My hot water baseboard system was bleeded two weeks ago when the water flowing noise was heard.

Now, every time when heat turn on, there are several big bang noise from the pipes on my second floor. Please tell me what is going on and what I should do?

I last week, I changed new expansion tank. When bleeding the water, the pressure readout is 10-12 psi. When bleeding is finished and the water is heating, the pressure readout is 20 psi.

Thanks!!

Your pressure readings are fine. You still have air in the lines unless this problem has always been there and you are just now trying to address it. If this is a new noise problem go for more air removal if not you will have to Goggle (water hammer) and pick your poison as to the cure you want to take.

In all the years of boiler service I never had to add a anti boiler water banging device to any system we installed so I would really take a hard look at the air in the line situation before you jump into other cures.

Many times you have to bleed the baseboards for a long time before the air will come out and whenever you add fresh makeup water to a system you automatically get more air and it will take awhile for the air to fall out of suspension with the water as it heats.

Dear HVAV1000:

I really appreciate your comments. I will try to bleed the system (for second floor) again to see if the problem can be solved. Please tell me usually how long does it take to complete bleeding. I have 3 bedrooms on the second floor. What is the pressure should I use to bleed the air?

Hope to see your answer soon.

Thanks again.

You can operate the boiler while you are bleeding. The higher pressure makes it easier to bleed the air. Make sure you use a catch pan or use rags so the water does not make a mess or ruin something.

Hi, HVAC1000:

I just finished bleed the air on the second floor where the banging noise happens.

During bleeding, the water pressure is at 20 psi. I let it run for about 20 minutes.
I turn the pressure back to 10-12 psi. Turn off the garden hose and turn off the pressure switch. Now the readout is 20 psi. Then I turn of power of heating furnace and let it go. When water is heated, the pressure is still at 20-22 psi.

There are still banging noise when the water heat turns on. Does it mean that the bleeding is not completed?

Enclosed with is the picture of my expansion which I bought new last week and installed.
There is a small metal cup on the top of expansion. There is a little cap on the cup (arrow points at). What is the cap? Should I close it tight or loose it?

I really appreciate all your help and comments.

HVAC1000:

Just a correction of the typo. "Then I turn of power..." should be "Then I turn on power ...".

Cap on top of tank should be loose and there is a shrader valve fitting under the cap. That is a auto vent. Can you send me the information off the tank you just bought? Name and all the numbers.
The vent looks to me to be a number 67 Maid O Mist valve see if you can check the number on that also.

HVAC1000:

Thanks for the response.

I can not find any information of the cap. I loose the cap as your instruction. When the cap is opened, I see a needle-liked mouth.

The expansion tank I just bought is: WATTS Hot Water Heating System Series ET Expansion Tank. Model ET-30. EDP#067411. I bought from Home Depot.

Let me know if you need any information.

Thanks.

HVAC1000:

I have two zone heating system. My first floor does not have such noise at all. Should I also bleed the air when I bleed air from baseboard on second floor?

It would not hurt to try and bleed the first floor. It may do no good but it will do no harm. Air has to be the problem.

I still have banging noise on my second floor hot water baseboard every time when the heat was tunred on.

The heating water is directly heated up through a gas furnace and then sent out to the first and the second floor. In addition, the gas furnace is also heating up the water for and send to water tank. Is there any realstionship between water tank and hot water baseboard?

I do no hear water running noise in the baseboard anymore, but still a big baning noise when heat turns on.

My first floor hot water baseboard does not have any problem at all.

Can you tell me what I should do? Do I still bleed the air which I have done several times already? At what extend can I stop bleeding and be sure that air completely out?

The bang is from expansion and air. You have done enough bleeding because if is not working correctly now it is something much more difficult to repair.
Next step is to call for a professional to look over your setup to see what can be done about the noise. You will probably need to have them install a water hammer device in your system.