Replacing Your Furnace in Ontario? Here's Why Your AC Should Go With It
Every fall I get the same question from homeowners: "My furnace is on its way out, but my AC is still running fine — can't I just replace the furnace?" Technically, sure. But in a lot of cases, replacing only half the system ends up costing more in the long run than doing both together. Here's how I walk customers through that decision.
Why This Question Comes Up So Often
Furnaces and air conditioners in most Ontario homes share the same air handler, ductwork, and often the same coil. When one half of that system fails, it's natural to want to replace just the broken part and leave the rest alone. The furnace salesperson in me gets it — nobody wants to spend more than they have to.
The Problem With Mixing Old and New
The catch is that a brand-new furnace paired with an air conditioner that's 10–15 years old rarely performs the way either unit was designed to. A few things go wrong in practice:
Mismatched efficiency ratings
New furnaces are built to work with a specific airflow and blower speed. An old AC coil sized for an older, less efficient furnace can choke that airflow, which means the new furnace never hits the efficiency numbers on its spec sheet.
Refrigerant compatibility
If your AC is old enough to run on R-22 refrigerant, it can't be serviced with modern parts going forward — R-22 was phased out in Canada, and once your existing refrigerant is gone, there's no legal way to top it up. That's a repair bill waiting to happen, independent of the furnace.
Warranty gaps
Most manufacturers only honour full-system warranties when the furnace and AC (or furnace and coil) are installed together as a matched set. Mix old and new, and you can end up with a shorter or void warranty on the new equipment.
The Cost Case for Doing Both at Once
This is the part that surprises people: replacing both at the same time is almost always cheaper than replacing them one at a time, a year or two apart. You pay one labour call instead of two, one service disruption instead of two, and you can bundle financing across the whole job instead of financing two separate smaller jobs. We're currently running 0% interest financing on featured furnaces and air conditioners, which makes bundling the two even easier to justify month to month.
When Replacing Just the Furnace Makes Sense
To be fair, there are cases where replacing only the furnace is the right call — mainly when your AC is newer (under 6–7 years old), already uses R-410A refrigerant, and is sized correctly for the furnace you're installing. A tech can confirm compatibility in a few minutes before you commit either way.
Get a Straight Answer Before You Decide
We've been installing and servicing furnaces and air conditioners across the GTA, Hamilton, London, the Tri-Cities, and Ottawa since 2012, with licensed, TSSA-certified technicians on every job and 5-star Google Reviews from homeowners who've been through this exact decision. Call 1-888-705-7368 and tell us your furnace and AC's age and model — we'll give you a straight answer on whether bundling makes sense for your specific setup, no pressure either way.
FAQs
Q: Is it against code to install a new furnace with an old air conditioner?
A: It's not against code, but it can void manufacturer warranties and lead to performance and reliability issues, especially if the refrigerant types or airflow requirements don't match.
Q: How much more does it cost to replace both at once versus just the furnace?
A: It varies by home, but the combined labour and bundled financing usually make the gap much smaller than people expect — often a better value than financing two separate jobs months apart.
Q: My AC is only 4 years old — do I still need to replace it with the furnace?
A: Usually not. If it's relatively new, uses current refrigerant, and is properly sized, a technician can typically confirm it'll pair fine with a new furnace.