How can a flame sensor be so sensitive?

[Justind97]

A couple of weeks ago I went into the garage and thought, WOW, it's really cold. I looked at the thermostat, set to 60 was reading 40 (40 is the lowest it will read, so it could have been colder).
Put the switch to off, then heat again and away I went.

Come this Saturday afternoon, same thing again. This time the furnace would cycle on for 8-10 seconds and then shutdown, did that 3 times and went into lock out.

Stumped and frustrated, went to google. Normal issue, you have to clean the flame sensor...

Sure enough, my flame sensor is mounted with the screws in the opposite direction and I had to take 1/2 the ****** thing apart to access it.

Cleaned the sensor with #0000 steel wool and it runs perfect.

Question is, is this sensor reading heat or what? The before/after cleaning looks near identical. I can't actually tell I cleaned anything off.

 

[Jim greengo]

Its reading milivolts,ok micro amps
Should be fine

 

[bonneyman]

A couple of weeks ago I went into the garage and thought, WOW, it's really cold. I looked at the thermostat, set to 60 was reading 40 (40 is the lowest it will read, so it could have been colder).
Put the switch to off, then heat again and away I went.

Come this Saturday afternoon, same thing again. This time the furnace would cycle on for 8-10 seconds and then shutdown, did that 3 times and went into lock out.

Stumped and frustrated, went to google. Normal issue, you have to clean the flame sensor...

Sure enough, my flame sensor is mounted with the screws in the opposite direction and I had to take 1/2 the ****** thing apart to access it.

Cleaned the sensor with #0000 steel wool and it runs perfect.

Question is, is this sensor reading heat or what? The before/after cleaning looks near identical. I can't actually tell I cleaned anything off.

One of the reasons why no repair on an HVAC system is never easy and cheap these days.
Just image if your cars oil drain plug was located under the exhaust manifold.

 

[Jackfre]

Your "flame sensor" does not read millivolts. A thermocouple reads MV and would be used with a standing pilot operation. If you have electronic ignition the flame sensor reads micro amps. Its purpose is to give a method of detecting flame on the burner and replacing the standing pilot which burns about 1000btu of gas per hour whether theere is a call for heat or not. With a flame sensor, in an no-call situation there is no gas consumption. When the unit stats operating, if everything goes well, you will likely get a pre-purge of the combustion chamber. Once that is proven, spark is generated and gas is released. At this point your flame sensor has to prove within a specific time or it goes into shut-down and a re-try. If it doesn't make it on the second try it is a hard lock-out as the unit thinks it has raw gas being sprayed into the combustion chamber. The flame sensor is isolated from the chassis of the unit and connected back to the PCB, which is grounded. The conductor is the flame and it is read with MICRO-AMPS. Be careful if you buy a multimeter for climate control stuff. A lot of guys see the MV setting on a meter and think they have it. Incorrect. When I start a gas unit up I will take a microamp reading and write it on the unit somewhere. If a problem develops I can take another reading and see if the signal has declined towards the minimum comparing it to the first reading. If it has then go ahead and clean the sensor, take another reading and if it is up to snuff, you have solved the symptom...but not the problem. What is the problem. Likely improper venting allowing condensation back into the combustion chamber, cross contamination of combustion air, fluctuating gas supply pressure, general dirty environment, etc. I used to use sand cloth and steel wool to clean rods too, but was told that scratching the surface just makes it easier to get it crapped up. I was told to clean it with a dollar bill. If it is a really expensive house use a 50

 

[nsula_country]

Your "flame sensor" does not read millivolts. A thermocouple reads MV and would be used with a standing pilot operation. If you have electronic ignition the flame sensor reads micro amps. Its purpose is to give a method of detecting flame on the burner and replacing the standing pilot which burns about 1000btu of gas per hour whether theere is a call for heat or not. With a flame sensor, in an no-call situation there is no gas consumption. When the unit stats operating, if everything goes well, you will likely get a pre-purge of the combustion chamber. Once that is proven, spark is generated and gas is released. At this point your flame sensor has to prove within a specific time or it goes into shut-down and a re-try. If it doesn't make it on the second try it is a hard lock-out as the unit thinks it has raw gas being sprayed into the combustion chamber. The flame sensor is isolated from the chassis of the unit and connected back to the PCB, which is grounded. The conductor is the flame and it is read with MICRO-VOLTS. Be careful if you buy a multimeter for climate control stuff. A lot of guys see the MV setting on a meter and think they have it. Incorrect. When I start a gas unit up I will take a microamp reading and write it on the unit somewhere. If a problem develops I can take another reading and see if the signal has declined towards the minimum. If it has then go ahead and clean the sensor, take another reading and if it is up to snuff, you have solved the symptom...but not the problem. What is the problem. Likely improper venting allowing condensation back into the combustion chamber, fluctuating gas supply pressure, general dirty environment, etc. I used to use sand cloth and steel wool to clean rods too, but was told that scratching the surface just makes it easier to get it crapped up. I was told to clean it with a dollar bill. If it is a really expensive house use a 50

$1 bill works wonders.

Also, they are polarity sensitive. If unit is not hardwired and has a ungrounded appliance cord, check it if it has been unplugged. If it is plugged in reverse polarity, it will cause proof of flame reject too.

Figured this out after a home inspector "inspected" a functional furnace at a house a friend recently purchased. Unit would light off then shutdown. Replaced flame sensor after cleaning had no change. Simply flipped the plug over and haven't looked back.

CT

 

[Jackfre]

Sometimes the flame sensor will get crapped up just because the manuf had a meathead who positioned it in a weak spot. At that point it is a maintenance issue.

 

[Justind97]

Sometimes the flame sensor will get crapped up just because the manuf had a meathead who positioned it in a weak spot. At that point it is a maintenance issue.

Maintenance issue for sure! Learnt something new that I have to do yearly-ish.. I've owned the furnace for 5 years

 

[Ohmthis]

Jackfre, nailed it! One thing to add the signal is actually rectified to DC micro amps. The flame rectifies it to the sensor. Look up Flame rectification. It’s pretty interesting. Maintenance is always a good thing for HVACR equipment.

 

[nsula_country]

Jackfre, nailed it! One thing to add the signal is actually rectified to DC micro amps. The flame rectifies it to the sensor. Look up Flame rectification. It’s pretty interesting. Maintenance is always a good thing for HVACR equipment.

This is correct.

Very cool tech. Old school, but neverless cool! AC ignitor arc is rectified in the flame and picked up by flame probe as DC miceoamps...

CT

 

[ddawg16]

Your "flame sensor" does not read millivolts. A thermocouple reads MV and would be used with a standing pilot operation. If you have electronic ignition the flame sensor reads micro amps. Its purpose is to give a method of detecting flame on the burner and replacing the standing pilot which burns about 1000btu of gas per hour whether theere is a call for heat or not. With a flame sensor, in an no-call situation there is no gas consumption. When the unit stats operating, if everything goes well, you will likely get a pre-purge of the combustion chamber. Once that is proven, spark is generated and gas is released. At this point your flame sensor has to prove within a specific time or it goes into shut-down and a re-try. If it doesn't make it on the second try it is a hard lock-out as the unit thinks it has raw gas being sprayed into the combustion chamber. The flame sensor is isolated from the chassis of the unit and connected back to the PCB, which is grounded. The conductor is the flame and it is read with MICRO-AMPS. Be careful if you buy a multimeter for climate control stuff. A lot of guys see the MV setting on a meter and think they have it. Incorrect. When I start a gas unit up I will take a microamp reading and write it on the unit somewhere. If a problem develops I can take another reading and see if the signal has declined towards the minimum comparing it to the first reading. If it has then go ahead and clean the sensor, take another reading and if it is up to snuff, you have solved the symptom...but not the problem. What is the problem. Likely improper venting allowing condensation back into the combustion chamber, cross contamination of combustion air, fluctuating gas supply pressure, general dirty environment, etc. I used to use sand cloth and steel wool to clean rods too, but was told that scratching the surface just makes it easier to get it crapped up. I was told to clean it with a dollar bill. If it is a really expensive house use a 50

^^Someone who understands it^^ I'm better you know who Eclipses and FNA is.

To add to this....is that sensor not a UV detector?

 

[nsula_country]

Not a UV proof of flame sensor. Usually those are on higher end resi and Industrial burners.

CT

 

[Jackfre]

Conventional Flame rods a basically a steel rod with a ceramic insulator like a spark plug. Some I think are tungsten. They rarely go bad, but are replaced frequently with the "might as well try this," and they are short money. One check you can make on them is to ohm out the sensor rod to the chassis. If you get a reading then the insulator may be cracked allowing it to short out to ground, but that is really rare in my experience. The sparker and the FR go hand in hand. If you are in there find out what the spec is for the gap in the sparker and set it correctly. Erratic spark can lead to delayed ignition or at least not a smooth ignition and that can show itself as a FR issue.