National Oilheat Research Alliance 5
This is somewhat complicated because steam
boilers are rated in B.T.U./HR and relief valves
are typically rated in LBS/HR. The conversion
factor is 970 BTU/HR = 1 LB/HR.
A boiler rated at 868,000 BTU/HR will need a
relief valve rated at 895LBS/HR
868,000÷970 = 895 LBS/HR
Low water cut-off (LWCO) - LWCO’s
protect boilers against damage, are required on
all steam boilers and are installed at, or above,
the boilers minimum safe water level.
There are two common types of steam
LWCO’s, float type and electronic probe type.
The float type has a device inside, similar to a
toilet tank float, that moves up and down as the
boiler’s water level changes. If the water level
gets too low, the LWCO activates a switch that
cuts power to the burner.
The electronic probe type utilizes an electronic
probe. When water is in contact with the
probe, it completes a circuit between the probe
tip and the boiler surface. If the water level
drops below the probe, the circuit opens and
shuts off power to the burner.
Electric water feeder - a LWCO can be
connected to an optional electric water feeder.
When the water level drops to an unsafe level,
the LWCO sends a feed signal to the water
feeder. The feeder then waits for a period of
time to allow condensate to return to boiler.
If make-up water is required after the delay
period, the feeder opens and allows water to
enter the system.
Boiler feed valve – This is typically a
hand valve that is opened manually to add
water to the system. Once the system is filled
the valve is turned off. When an electric water
feeder is piped in to the system, the manual
feed valve is piped in as a bypass.
Backflow preventer - These devices are
installed on boiler feed lines to prevent contaminants
in the heating system from flowing
back into the potable water supply.
Pressuretrol – A line voltage limit control
that controls steam pressure in the boiler by
starting and stopping the burner based on the
pressure in the boiler. Pressuretrols must be
installed above the boiler’s water level and a
pigtail (siphon) must be installed between the
boiler and the control.
There are two common types of pressuretrols
in the field, additive and subtractive:
An additive pressuretrol (AKA “Cut-in)
has 2 adjustments, “cut-in” and “differential.”
The cut-in setting is the pressure at which the
control’s contacts will close, sending power to
the burner.
The differential setting is adjusted to the
amount of pressure OVER the cut-in at which
power to the burner will be cut.
Figure 3