Way back in 1999 someone called sonofdawra wrote a very helpful
the lighting sequence of 58-type Carrier furnaces (see below).
All this [see below] makes senso to me, but what I do not understand is
why the design is
using pick AND hold signals for the pilot valve. Why could one not
just have a hold input that
simply controls the pilot valve? Or in other words: what would happen
the cold contact on the 3-wire pilot [connecting yellow to brown] was
permanently closed and the bimetal switch would only IN ADDITION supply
power to the main valve [connecting yellow to white] once flame was
The only downside I can see is that the spark circuit would be
the whole heat cycle, but since the pilot is lit all the time, no
POST from Nov 21 1998, 12:00 am
I will make an assumption here that may not be correct, but here
system that has the bimetal pilot safety switch with the 3 wires coming
white, yellow and green], then the following may help. Otherwise,
will be of some service to someone else.
The main burners will light and then go out after about 10-15 seconds,
and go out, over and over.
Below is a training handout I use to teach this technology to techs.
quite a bit of it on Bryant and Carrier furnaces in our area.
There are basically three reasons why this could be happening:
1. The pilot safety switch is going bad
2. The pilot orifice is partially plugged and needs cleaned or a new
assembly itself needs cleaned
3. The inlet gas pressure to the furnace is low. The pilot runs off of
pressure and with low gas pressure, when the main burner gas is
the inlet gas
pressure drops and there isn't enough gas to the pilot.
This system is a low tech intermittent pilot system that if your tech
before, it is a little strange. The pilot safety switch is a
double-throw switch. It
has a common lead that brings the 24 volts in and 2 output leads. When
calls for heat, the 24 volts is routed into this switch on the common
it goes out
through the contact that is closed when the bimetal is cool. When the
lights, the flame
hits the bimetal which makes it bend which causes the switch inside to
contact and close the "hot" contact. This then changes which lead the
is sent back
The gas valve that is used on this system is not a typical gas valve
instead it has three 24
volt operator coils in it. One is called the PICK, one is called the
the third is
called the MAIN. The HOLD is directly energized by the thermostat call
PICK and the MAIN are energized through the 3-wire bimetal pilot
there are 3 operator 24 volt coils, there are only 2 actual operators.
takes energizing both
the PICK and the HOLD operators to open the pilot operator in the gas
flow of pilot gas and allow internally for the gas to flow down to the
MAIN operator. But it takes only the HOLD being energized to keep the
open once it has opened.
I will below try to diagram the way the gas is controlled thru the gas
gas in---manual on/off operator----pilot operator---main operator---gas
The following is a sequence of operation for that style of
uses the three wire bimetal safety switch:
1. The thermostat calls for heat.
2. 24 volts goes to the HOLD coil in the gas valve and 24 volts goes to
3. The 3-wire pilot switch sends 24 volts out through the "cold"
that then produces the high voltage spark and at this time 24 volts
sent to the
4. With 24 volts now to the PICK and the HOLD coils on the pilot gas
in the gas
valve, then that operator opens and gas flows out to the pilot and
internally in the gas
valve, gas flows down to the inlet of the MAIN operator. To open
both the PICK and the HOLD coils must be energized, but to keep the
open it takes only the HOLD coil staying energized.
5. With the spark going [see step 3] and the pilot gas flowing [step
pilot now lights.
6. The bimetal in the pilot switch assembly is now heated by the pilot
and the bimetal
bends causing the switch to toggle. The "cold" contact is now
is made. This shuts off the 24 volts to the high voltage spark box
operator in the gas valve. At the same time, because the "hot"
made, 24 volts is
sent to the MAIN operator coil and the MAIN operator opens to send
manifold to the main burners.
7. The pilot light provides the source of ignition for the main burner
now the main
burners are light.
8. As long as the thermostat keeps calling and the bimetal switch works
contact stays closed], the HOLD and the MAIN operators stay
thermostat is satisfied.