Old Mora Two-System Stove: Pressure Rises to 3-3.5 BAR, Safety Valve Release, Radiators Hot

Hello
I have a problem, I have an old two-system Mora stove. When the furnace is started, the pressure on the furnace rises to 3-3.5 BAR until the safety valve is released. All radiators are hot water on the return is warm, it never happened. I also noticed that when I unscrew the tap with warm water, the pressure also increases. The increase in pressure at work in what happens quite quickly because within a dozen or so minutes. After the oven has cooled down, the pressure returns to normal. Nominal pressure on a cold oven is approx. 1 Bar.
I am asking for help, because I can not afford a new stove.
greetings

Search the electrode, for example
https://www.elektroda.pl/rtvforum/topic1208249.html

A linked thread can introduce confusion.
It is worth knowing the basics, because thanks to them you can determine yourself what the problem may be when the pressure rises.
The pressure can increase due to the heating of the water and the faulty diaphragm vessel.
In addition, the pressure can increase if we add water to the installation.

Your problem may be due to one of the above items or both at once.
In the case of problems with the diaphragm vessel, check the pressure and top up if necessary. It may turn out that you will need to replace the valve (the same as in the car), and in the worst case, the whole vessel.
In the event of a problem with topping up the water may be a faulty valve that allows water or if you have so-called Secondary plate heat exchanger it can be damaged and drains water into circulation What repair is basically the replacement of a defective element.

In any case, this requires checking and remedying the fault. If you do not have the knowledge and skills you need a visit from a service technician. If you have a technical jam and manual skills then you can try to solve the problem yourself if you have access to spare parts.
The easiest way is to check the pressure in the diaphragm vessel, alternatively replace the valve and inflate the vessel. I realize that some people can not change a light bulb in a car, it's hard to expect them to deal with a diaphragm vessel.
Replacing the boiler with a new one is unlikely to be needed, but for the repair you need to consider the amount of several hundred PLN for parts + labor.

You wrote that the radiators are hot and the return is warm, and it has never been like this before. Write how the installation was before. Does the boiler heat up to the set temperature, or does it not respond to the settings and heats up to the max?

So far, the stove has been working properly. The radiators were hot and returning too, but the pressure was not rising so rapidly, rising from 1.5 bar to max 1.9 bar, this was caused by the expansion of water under the influence of temperature.
Now the water pressure increases from 1.0 bar to 2.0-2.5 bar before the hot water returns to the furnace to heat the radiators well, the pressure rises to 3.5 bar until the free valve is released. After closing the water in the apartment and the water supply to the stove (even the tap water inlet) pressure continues to rise.
In this way, I probably eliminated the damaged exchanger and the water inlet valve to the furnace, because there was no water in the apartment.
greetings

So hot radiators were earlier and your comment that this was not the case only for the increase in pressure.
If before the test you cut off the main valve in the apartment and additionally the valve at the entrance to the boiler, and yet the pressure increased, you can assume that the exchanger and valve is OK, although it does not give us 100% certainty.
Focus now on the diaphragm vessel.

Check for the test of the pressure in the vessel on the cold installation.
I have almost 1.4 bar on a cold installation, then it shows 1.2 bar on the vessel.
Personally, I did not test what is the pressure with an empty vessel when the installation is flooded with water. In theory, with an empty vessel, the water will completely fill the vessel, so the manometer will show some pressure in the vessel, unless the valve completely lets through.

You should properly drain the water from the boiler and then check the pressure in the vessel.
Typically, manufacturers give the vessel pre-charge pressure of 0.75 bar and this is sufficient for single-level installations which, in the case of heating bunk houses, you have to count 0.1 bar for every 1m of the installation height, so the pre-pressure in the vessel can be higher.

I have 0.8 bar in my dish.

The system pressure must be at least 0.2 bar higher than the pre-pressure in the expansion vessel - I assume a value of 0.3 bar. Apparently 0.2 bar indicates some Polish standard, and 0.3 bar German standard.
If there is 0.8 bar in the vessel, it must be at least 0.8 + 0.2 = on the boiler with cold water 1.0 bar or according to what I am accepting 0.8 + 0.3 = 1.1 bar
As the upper value, I assume 1.5 bar, so at a pressure of 0.8 bar in the diaphragm vessel, the pressure on the boiler should oscillate in the range 1.1-1.5 bar with cold water. When the cookware is hot, after heating the water, the pressure jumps up by approx. 0.2-0.3 bar, so the working pressure in warm water should reach a maximum of approx. 1.3-1.8 bar .
The safety valve nominally has 3 bars, but 0.5 bar tolerance is assumed, so the installation should never exceed 2.5 bar, if we want to avoid the outgoing water from an efficient safety valve.

You are great and I'm happy.
On the vessel there was 0.1 bar after inflating the air to 0.9 bar and setting the cold water pressure to 1.2 bar I started the stove, and the successful stove heats the radiators, they get warm and the pressure gauge has 0.1 bar of water pressure.
Thank you very much for a wise hint.
Regards, prologue12

I'm glad that I could help

I am a Vaillant boiler user and how can I try to help others. Mechanisms of operation in some situations are similar regardless of the brand of the boiler, so it is universal knowledge.
Since you have given 0.9 bar on the vessel as a pre-pressure, the minimum pressure on the boiler for cold water is 1.1 / 1.2 bar depending on which standards you want to stick to.
Make sure that you never go below this value.

In your place I would give 1.3-1.4 bar on the boiler just in case you want to go through the venting procedure of the installation or have a microreservoir through the automatic vents. After heating, it will still be a safe value, far from water spilling through the safety valve.

If you are doing annual inspections, remember to check the pressure in the diaphragm vessel. Some vessels hold the pressure for a long time and it is enough to refill the pressure once every few years but there are those for whom refilling once a year is a necessity or even does not last a full year.

In addition, I will add that according to the instructions of the Reflex company (manufacturer of diaphragm vessels):

I. initial pressure of the vessel = minimum operating pressure of the installation, which, according to boiler manufacturers, is usually defined as 0.75 bar.

To calculate the initial pressure of a vessel, take into account:
1) static installation height
for every 1m of height, 0.1 bar counts, so for example, with a one-level installation in a 3m high, it gives 0.3 bar.
2) evaporation pressure - regarding the temperature above 100 degrees, so this case is not applicable and can be omitted
3) differential pressure on the circulating pump - regarding the situation when the vessel is on the pressure side, and usually in gas boilers is on the suction side, so in our case this value can be omitted.

As a result, for 3m heights we have 0.3 bar.
The calculated value is increased by 0.2 bar (according to Reflex instructions and the Polish standard) or 0.3 bar if we want to suggest a foreign standard.
Finally, we have 0.3 + 0.3 = 0.6 bar.
Manufacturers of boilers usually specify a minimum of 0.75 bar, which means that the pressure in the pot is sufficient for a single-level installation.

In the case of a two-level installation, counting 6 m in height, it gives us 0.6 + 0.3 = 0.9 bar in the diaphragm vessel as the initial pressure.

Reflex recommends 1 bar as the minimum initial pressure of the vessel, even for installations where the calculated value is lower.

II. The filling pressure of the co = initial pressure of the vessel (from point I) + 0.3 bar
- for one level with a height of 3 m, it gives us 0.75 + 0.3 = 1.05 bar on a cold installation after completing the venting
- for two 6m levels, 0.9 + 0.3 = 1.2 bar comes out
- if we would like to have 1.0 bar of initial pressure recommended by Reflex, then we have to fill the installation with min. 1.3 bar.

III. The working pressure should not exceed the pressure 0.5 bar lower than the nominal value of the safety valve, i.e. for the 3 bar safety valve, the maximum permissible operating pressure of the installation is 2.5 bar.
- for a 1-level installation with a height of 3m, the working pressure should be within the range of 1.05-2.5 bar
- for a 2-level installation with a height of 6m, it is 1.2-2.5 bar.
- if we want to have an initial pressure of 1.0 bar in the vessel, we have a range of 1.3-2.5 bar.

As you can see, the required minimum system pressure after filling and deaeration in cold water depends directly on the initial pressure in the diaphragm vessel.

A diaphragm vessel tends to lose pressure over time, so it seems reasonable to maintain a higher initial pressure. The value of 1.0 bar seems to be optimal, because it gives a reserve of pressure and there should be no need to top it up between reviews. On the other hand, the required minimum filling pressure of 1.3 bar is still a relatively low and safe value.

I struggled with a similar problem a few years ago. Ever since I have been calling a specialist once a year to let everything go, I have peace. However, it is gas, so it's better not to play with fire.

arkos123 wrote:

I struggled with a similar problem a few years ago. Ever since I have been calling a specialist once a year to let everything go, I have peace. However, it is gas, so it's better not to play with fire.

I do not know where you see the pressure relationship in the diaphragm vessel with natural gas to make it play with fire.
Supplementing the pressure in the diaphragm vessel can be done by itself and it is a relatively simple operation for those who think.
Not everyone has a predisposition to perform manual activities and if you are careless you can cut even with sheets of paper, then it is better to call a professional.