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HVAC Discussions - Heating, ventilation and air conditioning.
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Posted by on January 24, 2007, 11:23 am
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.
Thanks!
===
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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