Why Is My Furnace Running But Not Producing Heat?
When the temperatures drop in central Indiana and your furnace kicks on but your home stays cold, it’s frustrating — and concerning. You hear the blower running… but there’s no warm air. Before you reach for the thermostat or call for service, let’s walk through the most common causes of this issue and how to diagnose them safely.
🔥 1. Thermostat Settings or Malfunctions
Your thermostat is the command center for your HVAC system — and if it’s misconfigured or malfunctioning, your furnace may run without heating.
Things to check:
- Are you set to “Heat”, not “Fan” or “Cool”?
- Is the temperature set higher than the current room temp?
- Are the batteries fresh (for battery-powered models)?
- Is the thermostat level and securely mounted?
Tip for central Indiana winters: Thermostats can lose calibration over time — a quick reset or replacing old batteries often solves the issue.
🌬️ 2. Dirty or Clogged Air Filter
One of the simplest — yet most overlooked — causes of poor furnace performance is a dirty air filter.
When filters get clogged with dust and dander:
- airflow is restricted
- the furnace has to work harder
- heat exchange efficiency drops
Even if the blower is running, warm air might not circulate properly.
What to do:
Replace your filter every 30–90 days, especially during heavy heating months like January and February in central Indiana.
🔌 3. Ignition or Pilot Light Problems
Most modern furnaces use electronic ignition, but some older systems have a standing pilot light.
If your pilot:
- won’t light
- goes out frequently
- burns a yellow or flickering flame
…it may be the reason your furnace runs but doesn’t heat.
For electronically ignited systems, a faulty ignition module or flame sensor can also prevent the burners from firing.
Safety note: Never attempt to relight gas furnaces yourself unless you’re familiar with the manufacturer’s instructions.
🚪 4. Blocked or Closed Vents and Ductwork Issues
Even if your furnace generates heat:
- closed supply vents
- blocked return vents
- crushed ductwork
- disconnected air ducts
can prevent warm air from reaching your rooms.
This is especially common in older Indiana homes with additions or remodeled spaces.
Pro tip:
Check vents in every room — open them up! — and make sure furniture or curtains aren’t blocking airflow.
🛑 5. Limit Switch or Safety Controls Tripping
Your furnace is designed with safety switches that:
- prevent overheating
- protect the heat exchanger
- shut down burners if airflow is insufficient
If the limit switch keeps opening, it can shut off the burners while letting the blower continue running.
Causes might include:
- dirty blower wheels
- obstructed return air
- failing limit switches
This is a situation where a professional HVAC technician can provide the most reliable diagnosis.
❄️ 6. Frozen Condensate Line (High-Efficiency Furnaces)
Newer high-efficiency furnaces in the Midwest often have a condensate drain to remove moisture.
In freezing central Indiana weather, that line can freeze — triggering a safety shutdown.
Signs it’s frozen:
- water dripping near the furnace
- error codes or blinking lights
- furnace cycles but no heat output
Thawing the line safely and making sure the drain slopes correctly prevents recurring issues.
📍 When to Call Absolute Comfort (765)534-4328
Some furnace problems are easy fixes — thermostat adjustments, filter changes, vent checks — but others require trained hands.
Call a certified technician if:
✅ the furnace runs but never ignites
✅ there’s a strange odor or visible soot
✅ flames are yellow instead of blue
✅ the blower runs constantly
✅ you notice unusual noises
We serve homeowners in Indianapolis, Carmel, Fishers, Greenwood, Noblesville, and beyond, and we can diagnose furnace issues fast so your family stays warm and safe.
👍 Homeowner Checklist: Furnace Running But No Heat
| Check Item | Quick Fix |
| Thermostat | Ensure “Heat” mode & fresh batteries |
| Air Filter | Replace if dirty |
| Vents | Open & clear |
| Pilot/ Ignition | Call tech if stubborn |
| Condensate Line | Inspect & thaw (if high-efficiency) |
Stay Warm This Winter
If your furnace is running but not heating your home, you don’t have to guess what’s wrong. Many issues are common and solvable — but some are best handled by professionals trained in central Indiana heating systems.
📞 Need help? Contact a trusted local HVAC expert today to get your system back to reliable warmth.