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Will a fan limiting switch cause the pilot light flame to extinguish in addition to shutting the unit off entirely?

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Unlikely
it is more likely a problem photo electric cell that senses a flame
if it is faulty the sensor will not see a flame and will shut down all gas supply to prevent an explosion from unburnt gas
as gas is not a diy project, call a gas fitter to properly diagnose the fault

Posted on Jan 02, 2017

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Posted on Jan 02, 2017

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2 Answers

Pilot light starts then shuts off


The sensor is not working.

Oct 19, 2015 | Heating & Cooling

Tip

Heater check out list for heater failures


Almost all furnaces built after 1978 have intermittant type pilots.
Ca banned them requiring electronic spark or hot surface ignitors, followed by induced draft motors, pressure switches or centrifigal switches that "prove" a induced draft exists, allowing the pilot sequence to commence.
electronic spark type units have a high voltage low amperage spark that is AC. Fire rectifies electricity that passes through it meaning AC becomes DC ! Fire is a terrible conducter however as a result , the DC signal is only milliamps/volts signal strength. the DC prescence is proof a flame exists for pilot, and safe to commence to mainburners.

As long as pressure switches stay closed, safeties and limits stay closed
and the DC signal to the ignition module is not broken, the furnace will run.

The normal sequence of operation is upon a call for heat from the thermostat a 24 volt signal is sent to W white (heat circuit) at the low voltage terminal board on furnace.
from there depending on unit and age the heat circuit may include a circuit board, but it still has the typical limits and safetys reqired for operation
its just not using electromechanical relays to control fan and induced draft motor etc.

for the induced draft to begin ,all limits, safetys in heat circuit must be closed.

induced draft begins and a proving switch, either pressure or centrifigal close allowung the pilot to begin, a spark or hot surface ignitor is energized and a pilot flame established,modern types use flame rectification to prove a flame, older Carrier, Payne,types have a 3 wire switch in the piloyt assembly that is a bi metal that expands and contracts clicking open and shut, allowing pilot or main burners, depending if cold or hot.

pressure switches are attached with tubing to the induced draft wheel housing and sense pressure differential and close upon induced draft running, the orifices that the tube attaches to on induced draft housing often plugs or is restricted causing pressure switch to fail to close, or waver on open and close , open , close, .

if you can run the fan in the on postion , you have both high and low voltage to the furnace.

if you dont have fan control check power, breaker , fuses.

on Dec 04, 2009 | Heating & Cooling

1 Answer

If the pilot go out on a furnace would gas continue to flow to the pilot , or would it shut off with in a few seconds


The pilot flame heats a thermocouple, so if the pilot goes out, the thermocouple cools and shuts off the gas. It can take anywhere from 20 seconds to 2 minutes for it to shut off. You should hear an audible click when it happens.

Sep 14, 2014 | Heating & Cooling

3 Answers

I have a Intertherm gas furnace mod# MGHA 070ABFC 05. It comes on for 30 seconds then off for 30 then on and continues this.


hi,

check how far away the thermocouple is from the pilot flame, the end of the thermocouple probe should be in the flame, adjust it over. if its in the flame then it might need a thermocouple element it unbolts from the manifold. or the pilot flame may not be big enough to hold the valve open, have to research or use a wire the circumfrence of a bread tie to clean out the pilot orifice. always have a fire extinguisher in your rv..

jm

Jul 04, 2017 | Heating & Cooling

1 Answer

At 300F after half hour, pilot and main gas burner was going off. When re-lit would stay on only 5 minutes thereafter. Had the thermopile changed(bought locally- not original from Imperial but identical),...


Hi there.
The thermostat will only shut down burner flame but not pilot flame.
Try tightening all wiring connections.
By using an ohm meter, check the continuity across the hi limit terminals when the pilot goes off, your hi limit may be tripping prematurely causing pilot flame to shut down.
Regards.

Jul 29, 2011 | Kitchen Appliances - Others

1 Answer

I have 2 hvac units running an electric heating element for the heater. Both limit controls are set the same. One is ignited the flames and continuing with flames for close to 3 min. The other is taking...


Hi, these Honeywell combination fan/limit controls are used for turning the fan on after the combustion changer reaches the factory setting on the round dial that you see when you remove the cover. The high limit will turn the fan on when it reaches this temperature, and off when the stat is satisfied. If the blower motor fails due to a burn out or short, it shuts the main burners down on the high limit setting. These controls can be removed and you can clean the probe which has a set of contacts at the end. Sometimes this will fix the problem. Check the settings on the dial to see if they are set right to come on, the time it takes between cycles, and the temperature they are set to shut off. This sounds as though you need to check your thermostats also. They could be a problem. You say electric heat, but then you are saying flames? If these limits have the manual fan button, make sure they are on auto. Long delays could also mean you have a dirty flame sensor located by the pilot. You can clean the metal tip with steel wool or fine sanding cloth. Short heating cycles may be your thermostats like I said before. You shouldn't have to replace the fan/limits as I told you what they control. These will cause a fan to run and not shut off after the stat is done, so check to see if they are dirty. Please keep me posted.
Shastalaker7
A/C, & Heating Contractor

Feb 12, 2011 | Honeywell L4064B2236 Fan/Limit Control...

1 Answer

I HAVE A RUUD SILHOUETTE GAS FURNANCE. The pilot light keeps blowing out. When I light the pilot flame it seems to work perfectly at ignition. As the Manual instructs, I hold down the ignition switch for...


Short answer, yes. However you could have another problem. Replace the thermocouple that keeps the pilot lit. You can get a universal one at any good hardware or farm store. Make sure the new thermocouple is well into the pilot flame. If the blower knocks out the pilot flame again, I would be suspecting cross contamination from a crack in the heat exchanger. As the exchanger heats up, it may be causing a small crack to open and allows the blower to extinguish the flame. It may also be causing a leak of Carbon Monoxide into the heated room air. I would get it checked out very quickly.

Mar 07, 2010 | Ruud UBHC Air Conditioner

1 Answer

T-stat turns gas valve on,burner lightsburns long enough for fan stat to turn on fan,high limit shuts off gas valve,fan shuts off;repeats cycle


Probable causes include crack in heat exchanger, dirty pilot, bad high limit switch or bad gas valve. A crack can allow the fan to blow the flame off of the thermocouple or flame sensor. A dirty pilot will cause the same effect. The bad limit switch could cycle the system and a bad gas valve could be shutting off gas supply to unit.

Dec 02, 2009 | Off The Wall Heating & Cooling

2 Answers

Pilot light


Assuming that your make up air and exhaust hood are properly balanced, no negetive pressure conditions exist, your gas manifold pressure is at 4" wc, your air/gas mix on the burner set correctly, the your problem would be the pilot adjustment. If the flame is not intense enough flame to survive the sudden air shock, it will go out.

The pilot height should be about 1 1/2". Your Pilot orfice could also need cleaning. Eliminate excessive draft in your kitchen.

Here is the manual for your unit, it covers these topics.

http://www.frymaster.com/docs/uploaded/fry/products/OperationalManuals/819-5355.pdf

Jan 24, 2009 | Frymaster Dean H17-SC Deep Fryer

1 Answer

Pilot light won't stay lit.


most common cause of the described symptom overheating the thermocouple! a thermocouple produces millivolts of electricity based upon a differential of temperature between the tip and the base. If it gets too hot it will not work correctly. If the pilot flame is too large or the main burner flame is contacting the thermocouple or the thermocouple is too cheap or old all can cause this issue.

Dec 18, 2008 | Heating & Cooling

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