Why the gas heater turns off when mixing with cold water


Perhaps you got here because the gas heater turns off by mixing cold water with hot water at the time of use. Although it may not seem like it, this is a fairly typical failure in this type of equipment, and for this reason it is convenient to have some tips to help you understand what the problem is in these cases and how to solve it. Read this entire article, because it will be very useful to you.

What happens when the gas heater shuts off when mixing cold water

When the gas heater turns off when mixing hot and cold water, it may be because the opening of a second tap in the same service caused a change in pressure that the heater interpreted as the time to turn off. This, of course, occurs above all with traditional two-handle mixer taps, that is, those that have two controls or taps on the same tap.

By opening the cold water tap of the two-handle tap, the demand for hot water is reduced. And this supposed reduction in the demand for hot water (whose flow is replaced by cold) is a “false positive” for the heater, which will then turn off.

One possible solution for when the gas heater shuts off when mixing cold water, is to increase the pressure of the entire home hydraulic system. The two-handle tap can also be replaced by one of the innovative single-handle taps, which with a single valve, solves both the water flow control and the combination of hot and cold.

Other even more innovative variants that will avoid this problem are thermostatic mixer taps and infrared mixer taps.


Tips so that the gas heater does not turn off when mixing cold water

  1. The first tip is to avoid mixing cold water with hot water at the time of use. Mixing hot water with cold water is not the cheapest way. At that time the heater used all the fuel and did all the work, only for us to return the water to a lower temperature. It is better to regulate the temperature from the heater itself and choose the desired temperature directly.
  2. Replaces two-handle mixer taps with single-handle taps. Especially if it is a flow heater, the opening of a second way to combine the cold water with the hot water can cause the phenomenon that the gas heater turns off when mixing with cold water. This is due to the change in pressure that causes the opening of a second service in the same tap. The heater can interpret this as pressure reduction signaling that it is time to shut down. The solution for this is to increase the water pressure within the hydraulic system of the house, either by raising the tank higher or installing a pressurizing pump.
  3. Keep your hydraulic system free of blockages. Clogged pipes and taps (especially in old installations, even if the heater is new) generate pressure fluctuations and cause the heater to turn off when it shouldn’t. Normally a flow heater manages it’s on and off stages following changes in pressure, so it goes haywire if the pressure is not stable. Without clogging, the heater will not turn off even if you mix it with cold water when using the hot one.

Suner Frequently Asked Questions

In this section, we will briefly answer some of the frequently asked questions that users of heating and domestic hot water equipment ask themselves. As will be seen, these are questions related in some way to what we have been talking about in this article, but at the same time, they are sufficiently off-topic to be answered in a separate section.

What to do when the boiler is red?

Many brands of boilers do not include a red light to indicate faults. This is the case, for example, of the Baxi Roca, the Beretta, the Cointra, the Sime, the Tifell, the Vaillant, or the Viessmann. On the other hand, some brands of boilers do incorporate some red light signals (be it an LED light bulb, a change to red in the color on the notification screen, a Reset button with a red light, etc.). Chaffoteaux, the Domusa, the Fagor, the Ferroli or the Junkers.

In cases where the boiler includes a red light of any kind, if the red light comes on automatically, it usually indicates that a safety lockout has occurred. That is, the boiler microprocessor has received the signal from the sensors, and this indicates that there is some anomaly. In general, the red light will be accompanied by an error code, and it will be necessary to look in the manufacturer’s manual for what that error code means and what its possible solution is.

There can be many and very diverse breakdowns that cause the boiler to “be on red”. We don’t have space here to go into detail about that (we recommend you search Suner’s blog for our articles dedicated to errors for each brand of boiler ). Many times the anomalies are temporary, and the boiler itself resolves them, restarts automatically, and continues its normal work. Other times the anomalies require the user to restart or reset the boiler, and then everything will return to normal.

In the worst case (that is if the boiler does not resolve the fault by itself or after a manual reset by the user), it will be necessary to call a qualified technician to repair the fault. After that, the boiler will finally stop “being in red”, it will restart and it will work normally again.


When does a boiler not start?

A boiler may not start for many reasons:

  • Lack of general maintenance and excessive dirt and combustion residues. That is why the law establishes that the boiler receives general maintenance at least once a year.
  • Lack of gas or diesel, either because the fuel ran out or because the supply was cut off for any reason (closed tap, faulty regulator, clogged pipe, fuel leaks, etc.)
  • Poor water pressure, either due to lack of water in the heating circuit, because the water circulation pump is clogged, or due to blockages, or because the pressure of the house’s hydraulic system is low.
  • In cases of forced draft, sealed, or condensing boilers, if the gas outlet is obstructed or poorly installed, these units incorporate a sensor that, for safety reasons, prevents the boiler from starting.
  • By freezing in very low-temperature climates. If the water in the hydraulic system of the house or in the heating circuit is frozen, the boiler will not start. Some boilers include antifreeze systems and do not have problems of this type. But not all include systems like that.

(See details on this topic, and its solutions, in our Everything You Need to Know About Boiler Repair article .)

Why is my boiler not heating?

There are various reasons why a boiler does not heat (assuming that in this case, the equipment has started). Among the causes are:

  • That the heat exchanger is very dirty (from lime and minerals in the water) or is faulty, in which case it must be cleaned or replaced.
  • That the burner is dirty, clogged, or poorly regulated, which causes poor combustion. This also generates higher fuel costs and an increase in bills and pollution.
  • That the 3 or 5-way valve is broken (or as they say: “communicated”), so that, although the boiler heats the water, it mixes with cold water, and ends up being cold or lukewarm when you least expect it.

(To understand some more details on this subject, read our article Baxi boiler does not heat water .)

How to start the boiler?

Normally, to start the boiler you just have to press the power button, choose the desired temperature, and make sure that the pressure inside the heating circuit is correct (which you can check using the pressure gauge built into the boiler).

If the pressure is below the necessary parameter (for example, it is less than 1 bar), it is necessary to top up the heating water system, so that the equipment works correctly. It is best to look at the manufacturer’s manual, since for each brand of boiler the ignition procedure may be slightly different, as well as the normal working pressure.

(In this article of ours you can consult a table with the normal working pressure parameters for the most frequent brands of boilers sold on the market.)

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