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 replace the old boiler, the new one is usually 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 replacing a steam boiler, it is advisable to maintain the water level at the same height as the old boiler. 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 remain wet; otherwise, the radiators with the new dry returns will stop heating. A simple rule is that what was wet before should remain wet when the new boiler is installed.