Some Furnace Questions
I received an email from a podcast listener with some furnace related questions. Based on the nature of the questions I figured it would be better to ask an experienced furnace tech. Benoît Mongeau agreed to help by answering the questions.  My name is Matt and I am a newer tech (fully licensed this September, have […]
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Flame Sensing – The Basics
  Proving flame is an important part of the gas firing sequence. Without proof of flame, you risk dumping unspent gas into the heat exchanger, resulting in an explosion. There are many ways to “prove flame,” but we are focusing on the flame sensing rod (flame rectification) method here. Here are the facts: Flame sensing […]
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Combustion Air
To maintain combustion (burning), you need three things: fuel, heat, and oxygen. If you have all three in the proper proportion, you can maintain a continuous state of combustion. Remove one (or reduce one sufficiently), and the triangle of combustion can collapse. In a common NG gas furnace, the heat is the igniter, the fuel […]
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Testing a Thermocouple
First, a thermocouple is not a flame rectifier like a modern flame sensor. A thermocouple actually generates a millivolt potential difference when it is heated by a flame—just to get that out of the way for any of you newer techs who are used to modern flame sensors. With higher efficiency gas-fired equipment being the […]
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Inches of Water Column
Low pressures are often measured in inches of water column or “WC. Like most units of measure, it has a very simple origin; in a water manometer, 1″ of water column is literally the amount of force it takes to raise the column of water by 1”. While some water manometers (water tubes) are still […]
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Condensate Drainage in a High Efficiency Gas Furnace
This article was written by HVAC/Furnace technician Benoît Mongeau. Thank you, Ben.     High efficiency (or 90%, or condensing) furnaces use a set of two heat exchangers in order to retrieve more heat from the combustion products than their mid-efficiency counterparts. Because of this, they generate flue gases much colder than those of a […]
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Properly Diagnosing a Gas Furnace Pressure Switch
This article was written by boiler technician Justin Skinner. Big thanks to Justin for being one of the rare techs who cares enough to take the time to write something like this. Thank you, Justin! Pressure switches are used in a variety of applications. Generally, they are a normally open switch that closes with either […]
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TH, TR and TH/TR Gas Valve Terminals
Jesse Grandbois is one of the techs who reads the tech tips, and he wrote a few tips that he wanted to share on some gas furnace control basics. This tip is about the basic terminal designations on a typical 24v gas valve. Thanks, Jesse! Have you ever noticed the TH/TR terminal on a gas […]
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Combustion and Confined Spaces
All fuel-burning appliances require oxygen to burn. They require sufficient oxygen to burn clean and safe, without soot and CO (carbon monoxide). I live and work in Florida, where most of our fuel-burning appliances are 80% efficient with open combustion that utilizes air and oxygen from the space for combustion. With these low-efficiency appliances, whether […]
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