Some furnaces have variable speed fans, variable speed inducers, multiple pressure switches and a multiple stage or modulating gas valve. Some trouble shooting may vary for these furnaces. This troubleshooting guide does not diagnose modulating components.Guide is made of thick cardboard stock with metal corner rivets and measures 8.5 inches tall by 6 inches wide. All information on the chart in copyrighted and property of the author Nash Enterprise. The item "Gas Forced Air Furnace Troubleshooting Guide Slide Rule" is in sale since Thursday, April 24, 2014. This item is in the category "Business & Industrial\HVAC\HVAC Tools\Other HVAC Tools". The seller is "theheatcoolstore" and is located in Fargo, North Dakota. This item can be shipped to United States, all countries in Europe, all countries in continental Asia, Canada.