Just bought a house? Here's when to expect the big HVAC bill.

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Buying a home tends to be one of the biggest financial milestones most people will ever reach, especially in today's unusual housing market, where home prices remain high despite elevated mortgage rates. But once the closing documents are signed and the moving boxes are unpacked, a different phase of homeownership begins — one where unexpected home repair bills can compete with your mortgage payment for the top spot in your monthly budget.

And, heating and cooling systems are one of the most common sources of those surprise expenses. Unlike cosmetic upgrades that homeowners can delay until their finances allow it, an HVAC system doesn't give you much choice when it stops working. Whether it's an aging air conditioner that suddenly quits or a furnace that develops an expensive mechanical problem, these repairs tend to demand immediate attention — and immediate money.

The challenge is that many buyers don't know when those costs are most likely to arrive. A home inspection can provide valuable insight into a system's condition, but it can't predict exactly when a major component will fail. So when exactly should new homeowners expect to face a hefty HVAC bill? That's what we'll examine below.

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Most HVAC systems don't fail overnight. They gradually become less reliable and more expensive to maintain before replacement becomes the most practical option. Still, knowing what stage your system is in can help you estimate when a significant expense is approaching. Here's when it's more likely that you'll be faced with a large HVAC bill:

Buyers who purchase homes with systems that are already 12 to 15 years old or older may face a surprise HVAC bill shortly after they close on their homes — often within one to two years. While many furnaces and air conditioners can last 15 to 20 years with proper maintenance, age alone increases the likelihood of expensive repairs.

A home inspection may identify that the system is operational, but inspectors typically don't predict exactly how much useful life remains. That means a unit can work well enough to pass inspection but still require a major repair or replacement shortly after closing. So, if your inspection report noted an aging HVAC system, it may benefit you to start building a replacement fund immediately after purchasing your home rather than assuming you'll get several more years from the equipment.

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