 minutes, then kicks off. While it _does_ cool the house, his decision to install a more powerful unit has had some rather undesirable side effects: 1. Inefficiency. The compressor never really has time to warm up. And the frequent cycling is hard on the compressor. 2. Dampness. Most residential HVAC systems are _temperature_ controlled; there’s no RH measurement in the feedback loop. With too powerful a unit the system will kick-off when the lower temperature limit is reached (as designed) but the air will remain moist. More time is required to remove moisture. The only way to get rid of moisture with an oversized unit is to set the temperature lower than you want. This not only makes it too cold, but increases your utility bill. ---
**Author:** ![KnK](https://avatars.discourse-cdn.com/v4/letter/k/edb3f5/32.png) [@KnK](https://boards.straightdope.com/u/KnK)\ **Post date:** [May 7, 2003, 1:03pm UTC](https://boards.straightdope.com/t/central-a-c/173636/7 "2003-05-07T13:03:59Z")
Great info Dopers! Thanks:) ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [May 7, 2003, 1:04pm UTC](https://boards.straightdope.com/t/central-a-c/173636/8 "2003-05-07T13:04:51Z")
> [@](#): > > \*Originally posted by Crafter\_Man \* > **The only way to get rid of moisture with an oversized unit is to set the temperature lower than you want.** Actually, this isn’t entirely true now that I think about it. You could conceivably install a heater to run in parallel with the too-powerful AC system. But who wants to do _that_? ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [May 7, 2003, 1:10pm UTC](https://boards.straightdope.com/t/central-a-c/173636/9 "2003-05-07T13:10:18Z")
> [@](#): > > \*Originally posted by Crafter\_Man \* > \*\* > > Friend’s Tasks > Determine tonnage rating of unit > Procure all hardware (compressor unit, A-coil, tubing, etc.) > Custom sheet metal fabrication (if necessary) > Charge system w/ refrigerant > \*\* One more thing to add: my friend connected the refrigerant lines. If you have someone help you, or even if a contractor does the whole job, make sure the refrigerant lines are _ **brazed** _. I have heard that some installers opt to _solder_ the lines; it’s faster, cheaper, easier, and doesn’t require fancy equipment. Do not allow anyone to do this! Insist all lines are _brazed_… ---
**Author:** ![li\_l\_Dickie\_Dirtz](https://avatars.discourse-cdn.com/v4/letter/l/2bfe46/32.png) [@li\_l\_Dickie\_Dirtz](https://boards.straightdope.com/u/li_l_Dickie_Dirtz)\ **Post date:** [May 7, 2003, 2:26pm UTC](https://boards.straightdope.com/t/central-a-c/173636/10 "2003-05-07T14:26:34Z")
I don’t think brazing is strictly required. Certainly you want to avoid soft-soldered joints, but sil-phos is acceptable IMO.