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Standard Terms and Definitions
Switching Relay:
Transformer and relay in one box, which allows a low voltage thermostat to control a line voltage load (circulator),
with an isolated end switch that can energize the boiler.
Zone Valve Control:
Transformer and relay in one box, which allows a low voltage thermostat to control a low voltage zone valve, with
an isolated end switch that can energize the boiler and/or circulators.
Dry Contacts:
Set of relay contacts that make and beak, where no voltage is present. Usually can switch 24 or 120 VAC.
Isolated End Set Switch:
Set of dry contacts that make when a zone calls for heat. Usually connected to a boiler’s aquastat TT.
Mode Switch:
Switch set to
NORMAL
– When any zone calls for heat, the end switch will make and fire the boiler.
Switch set to
RESET
– When any zone calls for heat, a heat demand signal is given to external add-on reset control and it controls
boiler operation. Also when the priority zone calls for heat and is switched to priority, the end switch will override reset control and
fire boiler to high limit.
TT:
Where thermostat or aquastat connects on a heating control.
XX:
Denotes Isolated End Switch.
Low Voltage:
24 VAC for most HVAC systems.
Line Voltage:
Normally 120 VAC relative to ground.
H or L1:
Hot side of line voltage.
N or L2:
Neutral side of line voltage.
ZR:
120 VAC (Hot) signal from switching relay to boiler’s operating aquastat to fire the burner to high limit when any zone calls for
heat. Also ideal for controlling primary circulator.
ZC:
120 VAC (Hot) signal from boiler’s operating aquastat to switching relay allowing circulator(s) to run when aquastat is above the
set low limit.
Cold Start:
A boiler that fires only when there is a call for heat.
Tankless Coil:
A method of heating domestic water utilizing a heat exchanger inside a boiler.The boiler also maintains heat year round
to deliver hot water on demand.
DHW Heater:
A method of heating domestic water utilizing a heat exchanger and storage tank.
Transformer:
A component for increasing or reducing AC voltage. Rated in VA (volts x amps).
OHMS Law:
Example: A 24 volt, 40 VA transformer can produce how much current?
Current = VA / Voltage 40 / 24 = 1.6 Amps
I = E/R
where
I = Current in Amps
R = E/I
R = Resistance in Ohms
E = I x R
E = Volts
W = E x I
W = Watts or VA
I = W(VA)/E
RELAY CONTACTS:
N/C:
Normally closed connection of a relay, when not energized.
N/O:
Normally open connection of a relay, when not energized.
Com:
The common terminal for normally open and normally closed.
Pole:
The number of independent circuits of a switch or relay.
Throw:
The number of settings which a switch or relay can have.
DPDT = Double Pole Double Throw
N/O N/C N/C N/O
3 4 4 6 6 5
COM
COM
