A.O. Smith VB/VW- 750 User Manual

9

The “V(B/W)” models incorporate an outlet water probe consisting

of two limit controls:
1.  A Manual Reset High limit control that can be set as high as either 

 210°F (99°C) or 235°F (113°C), depending on the application.

2.  A  fixed  manual  high  limit,  factory  set  at  244°F  (118°C).  If 

  the manual reset should open due to high temperature, the  

  gas valves will close and unit will go into lockout. If lockout 

  occurs, push the SELECTION button on UIM to restart boiler.

The ON/OFF Switch is a single-pole, sin gle-throw rocker switch.  

This switch pro vides 120V from the line source to the boiler.

HOT WATER SUPPLY BOILER-VW, the circu lating pump is 

integral to the VW models. This pump has been lu bricated 

at the factory, and future lu brication should be in accordance 

with the motor manufacturer’s instructions provided as a 

supplement to this manual.
HOT WATER HEATING BOILERS-VB, the cir culating pump is NOT 

provided on standard models (optional) and must be obtained and

installed in the field.
NOTE: If a system pump is to be installed on a VB model, 

the maximum rating of pump motor must not exceed 1 hp. 

INLET / REMOTE 

TEMPERATURE 

PROBE

OUTLET

TEMPERATURE 

PROBE

INLET / REMOTE 

TEMPERATURE 

PROBE

OUTLET TEMPERATURE 

PROBE

Temperature probes are 3/4 inch male NPT threaded immersion 

probes, see Figure 7. Temperature probes have embedded 

temperature  sensors  (thermistors). The  boiler’s  control  system

monitors these sensors to determine water temperature at various 

points in the system. 
INLET AND OUTLET TEMPERATURE PROBES
All VF boilers have one Inlet and one Outlet Temperature Probe 

factory installed in the top of the heat exchanger to monitor the 

water temperature entering and leaving the boiler. The Inlet Probe 

is a temperature sensor only and has two leads. The Outlet probe 

also contains the manual reset high temperature limit switch and 

has four leads. The control system displays the Inlet and Outlet 

water temperatures sensed from these two probes on the default 

Temperatures screen.
REMOTE TEMPERATURE PROBE
All VF boilers are supplied from the factory with a Remote 

Temperature Probe. The supplied Remote Temperature Probe 

is used to control system water temperature for a single boiler 

in a domestic hot water storage tank or in the return line from a 

primary/secondary hydronic heating system. Use of the Remote 

Temperature Probe allows a boiler to sense the actual water 

temperature inside the storage tank or hydronic heating loop. 

The boiler will modulate its firing rate in response to the 

actual system temperature and load conditions. The control  

system displays the temperature sensed from the Remote 

Temperature Probe as the “Tank” temperature on the default

Temperatures screen.

QUAD THERMISTOR PROBE
When connecting up to 4 boilers to a single storage tank 

or one primary/secondary hydronic heating system the 

optional Quad Thermistor Probe should be used. The Quad  

Thermistor Probe is a remote temperature probe with four 

temperature sensors embedded in one device. The Quad Thermistor 

Probe allows up to 4 boilers to sense system temperature from 

same point in the system. Use of the Quad Thermistor Probe 

will allow each connected boiler to individually sense actual 

water temperature in the storage tank or hydronic heating loop. 

The temperatures sensed from each of the four temperature 

sensor circuits in a Quad Thermistor Probe are shown as “Tank”

temperature on each boiler’s default Temperatures screen.

NOTE: See the Field Wiring, Remote Temperature Probe 

Installation and the Primary System Control sections of this manual 

for operating and installation instructions.

If boiler is installed above radiation level, a Low Water Cutoff Device 

must be installed in boiler outlet at time of installation or, order 

pre-installed from the factory. If low water detection is required by 

authorities having jurisdiction, a low water cutoff switch should be 

installed in the boiler outlet water line. The switch should receive 

periodic (every six months) inspection to assure proper operation.

An ASME rated pressure relief valve is furnished with the boiler. 

Never operate the boiler if it is not filled with water and a properly

sized pressure relief valve is not installed.
The pressure rating of the relief valve should be equal to or less 

than the rated pressure capacity of any component in the system 

including the boiler. Should the valve need to be replaced, call 

the toll free phone number listed on the back of this manual for 

further technical assistance.

Relief Valve must comply with 

ASME code.

Properly sized Relief Valve must 

be installed.
Can  result  in  overheating  and

excessive  tank  pressure.
Can cause serious injury or death.

A discharge pipe from the relief valve should terminate at an adequate 

floor drain. Do not thread, plug, or cap the end of drain line.

Pressure Relief Valve discharge pipe must 

  terminate at adequate drain.