Electronic Gas Furnaces

February 7th, 2010

The most modern type of gas furnace is the electronic gas furnace. These are much more efficient, and therefore environmentally friendly than older standing pilot furnaces, but are more complicated to look after.

There are three types of electronic ignition furnaces, the hot surface ignition, intermittent hot surface ignition and the intermittent spark pilot. These three methods all offer higher efficiency than older furnaces and are controlled by circuits rather than mechanics. They will also most likely have high efficiency heat exchangers too which can provide up to 80% efficiency. The older standing pilot systems averaged around 60-70% efficiency if they were well maintained.

The hot surface ignition system has no pilot light. It uses a silicon-carbon element which it heats up to ignite the gas. This is why they are often referred to as direct ignition systems. After a call for heat, the furnace cycles up. First it purges, then applies power to the element, when it’s up to temperature it will glow red hot. Then the furnace will open an internal gas valve and pass it over the element, which ignites the gas. Once the gas ignites, the element powers off until the next cycle. The burners will continue to burn until the desired temperature is reached and the call for heat stops.

The intermittent hot surface ignition is for Honeywell systems. Their system is a middle ground between a hot surface ignition and intermittent pilot. They added a small hot surface igniter on a pilot light system along with a flame sensor. All the circuitry and flame sensor inside the gas valve and they called it the Honeywell Smart Valve. It works just like the intermittent pilot light, but uses the hot surface ignition technology.

The intermittent spark pilot is a simpler system, but one that uses slightly more gas than the hot surface method. On a call for heat, the system will light the pilot electronically using a 10,000 volt spark. Once the flame is proven using flame rectification, the gas begins to flow across the burners. The pilot lights the main burner and keeps burning until the desired temperature is reached, where the call for heat stops. The pilot light is then extinguished along with the burner until called again by the thermostat.

While the intermittent spark pilot uses slightly more gas, it is much simpler to use, troubleshoot and keep running. The hot surface ignition systems are very safe and efficient but the electronics are difficult to troubleshoot and are often cheaper to replace than repair.

All three use the latest in gas furnace technology to improve efficiency in both heating and fuel use, and couple with efficient heat exchangers can offer the highest HVAC efficiency ratings.