Replacing Your AC or Furnace? Fix Your Ducts First

Key Takeaways

1. Rated Efficiency vs. Delivered Efficiency

Every furnace and air conditioner carries an efficiency rating. That number describes how well the equipment turns fuel or electricity into heating or cooling at the unit. It doesn’t account for what happens after the air leaves the cabinet.

Delivered efficiency is what you actually feel in your rooms. If your ducts leak into the attic or crawl space, part of that heated or cooled air never reaches you. A heating efficiency upgrade on paper can deliver much less in practice when the ducts are leaky. Sealing the ducts raises delivered efficiency for whatever equipment you own, old or new.

2. Where New Equipment Capacity Goes

Here’s what leakage looks like in a real home. In an October 2026 test of a two-system home, the downstairs system was losing 561 CFM25, about 56% of its conditioned air. CFM25 is the leakage in cubic feet per minute when the ducts are held at 25 Pascals of pressure. The kitchen register delivered only 10 CFM. Upstairs lost 371 CFM25 (49%).

Swapping in new equipment on ducts like these would push more air into the same holes. The kitchen would likely stay uncomfortable. Across MaxSeal’s 38-system data, the average starting leakage was 400 CFM25, so this is a common situation in Bay Area homes.

3. Title 24 Testing When You Replace Equipment

In California, replacing a furnace, air conditioner or heat pump that connects to ducts generally triggers duct leakage testing under the Title 24 energy code. A third-party rater verifies the result. If the existing ducts can’t meet the leakage target, your contractor has to seal them or use one of the code’s alternatives.

That means the ducts get looked at either way. Testing before you sign a replacement contract lets you plan for sealing on your schedule instead of discovering a failed test at the end of the job. For the details, see Title 24 duct testing in California.

4. The Oversizing Trap

When some rooms never get comfortable, a common fix is to buy bigger equipment. If the real cause is leaky or restrictive ducts, a larger unit can make things worse. A U.S. Department of Energy fact sheet on equipment sizing notes that oversized equipment costs more up front, runs less efficiently and starts and stops more often, which adds wear. It recommends sizing from an ACCA Manual J load calculation rather than floor area or rules of thumb.

Fixing the ducts first gives the load calculation an accurate starting point. MaxSeal’s Manual J load calculation and full diagnostic starts at $495.

5. Decision Guide: Ducts, Equipment or Both

6. Cost Comparison

New equipment typically costs much more than testing and sealing, and sealing improves comfort with the equipment you already have. Spending a few hundred dollars to test first can tell you whether the money belongs in the ducts, the equipment or both. Multi-system discounts are available. See our price list.

7. How We Work Before a Replacement

Our Typical Pre-Replacement Duct Process

1. Free consultation. We talk about your comfort problems and your replacement plans.
2. Baseline test. We measure total duct leakage at 25 Pa and airflow at every register.
3. Optional full diagnostic. For sizing questions, we add static pressures and a room-by-room Manual J.
4. Written report. You get numbers you can share with your HVAC contractor.
5. Seal and verify. If sealing makes sense, we hand-repair large gaps, seal with Aeroseal and retest, usually in one day per system.

If the test shows your ducts are already tight, you’ve lost very little. You’ll know the equipment is the place to spend your money.

8. MaxSeal Works Alongside Your HVAC Contractor

MaxSeal does not sell, install or repair furnaces, air conditioners or heat pumps. We test, repair and seal ducts. That keeps our advice simple: we have no equipment to sell you.

Many homeowners bring us in before or after their HVAC contractor does the changeout. We can seal the ducts before the new system goes in, or after, in time for the Title 24 verification. HVAC contractors can also bring us in directly when an older duct system is too leaky to pass by hand sealing alone. Either way, your contractor handles the equipment and we handle the ducts. See our page for contractors.

9. Questions to Ask Before You Sign

A good HVAC contractor will welcome these questions. Clear answers help you compare bids on more than price, and they show which contractors look at the whole system.

• Has anyone measured my duct leakage and room-by-room airflow?
• Was the new equipment sized with a Manual J load calculation?
• Will the job include Title 24 duct testing, and who pays if the ducts fail?
• Are you reusing my existing ducts, and what condition are they in?
• What happens to rooms that are uncomfortable today?
• Will you measure static pressure and airflow after installation?