When something goes wrong with your HVAC system it can become very uncomfortable in your home, especially when you don’t need heat but your heating comes on even when you’ve put the thermostat off. Generally, you don’t want to call a technician before you have tried to solve the problem yourself. But where do you start to look for reasons causing the issue?
If heating comes on when the thermostat is off it could be due to thermostat issues, furnace issues or a boiler issue like a damaged non-return valve. If it comes on only for short periods once a day it might be an automatic overriding of the setting for necessary maintenance purposes. Some boiler-based heating systems activate automatically for a short period once every 24 hours to avoid problems like a pump sticking due to inactivity.
In this article, we’ll look at the most common “normal” reasons why your heating comes on when the thermostat is off. You will see in the article, it is normally the boiler-based systems that might come on automatically for maintenance purposes. We’ll also look at faults in the system that might cause the problem.
Two types of heating systems
To understand how the heating system in your home works, you have to know that there are in principle two types of home heating systems.
Your home can be heated by either a furnace or a boiler-based system.
- A furnace heats the air, and the warm air is then moved through the home’s duct system with a blower motor.
- With a boiler-based system, a boiler heats water which then flows through a network of pipes in the home. The home is then kept heated as long as the warm water is flowing through the pipes.
The reasons for the heating coming on when the thermostat is off or doesn’t turn off when it should, depend on which type of heating system you have in your home. Some issues, like thermostat malfunctioning, can occur in both systems, but some other issues can only appear in either a furnace system or a boiler system.
Sometimes it is “normal” for a boiler-based system to turn itself on and you don’t have to worry about it. But if you don’t have a boiler-based system or the heat comes on quite often there is most probably a fault in your HVAC system.
Possible reasons why heating comes on when the thermostat is off
There are various reasons why the heating comes on when the thermostat is off. Let’s look at possible thermostat-related reasons, then at furnace-based specific reasons and finally at boiler-based system reasons
Both furnace and boiler systems use thermostats to function. Thus, one of the most common places to start looking and checking for the reason why the heating comes on when the thermostat is off or doesn’t turn off at all is at the thermostat.
Some thermostat components such as the wiring, switches or heating sensors could have failed. If this is the case, you most probably have to replace the thermostat with a new model.
More than one HVAC thermostat
If you have more than one programmable thermostat in your HVAC system, it is possible that one thermostat in the group overrides the other thermostat(s) and put the heating on, while you’ve put it off on another thermostat.
Thermostats are generally not set on “off”
Most thermostats are normally set to a specific temperature, like 9 degrees Celsius, as the lowest temperature to detect. That level is commonly seen as the “off” position, but actually, the thermostat is never “off.” When the interior temperature then falls below that value (e.g. 9 degrees Celsius), the thermostat starts the heating process of your home automatically.
Specific furnace-based system issues
Faulty primary controller
Your furnace can also stay turned on if the primary controller is damaged. The primary controller monitors the presence and absence of a flame inside the burner and then reacts to the flame condition. When the controller is working correctly it starts the burner for a run cycle when the thermostat indicates that the temperature in the home is low.
This controller is the main heating element of the whole system, and when it stops functioning, the furnace can’t regulate the temperature anymore and the heat can come on uncontrolled.
Can’t find the reason?
If the issues persist after you’ve checked the thermostat, and the furnace keeps on coming on or stays on, you can turn the furnace off with the on/off switch at the furnace itself. This will cut all power to the furnace. Although this will not solve your problem, it will ensure that no further damage is done to your HVAC system while you wait for a technician to come to find the problem and rectify it.
Specific boiler-based system issues
Faulty check valve
If your heating is provided by a boiler system, there are mechanical issues to be checked. To send heated water through the radiators to heat your home, a boiler-based system uses a check valve.
The main function of a check valve, also known as a non-return valve, or a stop valve, is to allow steam flow in one direction and automatically and simultaneously prevent backflow.
A functioning valve circulates the water throughout the system, but a faulty check valve could be the reason that the circulation doesn’t stop when the system is turned off.
Override as result of freezing temperatures
Your boiler systems might have sensor overrides installed on some pipes. The overrides are for when the weather is below freezing and the pipes might be damaged by frozen water inside them. When freezing is imminent the sensors automatically override the thermostat and put on the heating system to prevent damage to the pipes. Thus, in freezing weather conditions it can be “normal” for the heating in your home to come on when the thermostat is off.
If the issue persists even when there are no freezing conditions anymore, one of the sensors could be faulty. A faulty sensor can keep on overriding the thermostat. This results in the heating coming on even when the thermostat is off.
Automatically activated for short periods
Some boiler-based heating systems activate automatically for a short period of time once every 24 hours. This is to avoid issues like a pump sticking due to inactivity for long periods. This also results in the heating coming on when the thermostat is off, but it is regarded as “normal.”
To know that when the heating system in your home sometimes comes on when the thermostat is off might be part of the system’s way of self-maintaining and is not due to a fault in the system is comforting. Fortunately, these overrides are for short periods and normally don’t make the house uncomfortably warm. Most of the time you are not even aware of these overriding periods.
However, when it becomes uncomfortably warm in the house when the heating comes on when the thermostat is off, there is a problem with the system. By using the information we’ve given you in this article you will be able to exclude some of the causes and even be able to fix the issue if it is not too technical.