compression tank surging at start up

So this past summer I got ambitious and removed the nine cast iron radiators in my two story house and took them to a powder coater for sandblasting and powdercoating. They are 1920s Corto design and look beautiful after removing 5-6 coats of paint. Reinstallation went well with no leaks.

The only change I made to the system involves the old Trane steam control valves on the radiator inlets(the original system was a Trane low pressure steam system, now converted to hot water) which I opened to their maximum opening after seeing how restrictive they were at the midpoint where they had previously been set. I think they were designed to throttle very low flows of steam.

I have a Burnham non condensing125,000 B hot water boiler about ten years old with a cylindrical(non bladder) expansion tank with a BG Airtrol fitting, plumbed to the top(outlet) of the boiler. The circulator pumps away from the boiler.

The boiler has always started up quietly, and only a very slight flow sound at the expansion tank at shutdown. When I started it up this fall, there were a lot of flow sounds (surging?)into or out of the expansion tank that persisted for about 15-20 seconds, then subsided. I have bled the air out of the radiators, as well as 3 convectors that are not as pretty as the radiators. I also drained the expansion tank and refilled it. Once it is up and running, all the radiators heat up uniformly. If I close the ball valve to the expansion tank, it starts up quietly.

Any ideas why It is noisy now? Does it need to go through a number of start up shut down cycles ? It really hasn't run much yet this season. Thank you for any advice offered.

Peter Ferguson

La Crosse, WI