How to reduce boiler pressure (Saunier Duval)


The Saunier Duval boiler is a precision-made piece of equipment that is made using high-tech parts. Given certain causes (which may be unrelated to the boiler itself), some operating parameters may be altered, such as temperature or pressure. In this article, we will show you, step by step, how to lower the pressure of a Saunier Duval boiler, as well as the possible reasons that the pressure has risen. We will also include a list of spare parts in case there are broken components to replace in your boiler.

Why is there too much pressure in the boiler?

The pressure of a boiler can sometimes exceed the normal limits for which it was manufactured. Excess pressure is one of the most frequent symptoms in Saunier Duval boilers, and it can be due to both simple and serious reasons.

In simple cases, you can fix it yourself. In serious cases, it is better to go to a technician, since excess pressure can cause major damage if it is not rectified in a timely way. Excess pressure arises for many different reasons. The main one may be that you have not turned on the boiler for a long time.

When the boiler remains idle for a long time (sometimes a whole year), the water pressure is affected. It may decrease to less than necessary, or it may build up in excess. In cases of excess, technically the boiler is supposed to block itself (you can tell because the red light turns on) and stop working. But it may not happen that way. You should check the pressure gauge and see the pressure level.

What is the maximum pressure of a Saunier Duval boiler?

The normal pressure of a Saunier Duval boiler is always between 1.2 and 1.5 bar. In other words, the maximum pressure is 1.5 bar. Bars are the unit of measurement of the manometer, which is the gauge (whether analog or digital) with which pressure is measured. But the exact pressure must be measured when cold, with the boiler off.

To be totally sure that you are observing the correct pressure on the gauge, the boiler must not only be off but also cold. It happens that the pressure measurement cannot be completely accurate while the equipment is in operation. This is common to almost any piece of pressure-operated equipment, not just boilers.

Correct Steps to Measure Boiler Pressure

So the steps to correctly measure the pressure of the boiler and understand if there is excess pressure are:

  1. Turn off the boiler.
  2. Wait a few minutes to give it time to cool down.
  3. Now it’s cold, look at the pressure gauge and check the pressure level.

If the cold pressure gauge reads more than 1.5 bars, you have excess pressure. To lower the pressure, you must apply some simple routine measures that we will now look at. But if you can’t do it yourself, you’ll need to call a technician.


Possible technical causes of excess pressure

A boiler is like a living organism. Your pressure can be unbalanced for many reasons. Among the possible technical causes of excess pressure in the boiler we have:

1. One of the valves is not closed properly

Sometimes without realizing it we incorrectly close the valves and this unbalances the pressure.

2. Failed or poorly maintained valves

The valves may be in poor condition or may have completely failed. They can be both the filling valves and the emptying valves. To replace them with spare parts, you must acquire the parts and go to a mechanic to have them replaced.

3. Expansion tank with punctured membrane

One of the mechanisms that boilers have to control pressure is the expansion tank (also called expansion vessel). Its function is to absorb pressure differences during system operation, to keep it stable. It does this through its membrane, which is a sensitive component.

For various routine reasons, this membrane can rupture, and then the vessel no longer fulfills its function and the pressure is unbalanced. In these cases, the expansion tank must be replaced. It is something that should be done as soon as possible, to avoid further damage. In any case, the boiler must not be used in the membrane is broken.


4. Plate heat exchanger with connected circuits

The plate heat exchanger is made up of several metal plates joined by a gasket. The plates form corridors connected to each other. Through these corridors, the water circulates. The objective is to transfer heat between 2 zones that are linked by a barrier, and thus save energy.

If the plate heat exchanger breaks down (and the circuits that must be kept separate contact each other), the pressure in the boiler can rise. It is another of the breakdowns that require replacing the part, so you will have to go to a technician.


5. Contact with water generated air

There may have been contact between the water and some of the installation materials, and this reaction may have generated air.

6. Radiators may be clogged

Radiators can become routinely clogged quite frequently, especially when the boiler has been out of operation for a while.

What to do when the pressure in my boiler is higher than expected?

Just as there can be more than one cause for the pressure to rise, there is also more than one solution to this problem. So when you notice on the gauge that the boiler has risen above 1.5 bar, you can do one of these things:

Open the drain valve

One of the solutions to lower the pressure in Saunier Duval boilers consists of using a screwdriver to open the drain valve and make the air (and even a few drops of water) come out until the pressure gauge reaches 1.2 to 1.5 bar (or 1 bar minimum). The best thing would be to have turned off the boiler before so that the water is not so hot. You will see that when you turn on the boiler again, the pressure will rise to 1.5 bar.

Open the valve for excess pressure

If your boiler does not have a heating water drain valve, then it must have a valve to expel excess pressure (overpressure). Find it, open it, and let the water run out until the pressure on the gauge drops. Preferably you should have turned off the boiler before, otherwise, the water will be hot.

Purge the radiator closest to the boiler


If after opening the drain or overpressure valve you are unable to lower the pressure, then use the screwdriver to bleed the radiators. Go to one of the radiators (preferably the one closest to the boiler), and open the purge valve until it whistles and drains some water. You can use a glass or a container to avoid spilling the water. You could do this with the boiler running. But the most advisable method is to turn off the boiler and wait for it to cool down a bit because otherwise, the water will be hot.


Don’t worry if when you bleed the radiator the pressure drops too low, because you can raise it again. It would be convenient for you to take advantage of this time and purge all the radiators in the same way. The radiators must be purged at least once a year, to avoid symptoms such as excess pressure.

Call service


If none of the previous solutions reduced the pressure of your boiler, then you should turn it off as soon as possible, and notify the technical service to arrange a visit. It is possible that you have some damaged parts and that they have to be replaced. Perhaps it is the expansion tank that is damaged (because the membrane is pitted), or perhaps it is the plate heat exchanger, which has connected circuits (as we mentioned above). These are things that you can no longer repair yourself unless you know about boilers in depth.

3 tips so that the pressure of the boiler does not decompensate

Here are some tips to keep the pressure of your Saunier Duval boiler balanced, extending the useful life of the equipment and avoiding unnecessary expenses.

1- Check the boiler pressure gauge once a week

It is worth checking the pressure gauge once a week and thus keeps the pressure monitored and controlled. In this way, we can balance or adjust it quickly whenever necessary. This can extend the useful life of the equipment and save us from spending on parts, or even on new equipment. The excessive pressure rise is a frequent symptom in Saunier Duval boilers and is one of those that cause the most serious breakdowns, with the frequent need to replace parts.

2- Turn on the boiler from time to time

Do not leave the boiler without turning it on for too long. In winter the boiler is normally used a lot. But if the summer or the hot season has arrived and you do not plan to use the boiler for a while, perform some control ignitions at regular intervals: turn on the boiler for at least 10 minutes and at least 3 times during the entire time in which it is not routinely used. This will prevent the pressure from rising higher than normal when you use it regularly again.

3- Conduct maintenance at least 1 time a year

Many of the pressure problems are based on failures that would never have occurred if the boiler had been serviced annually, or even twice a year. Prevention may seem like a major expense, but it may be minor.

Here is a list of spare parts that you may need for your diesel boiler.

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