When you are replacing an old boiler with a new higher efficiency boiler you may encounter water level problems, water hammer, short cycling and even boiler flooding. When you change the old boiler, usually the new boiler is shorter. This normally gives us a false sense of security that the A dimension will be correct.

When we lose our "A" (drawing) dimension we will encounter problems. When changing a steam boiler, it is a good idea to keep the water level close to where the old water level was. This is especially true when your system has a dry return, steam pipes run in a crawl space, or the old boiler, was in a pit and you bring the new boiler out of the pit.  A wet return can also create problems if the wet return goes uphill from the boiler towards the end of the steam main and has a short vertical rise between the return and the steam main. Any radiator returns that drop down to a wet return must stay wet or the radiators with the new dry returns will stop heating. A simple rule is what was wet before should remain wet when the new boiler is installed.