In the hierarchy of HVAC equipment, your thermostat is the top dog. Your furnace and air conditioner can't make decisions independently and instead rely on instructions from the thermostat to engage heating and cooling. Unfortunately, they also can't determine when the thermostat is making unreasonable or impossible requests.
While furnaces have plenty of ways they can fail on their own, a faulty thermostat can also potentially overtax your furnace and lead to some costly repairs. Below are three ways that an unaddressed faulty thermostat can potentially shorten your furnace's lifespan or lead to premature repairs.
1. Short Cycling
Short cycling can occur in both furnaces and air conditioning systems. A piece of HVAC equipment is short cycling if it turns on and off more than necessary to maintain an appropriate temperature. There's no strict definition of short cycling, so your system may cycle on and off rapidly, or it may cycle every few minutes. Faulty thermostats or dirty temperature sensors can sometimes cause this issue.
In general, short cycling isn't good for your furnace or your HVAC system as a whole. Each cycle places more wear on components such as the igniter and burners, and your blower motor is also more likely to wear out quickly. You'll also spend more money since a short cycling system will have trouble maintaining an adequate temperature.
2. Running Constantly
Of course, a faulty thermostat can cause the opposite problem to short cycling. Thermostats work by sampling the nearby air temperature and commanding the furnace to produce heat when the temperature falls below the user's setpoint. When the temperature increases above this level, the thermostat instructs the furnace to stop heating.
If your thermostat doesn't instruct the furnace to stop heating, it can do more than just make your home unbearably hot. Your blower may not be able to keep up with the increased temperature, causing your heat exchanger to overheat. In most cases, your furnace will eventually trigger its over-limit protection and shut down, but you'll still need to resolve this issue as soon as possible.
3. Tripping Breakers
A potentially unusual situation can occur if you use a smart thermostat with an older furnace. If your furnace lacks a common wire (often called a "C" wire), the smart thermostat may draw too much electricity and cause the furnace to short circuit or trip a breaker. This situation can potentially damage the furnace's electronic components.
While your thermostat might not seem critical, it can affect your HVAC system in a variety of ways. If you notice any of these issues, you should contact a furnace repair technician as soon as possible to evaluate both your furnace and thermostat.