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We currently have central heating. The unit is in the attic. The duct work is also in the attic. How would I go about adding central AC to my existing setup? How would I go about doing this my self?

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  • I do not think most places will allow do it yourself central AC setup. Some places might allow a DIY mini split system.
    – crip659
    Commented Jul 17, 2023 at 17:52
  • I did years ago when stuff was simpler. Today , likely it would be more practical to replace your old furnace with a new furnace/ AC unit. Commented Jul 17, 2023 at 18:51

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With a few exceptions, it's impossible to do it yourself because it involves work with refrigerants, and that requires training and certification and specialty equipment and a supply chain for disposal of refrigerants. At least, using bog-standard A/C equipment that is designed to work with forced-air furnaces.

And you may wonder why America uses forced-air furnaces with the godawfully fat Ductopus that takes so much space... instead of simple, easy-to-plumb hydronic or "radiator" systems as Europe uses widely. There is no reason, except for one: forced-air heat makes it easy to bolt on A/C.

However, this idea is dumb, and does not work well. The reason is balance (how much air goes to each room). In winter, heat needs are roughly even, since the sun is not a player during the long nights. But in summer, one side of the house is being absolutely mauled by solar gain (300 BTUs per square foot of sunlight), so you're being smoked out of some rooms but not others. So it's difficult to get the A/C even. You could wildly over-duct the south and west walls, but then in winter, they'd be overheated and other rooms would freeze.

It also matters a great deal where the thermostat is located. TLDR: Single-zone heat/cool sucks.

So you still want A/C, and you still want to DIY. There are two ways to side-step this - one practical and one theoretical.

Made-for-DIY mini-split heat pumps

There are a variety of "Mini-Split heat pumps" which are designed for DIY. They use one trick or another to solve the refrigerant-handling problem. One of them, MrCool, has "self-sealing" line-set couplers so the lines are pre-charged and you snap them together hopefully without any leakage nor creating a leak (because that would be BAD).

Other systems, such as Pioneer, have you build the line-set in the normal fashion (which means you need to learn that), then you need to use a gage set and vacuum pump (you need to buy that) to vacuum the line, then you pressurize the system with nitrogen (need to buy that) to test for leaks, and if it passes, you vacuum out the nitrogen and open up the main valves and the lines fill with the extra refrigerant they put in the compressor.

Both of these work fine, and Youtube is full of DIYer's showing how they were able to do it.

However, to the best of my knowledge, all of them are "split systems" where they want you to install a "lump on the wall" heat exchanger. They're quiet enough since they run at variable speed, no faster than necessary. But they don't "take advantage of" the ducts (which means they also don't have the balance problem). You also may need multiple heads to cover the whole house. (though it may suffice to put units on the hottest rooms, typically south/west side and top floor, and use the "fan mode" on the air furnace, assuming your return ducting is set up correctly. It's easy for ducting to be wrong.)

Note that heat pumps are good at heating too, which means you get to arbitrage the cost of gas vs the cost of electricity and choose the cheapest for your particular operating conditions. Spring and fall when mild heat is required, may be ideal for heat pumps).

And you an use the heat pumps (in heat mode) as "trim tabs" for the house heating, adding supplemental heat to rooms that aren't heated enough due to balancing issues.

Special unrestricted refrigerants

A few systems use special, unrestricted refrigerants. These do not require the certification, nor the supply chain for disposal of refrigerants (these can simply be PSSSST'd into the atmosphere). They still need the line-set fitting tools, gauge set, vacuum pump and skills.

You might find a "goes in the air handling stack" setup for this type of A/C unit, but honestly I can't think of a single product. They can exist.

Or DIY some of it

You can also go with a traditional A/C, and simply do some of the leg-work to make the work cheaper/faster for the professional. Generally there are three things which lend themselves to this.

First, there needs to be a "right-of-way" for an A/C line-set, between the furnace air handling stack and the future location of the condenser outside. I like shady locations for condensers, because they're more efficient on cooler air. And so you could do the carpentry work needed to establish this entire route and make it accessible, so this is easy work for the A/C guy.

Second, the A/C needs a dedicated electrical circuit to a disconnect near the A/C condenser. The disconnect does not need to be fused. It's probably best to assume 10 AWG wire. Remember NM cable cannot go outside, other than come through a wall directly into the back of a disconnect.

Third, there needs to be a general outdoor 120V receptacle within 25' cord distance of the air conditioning condenser. If you need one, place it where you'll find it useful for other uses like running an edger, Christmas lights, bouncy castle, level 1 EV charging, whatever. It does not need to be, and should not be, dedicated to the A/C.

With that finished, the A/C installer should have an easier time of it, and that should be reflected in lower bids for the work. Mind you much of that bid will be the physical equipment cost.

The difference between an "air conditioner" that only cools, vs. a heat pump that both cools and heats, is a $10 reversing valve. If I were king, these would be mandatory in all units. Shamefully, the North American A/C industry is very backwards-thinking, and charges a fortune for systems with this $10 valve added.

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You hire a professional to do it!

This is not a DIY project. It takes a lot of specialized knowledge. There is no gradual learning path for amateurs, and given that most people will only install new central air conditioners 1, 2, or 3 times in their lifetime, no opportunity to become good at it.

The way to add air conditioning to a home without professional help is to purchase window A/C units.

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    Or mini-splits. I DIY installed one myself for the first time a couple of months ago and it was pretty easy. Everything is pre-charged with refrigerant so as long as you follow the directions it turns out fine.
    – brhans
    Commented Jul 17, 2023 at 17:55
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    I shouldn't assume, but I'm assuming OP does not have the skills and experience to install a mini split. I would call that a very advanced DIY project. To contrast with what I wrote in my answer, yes you can learn all the skills you need .... but it's a long learning curve.
    – jay613
    Commented Jul 17, 2023 at 17:59

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