The ideal time to replace a furnace is typically when it's nearing the end of its expected lifespan, which is usually around 15-20 years for most furnaces. However, several factors can influence this decision. If your furnace requires frequent and costly repairs, it may be more cost-effective to replace it with a newer, more efficient model. Rising energy bills can also be a sign that your furnace is losing efficiency and needs an upgrade. Additionally, if your furnace no longer provides adequate heating or if it's producing unusual noises, inconsistent heat, or indoor air quality issues, these are indicators that it may be time for a replacement. Ultimately, consulting with an HVAC professional for an evaluation of your furnace's condition and efficiency can help determine the best time to replace it, taking into account both performance and cost considerations.