tures. Level III control system includes the Model S step
control and an outdoor thermostat. As outdoor tempera-
ture drops below a present point, the Model S step con-
trol is energized and operates the modules to maintain
the desired supply water temperatures.
CONTROL FUNCTIONS & DESCRIPTIONS
Intermittent Ignition Control:
Provides ignition
sequence, flame monitoring and safety shutoff for inter-
mittent pilot.
Transformer:
One per module is factory supplied. It's the
power source for the control circuit and cannot be used
as a power source for other controls.
Hi-Limit Aquastat:
One per module is factory-supplied
(well is factory-assembled on each module). Is a hi-limit
temperature safety control only; it must not be used as an
operating control. For mounting, see Section 5 in this
manual.
Manual Reset Hi-Limit:
Safety control used to shut off
all boilers if system temperature exceeds the setting,
usually 250° F. Must be reset manually by pushing red
button in cover. Locate in supply piping downstream of
connection of the last module, as previously discussed in
Section 3 in this manual.
Low-Water CutOff:
Electronic type or float type. Used to
shut off boilers if water level should fall below its sensor
location. Locate in supply header, as previously dis-
cussed in Section 3 in this manual.
Immersion Aquastats:
Single and dual aquastats are
used to open a control circuit in response to increasing
water temperature. May be used in multiples to provide
step firing in response to either supply or return water
temperature. Dual aquastat, controlled through one tem-
perature sensor probe, has two individually adjustable
temperature actuated switches; single aquastat has one
adjustable switch. Locate in return header; return water
temperature is preferred method for controlling system.
Outdoor Air Thermostat:
Switching device used to turn
on system or system components at a desired outdoor
temperature. Is also used as a heat starter to automati-
cally switch on system power to start heating plant. It is
generally set at an outdoor temperature of 65° F, has a
20-ft capillary tube with remote liquid filled bulb & comes
with sunshield for mounting on a north wall where sun's
effects are minimized; control box can be mounted any-
where within the length of the capillary tube.
System J:
The basic control system consists of an "AR"
reset module which accepts add-on "AA" winter/summer
circulator starter, "Y" transformer module, and "S" staging
modules. Optional add-on modules include "D" tempera-
ture display module and "CLK" night set-back program-
mable clock. Five pin connectors between modules
speed installation time and eliminate potential wiring
errors. The modules can be mounted on any wall surface
or an optional DIN rail.
Motorized Valves:
Commonly used on combination
space and service water heating applications. Externally
powered from a 24-volt source and controlled by a single-
pole double-throw switching device, for fully open or
closed positions. Typical application is to interrupt water
flow to the space heating section of the heating plant
whenever service water heating is in jeopardy, resulting
in a priority for service water heating; this is accom-
plished by installing an aquastat in the heat exchanger
shell, wired to the motorized valve control circuit so it will
close off boiler water flow to space heating when the heat
exchanger shell temperature falls below the aquastat set-
ting. For mounting of motorized valve, see Section 3 in
this manual.
Electronic Outdoor Reset Step Control:
(Hydrotherm
Model S Control) Designed specifically to reset system
supply water temperature in relation to outdoor tempera-
ture for large heating plants by step-firing the required
number of boiler modules for the desired system supply
temperature. Consists of all solid state components:
1. Control box for mounting in boiler room includes
microprocessor, L.E.D. indicating lights, and supply
and air temperature digital read out.
2. Outdoor temperature sensor & sunshield for mount-
ing on a north wall, and supply water temperature sen-
sor for mounting in supply header (no capillaries; both
wired electronically).
Optional equipment includes night set-back and time/
cycle monitor. Follow installation instructions packaged
with controls.
WIRING DIAGRAM EXAMPLES
This manual provides several examples of heating plant
control method applications for your use. For additional
information, contact your local Hydrotherm sales repre-
sentative or the Hydrotherm factory directly.
Space Heating Only Applications:
Level I Control,
Figure C2; Level II Control, Figure C3; Level III Control,
Figure C4.
Space & Volume Water Heating Applications With MC
Heat Exchanger:
Level I Control, Figure C5; Level II
Control, Figure C6; Level III Control, Figure C7.
23
