Before we begin, we must say only qualified persons should attempt working on a gas furnace. Please consult a professional service company for repairs or for correct diagnosing!
Remember when furnaces were pretty simple? There was a pilot and a thermocouple, a couple of safety controls, a blower assembly, a set of burners and of course the heat exchanger. The down side of these old furnaces was the cost to operate them as they were usually about 50% efficient. The modern furnaces of today run between 80% and 95% efficient.
We will talk about the typical 80% single stage heating gas furnace in this article as this is the most common out there these days.
On a signal from the room thermostat that heating is needed, the first thing that takes place is the draft inducer begins running and creating a draft in the heat exchanger. If there is no obstruction in the flue pipe of the furnace, the inducer will send a small pressure to the pressure switch via a small rubber tube, obviously, this tubing must be free of debris or any cracks. If there is sufficient draft, this will close contacts inside the pressure switch. The pressure switch then sends a signal down to the control board which is usually mounted in the blower section. There are various safety controls such as, but not limited to, over temperature limit switches, plenum thermostats, flame sensors, and roll out switches that must be closed, but if they are and the control board is good, a signal will be sent to the ignition device. There are many ignition systems used including intermittent spark ignition, direct spark ignition, hot surface ignition, electronic ignition, standing pilots and others. MOST furnaces at this time use hot surface ignition with a flame sensor. So we will concentrate this article on the hot surface ignition system that uses a flame sensor.
There are several different types of hot surface ignitors on the market. The most common is the Silicone Carbide type which is VERY brittle and breaks easily. Do not handle this type by the carbide section as the oil from your hands will shorten the life span. This type has a life expectancy of 3-5 years usually. There are flat ones and round spiral versions of these.
The next type is the Silicone Nitride which will last much longer and do not break easily, some of them are meant for 24 volt systems as in the Honeywell Smart Valve systems, and Trane 80 volt systems.
There are also older systems that use a hot surface ignitor that doesn't use a separate flame sensor but rather once they glow orange then switch over to sense the flame. These systems are called radiant systems and have been proven unreliable and sometimes unsafe as they are not sensing a flame on all burners, only the one the ignitor is mounted on. Most furnace manufacturers are requiring you to add a flame sensor when you change a control board.
Assuming the control board "sees" all the safety controls are closed and it has a signal from the pressure switch, it will then send 120 volts to the hot surface ignitor which is usually mounted on the right side of the burners. (Be aware that some ignition systems use 24 volts for lighting a pilot and Trane and some others use a 80 volt Silicone Nitride system on SOME of their models). Once the hot surface ignitor glows orange you should hear a click which is the gas valve opening to let gas flow to the burners. This should give almost instant ignition on all burners. The flame sensor is a small white ceramic electrode mounted on the left side of the burners. It's job is to "sense" a flame on the burner which is at the opposite side of the furnace from the hot surface ignitor. If it doesn't get a flame within about 10-15 seconds, it will shut the burners back off for safety. Usually if a furnace goes through (3) cycles like this, it will go into lock out. To get it out of lock out you will need to shut the power to the furnace off and back on. Flame sensors should be cleaned with sandpaper once a year as part of maintenance.
Once all the burners are lit and running, the control board will send a signal to the blower motor to start.
Be advised however, there are MANY control boards including intergrated control boards where all functions of the furnace are controlled by this board, also out there are ignition modules used for ignition only and a separate fan timer control board that only starts and stops the blower motor as well as monitoring safety controls.
Before you even THINK about working on your furnace, please remember this is a possible lethal piece of equipment which takes a great deal of training for a service person to work on. Folks may want to save money by doing their own work, but there is much to be said about a proper diagnoses by a reputable company let alone any dangers to bodily harm from inexperienced people working on this equipment. If in doubt "DON"T DO IT"
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