The loss of vacuum in a steam system is caused by any one of the following 3 conditions:

1. Loss of steam

2. Too much steam

3. Air Leaks

Below I will discuss the 3 conditions.

Firstly, a loss of Steam can be caused by a boiler shut down or the main steam valve being closed or not open enough to provide sufficient steam.

Secondly, if there is too much steam in the system, the steam will continually fill the radiators with steam.  As a result, no vacuum will occur in the system.

And lastly, air leaks tend to occur under various circumstances – the most common being the radiator valve.  The correct type of radiator valve should be a packless valve.  Very often, the packless valve is replaced with the packed valve.  Over a period of time the packing dry’s out allowing air to enter the system.

Another common cause of air leaks is pin holes in the piping.  This can occur when the pH level of the condensate is low causing acid to attack the bottom of the steam and return piping.  Often pin holes occur where the nipple from the radiator trap passes through a tile floor.

Air leaks also tend to occur in underground piping.

Most air leaks are not apparent when the system in operating due to the lower pressure inside the piping.  Sometimes they can be seen when the system is shut down.  Pressurizing the system will identify the majority of leaks, and is by far one of the best methods to detect any leaks.