Carrier Three Wire Pilots

[John Taylor]

Can anyone give me any insight into the normal operation of a Carrier
three wire pilot. Assembly was recently replaced with a new pilot
because it would not prove pilot flame and thus burner would not fire.
I believe it is supossed to cut out spark after pilot flame is proven
. Burner is now operating but spark ignitor is not cutting out.

Thanks in Advance,

John Taylor

[HVACMAN]

An interesting thread... I've noted (in a properly operationg"740" pilot) that the
spark box cuts out long before the switch makes... are we sure the electrode
doesn't sense flame to stop the spark? I would clean the electrode and make sure
the pilot switch is in it's original position... if the pilot flame misses the
electrode the spark will continue right through main burner light off...

Sonofdawra wrote:

> >Sounds
> >as if gas pressure might be the culprit since main burner is firing.
> >
> Not if main burner and spark both present at the same time.

> >
> >> Burner is now operating but spark ignitor is not cutting out.<<
>

> If the main burner is lighting and staying light, but the spark ignitor is no
> being shut off, it almost has to be a wiring problem. The COLD contacts feed
> the 24 volts to the spark producing module. When the pilot proves, this
> contact opens in the pilot safety switch and the HOT contact makes which
> energizes the MAIN operator in the gas valve to bring on the main furnace
> burners. Therefore, if it is wired up properly, there should be no way that
> the main burner and the spark can be on at the same time. The spark module
> should be wired in parallel with the gas valve's PICK operator. Sounds like
> the spark module is instead now wired in parallel with the gas valve's HOLD
> operator.
>
> If the main burner and the spark are both running at the same time, call back
> the service company that installed the new pilot switch. Sounds like a problem
> they should correct.
>
> Respectfully submitted, Ron

[Sonofdawra]

>Can anyone give me any insight into the normal operation of a Carrier
>three wire pilot.

Below comes from a post of mine from a couple of weeks ago. This post was
under the heading of "BDP/Carrier 3-wire pilot switch". Sounds like possibly
things were not wired back in properly when the 3-wire switch was replaced.
The ignitor spark should be cut off when the pilot is proved.

I am copying a response to the thread <<Subject: Re: Main Burners keep
cycling>> so some of you might read this when you were not following that
thread. I included an explanation of the 3-wire BDP pilot safety switch
handout that I use to train techs with. It might be of some use to you
personally or if you are a company owner, it might be something that you can
use in your training of new techs. If anyone sees where it can be improved,
I'm always open to steal other guy's good ideas:o). Hope it helps some, Ron

>>Brand: Payne

>>I have a problem with my home
>>gas furnace. The main burners will light and then go out after about 10-15
>>seconds, and then re-ignite and go out, over and over.

>The furnace has
>>an electronic pilot, and I think that may be the problem because perhaps
>>the sensor is telling the gas valve to shut down at the wrong time.

> (I had a
>>service guy out and he was worthless...long story)

We do not have Payne furnaces in this area, but do have Bryant and Carrier
which are both
coming out of the same BDP/Carrier/United Technologies group. I am not
familiar with
that Payne model number, but will take a stab at your problem.

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.

[John Taylor]

Sonofdawra wrote:

Thanks much for your time and consideration. Forums such as this and
individuals who are willing to help are an invaluable resource. Sounds

as if gas pressure might be the culprit since main burner is firing.

John Taylor

[Sonofdawra]

>Sounds
>as if gas pressure might be the culprit since main burner is firing.
>

[Bob Pietrangelo]

Carrier three wire pilot is a plug in device there is no miswiring possible unless
plug has been cut off. Even at that it is color coded green 3, white 2, and yellow
1. 1-3 is nc and 1-2 is no. When 1-2 closes mgv should cut in. When 1-3 opens
it should cut out spark. These are all bi metal contacts. I have seen 1-3 stay
closed a while longer after 1-2 closes. I wouldn't lose much sleep over it.
Bob P.

[peter.c...@gmail.com]

Not sure if you could help me on the similar issue. I will post my issues after hearing from you. Thanks a lot

On Sunday, November 29, 1998 at 3:00:00 AM UTC-5, HVACMAN wrote:
> An interesting thread... I've noted (in a properly operationg"740" pilot) that the
> spark box cuts out long before the switch makes... are we sure the electrode
> doesn't sense flame to stop the spark? I would clean the electrode and make sure
> the pilot switch is in it's original position... if the pilot flame misses the
> electrode the spark will continue right through main burner light off...
>
> Sonofdawra wrote:
>

> > >Sounds
> > >as if gas pressure might be the culprit since main burner is firing.
> > >

[mar9...@msn.com]

On Sunday, November 29, 1998 at 12:00:00 AM UTC-8, John Taylor wrote:
> Can anyone give me any insight into the normal operation of a Carrier

> three wire pilot. Assembly was recently replaced with a new pilot
> because it would not prove pilot flame and thus burner would not fire.
> I believe it is supossed to cut out spark after pilot flame is proven

> . Burner is now operating but spark ignitor is not cutting out.

> Thanks in Advance,
> John Taylor

I have worked on these 3 wire pilot assemblies for years. They are inherently flawed in their design and very unreliable. Both the LH680005 and other clones use the same design. I have taken these pilots apart and they all rely on a springy copper metal piece to precisely contact the pilot contact and later on the main burner contact at the precise time. If the pilot keeps ticking through the whole cycle it means that the contact internally for the pilot is still engaged because the springy metal has changed slightly internally and thus not breaking contact with the spark box. The other thing I have found is the warp metal that controls the warp switch on the other end often can change slightly in even 2 to 4 weeks and supply too much gas right away to the main burners which causes a major explosion when the main burners are turned on and initial ignition takes place. There is no way I would recommend to anyone that they can take these pilots apart and repair them. If you open the pilot valve to increase or decrease the pilot flame you need to disconnect the 1/4" aluminum pilot line from the pilot and reconnect it before you run your furnace again or otherwise there is a high probability that the furnace main burners will again explode when ignited. The better idea would be to get a new furnace which does NOT use this 3 wire pilot as it is totally flawed in its design.