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Repair or Replace a Furnace? Age, the 25% Rule and Efficiency
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The signs (Minnesota Commerce)
| Sign | What the guide says |
|---|---|
| Age | Most furnaces have a useful life of 15 to 20 years, depending on conditions. As yours approaches the end, start planning. |
| Expensive repairs | Spending 25% or more of the cost of a new furnace on repairs may indicate it is time to replace. |
| Low efficiency | Furnaces at 80% AFUE or less cost real dollars in fuel. Going to 95% may easily pay for itself over the new furnace's life. |
Source: Minnesota Department of Commerce Home Energy Guide, Furnace Replacement.
The guide says that buying a new furnace in January after the old one died unexpectedly may not lead to the best choices, so start research early.
What to look for in a new furnace
- Efficiency. ENERGY STAR sets 95% AFUE or greater for gas furnaces in the South and 97% or greater in the North, and 87% for oil. Air leakage must be 2.0% or less.
- Sizing. Minnesota Commerce warns against a unit larger than the old one, because oversized furnaces cycle more frequently. It says to insist on a heat-loss calculation worksheet in the contract.
- Ductwork. The guide says to examine ducts so airflow is not restricted.
- Sealed combustion. High-efficiency furnaces draw combustion air through a plastic pipe and vent exhaust directly outside, which the guide says essentially eliminates the danger of backdrafting.
- Installation. ENERGY STAR says improper installation can reduce system efficiency by up to 30 percent.
The example (not your house)
For a sample 2,500 to 3,000 sq ft Twin Cities home and $0.77 per therm gas, the guide shows about $291 a year saved going from 80% to 95% AFUE. Your savings depend on your home and gas price. That is an example, not a promise.
Prices
We found no sourced figure for repair or installed replacement cost, so we give none. Get at least two written quotes that include the heat-loss calculation.
Want quotes from licensed pros?
Gas, burner, ignition and venting problems are for a licensed HVAC technician. Our guide covers which signs mean call now.
When to call an HVAC technicianFind a licensed HVAC pro near youCommon questions
How long does a furnace last?
Minnesota Commerce says most furnaces have a useful life of 15 to 20 years, depending on conditions.
What is the 25% rule?
Minnesota Commerce says spending 25% or more of the cost of a new furnace on repairs may be an indication it is time to replace rather than repair.
Is a bigger furnace better?
No. Minnesota Commerce says oversized furnaces cycle more frequently, cause extra wear and cost more to buy.