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Ignition Troubleshooting and Replacement for Furnaces and Hot Water Heaters

How to replace your hot surface ignitor, thermocouple, or intermittent pilot on a furnace or hot water heater.

The electronic ignition system in a gas furnace or hot water heater is a development that allows more reliable performance than standing pilot furnaces provides energy savings and contributes to better furnace efficiency (AFUE). The AFUE is the most widely used measure of a furnace's heating efficiency. It measures the amount of heat actually delivered to your house compared to the amount of fuel that you must supply to the furnace. For instance, a furnace that has an 80% AFUE rating converts 80% of the fuel that you supply to heat; the other 20% is lost from heat escaping up through the chimney, heat loss through the shell of the appliance, or standing pilot losses. Standing pilots are common on older low efficiency furnaces where an AFUE of 55% to 65% is common; the small gas flame is referred to as a "pilot light". Obviously this type of ignition wastes energy by constantly burning gas and can be unreliable.

These issues have led to the development of electronic ignition systems for mid to high efficiency furnaces that exceed the U.S. government’s established minimum AFUE rating of 78%. The electronic ignition occurs typically in one of two ways:

• Intermittent Pilot, or

• Hot Surface Ignition

The intermittent pilot system uses an electronically controlled high voltage electrical spark to ignite the gas pilot and then subsequently the main burners, when the thermostat calls for heat. This is similar to a gas range where you hear a clicking sound when you light the burner.

The hot surface ignition system uses an electronically controlled resistance heating element, similar to a light bulb filament, to ignite the gas burner.

Silicon Carbide Igniter

Silicon Nitride Igniter

Types of Gas Furnaces

There are 3 basic types of gas furnace designs:

• Conventional Warm Air Furnace,

• Induced Draft Furnace, and

• Condensing Furnace

Conventional furnaces can have any of the 3 types of igniters, with the age determining what type is used.

Standing Pilot

A standing pilot uses a thermocouple, technically a thermocouple junction, to keep the gas on for the burner to use. The thermocouple contains two metal wires welded at the ends and placed inside a protective metal case. The thermocouple sensor is located on an angled bracket near the pilot flame and is designed to be placed in the hottest part of the flame. The other end is connected to the pilot valve body. As the thermocouple heats up, it produces a small amount of electricity and when it gets hot enough from the pilot flame the voltage will create a signal to open the gas valve by using a solenoid operated by a 24 volt transformer. The thermocouple generates a small DC voltage up to 30 millivolts (mV). A gas valve normally requires 14 – 18 mV to open.

Thermocouple

Once the gas valve is open, gas is then constantly supplied to the pilot and as required for the gas burner, called for by the thermostat. If the pilot goes out, then the thermocouple gets cold and produces no electric signal to open the gas valve's solenoid and the gas valve shuts off the gas supply to the pilot and burners.

To replace a thermocouple, you will need to take off the old one and determine how long it is; 18, 24, or 30 inches are common for residential appliances.

Replacement

1. Turn off the gas to the unit.

2. You will need a small adjustable wrench or box wrench to remove the fitting from the valve body. Loosen the fitting and remove it from the valve.

3. Unscrew the sensor from the bracket near the pilot flame. Sometimes there is a barbed fitting that slides over the sensor and is pushed into the hole on the bracket. You should be able to remove this by hand.

4. Be careful not to kink the copper tubing and run the new thermocouple from the valve body to the pilot flame bracket.

5. Screw or press fit the sensor on the bracket.

6. Tighten the fitting into the valve body with the wrench. Make sure that it is secure, but do not over tighten.

7. Turn gas back on and light the pilot by following the instructions on the unit.

A natural gas flame should be a bright blue with a small yellow tip. A propane flame should have a bluish green flame with a tinge of yellow at the tip. The flame should be strong enough to hit the thermocouple tip about 1/2 inch from the tip end. If the flame is weak or shaky looking, check to see that a breeze or draft is not blowing on it.

Adjusting the Flame

There is usually a small screw on the pilot valve body that will adjust the flame. You may have to refer to the manufacturer's instructions to find the screw. Turn the screw as needed to adjust the flame throw.

Yellow Flame

A yellow flame is caused by lack of air and incomplete combustion. It can be caused by a dirty pilot tube tip.

Split Flame

This is caused by dirt in the pilot gas tube. Take a needle or small nail and gently clean the tube.

Flickering Flame

Air is blowing across the flame, check for drafts in the area.

Intermittent Pilot

Most usually found on some induced draft furnaces, this pilot ignites with a high voltage spark only when the thermostat calls for heat. Once the intermittent pilot (IP) is lit and the main burner senses the pilot flame (flame sensing rod), the main burner will ignite.

For an intermittent system, a pilot problem will usually mean the replacement of the entire control box, which can be as high as $100 and most models will have proprietary units, meaning that there are few universal replacements. It may be best to contact an HVAC repair company to conduct this repair. If you feel confident in your abilities, you will need the check the part number from the pilot assembly and locate a dealer nearby that stocks the part.

Hot Surface Ignition

The hot surface igniter is the most commonly used electronic ignition system used today since they are reliable, inexpensive and the electronic controls they require are also less expensive. All they require is a flame sensor to detect the igniter glow and open the main burner valve. This is also one of the easiest repairs to perform; all you need is a screwdriver and your tow hands.

Electricity passes through the Silicon Carbide or the newer Silicon Nitride igniter and makes it glow red hot. Operating under normal conditions a hot surface igniter will last for 3 to 5 years, but the Silicone Nitride can lasts twice as long. During that time igniters will eventually crack and need to be replaced. Like a light bulb, they are a regular replacement item. Most igniters are interchangeable as long as they fit in the furnace space provided.

These hot surface igniters are between $25 to $50 depending upon the model of furnace you have. It is a good idea to purchase a spare and keep it near the furnace.

Possible Reasons for HSI Failure

• Premature failure due to improper handling; oil from skin or dropped igniter.

• Improper igniter

• End of normal usage life cycle

• Electric current in the home is too high (over 125 VAC)

• Dirty filter or squirrel cage for burner draft.

Replacement

Note: If you are uncomfortable performing this work, hire a qualified HVAC technician.

1. Shut off the power and gas to the unit.

2. Locate the burner section of the furnace and remove the cover.

3. Use a Phillips-head screwdriver to remove the two screws that secure the furnace igniter to the furnace with a Phillips-head screwdriver. Carefully, pull the igniter out of the furnace.

4. Take the igniter with you to home center or a HVAC dealer for a replacement. Some models come with universal mounting brackets and can be used is dozens of furnaces.

5. Insert the new igniter into the furnace. Do not touch the part of the igniter that fits inside the furnace. The natural oils from your skin will cause the igniter to wear out prematurely.

6. Secure the igniter to the furnace with the screws provided or the old screws if they fit. Connect the wiring harness to the wire terminal block on the new igniter by pushing the two pieces together. Place the access panel back into position and slide the access panel down to secure the panel to the furnace.

7. Turn the power back on to the furnace and turn the gas back on at the gas valve.

 

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