Sail Switch; Limit Switch; Dual Solenoid Gas Valve - Atwood AFS Series Training Manual

Sail Switch

The sail switch is an air proving device. It is used as a safety component that will not let ignition occur until it sees air
produced from the motor wheel. It insures that the combustion wheel is rotating fast enough so that there is a proper air for
gas mixture for smooth ignition.
There are different size sail switches. The noticeable difference is the size of the paddle and the switch. Each switch is
matched to the size of the motor it must respond to on older models. New 7900-II and 8012-II series units have two
switches one for all the 7900 series and one for the 8012 unit. The new 8500-IV and 8900-III series have only one switch
for both the 8500 series and one for the 8900 series.
When the paddle of the switch is depressed, there should be continuity through the switch. If a sail switch needs to be
replaced, it can be replaced with the current series switch. If a larger switch than the original is used, it will probably not
close when the motor produces the required air flow and therefore keeping ignition from occurring.
The most common problems with these switches are bent paddles, loose wire connections or an obstruction between the
paddle and switch contact. If continuity cannot be read through the switch, the switch must be replaced.

Limit Switch

The limit switch is a very important safety device on a furnace. The function of a limit switch is to protect the furnace from overheating. These
switches come in a variety of temperature ratings and are located at critical locations above the heat chamber on the various models of the
furnaces. Therefore, it is very important that when one of these switches is replaced, you do so with the properly temperature rated switch.
Failure to do so could causes an unsafe condition with the heating system. When in doubt as to whether you are using the proper limit switch
for a furnace, look at the temperature setting located at the base of the switch and match it to the temperature and related switch noted below.
Part Number 36205 was a different type of limit switch was use for a short period. This switch does not reset itself it is a manually reset limit
requiring resting by the operator and was only used on 89-II furnaces. It was located above the chamber and tripped in the event of a burn
through the chamber area.

Dual Solenoid Gas Valve

We use a dual solenoid gas valve on all or our furnaces. The valve requires a
minimum of 9.0 VDC in order to open and when open, should draw no more than
.48 amps.
Later model furnaces have an ON/OFF switch on the wire to the gas valve. This
replaces the manual shut-off that was used on earlier model furnaces. You can now
electrically shut off the gas to the valve with this switch before servicing the
furnace.
When these valves become inoperative, it is typically one or both of the coils that
fail. In order to determine which coil is a fault, you need to conduct a continuity test
on both leads of each coil. The resistance on a good coil will be 30-50 ohms. If the
resistance is not in this range, the coil is defective and must be replaced.
The coils on these valves are wired in parallel. Therefore, when replacing the wires
on these coils, it is important that they be attached per the diagram. The red
12VDC supply wires must be attached to terminals 1 and 4 and the ground wires
must be attached to terminal 2 and 3. If the coils are mistakenly wired in series and
there is a marginal supply voltage, there will be a voltage drop from coil to coil and
the will not open.
39