What is Boiler Interlock?

A boiler interlock is a safety measure in a boiler or a heating system to help prevent it from running around all the time and consuming extra energy. It essentially will make sure the protection and security of the boiler’s functioning and for optimal central heating. It is not an actual device per see, but a combination of controls and pre-programmed command to prevent accidents.

Excessive levels of hot water and high pressure can be dangerous. The boiler interlock also prevents the boiler from further heating up space around when the optimal temperature that you hardwired has been reached. 

The interlocks also help prevent the unnecessary firing up or start of your central heating system when the parameters you set around are not matched. Boiler interlock can be achieved by using a series of fixes and hardwiring {{{and end your footnote with this short code:}}} done to your boiler along with your thermostat to prevent unneeded usage. You can think of them as part of your boiler safety settings.

Some central heating systems, like a combi boiler, use a thermostat to effectively interlock the heating system. To know the kind of interlock your boiler system requires will depend on its functions, programming, and hardwiring.

Need for A Boiler Interlock In A Heating System

Boiler interlocks are required as they are protective interlocks, and everyone using heating and boiler system needs them. The interlock will not only prevent your boilers from firing up every time when not needed, but it will also point towards any faults in the system. It also prevents the steam pressure from rising to dangerous levels.

If your required interlock is not in place or it is not functional in any part, then it may point towards problems in your boilers. Here are some of the causes for safety concerns in a boiler system.

Water Level

If the water level is too low or too high, then the boiler could get damaged and even explode.  Especially in the case of low water level as it could uncover the surfaces that are heating. Boilers come with controls that immediately shut down the access to the fuel or gas if the level gets under the optimal level.


Forced draft fans are important for supplying pressurized air. If the forced draft fan is damaged or not starting up due to less energy flow, then the interlocks will cause the alarm to sound. This would indicate that the fan may need to be inspected or replaced. Draft fans create a vacuum o negative air pressure. This negative air pressure vacuum is used to remove flue gasses.

Tools Used For Boiler Interlock

Here are some tools and controls that are used to achieve the bowler interlocking mechanism in systems.

A Room Thermostat

Room thermostats usually are the way that combi boilers are interlocked. A room thermostat essentially picks up the room temperature and signals to the boilers to stop the hot water and gas energy flow if the optimal temperature for the rooms has been achieved.

The optimal room temperature will be set by you with the controls and settings of the boilers. The room thermostat analyses the temperature of the air flowing in the room. If the temperature set has been attained, then it signals the boiler firing system and the central heating pump to stop further heating. This sure keeps the room from overheating. In the opposite case, it helps to warm up the room well when needed.

A room thermostat is very crucial for a boiler interlock to function properly. So, the location of the room thermostat also should be taken into consideration. Ideally, the room thermostat could simply be placed in the living room or hallways. However, a warm space or building room like a kitchen or bathroom should be avoided.

Thermostatic Radiator Valves

Thermostatic radiator valves or TRV are water regulation valves in the boiler firing systems. The TRVs are self-regulating and are used to adjust and control the volume of water in the radiator. The TRV ideally would also need to be installed with the thermostat, which would make sure the boiler’s interlock.

Like thermostats, it is important to make a good pick for a spot to install the TRV. TRVs should not be placed in a kitchen or bathroom as such spaces are warm and humid. This can lead the TRVs to switch or turn off the flow of water to the radiator even when not needed. At such times, the TRVs radiator heat is required to remove condensation, but the humidity of the space may confuse the TRVs. 

Electronic TRVs are even handier as they give one the control to set different temperatures for different rooms, depending on your preference. One can simply set the temperature for your living room to be completely different from your bedroom and so on.

Hot Water Thermostat

Hot water cylinder thermostats are an important boiler interlock tool. They sense the water temperature and pressure in the cylinder and are set to signal the boilers to switch on and off depending on the optimized temperature.

However, if the temperature, gas, and pressure controls that you set in the cylinder are not met, then the hot water thermostat would signal the boiler to turn on the switch. If the temperature you set the controls for in the boiler’s reached, then the hot water thermostat signals it to be turned off.

It is important to note that setting high temperatures on the thermostat valves for the hot water cylinder is not ideal if you want faster heating in your home. Instead, set the timer for the cylinder in advance according to your need so that you won’t need to wait and don’t have to make haste. You can also save time in this manner.

Interlock Switches

Interlock switches are a little different from boiler interlocks but work in a similar fashion. They are mostly used as a heat protection device on a boiler firing system or even industrial-strength machinery to control electric current flow. They work by interrupting the flow of electricity in the circuit. This keeps the machine and your home heat-safe.

What is interlock and protection?

An interlock is a kind of protection mechanism put in place to improve the safety and efficiency of the boiler heat system. Usually, boiler systems have to be hardwired and programmed with boiler’s interlock mechanisms.

Interlock is not a switch or a device control but is related to combinations of controls and settings. They can be used to shut the power in case of fluctuations in levels of heat and water or to prevent the boilers from working once the desired setting has been reached.

They can also alert you to let you know if the draft fan for the central system is not working. They are often mentioned in the building regulations and are needed to be programmed in for a safer home.

What is a permissive interlock?

Permissive interlocks are a kind of system safety interlock that prevents starting some activities or functions before the criteria set by your heat controls are met. For example, they would prevent the boiler from starting up and firing at your home if the water quantity is not optimal.

Or they would prevent the pump from working if, for example, the suction valve is not closed. Another example can be if the water temperature inside the cylinder is not according to the criteria. The boiler would be switched on or depending on the settings.

What does override mean on a boiler?

An override function or button is mainly used to increase your heating or hot water levels if they are not up to your liking. This way, you can achieve the desired temperature of hot water or heat controls that is different from the one that you may have programmed in the first place.

The override function can also be used if you are facing any faults in your boiler. This way, you may use the boiler in a different setting if your regular controls and settings are not working.

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Last Updated on May 3, 2021


Author: Indra Gupta

Indra is an in-house writer with a love of Newcastle United and all things sustainable.

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