SAUNIER DUVAL THEMA C AS 23 E Furnace: Rapid Pressure Increase to 3.5 bar & Radiator Venting

Hello
Yesterday, the pressure in the Saunier Duval THEMA C AS 23 E furnace increased to 3.5 bar while heating CO, the failure indicators in the air system and in the gas system came on and the stove turned off. I vented the radiators, released the water to a pressure of 1.5 bar. I fired up the stove - everything ok, the radiators work, but the pressure rises INSTANTLY, about 1 bar for half an hour and the stove turns off unless I drain the water. City water inlet valve closed.
Do you have any idea what it might be?

Buddy, you either have a defective release valve, or the temperature in the boiler rises too high.
One more thing, is your diaphragm tank in working order?

The water release valve seems to be the case. The expansion vessel (oval, metal, red at the back of the stove) was "vented" by a roommate - some gas flew out and then water. However, the problem has not gone away. The diaphragm tank seems empty. I don't know if it should be like that. The problem suddenly appeared yesterday, everything was fine until then.
Added after 7 [minutes]:

Added after 7 [minutes]:

edit. With the stove turned off, the pressure also increases - so I think it's a water supply valve?

Maciej Zelechowski wrote:

The water release valve seems to be the case. The expansion vessel (oval, metal, red at the back of the stove) was "vented" by a roommate - some gas flew out and then water. However, the problem has not gone away. The diaphragm tank seems empty. I don't know if it should be like that. The problem suddenly appeared yesterday, everything was fine until then.
Added after 7 [minutes]:
Added after 7 [minutes]:
edit. with the stove turned off, the pressure also increases - so I think it's a water supply valve?

Buddy, is this an expansion vessel, as you write, is not a diaphragm vessel that protects the installation against pressure increase?

it's about the orange part.

It was similar for me, the pressure increased during heating, but when the stove was not working - it decreased. The expansion vessel was damaged, after its replacement the problem disappeared. In your case, it could be a leaky valve.

Hello, I had such a case once with my Duval Isotwin Condens F 30 E sauna - the cause was corroding inside the heat exchanger - this was the way the water was squeezing from one system to another. After replacing the exchanger, the problem ceased.

Of course, by the way, because of this, the membrane in the expansion vessel was damaged, but the stove was working. The service technician did not replace this membrane for me, saying that he did not take it with him, and this is only the loss of a glass of water a day, so it is not worth the trouble and now, after 4 years, I bake, when I replaced the seals in the three-way valve (it keeps tightness), when switching heating from DHW on CO, turns off the burner. When switching from DHW to CH heating, the pressure increases, the non-functioning membrane does not cushion, the pressure rises momentarily and the pressure sensor turns off the flame.