Carrier 58 furnace pilot gas valve

Way back in 1999 someone called sonofdawra wrote a very helpful description of 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 if 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 proved? The only downside I can see is that the spark circuit would be energized through the whole heat cycle, but since the pilot is lit all the time, no sparking occurs.
== POST from Nov 21 1998, 12:00 am
I will make an assumption here that may not be correct, but here goes....if you have the system that has the bimetal pilot safety switch with the 3 wires coming from it [usually white, yellow and green], then the following may help. Otherwise, maybe the description will be of some service to someone else.
The main burners will light and then go out after about 10-15 seconds, and
then re-ignite and go out, over and over.
Below is a training handout I use to teach this technology to techs. We have 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 orifice or the pilot assembly itself needs cleaned 3. The inlet gas pressure to the furnace is low. The pilot runs off of unregulated gas pressure and with low gas pressure, when the main burner gas is turned on, 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 had never seen one before, it is a little strange. The pilot safety switch is a single-pole double-throw switch. It has a common lead that brings the 24 volts in and 2 output leads. When the thermostat calls for heat, the 24 volts is routed into this switch on the common lead and it goes out through the contact that is closed when the bimetal is cool. When the pilot lights, the flame hits the bimetal which makes it bend which causes the switch inside to open the "cold" contact and close the "hot" contact. This then changes which lead the 24 volts is sent back out on.
The gas valve that is used on this system is not a typical gas valve but instead it has three 24 volt operator coils in it. One is called the PICK, one is called the HOLD and the third is called the MAIN. The HOLD is directly energized by the thermostat call for heat. The PICK and the MAIN are energized through the 3-wire bimetal pilot switch. Even though there are 3 operator 24 volt coils, there are only 2 actual operators. It takes energizing both the PICK and the HOLD operators to open the pilot operator in the gas valve to start the flow of pilot gas and allow internally for the gas to flow down to the inlet of the internal MAIN operator. But it takes only the HOLD being energized to keep the pilot operator open once it has opened.
I will below try to diagram the way the gas is controlled thru the gas valve.
gas in---manual on/off operator----pilot operator---main operator---gas out to main burners
The following is a sequence of operation for that style of Bryant/Payne/Carrier furnace that 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 the 3-wire pilot switch.
3. The 3-wire pilot switch sends 24 volts out through the "cold" contact to the spark module that then produces the high voltage spark and at this time 24 volts is also sent to the PICK operator.
4. With 24 volts now to the PICK and the HOLD coils on the pilot gas operator 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 the pilot operator, both the PICK and the HOLD coils must be energized, but to keep the pilot operator open it takes only the HOLD coil staying energized.
5. With the spark going [see step 3] and the pilot gas flowing [step 4], the pilot now lights.
6. The bimetal in the pilot switch assembly is now heated by the pilot flame and the bimetal bends causing the switch to toggle. The "cold" contact is now opened and "hot" contact is made. This shuts off the 24 volts to the high voltage spark box and to the PICK operator in the gas valve. At the same time, because the "hot" contact made, 24 volts is sent to the MAIN operator coil and the MAIN operator opens to send gas down the manifold to the main burners.
7. The pilot light provides the source of ignition for the main burner gas and now the main burners are light.
8. As long as the thermostat keeps calling and the bimetal switch works correctly ["hot" contact stays closed], the HOLD and the MAIN operators stay energized until the thermostat is satisfied.
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