Why your gas heater won’t start when you turn on the faucet


One of the most common and frequent problems with domestic hot water (DHW) is that the gas heater does not turn on when you open the faucet; you only realize it when the water is already cold, and you have been deprived of the anticipated pleasure of hot running water.

So, if this has happened to you, don’t worry, it’s not as serious as it seems and it has happened to many users of water heaters and instant hot water systems. In this article we will explain the causes of this malfunction, and above all we will explain the most effective solutions that have been tested in practice by technicians and users.

How instant gas heaters work

The gas water heater is one of the most beloved appliances in the home. It provides the comfort of being able to bathe in warm or hot water, and in general facilitates the use of DHW in all the installations we need. When the heater stops working, that’s when we really appreciate the service it provides, especially if the failure happens all of a sudden, at the very moment when we turn on the tap to get hot water.

To understand this problem, we must first understand the structure and the way in which these types of heaters, called “instant heaters”, work, and how they differ from the so-called “flow heaters”, which have a more traditional and less sophisticated design.

Traditional gas heaters basically consist of a tank (either a stand-alone tank with its own resistance or fuel, or a tank linked to a boiler), which is responsible for heating and storing the water until there’s a demand for its use. In the event that the hot water runs out, we need to wait for its entire volume to be heated again. And then there’s the matter of theLegionella bacteria, which can incubate in any water storage tank, and so requires special filters and rules and laws to follow.

On the other hand, instant heaters (as opposed to traditional ones) do not come with a water tank incorporated, and thus they don’t have any of the risks or limits that come with having a tank. They are, in fact, designed to feature a coil that heats the water as soon as it needs to be used. Hence the name “instant” heater. The operating steps of an instant heater are as follows:

  1. There’s a demand for hot water in the home. The first step is turning the tap on in any spot inside the house (kitchen, bathroom, shower, etc)
  2. The electronic ignition is activated. When water flows through the valve, the ignition included in this equipment is automatically activated, the gas valve becomes active and the burners start up. Automatic ignition varies from one brand to another. There are units which include batteries to provide the initial energy, and there are those which include a small hydrogenerator, which creates the initial electrical energy when the water circulates inside the unit.
  3. The water is heated through the coil. As mentioned before, every instant heater has a built-in coil, through which the water circulates and heats up instantly.
  4. The hot water comes out of the tap we open. This is the climatic moment, when the magic of the instantaneous water heater allows us to enjoy hot water without having to wait and without having a DHW tank in between.

If you look closely, you probably have figured out that the most likely problem is in step 2. That is, in the automatic ignition, in that magic step that links our need for hot water with its effective output through the faucet. This malfunction, of the gas water heater not turning on when the tap is opened, of course, can be due to several reasons. Let’s look at some of them and their possible solutions.

Reasons why a gas heater won’t start when you open the tap

1- The batteries have run out

This is the simplest explanation, and one with the easiest solution: swap the old batteries for new ones. Some users of Cointra heaters (although this might also happen with other brands) have reported that sometimes the spark plug does work, but the heater still won’t provide hot water. After much investigation, they finally discovered that the problem was indeed the unit had run out of battery. In the case of this type of water heater, that’s not easy to spot at first glance. It’s something to take into account to avoid unnecessary worry.

2- A problem with the gas supply

Gas supply failure is a generic problem which can have several causes and several solutions. It may be caused by an empty fuel tank, a faulty gas regulator, or an unintentionally closed fuel tank tap, etc. In any case, you need to determine which of these faults has cut off the gas supply and fix the issue.


One more thing. If you live in a very cold area, including any with a chance of frost, and you use butane gas as fuel, keep in mind that from ambient temperatures of 0°C downwards, the butane tank will match the outside pressure, so the gas won’t flow out and it will be as if the tank was empty. If this is the case for you, consider switching to propane which can withstand temperatures as low as -40°C without stopping the flow.

3- There is not enough water flow to produce ignition

The automatic water heater needs sufficient water flow to turn on. The flow rate is the amount of water flowing through the circuit. There may be good pressure and not enough flow, so the flow will have to be generated.

4- The membrane is hardened or damaged

Sometimes the membrane, which reacts to the passage of water and acts on the microswitch to turn on the heater, hardens and stops working. If this is the case, it must be replaced.

5- The control module is blocked

Another of the simplest reasons why a gas heater does not ignite when the tap is turned on is that the electronic module responsible for the automatic ignition is blocked or faulty. It needs to be repaired or replaced.

6- The ignition microswitch is blocked or faulty

In the case of battery-operated heaters, they include a small switch that is activated when the membrane reacts due to the demand for hot water in the house. The signal from the activated microswitch instructs the control module to turn the heater on. If this part is faulty, the automatic ignition process will not occur. The solution is to replace the microswitch.

7- The ignition regulator screw is not correctly adjusted

Sometimes automatic gas heaters include a screw which you can use to regulate the water flow rate that turns on the heater. There can be an imbalance between this screw and the actual water flow rate. The solution is to regulate this screw or vary the water flow rate as needed. These things should only be handled by qualified personnel.


8- The hydrogenerator is not sending power to the control module

The hydrogenerator (if your model includes one) is responsible for producing electrical energy when water passes through it, which allows the burner to ignite. If this part is dirty or faulty, it must be cleaned or replaced with a spare so that the heater can ignite properly.


Mystery solved: the heater won’t start if one particular faucet is turned on

There have been cases where the heater stops working only when a certain faucet is opened, but it will start with any other faucet. This happens when the installation related to that faucet, or the faucet itself, has been filled or clogged with water minerals, so it doesn’t allow a sufficient flow for the heater to detect a demand for hot water. The solution is to thoroughly clean the tap and its installation (hoses, filter, etc.).

Check out heaters and heater parts at Suner

We sell heaters and spare parts at the Suner store. We work exclusively with original products of the highest quality. Our store has a specialized search engine, and shipments are carried out within 24 hours.

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