If you’ve suddenly found yourself in charge of fire safety and all the different types of fire detection systems have you feeling confused, then you’d probably want to know if you need to have heat detectors or smoke detectors, or maybe both. So, let’s talk about both of these in a little bit more detail to find out how they work, what they are used for and where you should have them installed.
Heat Detectors vs Smoke Detectors: How do they work?
All major differences between heat detectors and smoke detectors stem from the difference in the way that they work. To understand what function each of them serves and how they can be useful, we’ll first take a look at the various types of detectors and the working principles of each type.
How do heat detectors work?
Heat detectors (also known as heat alarms) detect a rise in temperature and sound the fire alarm when a certain temperature limit is reached. There are two main types of heat detectors:
Fixed Temperature Heat Detectors
A fixed temperature detector measures the temperature in the room and when it passes a certain level the alarm is sounded. Usually, these heat detectors are set to be triggered at 58°C.
The working principle involves a bimetallic switch element composed of two different metals – one of a higher temperature coefficient of expansion than the other. The metal with the higher coefficient is heated during a fire and expands when the temperature limit is reached, causing the switch to bend and closing the circuit, which activates the alarm.
Rate-of-Rise (ROR) Heat Detector
Rate-of-rise detectors measure the rate of change in the room temperature. They have two thermal sensors (thermocouples or thermistors) – one detecting heat caused by convection or radiation and the other – the ambient temperature in the room. If the first one rises in temperature relative to the other at a rate of at least 6.7° to 8.3°C per minute, the alarm is triggered.
Here’s how it works: The ambient temperature thermistor is located within an air chamber exposed to room temperature. The other sensor is separated from it by a flexible diaphragm and sealed in a separate space. Between the two there is a small vent which allows for a calibrated leak of air to happen, making sure that the alarm isn’t triggered by normal shifts in the room temperature. If a high-energy, high-heat fire occurs, the ambient temperature rises so quickly that the leak can’t compensate for it and the air expands to the point where it pushes the diaphragm and activates a pressure switch in the sealed area which sounds the alarm.
In case the fire is a slow-burning one, the ROR heat detector will sound the alarm when both thermistors reach a pre-set temperature.
How do smoke detectors work?
Smoke detectors or smoke alarms are designed to detect the presence of smoke or fumes, which are some of the initial signs of combustion. We have a guide that teaches you more about smoke detectors and how they work but for the purposes of this comparison, you should know that they also have two main types:
Optical (Photoelectronic) Smoke Detectors
Those are triggered by changes to a light source due to smoke. Either optical beams are blocked by the presence of smoke and the alarm is triggered or a photoelectric receiver is triggered when the light is scattered by particles.
Ionisation Smoke Detectors
Those use a radioactive source to ionise the air molecules and produce an electric current which, if disrupted by the presence of smoke particles, triggers the alarm.
Where should you use a heat detector and where – a smoke detector?
Every residential property should have smoke detectors installed because they are faster to detect possible fires than heat detectors. This is the major advantage of smoke detectors over thermal detectors. Not all fires burn fast, so by the time the temperature has risen sufficiently to trigger a heat alarm, a smouldering fire could have created so much smoke in the air that it could put people at risk. When high levels of CO (Carbon Monoxide) are inhaled, a person can suffocate or lose consciousness within minutes. This is why where people’s lives are involved, smoke detectors are a must-have part of your fire detection system.
Now, you might wonder, if smoke detectors are better than heat detectors, why do you need a heat alarm? The answer is to protect your property and avoid false alarms. The heat detector will only react when there is enough heat to indicate fire and it won’t be triggered by environmental factors, such as dust particles, steam or smoke that’s not caused by a fire incident.
When you have precious items stored in a dusty area, such as a cellar, attic or garage, a smoke detector isn’t a very practical solution, as it’s prone to false alarms. Another example are smoking areas. The amount of smoke generated in a designated smoking area would simply turn a smoke detector into a nuisance. However, cigarette butts that haven’t been properly put out can cause a fire and that’s a situation where a heat alarm would be better suited to give an adequate indication of the possible danger. Heat alarms are also needed in areas that are at risk of high-intensity fires that produce little smoke, such as places where highly flammable chemicals are stored
So, which is better: a smoke detector or a heat detector?
At the end of the day, how we answer this question depends on the individual situation and on a combination of factors that have to be carefully analysed. When choosing between smoke detectors vs heat detectors, you have to take into account the area where the detectors will be used and how likely it is to be affected by constant false alarms, as well as the function of the room and the danger of people getting hurt. In many cases, your fire alarm system will likely have to feature both in different areas of the building.
Interested in more information about fire alarm systems and their components? Check out our article on fire alarms vs smoke detectors next!
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