Furnaces & Boilers
Complete Guide for HVAC Technicians
🔥Understanding Heating Systems: How Furnaces & Boilers Work 🔥
What are Furnaces & Boilers?
Furnaces and boilers are the two main types of heating systems found in residential and commercial buildings. They heat indoor spaces by using fuel sources such as natural gas, propane, oil, or electricity.
What are Furnaces & Boilers?
Furnaces and boilers are the two main types of heating systems found in residential and commercial buildings. They heat indoor spaces by using fuel sources such as natural gas, propane, oil, or electricity.
Key Differences Between Furnaces & Boilers?
| System Type | How It Works | Distribution Method | Common Fuel Types | Efficiency Range (AFUE) |
| Furnace | Heats air and distributes it through ductwork | Forced air system (vents/registers) | Gas, propane, oil, electricity | 80%-98% |
| Boiler | Heats water and circulates it through pipes/radiators | Hydronic heating (radiators / baseboard units) | Gas, oil, propane, electricity | 80%-95% |
💡 Pro Tip: Furnaces use air filters & blowers, while boilers rely on pumps & pipes for heat distribution.
How Furnaces Work 🔥
Furnaces operate using a forced-air heating system. Here’s the step-by-step process:
- 1️⃣ Thermostat calls for heat → Signals the furnace to turn on.
- 2️⃣ Burner ignites fuel (gas/propane/oil) or electric elements activate.
- 3️⃣ Heat exchanger warms up (Gas furnaces: Heat exchanger prevents combustion gases from mixing with household air).
- 4️⃣ Blower fan pushes warm air into ductwork.
- 5️⃣ Air travels through supply vents and heats the space.
- 6️⃣ Cool air returns to the furnace via return ducts and the cycle repeats.
Common Furnaces Types
- ✅ Gas Furnace – Uses natural gas or propane; most common.
- ✅ Oil Furnace – Used in colder regions where natural gas is unavailable.
- ✅ Electric Furnace – Uses resistance heating (expensive but simple).
- ✅ Modulating Furnace – Adjusts heat output for high efficiency.
How Boilers Work 💧🔥
Boilers heat water and distribute warmth through pipes and and radiators. Here’s how it works:
- 1️⃣ Thermostat activates the boiler.
- 2️⃣ Burner heats the water inside the heat exchanger.
- 3️⃣ Circulator pumps move hot water to radiators or baseboard heaters.
- 4️⃣ Radiators heat the surrounding air via conduction.
- 5️⃣ Cooled water returns to the boiler to be reheated.
Types of Boilers
- ✅ Hot Water Boiler – Heats water and circulates it at 180°F.
- ✅ Steam Boiler – Produces steam and distributes it at 212°F via radiators.
- ✅ Condensing Boiler – Captures exhaust heat for extra efficiency (95%+ AFUE).
- ✅ Combination (Combi) Boiler – Provides hot water & heating in one system.
Furnace vs. Boiler: Pros & Cons
| Feature | Furnace (Forced Air) | Boiler (Hydronic Heating) |
| Efficiency | Lower AFUE, can lose heat via ducts | Higher AFUE, retains heat better |
| Air Quality | Can spread dust/allergens | No forced air = better for allergies |
| Installation Cost | Cheaper to install | More expensive due to piping |
| Heating Type | Heats Quickly | More even & comfortable heat |
| Maintenance | Requires filter changes | Requires annual water treatment |
| Cooling Compatibility | Can integrate with AC | No AC compatibility |
💡 Pro Tip: Boilers are quieter & more efficient for heating, but furnaces are cheaper & easier to install.
Furnace & Boiler Efficiency (AFUE Ratings)
💡 Annual Fuel Utilization Efficiency (AFUE) measures how efficiently a system converts fuel into heat.
- Standard Efficiency: 80% AFUE (20% heat loss).
- High-Efficiency Gas Furnace: 90-98% AFUE (Condensing furnaces).
- High-Efficiency Boiler: 90-95% AFUE (Condensing boilers capture exhaust heat).
🔥 Regulations:
- Minimum AFUE for gas furnaces: 80% (in most states)
- High-efficiency furnaces required in cold climates: 90%+
- Energy Star-certified systems: 90-98% AFUE
Common Furnace & Boiler Problems (Troubleshooting Guide)
| Issue | Furnace Cause | Boiler | Fix |
| No Heat | Thermostat issue, dirty flame sensor, igniter failure | No pilot light, failed circulator pump | Check thermostat, clean/replace igniter or sensor |
| Short Cycling | Dirty air filter, overheating, faulty limit switch | Pressure issue, thermostat malfunction | Change air filter, inspect limit switch or pump |
| Strange Noises | Blower motor failure, duct expansion | Water hammer (steam), air in pipes | Lubricate blower, bleed radiators |
| Blowing Cold Air | Clogged condensate drain, flame sensor issue | Circulator pump failure | Clean drain, replace faulty components |
| Leaking Water | Condensate pump failure | Pressure relief valve leaking | Replace pump, check system pressure |