Gas Furnace Class w/ Bert
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Furnaces come in multiple types. One of the more rudimentary types is the natural draft furnace, which doesn’t have an exhaust blower and relies on the rising hot air to draw in more air for open combustion. Induced draft furnaces also draw in air somewhat similarly to natural draft furnaces, but they have an inducer fan motor that pulls air in. High-efficiency condensing furnaces create their own condensation and have closed combustion.
Unlike heat pumps, furnaces have fire. To create a fire, we need fuel (like natural gas or oil), oxygen, and heat. We get a flame when there is the rapid oxidation of fuel, resulting in the release of heat. Since furnaces bring fire into our customers’ homes, we need to keep safety in mind.
Furnaces have many parts that are integral to proper operation. One is the inducer fan blower, which is powered by 120v and pulls in air for combustion. It also allows negative pressure to “push” air out of the exhaust. The pressure or safety switch proves that there is adequate airflow by closing under a vacuum created by the inducer blower. Limit switches are normally closed and open under abnormally high heat; you may have to reset these manually, but bimetal limit switches reset automatically.
There are a few different ignition strategies, including hot-surface (HSI), intermittent-spark (ISI), and direct-spark (DS). HSI tends to be the most common, though some manufacturers still make systems with direct-spark igniters. In the Central Florida market, ISI is common on pool heaters.
Gas valves, which supply the gas, are normally closed and open when 24v power is applied to the coil. The gas valve also drops gas pressure to the manifold. Flame sensors detect if there is a flame; when a flame is present, a small DC current passes from the rod to ground via the flame, and that signal gets picked up by the board. Flame sensors need to stay clean to work properly, so we should avoid touching the metal.
Each part has a role in the sequence of operations. First, the furnace receives a W call on the control board from the thermostat. The control board then confirms that the pressure switch is open to ensure that the switch hasn’t failed. Then, the inducer fan starts, causing the pressure switch to close. All the while, the board checks to make sure the safety circuits are closed. Then, the ignition sequence begins; some ignition sequences have delays. Then, the gas valve opens, and the flame sensor proves the flame. Once the flame has been proven, the blower starts. Bert then demonstrates the sequence of operations on a training unit at the Kalos HQ.
When getting into diagnostics, a diagnostic chart will be your best friend. It will contain error codes and a list of possible causes, as well as things to look out for when solving the problem. Furnaces need an adequate combustion air supply, and we need to look at the infrastructure to make sure there aren’t any possible pressure or venting problems that could cause dangerous conditions like carbon monoxide production or flame rollout.
Some of the most common diagnostic issues include a blocked flue vent pipe, a jammed blower wheel, a damaged or loose hose, a failed inducer blower, a failed switch, loose wires, or a failed board. Limit lockout error codes are also common, and there are a number of things we should be checking, including flame rollout conditions, broken boards, loose wires, and partially blocked exhaust pipes. Failure to prove ignition is another common problem, and we have to look for issues with our gas valve, flame rod, damaged wires, and the igniter itself. Many of the same diagnostic concerns, especially regarding flame rollout and gas valve failures, are also applicable to gas pool heaters.
When working on gas appliances, some of the most common safety concerns include gas leaks, carbon monoxide, and flame rollout. When someone smells gas, you should take those concerns seriously. Carbon monoxide, on the other hand, is odorless and can be a silent killer; we can reduce CO in a house by advising customers to have a heat pump installed and use a CO detector. We can also make sure the heat exchangers are in good condition—not rusted out or cracked.