• Too long a nipple (anything longer than a
“close” nipple) at the Hartford Loop, causing
water hammer as the boiler water moves
up and down.
• Improperly pitched return line to the boiler
feed system or receiver. A water leg here will
cause hammer and prevent proper venting of
air.
• An added “master” trap at the receiver of
feed system. Double trapping will cause
hammer in the piping between the steam
main drip trap and the trap at the tank.
• Problems with the pumped return piping.
When pumping into a Hartford loop connection
hammer will occur if the feed rate is too
high and the equalizer isn’t large enough
Shut-down water hammer – if water hammer
occurs on shut-down, check for:
• Improperly Hartford Loop horizontal connection..
The top of the close nipple should be
2" below the water line, if it is too close to the
water line, steam can enter the horizontal line
as the boiler shuts down and the water level
drops.
50 National Oilheat Research Alliance
• Uninsulated piping in the boiler area. If
the piping in the building is insulated but the
piping in the boiler room is not, the system
piping will be kept warm by the insulation and
cool more slowly. The cooler boiler room piping
will cause a quick vacuum on shutdown as
the steam condenses. The rapid vacuum will
cause lifting of the boiler water into the header,
bouncing of the water level and water hammer.
Main Vent Problems
The Main Vent, and Riser Vents, are the heart
of a one-pipe system. They must work to provide
uniform heat in the building by removing
air from the steam distribution piping. When
they work properly, all the radiators receive
steam at the same time.
Main steam vents are often piped as shown
on the right. This allows water hammer at the
end of the main to slam into the vent, causing
the float to quickly collapse, causing the vent to
fail in the closed position.
The main vent should be piped as shown on
the left. This piping method protects the vent
from water hammer at the end of the main and
provides an air cushion against shock.
If the main vent doesn’t work, the heat emit-
If water hammer occurs mid-cycle,
check the return piping and the
boiler feed piping. Use a level
to make sure that the piping is
pitched properly.
The boiler level drops on shut-down.
This causes a bouncing water level
in the equalizer line. If the Hartford
Loop connection is too close to the
water line the horizontal connection
can be exposed to steam, causing
hammering.