Heating bills that won’t go down or rooms that never seem to warm up evenly are both signs your furnace is operating way below peak efficiency. While some conditions develop gradually over time (such as dirty filters or leaky ducts), others will appear suddenly before total failure occurs. Knowing these early warning signs can save you on energy costs and keep you from needing costly repairs.
Here’s what you need to know:
Warnings that Your Furnace Is Losing Efficiency
Furnaces are rated by AFUE, which stands for Annual Fuel Utilization Efficiency. The AFUE rating is supposed to represent the fraction of fuel converted to usable heat.
The average efficiency ratings you’ll find are:
- 95–98% AFUE – high-efficiency condensing furnaces
- 80–85% AFUE – mid-efficiency models
- 56–70% AFUE – older furnaces (installed before the 1990s)
Replace an older furnace with 70% AFUE efficiency with a new 95% AFUE furnace and you could see your fuel consumption drop by about 26%.
It’s important to note that the real-world AFUE can be much lower as a result of:
- Duct leakage
- Short cycling
- Improper installation
- Restricted airflow
Let’s take a closer look at some signs your furnace may be losing efficiency.
Sign #1: Rising Heating Bills
Are your utility bills slowly creeping higher every month?
If your heating bills increase by 10–30% over last year’s bill for the same month then your furnace could be losing efficiency.
Say your heating bill averages around $200/month throughout the winter. Between $50-$80 of that could be from wasted energy if your furnace has trouble heating your home.
Common conditions that lead to high heating bills:
- Dirty air filters
- Malfunctioning flame sensors
- Duct leakage
- Aging heat exchangers
The easiest way to notice a loss in furnace efficiency is by comparing your yearly utility bills.
Sign #2: Rooms That Won’t Warm Up
Warm air doesn’t reach some rooms or certain areas of rooms when your furnace runs.
Causes of uneven heating:
- Leaky ductwork (lose 20–30% of warm air)
- Closed vents or registers
- Restricted airflow
- Improper thermostat location
When airflow is restricted, uneven, or interrupted, the furnace runs longer to achieve the desired temperature on the thermostat.
Sign #3: Short Cycling Furnace
Short cycling means your furnace turns on and off several times without completing a heating cycle.
Reasons your furnace may start short cycling:
- Too large of a furnace for your home
- The air filter needs to be replaced
- Restricted airflow causing furnace overheating
- Faulty limit switch
Just like your car burning excess fuel when it idles, short cycling wastes energy every time it starts back up.
Sign #4: Strange Sounds & Odors
Strange noises coming from your furnace often point to issues that also impact efficiency.
Listen for these sounds:
- Banging noise = ignition occurring too late in the cycle
- Rattling noise = loose parts
- High-pitched squealing = blower motor problem
Smells coming from your furnace aren’t normal either.
Troublesome smells include:
- Yellow burner flame = improper combustion
- Visible soot on walls/grate = incomplete combustion
- Odor of rotten eggs or gas = carbon monoxide leak or gas leak Call an HVAC technician. ASAP!
Strange noises and odors aren’t only costing you money on wasted fuel, they can also be dangerous.
Sign #5: Multiple Repairs Each Season
Furnaces that need frequent repairs every year are often throwing away money.
Your furnace may require a few repairs over its lifetime, but if your technician is constantly at your house taking care of issues, it may be time to replace your furnace.
Average furnace repair prices include:
- Furnace ignitor: $150–$350
- Furnace blower motor repair: $400–$900
- Furnace heat exchanger replacement costs: $1,000+
Once repair costs start to reach 50% of what a new furnace would cost, replacement is recommended.
Common Furnace Problems that Increase Energy Costs
Dirty filters. Dust buildup inside your furnace filter reduces airflow and causes your blower motor to work harder than necessary.
Not only can dirty filters cause overheating or short cycling, but they can increase your energy bills by 5–15%.
It’s recommended you change your air filter every 1–3 months depending on your filter's MERV rating and if you have pets or allergies.
Leaky ducts. Did you know leaky ducts are the leading culprit for heating loss?
Depending on how well sealed your ducts are, you can lose 20–30% of air that’s supposed to heat your home.
This is because your furnace is forced to work longer (runs more) to reach your thermostat’s set temperature.
Sealing ducts with mastic or aluminum HVAC tape is one of the easiest ways to lower your heating costs by 10–20%.
Repair vs. Replacement: Should You Replace Your Furnace?
You should consider replacing your furnace if:
- It’s older than 15 years and inefficient.
- You notice big jumps in heating costs.
- Repair costs start adding up to exceed the cost of a new furnace.
Here’s an example of the savings you could realize by upgrading your furnace.
Let’s say you currently have an older furnace that’s only 70% efficient. If you upgrade to a newer, energy-efficient furnace that’s 95% AFUE, you could save about $470/year on fuel costs if your annual heating bill was $1,800.
You also may be able to take advantage of rebates from your utility or promotional deals from manufacturers that lower your installation cost by $500 – $1,500.
Should I Repair or Replace My Furnace?
When To Call a Professional HVAC Tech
Not sure if you should call for help with your furnace?
Here’s when you need to contact an expert:
- Smell gas or carbon monoxide alarms go off
- Burner flame is yellow/or produces soot
- Loud boom when furnace turns on
- Water leak coming from furnace
- Heating bill suddenly spikes higher than normal
Furnace maintenance you can perform:
[how to lower furnace bills]
Some energy efficiency upgrades that help lower heating costs:
- Install a programmable thermostat or smart thermostat
- Seal and insulate air leaks
- Improve your home’s insulation
Final Thoughts on High Furnace Bills
If you notice your heating bills creeping higher each month, your furnace running for long periods of time, short cycling, or strange behavior from your furnace, give it a little checkup.
Many of these conditions can be easily fixed with routine maintenance or by correcting common issues. By keeping up with maintenance, you’ll save on your monthly utility bills and extend the life of your furnace.
Frequently Asked Questions
Q1: How do you know if your furnace is running efficiently?
A: Look for rising heating bills, longer furnace run times, uneven heating throughout your home, short cycling, and strange behavior from your furnace.
Q2: How often should I replace my furnace filter?
A: Every 1–3 months is ideal for most homes. If you have pets or allergies, you might need to change it more frequently.
Q3: Is it worth replacing an old furnace?
A: If your furnace has an AFUE rating of 70%, replacing it with a newer furnace that’s 95% AFUE can save you about 25–30% on heating costs.
Q4: Do duct leaks make your furnace run more?
A: Since leaky ducts can waste 20-30% of your heated air, your furnace will need to run longer to achieve the temperature your thermostat is calling for.
Q5: How can I improve my furnace’s efficiency?
A: Annual furnace tune-ups, changing air filters regularly, cleaning your burner assembly, sealing ductwork, adding insulation, and upgrading your thermostat can all help improve your furnace efficiency and save you money on energy bills each year.