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There's no heat in part of the system

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Published
July 10, 2009 Subscribe to RSS feed

The steam traps have failed in the closed position.
If a thermostatic radiator trap fails in the closed position, no air will pass through it, so little or no steam will arrive at the radiator.
Open the top of the trap and examine the bellows. If it's cool and fully distended, the trap has probably failed in the closed position. If the system is running when you remove the top of the trap, notice whether air rushes from the radiator. This is another good indication that the trap has failed in the closed position.
Repair or replace the trap.

The steam traps have failed in the open position.
A two-pipe steam system is like a ladder. One side of the ladder is the supply line; the other side is the return line. The rungs of the "ladder" are the radiators, and at the end of each rung there is a steam trap.
The trap's job is to pass air into the return side (the no-pressure) of the ladder, to close when steam arrives, and to reopen when condensate forms. If a trap fails in the open position, steam will pass into the return side of the "ladder," causing water hammer as it meets the condensate. Repair or replace the steam traps.

There's a water leg before the condensate- or boiler-feed pump receiver.
In two-pipe systems, you have steam traps on the radiators instead of air vents. The steam pushes the air through the traps, into the dry returns and toward the condensate- or boiler-feed pump's receiver. Since the receiver is vented to the atmosphere, that's where the air is heading. If your return line drops below the inlet to the receiver, however, you'll have problems. That line isn't under pressure because it's downstream of the radiator and end-of-main F&T traps. As condensate drains from the radiators and pipes, it will pool in that water leg and form a seal. Air can't vent through that water seal, so the building will take forever to heat.
If the traps are defective, they'll mask this problem because there will be enough pressure to force the condensate out of the water leg. A lot of water hammer usually accompanies this evacuation of the water. That encourages the building owner to have his traps fixed. When you fix the traps he'll have no heat because the air can't get out. At this point, you have two choices. Raise the return line to eliminate the water seal, or install main vents at the outlet side of the end-of-main F&T traps.