Trane furnace with white-rodgers 50A50-571 control NO FAN

[hahawil]

hello   i have a Trane furnace with a White-Rodgers control   50A50-571    the furnace fires but the circulation fan does not spin, after a short time the hi-limit will do its job and shut it down.  I jumper the red and green wire and the fan will run.  was thinking the control board was bad but if jumping the two wires from the t-stat makes it work seems as tho the board is good and the t-stat is bad???
thanx  Alan

[JWWebster]

The board should bring the main blower on line after flame verification. We know the fan is fine. I am leaning toward a bad relay on the board. Anybody else?

[hahawil]

The board should bring the main blower on line after flame verification. We know the fan is fine. I am leaning toward a bad relay on the board. Anybody else?

Thanx  i installed a new board 50M56u-843   I am having trouble now with a timing issue   I get my pre-purge, the ignitor starts and just as it is at the end of its full glow i get the gas valve energized,but by the time the gas get down to the ignitor it has started to cool off and it wont ignite. then of course the flame sensor shuts it down.  If I light the gas with a match it runs as normal.  the direction say the ignitor has a 17 second time limit.  one spot it reads that the 17 is for "the first 64 warm-up periods following power"  earlier it stats that the time will be increased to a max of 19 seconds depending on whether or not flame is achieved.
Can I adjust the timing or do I need to babysit it thru the first 64 cycles??


http://www.emersonclimate.com/Documents/White-Rodgers/instruction_sheets/0037-7042.pdf

page 6


When heat is required, the thermostat will send a call for heat
to the control. This starts the controls heating sequence. The
ignitor and humidifier (optional) are powered. The ignitor is
powered after the pre-purge period.
Upon initial application of power, the warm-up time is 17
seconds. The ignitor on-time will then be increased depending
on whether or not flame is achieved. The warm-up time is
limited to a maximum of 19 seconds. During the first 64 warmup
periods following power-up, the warm-up time may not be
less than 17 seconds.
In the event of a retry, the warm-up time will be increased by
one second and locked in at that duration. Once the warm-up
time is locked, it remains fixed until another call for heat results
in a retry, in which case the warm-up time is again increased
by one second and remains locked.
In the event of two successive retry attempts, the warm-up
time will be unlocked and set to 19 seconds. If flame is then
achieved, the warm-up time will begin adapting again with the
next call for heat. If, however, this third attempt fails to achieve
flame, the control will go into system lockout.
After the ignitor warm-up period, MV LO (first stage) and MV
HI (second stage) are both energized to the gas valve. Flame
must be detected within 4 seconds. If flame is detected, the
30-second HEAT delay-to-fan-on period begins. The circulator
and electronic air cleaner (optional) will also energize at this
time.

[Icehouse]

Furnace short cycling is very hard on the furnace and hard on your energy bill. Furnace short cycling can be caused by (1) the thermostat anticipator (if equipped) not set high enough, (2) gas valve gas pressure set too high, (3) blower speed set too low or weak blower motor capacitor (4) duct work too small to provide enough air flow for the furnace,  (5) evaporator coil stopped up with dirt or lint or (6) a furnace that is sized too large for your home. I hope this helps you find out why your furnace is cycling too much.
 I hope you have a blessed day! I also hope that you and your family have a Happy, Healthful and Prosperous Holiday Season! God bless you and your family.

[JWWebster]

Do a pressure tap

[Icehouse]

Do a pressure tap

Thanks JW, some people do not read. # 2 in my response "Gas Pressure" to high.
What do I know I'm just old. 

[niobrara]

From what I have read,you seam to know a lot.You gave a great breakdown.

[Icehouse]

Try this if nothing else works          ;

Problem: Your furnace will not ignite the gas to produce heat for your home. When a furnace has a bad ignitor what I see most of the time is the following sequence of operation:
 

1. Thermostat calls for heat.
2. Draft inducer motor starts.
3. Pressure switch attached by a small plastic or rubber tube senses the negative pressure  produced by the draft inducer and closes.
4. Draft inducer runs for 30 seconds to a minute before you hear a gas hissing sound. The ignitor did not glow, the flame sensor (a small metal probe about 1/8" in diameter, with a white porcelain base) does not sense the flame, so after 8 to 10 seconds the hissing sounds stops with no ignition of gas to heat your home. Your furnace shuts down and goes into a lock out condition until you turn your power switch back off and on again. Then the sequence starts all over again with no ignition of the gas.
Solution: You probably need to purchase and install a new ignitor. I would suggest that you inspect your ignitor closely for cracks. Make sure you do not touch the ignitor with your bare hands. If you do not visually see a crack, then you could have a furnace control board problem or a limit, rollout switch problem.  The furnace's control board might not be supplying the voltage to the ignitor.  If your furnace lights and the gas stays on for 8 to 10 seconds, then shuts right back off, then you need to clean your flame sensor with light sand paper or steel wool. You might need a new flame sensor, but most of the time they can be cleaned an will work well after cleaning.
 [/b]