Boiler wiring is comprised of some basic components that are universal to both
gas and oil fired as well as steam or hot water. They all have a power source
and boiler controls. Boiler controls serve two purposes, safety and sequence of
operation. The wiring for boilers is usually a combination of line voltage and low
voltage. The boilers I will try to cover here are residential type systems although
the principles are the same for larger boilers. Systems include gas-fired hot
water, gas-fired steam, oil-fired hot water and oil-fired steam.
Boiler wiring can seem complicated but it really is not when you break it down.
It still always comes down to a combination of switches controlling something. In
the case of boilers, much of the control wiring is safety related. Most boilers
today have some common components.
First is a 120 volt power source. This source will feed a transformer, for all the
low voltage controls, and the circulators which are 120 volts as well. Most of the
remaining controls are low voltage with power supplied from the transformer.
Here are some coomon controls and how they function:
The spill switch is installed on the draft hood of a central heating system or
water heater and shuts off gas to the main burner in case of a sustained back
draft or flue gas spillage. Once shut off, the burner cannot be reactivated until
the switch is reset manually. Repeated trippings of the switch is a clear signal of
a chimney or vent problem.
A low water cut-off will shut down the boiler in the event there is not enough
water in the coils. This will prevent a boiler core from cracking.
The High/Low water temperature switch will only allow the boiler to turn on if
the temperature is with a range that can be set manually.
There is a damper switch on the damper motor that will not let the boiler start
unless the damper is completely open.
Some basic tips should be noted when troubleshooting.
· Voltage tester that can test 120 volt and 24 volt
· Continuity tester
· Slotted screwdriver
· Phillips screwdriver
· Piece of wire for jumper #14 awg minimum
Before you begin
· Be sure the power is on to the boiler, check switch and circuit breaker
· Be sure thermostat is turned up and is working
· Look for obvious loose connections or connectors that may have come loose
Understanding the basic boiler circuit
It is important to first understand the basic operation of a boiler. There is always
a sequence of operation that must be followed. Most times on newer boilers you
can find this either on the inside of the front panel on the unit or in a manual
included with the boiler. A common sequence would go like this.
1.Thermostat calls for heat
2.Relay closes and energizes circulator and the control relay in the boiler
3.The damper motor is energized
4.When damper opens fully end switch closes control relay
5.Gas valve opens
6.Boiler fires and runs until temperature limit switch turns boiler off or
thermostat reaches desired temperature.
That’s the basic idea and what you need to find out for the boiler you are dealing
with. If you take a look at the following illustration you can see there is quite a
few other components in the system. These all have to be in the desired state for
the system to operate. If any one of these detects a problem or does not meet
desired levels the system will not operate.
This is just a one line diagram to show all the items that work to fire the boiler.
This is not a wiring diagram
Most boilers today come pre-wired or as a package. What this means is some of
the boiler controls are already wired or have plug in connectors that just need to
be put together. Some packaged boilers need a little more electric assembly than
This is an Argo 4 zone relay. This unit can be used to control 4 seperate zones or
3 zones and a hot water tank. The hot water tank would be the priority zone. A
priority zone on a relay means that zone takes presedence over all other zones.
Here is some pictures of equipment we have installed
|(C)Copyright 2005 Forte Electric Inc.