Heat exchanger. This is the heart of the furnace and it's impractical to repair it. Furnace replacement is often the only option if the heat exchanger is at the end of its useful life.
Efficiency. A new high-efficiency furnace can more than pay for itself over time. Moving from a furnace with an overall efficiency of 75 per cent to a new furnace that delivers 95 per cent efficiency saves you more than $20 per month for every $100 a month you spend on fuel.
Fuel type. If you've got natural gas available where you live but your furnace uses something else, then consider this when you switch to a new furnace. Natural gas is currently much less expensive than any other home heating fuel, and this is unlikely to change in the future.
Renovation plans. If you're thinking of finishing your basement in the near future, this is another good reason to consider replacing an older furnace. It's a lot easier to get the old furnace out and a new one in before your basement is finished, with less risk to newly finished floors and walls. New furnaces can also be substantially smaller than old ones. Finishing your basement after furnace replacement lets you make full use of any extra space freed up.
Make sure that the heating contractor you hire is properly licensed and provides a complete written contract for the work to be done. The Canadian Home Builder's Association offers free, unbiased information on how to hire a contractor the smart and safe way. Find more information at www.getitinwriting.ca.