Why does my gas boiler not switch on when I open the tap?


One of the most common and frequent DHW problems is the gas boiler not turning on when you turn on the tap. You immediately know that this has happened when you feel cold water against your skin when you were expecting the comforting warmth of hot water. If this has happened to you, don’t worry, the problem is not as serious as it seems and many water boiler users have experienced the same thing. In this article, we will explain the causes of this boiler problem and we will give you some effective solutions from boiler technicians that have been proven to work.

How an Instant Gas Boiler Works

Gas boilers are one of the most popular appliances in the home. They are responsible for providing you with the comfort of hot water for bathing, and ensure that DHW appliances work whenever you want. When the boiler stops working, especially when it happens suddenly and just as we turn on the hot tap, this is when we realize how much it helps us in our daily lives.

To understand this problem, we first need to understand the structure and function of these kinds of “instant” boilers, and how they differ from the so-called “passage boilers”, which have a more traditional and less sophisticated design.

Traditional gas boilers consist mainly of a tank (either an independent tank with its own fuel, or a tank that is connected to a boiler), which is responsible for heating and storing the water until there is a demand for hot water. In the event that the hot water runs out, you will need to wait for an entire tank full of water to heat up. Of course, there is also the danger of Legionella bacteria, which can be present in any tank with sitting water, and this requires special filters and compliance with regulations.

In comparison, instant boilers (unlike traditional boilers) do not have a water tank or any of its associated risks or limitations, and they have been designed with a coil that heats up the water when needed. Hence the name of “instant” boiler.

An instant boiler works as follows:

  1. There is a demand for DHW in the home. Opening a hot tap anywhere in the house (kitchen, sink, shower, etc) is the first thing that happens.
  2. The electronic ignition is activated. When water circulates through the valve, the in-built electronic ignition is automatically activated, the gas valve activates, and the burners start working. Automatic ignition varies from brand to brand. Some units have batteries that provide the initial energy, while others have a small hydrogenerator, which creates the initial electrical energy when the water circulates within it.
  3. The water is heated in the coil. As mentioned above, all instant boilers have a coil through which the water circulates and is instantly heated.
  4. The hot water comes out of the tap when opened. This is the climax and the moment that the instant boiler’s magic allows us to enjoy hot water without having to wait. There is also no DHW tank anywhere in this process.

When paying close attention, you will probably notice that the step most likely to experience a problem is step 2. This is related to the automatic ignition, the magical step linking our need for hot water with its effective production through the tap. Problems when the gas boiler doesn’t turn on when the tap is opened can be due to various causes. Let’s take a look at a few of these and their possible solutions.

The gas boiler doesn’t turn on when the tap is opened (Table)

The batteries are depleted (or almost), or the contact points are sulphated.Replace the batteries with new ones that are properly charged, or sand the contacts down with fine sandpaper.
Gas supply failureCheck if there is gas in the cylinder and if there is sufficient gas supply. Check the unit and check if the gas regulator is working well or if there is a problem with it.
There is insufficient pressure in the home’s water systemClear any blockages, elevate the water tank more, make diameter reductions in the water system, or install a pressure pump.
The ignition membrane is hardened or crackedReplace the ignition membrane
The control module is lockedRepair or replace the control module (as applicable)
The automatic ignition microswitch is stuck or defectiveUnblock and lubricate it, or replace the microswitch
The ignition regulator screw is poorly adjusted (on models that include it)Adjust the screw or change the water flow, depending on what is needed
The water generator (on models that have one) is not sending a current to the control module as it is dirty or brokenClean or replace the water generator
General or local blockage in the home’s water systemClear up the blockages so that they don’t prevent water flow

Reasons a gas boiler doesn’t turn on when you open the tap

If you have ever asked yourself questions such as: What do I do if my boiler doesn’t turn on? Why doesn’t my gas boiler turn on?, or why is the water not heating up?, we will discuss the answers to these questions and their solutions in the summary table.

1 – The batteries are flat

This is the simplest cause of all. The solution is, of course, to replace the batteries. Some Cointra boiler users (although it is not set in stone that this may not occur in other brands) have reported that sometimes even though the spark plug works, the boiler doesn’t supply any hot water. After much research, it was finally discovered that the problem was deflated batteries. However, when it comes to these kinds of boilers, there is often another cause. This is something to take into account to avoid headaches.

2 – There is a gas supply failure

A gas supply failure is a generic problem, which in itself, can have many causes and many solutions. It might be because the fuel tank is empty, the gas regulator is faulty, or the tank valve has been accidentally closed, etc. Either way, it’s important to find out which of these faults has resulted in the lack of gas supply and how you can solve it.

Another thing to consider is that if you live in a very cold area where there is frost and you use butane gas for fuel, ambient temperatures below 0 degrees celsius will result in the gas equalizing the pressure from the outside and into the tank so that the gas cannot flow out, making the tank seem empty. If this sounds familiar, you should consider switching to propane, which can tolerate temperatures up to -40 degrees C without stopping the gas flow.


3 – The boiler isn’t turning on due to low pressure

As awe have already seen, the bypass boiler’s ignition mechanism (whether it is an automatic or instant boiler), depends entirely on the water pressure. This units need enough water to be flowing in order to turn on. The water flow rate is the amount of water circulating through the circuit at any given time.

If the flow is not sufficient, the pressure will also not be sufficient, and it will need to be generated. Some boilers are not sensitive to this problem thanks to the manufacturing design that ensures they are able to ignite at almost any pressure, regardless of how low. However, this is not always the case.

There are many boiler models and brands that only require very minimal pressure or water flow to be able to turn on automatically. When the water pressure is insufficient, the unit simply cannot turn on even if a hot tap or a shower is turned on.

If there is not sufficient water flow to produce ignition, the first thing to consider is whether or not there may be general or local blockages in the home’s water system. A blocked or semi-blocked pipe can reduce the water flow until it is almost non-existant. A good plumber will be able to determine if this is a problem and how it can be solved.

If there are other reasons for insufficient pressure, there are many ways to increase the pressure within the home’s water system. The first one is to elevate the water tank to a point where the gravity of the falling water can reach the required pressure. So, what height should you place the water tank so that it can reach the required pressure? There is no simple answer, and the problem is best investigated by a plumber or a hydraulic engineer.


Another way to increase the water flow of pressure is to redesign the unit itself, meaning to properly manage the pipe diameters, and make reductions in important areas of the circuit so that the water is forced to reach a higher pressure.

Finally, if non of the above methods work to help increase the water pressure, either because the tank can’t be elevated, or because, for some reason, it is not advisable to alter your home’s water system, you could choose to install a pressure pump. This type of pump is able to increase or decrease the water pressure as needed. Booster pumps are compact and quiet when operating, and come in multiple designs.

The next logical question to ask yourself is: What water pressure does a gas boiler need? Logically, this will depend on the boiler’s make and model. However, let’s say that, generally speaking, the components that require pressure should have a minimum pressure parameter of 0.25 bars, and the operating flow should be about 3 liters per minute.

4 – The ignition membrane has hardened or is damaged.

Sometimes the membrane reacting to the water flow and acting on the microswitch to turn on the boiler, hardens and stops working. In such cases, it should be replaced. Sometimes, the membrane may just be dry and can be lubricated. However, this part is really not expensive, and the best solution is almost always to replace it.

5 – The control module is locked.

Another simple reason why your gas boiler may not be turning on when you open the tap is that the electronic module producing the automatic ignition is blocked or broken. It should therefore be repaired or replaced.

6 – The starter microswitch is blocked or faulty

Battery-powered boilers have a small switch (a microswitch) that is activated when the membrane activates a stem that, in turn, presses the microswitch when there is a demand for hot water in the home. All these components form part of the bypass boiler’s automatic ignition function.

The activated microswitch signal gives the command to the control module to switch on the boiler (from the perspective of the gas opening and the ignition spark). If the microswitch is faulty or stuck, the automatic ignition process cannot take place. The solution is to clean and lubricate the microswitch, or replace it if needed.


7 – The ignition regulator screw has been incorrectly adjusted

Sometimes automatic gas boilers have a screw that allows you to regulate the water flow when you turn the unit on. Sometimes the link between the screw’s setting and the actual water flow becomes unbalanced. The solution here is to adjust the screw or change the water flow as needed. These things should only be carried out by a qualified technician.

8 – The water generator is not sending a current to the control module

The water generator (if the brand or model includes one) is responsible for producing electrical energy when the water passes through it, and allows the burner to ignite. If this component is dirty or damaged, it will need to be cleaned or replaced with a new part in order for the boiler to ignite properly.

Mystery solved: the boiler doesn’t only turn on when a tap is opened

There have been cases where the boiler doesn’t switch on only when a certain tap is turned on, but it works with other taps perfectly. This happens when the unit associated with the tap, or the tap itself, is full of minerals in the water, becomes congested, and does not allow sufficient water flow to the boiler to detect a demand for hot water. The solution is the clean the tap and the unit (spouts, filters, etc) well.

If your Ariston boiler isn’t turning on…

This section discusses Ariston boilers not turning on (as this is a frequently asked question). However, the following comments may actually apply to other boiler brands as well.

Sometimes an Ariston boiler won’t turn on and also behaves strangely. When there is a demand for DHW in the home as a hot water tap or shower has been turned on, the Ariston boiler doesn’t switch on. Sometimes where there is no demand, it can make the characteristic sparking sound on its own. When it makes this sparking sound, we open the hot tap and the water coming out is hot.

The secret behind this strange behavior of Ariston boilers is the microwswitch is broken or stuck between on and off. The solution can be to unblock and lubricate the microswitch, but you should not rule out replacing it.

How do I light an Ariston boiler? If it is an Ariston gas flow-through boiler, you don’t have to do anything to turn it on as it will switch on automatically when a hot tap is opened. Otherwise, if it’s a gas model with a pilot flame, follow these steps:

  1. Turn the knob to the On/Pilot position.
  2. Keep the Pilot button depressed, and press the piezoelectric button until it lights up
  3. When the pilot lights up, release the piezo button but hold the pilot button down for about 30 seconds more.
  4. If, when you release the pilot button, the pilot flame goes out, start the process again from step 2.
  5. If it stays on, you can now select your desired water temperature.

Important: if you have an electric Ariston boiler with a water tank, in order to turn it on you must wait until it is full of water, otherwise it will burn out.

If you need to research other causes of an Ariston boiler not turning on, there is a good tutorial in this article that also applies to Ariston step boilers and others.


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We have boilers and boiler spare parts available for you in the Suner store. These are always original parts and have been made to the highest standards. Our store also have a specialized search engine, and we ship orders within 24 hours.


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