I have a townhouse built in 1975 with an air conditioner on the roof. I have no idea how old it is but I can imagine it is on it's last legs. What's more, this last month's energy bill was one of our highest so I'm looking to see how much I'd save a month if it were replaced? I know ACs can be expensive but they have improved so much in their efficiency I figure at some point it'll be worth replacing the old one.

How long would I have to own the townhouse before I start to make money on a new AC?

Are there any other tips/tricks to maximize efficient usage of the old one in the meantime?

5 Answers 5


Let's see.. Maybe $50/mo on average house with high efficiency unit? So, let's say 6 month by $50 - $300 per year. Given $5000 unit - it will take unit's lifetime to pay for itself :)

I have friends in HVAC business and to be honest - if your unit works - I would keep on using it. My other house have 30+ year unit and with maintenance it still works good. Yes, maybe not as efficient but it works. And I don't have to spend $5000.

  • 1
    Obviously very general guesses regarding figures, but I think they make the important point, the payback period will be substantial.
    – auujay
    Commented Sep 19, 2011 at 4:59
  • @auujay You absolutely correct. It's just rough guesses. Last year we replaced windows. I see savings but with Marvin windows - they never going to pay off.
    – katit
    Commented Sep 19, 2011 at 14:03
  • You are right with the math, though I want to add that there are environmental reasons that would favour a replacement with a more energy efficient device. Unfortunately, the environment has no price tag.
    – daraos
    Commented May 30, 2014 at 12:44
  • And it's really hard to figure out what it takes to produce new unit and dispose of an old unit (environment-wise), so who knows. Some people replace every 10 years thinking it's good. Some people put solar batteries on roof..
    – katit
    Commented May 30, 2014 at 17:27

It'll depend on the SEER of the current model vs. the one you're changing to, how much you run your A/C, and the total cost of the installation.

When was the last time the current unit was inspected? It's possible that it might have lost pressure, which will affect its efficiency, but you typically notice that the system's not able to keep up with its cooling. Of course, depending on the coolant used, it might not be that easy to get a recharge, as the regulations on freon have changed over the years.

(note -- my knowledge of A/C recharging is based on dealing with cars, I've never done a whole house, as I have window rattlers, not a centralized system)

What I'd personally do is try to identify the model & SEER of the current unit, so you at least have an idea of how much of an improvement you'd get. I'd also contact your local utility company, as most either have a program to check for leaks & insulation problems, or can tell you what companies in your area can provide the service.

Sealing drafts is likely going to give you the quickest ROI as it's a DIY type thing, and it's going to be useful if you change out the unit or not.

  • There are other factors to keep in mind as well...future power costs, quality of insulation in the house, types/quality of windows etc.
    – DA01
    Commented Sep 19, 2011 at 4:31
  • 2
    @DA01 : well, there's two issues here ... differential cost, and improving the insulation in the house actually makes it longer to get back the value from changing the HVAC (as there is less waste, the total energy used is less, so the savings are educed) ... however, changing out the insulation & windows first sometimes means that you can downsize the HVAC, resulting in it costing less to install.
    – Joe
    Commented Sep 19, 2011 at 13:35

I do like your post. But your numbers and estimates are a little off.

I am in the HVAC business, and what's a little more realistic in the summer is when someone is complaining about high power bills. It's usually in the naiborhood of $350 - $600 a month. Now if A new unit can knock 30% - 50% out of that, that's a little more dramatic than the estimates of $10 or $20 a month. Grant it 50% doesn't happen as often, but it does happen occasionally. Because we now have the technology to speed up and slow down the capacity to better match usage to the actual demand in real time as it changes in addition to higher seer ratings. Therefore only using the energy needed at the time to satisfy the demand.

Unlike the old technology were it was just on and off with no speed adjustment, so it was always all or nothing. For example, That creates waste because you are paying for 3 tons of air when it kicks on even though you might only need half of that on an 80 degree day and then need the full 3 tons on that 97 degree day. They will also save money on repairs, because most new units now come with 10 year all parts warranty. And if done right should deliver much better comfort than that old worn out 15 or 20 year old unit as well.

What we usually see in a situation like that is most people don't realize how bad the unit is under performing until it gets replaced because it has degraded so gradual over time. The rule of thumb with good maintenance being done on a regular basis is a 10% loss of efficiency and performance every 10 years and worse if not regularly maintained twice a year. Combine that with advances in technology, and if done right you can usually create a situation where it will pay for itself much quicker while delivering all the other benefits like better comfort, less breakdowns, property value increases etc. In addition, a lot of houses are not sized correctly. Duct-work not designed and balanced for maximum efficiency etc. All of these things and more have to be considered because they effect the finial outcome of the situation.

Just food for thought. Little more to it than what meets the eye usually. Of course everyone's situations are different, and these are only estimated examples. But everyone has the ability to improve their situation if they get the right company that knows what they are doing and that cares about their clients. We have actually created some situations that put extra money in our clients pockets while the new heating and air system pays for itself. Example, system might save $150 a month off of the utility bill while we got them financed for no money down and only $75 a month therefore allowing them to put the extra money in their pocket every month. Of course this doesn't happen every single time. But similar results can and do.

My two cents worth.


Assuming that your new AC is not impossibly more efficient than your current unit, it can't really "pay for itself."

Say your brand new unit saves you $10 a month. If you're already spending $50/month that's a 20% decrease. A 20% increase in efficiency is probably on the high side if not unrealistic. If your new unit costs $5000 then it will take you 5000/(10x12) = 42 years to "pay for itself."

Let's say that a good AC unit lasts 10 years. You will need to save $500 a year or $60/month for your unit to pay for itself.

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    Where do you get the $10 and $50/month figures from?
    – Niall C.
    Commented Jan 7, 2014 at 17:28

in summary don't install a new AC unit to "save money". install a new AC unit to replace a unit that stopped working, and you WANT a cool home. That is, run a unit until it won't run anymore, just like cars, AC units are a net LOSS of money. But they are a GAIN in... comfort! When you do have to replace a failed unit, buy a cheap one, and fix it over and over again unit it stops working. Only way to "save" money is to not have AC, no thanks, I'll pay that price. of course as you can tell I don't give a damned about my fossil fuel footprint, just $ in my wallet and no sweat on my head. Of course politics will dictate whether or not it is economical to choose an efficient unit, though taxes and penalties. Remember "smart meters" are for that exact purpose, specific, personalized taxation. At that point, I might just say screw it, I'll sweat it out!

  • Wow! This thread is obviously ancient, but I hope no one reading this follows this advice. Commented May 30, 2014 at 10:17
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    A cheap inefficient unit will not only cost more to run, but will cost more in the long run as well. Buying something better quality and of higher efficiency only makes good sense in the long run. Spend a bit more now and save later. At the same time, to the OP from 2011, you'll NEVER "make" money. You WILL however eventually make your investment back with a higher efficiency unit. Commented May 30, 2014 at 10:20

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