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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?

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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.

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Obviously very general guesses regarding figures, but I think they make the important point, the payback period will be substantial. – auujay Sep 19 '11 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 Sep 19 '11 at 14:03
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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.

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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 Sep 19 '11 at 4:31
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@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 Sep 19 '11 at 13:35

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