Gas boiler draft sensor: how it works and works + subtleties of checking functionality
Modern gas equipment, as a rule, can operate fully automatically.Thanks to built-in design components for monitoring the safe operation of equipment, the reliability of the system as a whole is ensured. One such device is a gas boiler draft sensor.
Agree that using equipment that does not require constant human presence is quite convenient. But on what principle does the traction sensor work and is it reliable?
We will consider these questions in our publication - we will talk about the design of the traction sensor, its functionality and features of testing its functionality. We will supplement the presented material with thematic photos and video materials.
The content of the article:
Design and principle of operation of the sensor
Considering the variety of designs of gas boilers, it should be noted that draft control sensors are also found in different designs. If we consider their design in an extremely general way, we will talk about a fairly simple mechanism of devices.
The basis of almost any gas boiler draft control sensor is a bimetallic element that changes shape with changes in the temperature background. In fact, it is a simple bimetallic strip that bends when heated or cooled.
Changing the shape of the plate is controlled by the contact group, which transfers the state of the contacts to "on" or "turned off". The switching signal of the contact group is transmitted to the gas boiler controller or to a simpler gas supply control mechanism.
The type of sensor that controls draft in the smoke channel depends on the boiler used.
So, there are two types of gas boilers that exist and are used in practice:
- Structures equipped with a simple chimney (with natural draft).
- Structures equipped with a chimney with a turbine (with forced draft).
These designs differ from one another and the traction sensors used for them also differ.
Devices for natural draft boilers
Natural draft boilers use a so-called flue gas hood, into the body of which a simple miniature thermostat is built, as shown in the picture below.
A thermostat of a simple design in a miniature design is usually equipped with a corresponding temperature mark directly on the body (on the metal shell). This mark (eg 75º) indicates temperature limit of contact group operation sensor
This device works simply. If the flue gases passing through the hood with the installed sensor heat the device above the set temperature parameter (which indicates a violation of the draft mode), the contacts will open the circuit.
Accordingly, due to an open circuit, the gas supply system to the boiler will be switched off (blocked). The equipment will restart only after the sensor has cooled down and the open contact has been restored.
Turbine boiler sensor designs
Boilers equipped with a chimney with a turbine have a slightly different sensor for determining the draft of a gas boiler with an operating principle that differs functionally. First of all, the difference is that the sensor actually controls the boiler turbine fan. In other words, the optimal draft of flue gases by the fan is controlled.
That is why the design of draft sensors for turbine gas boilers is made not for temperature control, but for control of the volume of passing carbon monoxide.
Such sensors operate based on the presence of optimal vacuum inside the combustion chamber and have a contact group of three elements:
- contact COM;
- normally open (NO);
- normally closed (NC).
Structurally, the devices are made in different shapes, but their principle of operation remains unchanged. Upon the formation of operating conditions inside the gas boiler chamber (optimal vacuum), the supplied air pressure closes the contact group, sending a signal to supply gas.
How to check the functionality of the sensor?
Disturbances in the draft of a gas boiler are often compared precisely with sensors. In any case, many repairmen traditionally point to a faulty traction sensor.
It is quite simple to check how the draft detection sensor works on a household gas boiler. It should be noted that periodic inspection of such structural components is essentially routine. Especially for boilers equipped with a fan.
Stage #1 - Performing a test of control sensors
Almost every equipment with a fan has special test points, using which the sensor is tested.
Test points (fittings), as a rule, are located in chimney area (top of the boiler). An example of the location of such elements is shown in the picture below. Both fittings are marked accordingly. That is, they have the designations “+” and “-”, indicating the flow path.
Next to the control fittings there is usually another control interface (on the left, closed with a lid), through which it is possible to measure the temperature of the gases and the efficiency of the equipment.
The procedure for taking measurements is as follows:
- Twist off the protective caps on the fittings.
- Connect the pressure gauge tubes to the fittings.
- Observe the accuracy of the connection at the “+” and “-” points.
- Turn on the “Chimney Sweep” mode on the boiler.
- Wait for the equipment to reach maximum capacity.
After the equipment reaches maximum power, check the pressure gauge readings. The device should show permissible vacuum level, not falling outside the established range for a specific brand of gas boiler. The required range can be found in the equipment documentation.
The procedure demonstrating how to check the draft sensor on a household gas water heater, in addition to measurements with a pressure gauge, also includes one more necessary action - checking the boiler pressure switch.
The gas boiler fan is traditionally equipped with a device called a pressure switch.Thanks to this device, it is possible to control of fan operation and actual burner control gas boiler.
The pressure switch is connected to the air channel by rubber tubes. However, to check this element of the circuit, you will have to open the body of the gas boiler.
The operating principle of this technical pair is quite simple. From the air channel through a rubber tube, pressure (negative relative to the pressure in the second tube) is taken by a pressure switch.
If the pressure selection is normal, the contact circuit of the pressure switch is closed - the gas boiler is functioning normally. In the event of a change (deviation) in the vacuum level, the pressure difference changes, which leads to a rupture of the contact group of the pressure switch. Accordingly, the equipment is taken out of operation (boiler blocking).
Any branded pressure switch always has operating parameters marked on its body. In particular, the device response pressure parameter for turning on and off is indicated (for example, for the pressure switch shown in the photo above, this is 70/45 Pa). In other words: in this case, the gas burner operates at a pressure of 70 Pa and locks out at a pressure of 45 Pa.
Stage #2 - checking the functionality of the boiler pressure switch
To check the pressure switch, you need to perform a simple action - determine the quality of switching of the electrical circuit of the device. The switching element of the pressure switch is a conventional microswitch, built into the design of the device.
The microswitch is controlled (the contacts are closed or opened) by a plate, which is affected by the force of air pressure entering the device through the tubes.
The microswitch contacts are located on the outside of the device body. Accordingly, to check, you need to connect a measuring device (multimeter) configured to measure Ohm resistance to the contact group.
Each branded device is equipped with an electrical circuit indicated on the case. In accordance with this diagram, the probes of the multimeter and the contacts of the device are connected.
After connecting the multimeter probes, a piece of silicone tube is connected to the negative pressure channel of the pressure switch. Through a connected tube, negative pressure is created on the device (simply by sucking in air with your mouth) and at the same time monitoring the readings of the multimeter.
During normal switching, the device needle will show minimal resistance or will not react at all, depending on the pressure created in the tube. If the microswitch is faulty (the commutation channel is broken), the multimeter will not show any response. In this case, the pressure switch must be replaced with a new one.
We recommend that you familiarize yourself with the intricacies of inspection and gas boiler maintenance.
Stage #3 - identifying the cause of decreased traction
The reason for the decrease in traction is not always a sensor failure.
Thus, practice demonstrates that insufficient traction can be caused by many other factors:
- clogging of air transmission tubes;
- clogging of the internal area of the fan cochlea;
- the formation of condensation inside silicone tubes;
- foreign objects entering the tubes.
One of the common reasons for a decrease in boiler draft is often clogging of the internal area of the fan scroll. Cleaning this area restores full traction.
After prolonged operation of the gas boiler, a large amount of dust and fumes collects on the blades of the fan impeller and the walls of the volute. Over time, these deposits become compacted, acquire a rigid structure and, as a result, create significant resistance to air flow. This is one of the common reasons for a boiler to lose draft.
The boiler fan will, of course, have to be removed before cleaning the interior area. Most boiler designs provide for easy removal/installation of the fan. Usually it is enough to remove two or three mounting screws to detach the component from the chassis. You must first disconnect the gas boiler from the power supply.
Washing with water should be carried out in such a way that moisture does not get on the stator winding of the electric motor and other electrical elements. The best option seems to be cleaning by blowing the inner area of the volute and blades with compressed air. True, at home this option is often impossible.
We have provided recommendations for cleaning and manual maintenance of the gas water heater in next article.
Step #4 - Retest Traction
Upon completion of the procedure for cleaning the gas boiler turbine fan and installing this structural component in its workplace, it is necessary to repeat testing of the equipment for the level of flue gas draft.
That is, again, you should perform the operation described above - checking the vacuum level inside the combustion chamber. The previously disassembled body of the gas boiler must be installed in place to bring the boiler into full working order.
As a rule, the test results show a slight increase in the pressure gauge readings, which indicates the normal operating condition of the flue gas exhaust channel. Taking into account this practice, we can draw the appropriate conclusion that the primary reason for the violation of the draft mode of a gas boiler is not always the temperature sensor or pressure switch.
Therefore, initially you need to check all the equipment and accessories involved in the smoke channel circuit. Indeed, in this case, the problem was a clogged gas boiler turbine fan.
Possible reasons for the sensor to trigger
Frequent operation of the gas boiler draft sensor is often observed immediately after the installation of new equipment and subsequent commissioning.
Malfunctions of the boiler with this option are usually due to:
- incorrect channel construction scheme smoke removal;
- unusual weather conditions in the region;
- violation of the traction characteristics of the equipment;
- incorrect settings of the control controller.
In regions where strong winds prevail, the reason for the sensor to trigger may be trivial - wind entering the flue gas exhaust channel. For such cases, it is recommended to additionally install on the pipe traction stabilizer.
The traction characteristics were noted above, and specialists should be involved in setting up the gas water heater controller.
Conclusions and useful video on the topic
The video discusses the structural details of the traction sensors, the location of these components and their operating principle:
If professional craftsmen are quite familiar with gas equipment, for the average user, troubleshooting a gas boiler is a “dark forest”. In addition, handling gas systems in the absence of appropriate knowledge is fraught with serious consequences.
Therefore, when there is a desire to independently replace or repair the same draft sensor or some other equipment of a gas water heater, you first need to at least study the system. But the best way to eliminate gas system defects is to contact specialists.
Would you like to supplement the above material with useful comments on the principle of operation of the traction sensor? Or would you like to share your sensor testing experience with other users? Write your remarks and comments in the block below, add unique photos of your own testing.
