Concepts of Operation.
gas heaters control the flow of gas with the main gas valve, item 3
above. This valve is powered with a very low voltage supply
source, called the pilot generator. The original name for
this device is called a 'thermopile', and is simply an
encased, twisted pair of wires made from dissimilar metals, such as
copper and zinc. When this type of twisted wiring is heated,
it generates a small voltage.
The gas heater uses this voltage to operate 2 separate
electro-magnetically operated valves inside of it. These
precision devices are designed to open and allow the flow of gas, with
as little as 250 millivolts applied to them, however, the valves are
most effective with approximately 450 millivolts of power.
Most pilot generators will generate as much as .850 volts (850
millivolts), when new. Over time, this voltage level will
deteriorate, and the valve's operation may become erratic, and
sometimes just stop working altogether. At that point, if the
voltage output of the pilot generator is less than .250 volts underload
from the gas valve, then it's time to replace the generator.
The Pilot Valve
The first valve is
called the pilot valve. When you initially ignite a pilot
light flame, you must hold down the rotating valve select switch for a
short while to get the pilot to stay lit. What is happening,
is that while you are holding down the red knob, you are bypassing the
electrically activated pilot valve, and allowing the thermopile/pilot
generator to heat up sufficiently, to generate enough voltage to keep
the pilot valve open. Once this happens, the valve is opened,
and then you can release the control knob.
This provides a critical safe operation feature, in that if the pilot
light ever goes off, the pilot generator cools off, and subsequently,
without enough voltage to keep it open, the pilot valve will turn
itself off, preventing flooding of gas in the heater or the room where
it is located.
Main Gas Valve
The main gas
valve, simply supplies gas to the burner manifold. Also
operated by the pilot generator, the voltage/current flow from the
generator goes through a number of devices in a daisy chain
fashion. If any of these devices break the circuit, the main
gas valve will shut down, stopping the heater from operating.
These devices almost always incorporate a thermostat, on/off switch,
high temperature limit switches located in the water manifolds,
pressure switch, and a flame switch which is usually located near the
front opening of the burner tray compartment.
What's a Fireman Switch?
is also a connection block usually colored red, called the 'fireman'
switch connection. What this is normally used for, is for a
connection to a remote thermostat, and to a micro-switch contained
inside the pool/spa timer.
In the latter, let's say that you've got a pool or spa pump set to turn
on at 4:00, and turn off at 8:00. The position of the
microswitch on the timer, shuts down the heater operation approximately
15 minutes prior to pump shut down, to give the heater time to cool
down with the water continuing to run through it. On some
heaters, without this switch, it is possible to cause problems with the
heat exchanger, and/or heater manifolds due to excess heat build up
after the pump has shut down.
The millivolt heater is quite an efficient system, and has been proven
for many many years, and as such, is still being produced by many
Never under any
circumstances, disassemble a gas valve and attempt to repair
it. These are extremely precision devices, and the slightest
error in reassembly, including scratching the milled surfaces or
warping the control rods, could result in a disaster that could burn
your house down... or worse. Always replace the gas valve as
a complete assembly.