Explain that when the unit goes bad, she can claim the additional depreciation which will lower the basis in the home. Then explain that the new unit installed at that point, will start the process all over. Use lots of numbers, percentages, number of years the average homeowner stays, etc.... Ask her several times if she understands, and if she says yes, ask her to explain it to you so you can be sure that she does. This conversation will last 10 minutes max...
A tried and proven method. Also, bill her for the time.