Filling 12L DHW Expansion Vessel with Air Connected to 150L Hot Water Tank: Overcoming Challenges

I have a question, how to fill the expansion vessel with air?
I have a 12l expansion vessel connected to a 150l hot water tank.
I noticed that the vessel tank is empty (the pressure is 0, when I press the valve there is no air hissing).
I tried to pump up the air with a pump, but it cannot be done. I feel resistance pumping air, I have no feeling that wherever the air is escaping and the tank is not filling up. The pressure is zero all the time. I also replaced the air valve insert.
What am I doing wrong?

Can the vessel be damaged? (he is already 9 years old but what can he get damaged in?)

Hello friend. I guess it's the diaphragm's fault. Try to fill with pressurized water or it won't flow out through the air valve.

And the vessel is not under water pressure all the time?
It is mounted on a pipe that supplies cold water to the domestic hot water tank
Should I disassemble it and connect pressurized water to it?

I attach a picture of the vessel
https://obrazki.elektroda.pl/8422634300_1400359445.jpg

I think I must have emptied the diaphragm vessel myself. A few months ago, I replaced the magnesium anode in the tank. When I unscrewed the screws from the tank lid to get to the anode, there was a hiss of air for a long time.
Do I need to refill the expansion vessel after installing the anode?

Tomi_ wrote:

I think I must have emptied the diaphragm vessel myself. A few months ago, I replaced the magnesium anode in the tank. When I unscrewed the screws from the tank lid to get to the anode, there was a hiss of air for a long time.
Do I need to refill the expansion vessel after installing the anode?

The anode has nothing to do with the vessel. Except for the water, of course.
If you want to pump up effectively, you need to close the water supply to the tank and release (remove the pressure) the water by turning on the nearest tap. The warm one, of course. Then you connect the pump and blow until it works, necessarily with the tap open. The water is stiff and the pressure will build up quickly making it difficult to pump. Inflate to ~ 0.15mPa.

piotr_boncza wrote:

Inflate to ~ 0.15mPa.

It is a domestic hot water vessel, not what, so you need to pump it up to about 4 bars.

Magister_123 wrote:

piotr_boncza wrote:

Inflate to ~ 0.15mPa.

It is a domestic hot water vessel, not what, which means that you need to pump it up to about 4 bars.

Thanks, I was late unconscious.

Thank you for all your help.
It worked.
I will only add that I also had to turn on the cold water tap to fill the expansion vessel. With only the hot water tap turned on, there was no pressure in the vessel.

This thing is bunched. Check whether the check valve is in front of or behind the vessel. And the best way is to draw an installation or take a photo with a description.

Well, the problem turns out to be more complex than I thought because in the installation (it is 9 years old and there have been no problems so far) miracles are happening to me.

I became interested in the DHW expansion vessel because one evening in a single-function gas furnace (Vaillant VU PL 202 / 2-5 R3) with a reservoir, the pressure rose to over 3 bar and the safety valve was activated.
I drained the excess water to the sewage system using a hose connected to the valve and checked the pressure in the diaphragm tank in the furnace (it was OK) and in the diaphragm DHW tank (it was zero). It may not be logical, but I found that the lack of air in the domestic hot water vessel may increase the water pressure in the stove.
I pumped the tank up to 4 bar following your advice and after a while the problem with too high pressure reappears and the pressure in the furnace keeps rising. What I discharge them by draining the water with the safety valve, after some time the pressure increases again.

And now for the best. The stove is off power. The main water valve is closed, the hot water in the hot water tank (Galmet Rondo 150l) is 52 degrees Celsius, the pressure in the furnace is set to 1 bar (I drained the water through the valve until 1 bar was left) and it is slowly rising all the time.
When I turned off both valves (inlet and outlet) of the water from the furnace to the coil, the pressure does not increase.
As soon as I unscrew one valve (it does not matter whether the inflow or outflow) that supplies hot water from the furnace to the coil, I hear a gentle noise as if liquid was flowing (the furnace is turned off, the main water valve is closed) and the pressure on the pressure gauge in the furnace increases until it finally stabilizes but at a fairly high level. When I open the main water valve, there is a lot of flow - you can see that the water loss is being replenished.

It looks as if the coil has a hole and the water from the domestic hot water tank is being pumped into the furnace installation. Is it physically possible?
The stove works normally, heats the water, the water (hot and cold) flows without problems.

Is the domestic hot water tank to be replaced?

Tomi_ wrote:

Thanks to everyone for your help.
It worked.
I will only add that I also had to turn on the cold water tap to fill the expansion vessel. With only the hot water tap turned on, there was no pressure in the vessel.

What if I have a hot water expansion vessel? how to inflate, turn on the hot tap and wait for the pressure to go down and pump? My plumber said that he had bought a vessel that can be used with warm water, and that the stove was tightly placed around it, so he installed it on a warm one. Until now, it worked as it should, but after a year I see a slight increase in pressure when heating the tap water.