There are different types of gas furnaces which are available for purchase today and the older models which were installed many years ago still in operation. The older models were typically atmospheric type gas furnaces and used a lot of gas to produce heat much of which went up the flue because efficiencies were low. The standing pilot atmospheric type gas furnaces still exist today but they are slowly going the way of the dinosaur as they are being replaced by newer more efficient furnaces. We will take the typical modern gas furnace and break it down into different components and parts and then put it all back together again so that you will understand each part and how a gas furnace works. Each manufacturer has their own way of engineering their products but the principles and gas furnace components are basically the same. We’ll describe each gas furnace component necessary for the proper functioning of the gas furnace. Gas appliances require piping and gas piping in some states or provinces may be regulated by the plumber’s board but many HVAC contractors are qualified for running gas pipe. Many HVACCompanies have a plumbing department or the Plumbing Company has an HVAC department. Either way the gas piping should be done by a professional who is familiar with gas piping, gas pipe sizing, and the Fuel-Gas Code (other local codes may apply, consult with your local code authority before beginning a new piping job). Safety is the primary factor with any heating and we will stress this as we go along covering all gas furnace components. Of course we don’t know everything but we do know a lot about HVAC so if we leave something out I’m sure the High Performance HVAC readers will let us know as they always do. Suggestions and comments are welcome about gas furnace components and anything else you want to contribute concerning HVAC.
Natural Gas or Propane Details
For natural gas the meter is where your gas feed begins to your gas furnace and is considered an integral part of your gas furnace. For LP or liquid propane the gas comes from a tank somewhere close to your gas furnace. Both systems typically will have a pressure regulator somewhere between the meter or tank and your gas appliances including the gas furnace. A typical problem which can develop with natural gas pressure regulators in the winter time is they can freeze up especially when they are at the end of a long gas line run which is open to the outside ambient air temperatures. Natural Gas contains a small amount of moisture (approximately < 5%) and this moisture will freeze in the winter in long exposed piping runs. Some of this moisture ends up in the gas pressure regulator and will freeze the diaphragm inside the pressure regulator causing it to malfunction or close off the gas feed. This often results in a call to your local HVAC contractor or even Gas Company. Many HVAC contractors and Gas Companies are aware of this and take precautions against this problem upon installation of your system. Depending on the number of appliances you have and the pressure requirements of these appliances will depend on whether you have the pressure regulator located at the meter or tank or whether each appliance has their own pressure regulator. Some systems have both pressure regulators at the (meter or tank) and pressure regulators for each appliance. This will depend on a few variables like pressure of gas at the delivery point (meter or tank), the length of piping run, and the pressure needs of the appliances. The sizing of the gas pipe which feeds all gas appliances will also depend on these variables. The size of the gas pipe is based on BTU/h requirements of appliances, pressure of gas delivery, length of piping run, and specific gravity of the gas. HVAC or Gas contractors use a formula or a piping chart and different methods of sizing pipe for whatever BTU/h requirements are for gas appliances that are present, length of gas pipe run, gas pressure at delivery point, and the specific gravity of the gas. Gas piping support and gas piping protection from damage is also necessary for the proper installation of a gas line. This gas line is an integral component of your gas furnace. Without the gas line and the proper functioning of the gas line your gas furnace will not function. Occasional maintenance of steel gas lines and piping support is necessary in the way of rust prevention. A good coat of rust preventing paint is necessary for the long term as these steel gas lines have been known to develop leaks especially at the joints from rusting and pipe supports from becoming weak. From time to time the Gas Company sends out technicians with gas detectors. They walk around neighborhoods and scan for gas leaks not only in the Gas Company’s pipelines but also in exterior piping around homes and businesses. If a leak is detected in the gas pipe around your home or business the Gas Company will lock out the meter supplying your gas appliances. You will need to call an HVAC or Gas Qualified company to repair the leak before the Gas Company will restore the gas service to your home or business so maintenance of the gas line is recommended. Additionally there should be gas cut-off valves installed in these lines near the furnace and at the meter. If you ever smell gas it is a good idea to turn these valves off and call the Gas Company or your local HVAC contractor immediately.
The Gas Valve
The gas valve component of your gas furnace connects the gas line to the manifold and controls the gas feed to the manifold and burners. The gas valve component of the gas furnace is considered a gas control. The components inside the gas valve are slightly different from the natural gas gas valve to the propane gas gas valve. Many new gas furnaces come equipped with a natural gas valve but have a propane conversion kit with the new natural gas valve so it can be quickly converted in the field depending on the type of fuel delivered to the gas furnace. After a new installation of a gas furnace the furnace should be inspected by the local inspections department. The Fuel-Gas Code requires all new installations of gas piping receive a pressure test in accordance with the guidelines in the Fuel-Gas Code book. The gas line near the furnace should be disconnected before this gas line pressure test is performed. If not the gas valve can be damaged as result of the excessive pressures placed on it during this gas line pressure test. This is also in accordance with many furnace manufacturers and gas valve manufacturer’s installation instructions which usually take precedence with many code officials and inspectors over the code book. Too much pressure on the gas valve and the diaphragm could rupture or other components inside the gas valve will be damaged because they are not rated for the higher pressures involved with testing the gas line. It is also in the Fuel-Gas Code that the gas line be disconnected from the meter or a live gas connection before this pressure test is performed. It is necessary after the gas furnace is put into operation that the gas valve have the proper pressure drop across the valve. This is dependent on the gas pressure supplied to the gas valve and a small adjustment on the gas valve which requires a special instrument for properly setting the right pressure drop across the gas valve. The HVAC installer or HVAC start up technician should have this instrument and the ability to make this adjustment. The gas valve gets its signal to open from a gas furnace control board or gas furnace control module. There is a sequence of operation that must occur before the gas valve will open allowing gas to get to the manifold and the main burner. This sequence of operation includes proving all safety switches in the gas furnace safety circuit are closed and ready for main burners firing. The gas valve usually requires no maintenance but if it is ever submerged under water or becomes excessively wet from moisture it is recommended it be replaced and precautions taken to prevent the new gas valve from getting wet.
Gas Furnace Components 2
Gas Furnace Components 3