Reducers
If start-up water hammer is
present after a new boiler
installation, the water level on the
new boiler may be higher than on
the old boiler and “dry” returns
may now be partially wetted.
National Oilheat Research Alliance 49
Chapter 11
NORA Steam
Troubleshooting
In this section, common challenges encountered
by service technicians working on steam
heating systems will be explained.
Water Hammer
There are a number of reasons why water
hammer occurs in a steam system, including:
• When steam encounters condensate that is
too deep in the horizontal piping.
• When hot condensate flashes into steam in
return piping.
• When the boiler is oversized for the connected
radiation, causing water to be carried
over into the system piping.
• When an over-sized tankless coil is installed
in the boiler. When cold water flows
through the coil it can cause the water level
to collapse quickly, pulling down the water
level in the area surrounding the coil. The
water from the rest of the boiler causes hammer
as it flows into the void.
Water hammer can occur on start-up, during
a heating cycle or on shut down.
Start-up water hammer – when water hammer
is present when the unit fires, check for:
• Improper near-boiler piping.
• Sagging or insufficiently pitched steam
mains and returns.
• Improperly installed concentric reducers
in steam carrying
pipes and returns
(they should always be
flat at the bottom)
• Drip traps that are not removing condensate
or that don’t have enough static head on
the inlet to drain condensate without pressure
upstream
Mid-cycle water hammer – if water hammer
occurs mid-cycle, after the system starts to
make condensate, check for:
• Clogged returns on gravity systems, which
cause water to back up the return riser into
steam pipes or dry returns.
• An oversized boiler which will cause too
high a flow in the steam lines and too much
pressure drop, forcing water to back up the
riser into the main or dry returns.
• Improper near boiler piping which can
cause wet steam to be carried into the system.
• Defective steam traps, failed open, allowing
steam to pass into the returns.
• Large one-pipe heat emitters that are venting
too quickly – use a smaller vent or two
smaller vents, one mounted above the other.