My furnace is going to 30 PSI when the heat runs. I replaced the expansion tank and it is still going to 30 PSI. The auto fill valve is only two years old. What else could it be making the psi go to 30. When the boiler is cold the psi is at 15.

Usually too much water in the system. The system water tempersture will also cause a rise in water pressure. I would revisit the expansion tank install. Is it just a tank or does it have a bladder/with a air valve on the bottom.

It has a bladder/with a air valve on the bottom. I put 12 psi in it.

When the boiler is cold the psi is at 15. Try lowering the cold start pressure to 10 and see what happens.

I did let some water out and lowered the psi to 10 and it went back up to 15 cold start pressure.

Are you sure your expansion tank is sized correctly for your system. What type of radiators do you have. How many. How many levels is your house. What size piping is supplying your radiators? The more hot water you have, the more expansion you will have. You will need to make sure that your expansion tank can handle the amount of expansion taking place in your system.

I did let some water out and lowered the psi to 10 and it went back up to 15 cold start pressure.

If you just let water out to lower pressure, the auto water feed(pressure reducing valve) will just let water back in. You will need to adjust the pressure reducing valve to lower water inlet pressure. Can you post a pic of the water inlet valve? If you can, we will tell you how to adjust it.

The expansion is the correct one. I don't have a picture of my pressure reducing valve but here is a link of what mine LOOKS like. It looks like the one on the RIGHT? Here is the link. http://i152.photobucket.com/albums/s194/yiming__bucket/IMGP0300.jpg How do I adjust it? Thanks.

You are supposed to close the isloation valve on the water line before the auto feed to temp lower the pressure. This test is to help tell where in the system the problem is at.

if you just let water out to lower pressure,, the auto water feed(pressure reducing valve) will just let water back in. You will need to adjust the pressure reducing valve to lower water inlet pressure. Can you post a pic of the water inlet valve? If you can, we will tell you how to adjust it.

Here is the link. http://i152.photobucket.com/albums/s...t/IMGP0300.jpg How do I adjust it? Thanks.