My home has central AC with a single condensing unit outside, in the shade, that struggles during the summer. By struggle, I mean that it runs constantly and fails to cool to the set temperature. I like to stay at 72 degrees if possible but my AC will cool the home only 10 or 12 degrees cooler than the outside temp.

It is a Carrier brand unit and I cannot see any BTU rating on the information stickers attached to it. My house is about 2500 sf (two-story with a full basement) and I have noticed most of the other similar houses in my neighborhood have two of the AC condensers that my house has.

My question is whether there is an advantage to just adding a second similar AC or should I take it out and just get a single powerful unit? I had a technician out about two years ago to figure out why the system would shut itself off. He checked the system over at that time and it had no issues other than the filters were too good and actually slowed the airflow down tripping the shutdown. I have another technician coming out in a few days to give recommendations but I thought I would ask the community first.

  • Is your AC unit in the shade? Most installers don't give a crap about that, but that adds a significant amount of cooling because it doesn't have to fight direct heat from the sun.
    – Nelson
    Jul 28, 2021 at 16:39
  • By your having a single unit do you mean you have a central a/c with a single evaporator coil/air handler inside and a single condensing unit outside? And your neighbors have two condensing units outside? Jul 28, 2021 at 16:43
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    What's is the BTU (ton) rating of your A/C? "small" is a useless descriptor because a high seer 2 ton unit is easily larger than a 4-5 ton low seer unit. Is the refrigerant properly charged? It sounds like you need professional and on-site HVAC attention and that is simply not something easily done over the Internet.
    – MonkeyZeus
    Jul 28, 2021 at 16:54
  • When you say "AC" could you edit and add some more words to that? Like "1-hose portable AC", "8000 BTU window A/C", "central A/C with outdoor unit on a pad", words like that. Jul 28, 2021 at 17:34
  • Is your's a one or two story house?
    – SteveSh
    Jul 28, 2021 at 18:38

3 Answers 3


I would consider a Heat Pump

Your existing unit is already a heat pump, it just has the $10 reversing valve deleted so that it can only run in A/C mode. Because American manufacturers like to do that, and American customers put up with it.

I mean, you know how the inside A/C gets cold and the outside unit gets hot. The A/C isn't making that heat... it's just moving that heat from inside your house. The reversing valve just flips that so it brings heat into the house.

And in above-freezing weather, it can do this at 200-500% efficiency. i.e. spending 1 unit of electricity moves 2-5 units worth of heat.

Heat pumps also tend to be the most efficient air conditioners too.


They don't have absolute ratings, because operating conditions vary. But all units are tested to a standard, so you can accurately compare units to each other.

For cooling, the rating is called SEER. Your old air conditioner is probably 5-8 SEER... the cheapest junkiest "air conditioner only" unit they'll sell today is 13 SEER (twice as efficient/half the electric bill). The better heat pumps are typically 20-24 SEER in cooling mode, so the electric bill almost halves again.

For heat pump heating, the rating is called COP. Again bigger is better.

Look to replace your old built-in A/C if it's obsolete

If it use R-22 refrigerant, the unit is "on borrowed time" as R-22 can no longer be obtained. Next time it needs servicing, you will get a "surprise" as you suddenly need to replace the whole system. Nothing can be salvaged except the concrete pad.

If it has a poor SEER number, then it's driving up your electric bill.

However, you can "limp along" the old A/C for awhile. But in that case I would install a second newer system alongside it both to increase capacity and have redundancy. Meaning if one system quits you still have the other, so you're not boiling or freezing.

The second system could be a heat pump.

It could also be a new family of heat pumps called "Mini-splits". These put a unit outside on a pad, and multiple individual heater/cooler heads in each room. They add noise and take wall space, but they can be cheaper.


If you have a window unit in a room, adding a 2nd similar window unit will double your cooling. But at 2,500 square feet, you are typically using a ducted HVAC system. Putting a larger main unit (compressor, air handler, etc.) will increase the cooling capacity, though even that has to be considered carefully as a system that is too large may provide sufficient cooling but cause other problems.

Putting in a second main unit is quite a bit more complicated - typically involving splitting the duct work to serve different zones of the building. Not impossible, but not nearly as simple as a replacement/capacity upgrade.

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    Thank you for this, it explains what I needed to know. It is further complicated because I am planning on finishing the basement. I could always add the larger main unit for the main floors and keep the smaller unit to route to the basement. Thanks again.
    – SDH
    Jul 28, 2021 at 16:24

Depends on location, or how much hot weather you have . For a location with a several very hot days per year , one or two window units for occasional supplemental capacity is likely the lowest cost. I have 2500 sf in hot, humid weather for 6+ months so we have a 5 ton ( large ) single unit.

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