Hydronic System
Functions
Space temperature sensing
Over-temperature protection
Relay control of circulator motor
by thermostat
Minimum water temperature
maintenance
Prevention of circulator operation
with low limit made
Control
THERMOSTAT
HIGH LIMIT
CIRCULATOR RELAY
LOW LIMIT
CIRCULATOR CONTROL
Figure 2
Types of Hydronic Controllers
Figure 2 shows the control functions (including
domestic hot water) used in hydronic systems.
The thermostat senses temperature, calling for
heat when it is below the set point and turning off the
system when the temperature reaches set point.
The high limit, electrically connected to the burner
circuit, stops the burner if water temperature or
pressure is too high. The high limit may reset
automatically or it may require manual reset.
The circulator relay switches the circulator pump on
and off. This relay enables a low voltage thermostat
circuit to switch the line voltage circulator circuit.
The low limit switches the burner on in response
to a fall in boiler water temperature and switches it
off when the boiler water reaches the
desired minimum temperature. The low
limit is required in heating systems that
also provide domestic hot water. With
the low limit, the boiler water is kept hot
all the time even if the thermostat is not
calling for heat.
The circulator control is also required
in systems that provide both heating
and domestic hot water. The circulator
control prevents the circulator pump
from coming on if the burner is
operating in response to the low limit.
The circulator control grants priority
to domestic hot water because the
circulator cannot run until the low limit
opens.
In some systems, a pressure-regulating
valve is required to reduce line pressure
to system pressure.
Steam systems and some hot water
systems require a low-water cutoff. This
device shuts off the burner if the boiler
water level is too low. It is an important
safety feature that prevents firing the
boiler with insufficient water.
Types of Hydronic Controllers
Hydronic control functions may be
performed by separate controls or they
may be combined with Aquastat controllers and relays.
These common terms are used to describe the different
types of Aquastat controllers:
• Aquastat controllers may act as a high limit only, a
low limit only, a circulator control only or a combination
low limit and circulator control.
• Aquastat relays combine high limit control with
a switching relay to permit low voltage thermostat
control of burner and circulator. Low limit/circulator
control must be added if domestic hot water is required.
Aquastat relays may include a provision for connecting
zone valves.
• Dual Aquastat controllers perform the functions of
high limit and either low limit or circulator control.
• Triple Aquastat relays perform the functions of
high limit, low limit and circulator control and include
circulator/burner relays. They may include provision
for connecting zone valves or additional relays for zone
circulators.
Hot Water Heating Systems
A hot water heating system is one in which water is
used to convey heat by flowing through pipes connecting
a boiler or water heater with baseboard, radiators,
convectors, radiant or other suitable heat distributing
means. The systems we will discuss have supply water
temperatures less than 250°F and 30 pounds pressure
and are classified as low temperature systems.
• The gravity system in which circulation of the
water is due to the difference in weight between the
supply and return water columns of any circuit or
system, and
• The forced system in which a pump, usually driven
by an electric motor, maintains the necessary flow.
Figure 3: Hot Water Systems
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