The burner short-cycles
The pressure is too high.
If it's a gravity-return, two-pipe system, high boiler
pressure will back water up into the vents, and trap air in the system.
Many of the radiators won't heat. And since air is a compressible gas
(just like steam), the burner will begin to short-cycle as the steam
squeezes the air into a corner. You may be tempted to increase the
pressure, thinking this will cure the short-cycling. But don't do it;
it will only make things worse. Instead of cranking the pressure up,
crank it down. And think like air. Walk along the piping and ask
yourself, If I were air, could I get out? If you can't get out, neither
can the air.
Another thing: overfiring can also cause the burner to short cycle.
Fire only to the connected piping and radiation load - no more and no
less.
Wet steam also can cause short-cycling. The steam leaves the boiler,
the pressuretrol hits its high limit, the steam rapidly condenses and
causes the burner to turn back on.
Dirty water is another common cause of burner short-cycling. Clean the system with trisodium phosphate.
