Why Is My Furnace Consuming More Gas than Usual?

A sudden jump in gas consumption is almost never the gas meter’s fault. It is a signal that something in your furnace has changed. In my 25 years, I have traced dozens of these cases. The cause is usually one of seven things: a misadjusted burner, fluctuating gas pressure, a failed recuperator, damaged lining, leaky door seals, a drifting thermocouple, or a change in operating practice.

Here is how I diagnose it, step by step.

First, Confirm the Increase Is Real

Before you touch the furnace, check your records. Compare this month’s gas consumption per tonne of aluminium melted with the same month last year. If production has increased, higher total consumption is normal. What matters is consumption per tonne.

Also check the weather. In colder months, the furnace loses more heat through the shell and open doors. A 10% increase in winter is not unusual.

If consumption per tonne has risen by more than 10%, and production is unchanged, then you have a problem.

Cause 1: Burner Air-Fuel Ratio Has Drifted

This is the most common cause I see. A burner that was tuned perfectly last year is not tuned today. Nozzles wear. Linkages slip. Air dampers move.

Symptoms: Flame looks yellow or lazy. Flue gas temperature is higher than normal. You may see soot around the burner.

Diagnosis: Use a portable flue gas analyser. Measure oxygen (O₂) and carbon monoxide (CO). If O₂ is above 5%, you have too much excess air. If CO is above 50 ppm, you have incomplete combustion. Both waste gas.

Fix: Adjust the air damper until O₂ is between 2% and 4%, and CO is below 50 ppm. Clean the burner nozzle while you are at it. I recommend doing this every three months.

I once visited a foundry in Malaysia where gas use had risen 18% over six months. The burner had never been tuned since installation. We adjusted the air-fuel ratio and cleaned the nozzle. Consumption dropped back to normal within a week. The cost of the fix was two hours of my time.

Cause 2: Gas Pressure or Gas Quality Has Changed

Gas pressure that is too low or too high affects combustion. Low pressure means less heat input. High pressure can cause over-firing and wasted gas. Gas quality — the calorific value — also matters. If your supplier changes the mix, your burner needs re-tuning.

Symptoms: Flame size changes during the day. Gas pressure gauge fluctuates.

Diagnosis: Install a pressure gauge near the burner. Log pressure every hour for a few days. If it varies by more than 10%, call your gas supplier.

Fix: If pressure is unstable, ask the supplier to investigate. You may need a buffer tank or a pressure regulator. If gas quality has changed, re-tune the burner.

I remember a die casting plant in Thailand that had random gas spikes every afternoon. The supplier was diverting gas to another customer. A buffer tank solved it.

Cause 3: Recuperator Leak or Blockage

If you have a heat recovery system, it can fail. A leaking recuperator lets cold air into the flue, reducing preheat. A blocked recuperator restricts flow, increasing back pressure. Both waste gas.

Symptoms: Preheat air temperature is lower than normal. Flue gas temperature after the recuperator is higher than usual. You may hear whistling or see soot.

Diagnosis: Measure the temperature of the combustion air entering the burner. Compare with the design value. Check for leaks with a smoke pencil.

Fix: Repair or replace the leaking section. Clean the recuperator if blocked. I have seen a simple gasket replacement save 8% on gas.

Cause 4: Refractory Lining Is Damaged

A cracked or thin lining lets heat escape through the shell. The burner runs longer to maintain temperature. Gas consumption rises.

Symptoms: Furnace shell feels unusually hot. Melt time increases. Energy per tonne rises.

Diagnosis: Use a thermal camera. Look for hot spots on the shell. Inspect the lining visually for cracks, spalling, or thin areas.

Fix: Repair or replace the damaged lining. A small patch can be done in a day. A full reline takes longer but restores efficiency.

In 2019, a foundry in India had a 15% gas increase. The lining was 8 years old. A thermal scan showed hot spots along the sidewall. We replaced the lining. Gas consumption dropped by 20% — better than before.

Cause 5: Door Seals or Openings Are Leaking

Every time a furnace door opens, heat escapes. If the seals are worn, heat leaks even when the door is closed. Operators may also leave doors open longer than necessary.

Symptoms: You feel heat radiating from the door area. The door seal looks flattened or cracked.

Diagnosis: Run your hand around the door edge (carefully). If you feel hot air, the seal is leaking. Check for gaps with a feeler gauge.

Fix: Replace the door seals. Train operators to keep doors closed except when charging. A simple seal replacement can cut gas use by 5–10%.

I once saw a foundry in Vietnam that had a 12% gas increase simply because the door seal was worn. The replacement cost $200. The annual saving was over $15,000.

Cause 6: Thermocouple or Controller Drift

If the thermocouple reads low, the controller thinks the furnace is cooler than it is. It keeps firing the burner. The furnace overheats. Gas is wasted.

Symptoms: Actual melt temperature is higher than setpoint. Metal quality suffers. Energy use rises.

Diagnosis: Check the controller reading against a handheld pyrometer. If they differ by more than 10°C, calibrate or replace the thermocouple.

Fix: Calibrate or replace the thermocouple. Verify the controller is configured for the correct thermocouple type. This is a five-minute fix that can save thousands.

Cause 7: Changes in Operating Practice

Sometimes the furnace is fine. The way it is being run has changed.

Wet scrap. Moisture absorbs heat. The burner runs longer.

Overcharging. Too much metal blocks the flame. Melt time increases.

Holding metal longer. Idle holding at high temperature wastes gas.

Different alloy. Some alloys need higher temperatures.

Night shift habits. Operators may skip maintenance or change settings.

Diagnosis: Talk to the operators. Review the production log. Compare current practice with six months ago.

Fix: Train operators. Pre-dry scrap. Match melting to casting. Avoid unnecessary holding.

A Real Case from the UK

In 2020, a foundry in Birmingham had a gas consumption increase of 22%. They had checked the burner, the gas pressure, and the lining. Everything looked fine.

I spent a day on site. The burner was well-tuned. The lining was good. The recuperator was working. Then I watched the night shift. The operator was charging wet scrap directly from an outdoor pile. It had rained that week. The moisture was sapping heat.

We built a simple drying rack near the furnace. The scrap dried before charging. Gas consumption dropped back to normal within two weeks. The fix cost almost nothing.

A Simple Checklist to Find the Cause

Compare gas consumption per tonne with last year.

Check burner air-fuel ratio with a flue gas analyser.

Log gas pressure for a few days.

Measure recuperator preheat temperature.

Scan the shell with a thermal camera.

Inspect door seals.

Verify thermocouple accuracy with a pyrometer.

Review operating practices with the team.

Work through these in order. Most problems will be found in steps 1–3.

When to Call a Professional

Some checks are safe for maintenance staff: reading a pressure gauge, cleaning a burner nozzle, checking a thermocouple. Others require a qualified engineer: gas train repairs, refractory work, control system calibration. If you are not sure, call someone who knows.

Final Thoughts

A furnace that uses more gas is not a mystery. It is a symptom. Find the cause, fix it, and your gas bill will return to normal.

In my experience, 80% of gas increases come from three things: a misadjusted burner, a damaged lining, and leaky door seals. Check those first. Then look deeper.

And remember: the best troubleshooting is prevention. A good maintenance checklist — like the one I’ve written about — will prevent most of these problems before they happen.

Internal links: Furnace Maintenance Checklist, How to Optimize Burner Efficiency in Gas-Fired Aluminium Furnaces