As we enter the heart of winter and turn our attention to- ward cold weather conditions where freezing water can
wreak havoc on commercial and residential property, the threat of catastrophic
failure to boilers looms large (as seen in
Figure 1). Although failsafes against significant boiler failures do exist and continue to improve, they remain far from
One particularly susceptible failsafe
is a boiler’s low water cutoff (LWCO).
An LWCO is designed to alert the boiler
that it does not contain sufficient water
to “fire,” and prevents the boiler’s burn-
ers from igniting until a safe water level
has been restored. Because these types of
failures can lead to injury or death, most
state and municipality codes require the
installation of low water cutoffs for both
residential and commercial boiler appli-
LWCOs are found in one of two designs: Electronic-probe (a more recent
technological introduction) and mechanical-float (a traditional, lower-tech version, seen in Figure 2). For this article,
we’ll focus our attention on the latter.
What is a mechanical-float?
Mechanical-float low water cutoffs are
Common issues with LWCO
the most widely used water-level safety
device in residential and commercial
heating boiler applications. These LW-
COs are simple in design and function
much like fill arms found in residential
toilet tanks. As the water level drops in a
boiler reservoir, the buoyant arm “floats”
down until it reaches a point at which the
boiler is unsafe to operate, and the switch
turns the heating system off as illustrated
in Figure 3.
Despite their simplistic design, mechanical-float LWCOs are prone to several issues, which can have catastrophic impact
if not addressed. These issues include:
1. Stuck valves: Mechanical-float
LWCO valves are in constant contact
with supply water, and are exposed
to minerals, dirt, rust and other total
dissolved solids (TDSs). This prolonged exposure to TDSs can interfere with the designed function of
the float, preventing it from moving
freely up and down in line with the
actual water level.
2. Clogged LWCOs: Sometimes, TDSs
and other impurities can prevent
the free-flow of water through the
LWCO, and can “trick” the LWCO
into reporting that an adequate supply of water is present in the boiler,
when in fact the minimum safe water level has been breached.
Consequences of LWCO failure
If the float is prevented from moving
freely and from sending the appropriate signal to the boiler controls (and the
burner) to shut down the boiler when
the minimum safe water supply level has
been breached, the boiler may continue
to run without water (or may be allowed
to start without water present).
If a boiler operates without the necessary
water supply, the heat generated in the boiler’s combustion chamber does not transfer
to the water supply, and causes the burner,
heat exchanger, and/or boiler tube(s) to
overheat. With prolonged or repeated exposure to this condition, the walls of the burner, boiler or heat exchanger may weaken and
ultimately crack, as seen in Figure 4.
The Dangers of Freezing
Water & Boilers
H V A C i INSIDE OF A STUCK LOW WATER CUTOFF