There's no denying that modern gas furnaces are safe appliances. These devices include numerous safeties and fail-safes that prevent them from operating in an unsafe fashion or, in a worst-case scenario, prevent the furnace from operating at all. However, it's still important to understand these safety mechanisms and to recognize when they're trying to protect you.
In many cases, your furnace won't stop working because it can no longer run, but instead because a safety switch recognized a hazard and stopped the system from working. While this situation can be frustrating on a cold night, it's critical to keeping you and your family safe. If your furnace suffers from any of these three issues, it's important to contact a professional to avoid creating a hazardous situation.
1. Heat Overlimit
Your furnace needs to know the temperature around the heat exchanger for two different purposes. First, avoiding turning on your blower while the heat exchanger is still cold is important. Waiting to turn the blower on ensures that your vents don't produce a blast of cold air when the furnace turns on. Secondly, it's important to shut the furnace off when the heat exchanger becomes too hot.
While it may seem counterintuitive that your furnace can be too hot, high temperatures can cause the heat exchanger to crack. A cracked heat exchanger will release dangerous exhaust gases, posing a potentially deadly hazard. When your furnace triggers its overlimit switch, try changing the air filter. If that approach doesn't work, it's time to call in a professional.
2. Flame Roll-Out
While an overlimit trigger warns you to take action to avoid more serious problems, the flame roll-out switch tells you that a problem already exists. Most modern furnaces will include multiple roll-out switches that detect flames inside the furnace cabinet. Your furnace's flame should always remain contained to the combustion chamber, so roll-out is a sign of a serious issue.
When flames roll out from the combustion chamber, they typically do so because they're looking for more oxygen to burn. This situation may indicate that there's already a crack in your heat exchanger. Whatever the case, these flames can produce harmful carbon monoxide that may escape into your home. If your furnace triggers a roll-out sensor, shut the unit off and call in an expert.
3. Draft Problems
While older furnaces relied entirely on gravity to move exhaust gases outside, modern units use a draft blower to create negative pressure in the combustion chamber. This negative pressure ("draft") pulls gases through the flue, allowing modern furnaces to use exhaust flues that run nearly horizontally instead of up and through the roof.
Your furnace uses a pressure switch to ensure the blower is creating enough negative pressure, and this switch will cause your furnace to shut down if the blower isn't working or there's an obstruction in the flue. Since these problems can potentially allow exhaust gases to back-feed into your home, you'll want an expert to take a look as soon as possible.
Speak to a gas furnace repair professional to learn more.