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Does the pilot flame of a gas furnace stay on 4 the duration of a heating cycle?

Does the pilot flame of a gas furnace stay on 4 the duration of a heating cycle?

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Old 12-15-14, 04:01 PM
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Does the pilot flame of a gas furnace stay on 4 the duration of a heating cycle?

I was looking at my furnace a couple of days ago and I notice one thing: there is something red hot there where the spark igniter and the thermocouple assembly is. I can hardly see it and I am not sure what component is that but I was just wondering if this is normal.

This is what I have for thermocouple and spark igniter


here is what is hot red. I looked again from a different angle. I think that the pilot light stays on for as long as the thermostat asks for heat. After that it turns off, that does happen I am not concerned about that. I am just wondering if, after the burners are lit, the pilot light should still stay on. I see no sense in that.

Here is the electrical scheme for my igniter
 
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Old 12-15-14, 04:07 PM
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You don't have a thermocouple. You have a spark igniter that doubles as a flame sense rod. The pilot light remains on during the burner cycle.

I see the picture of a meter in your diagram. Not sure what they want you to test for but I don't recommend connecting a meter like that as the spark ignition voltage is high enough to fry many meters.
 
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Old 12-15-14, 04:36 PM
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The picture was taken from this article, slightly connected with the topic

Troubleshooting Intermittent Ignition

Thanks for the reply, so it is normal to have that red hot, it is part of how it works. From what I understand the loop closes via the pilot flame (goes from the control board via the cable, the flame makes connection between the igniter and the sensing rod and from there to the ground of the furnace)
I the sensing rod is designed to stay red hot while the flame is on, right?
 
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Old 12-15-14, 04:53 PM
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The flame makes contact from the igniter/flame sensor to that grounded "hood like" thing above the flame.

The rod will glow red during operation.
 
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Old 12-15-14, 05:04 PM
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Yeah that is what I read about it too but I was not sure if sensing the pilot flame was needed only when the burners get lit or after that too.

I guess it works like this:
1. the thermostat calls for heat
2. the controllers send the signal and generates the spark while it opens the pilot valve
3. If everything goes well the pilot flame is on and the controller senses the flame
4. If #3 happens then the controller knows that it can open the main valve

from this point forward I am not sure about the correct sequence.
Common sense logic tells me that the pilot valve should be switched off but I guess the controller needs a way to know that the burners are lit and everything is OK. I am not sure how the pilot valve contributes to that. There should be a way for the controller to learn when the burners are off so it could shut down the main valve
I am not talking here about the situation when the thermostat calls for a heat stop.
I am talking here about an accidental situation when the burners go off (no flame) but the main valve is still open so the gas is flowing into the room!! I guess that in such a case having the pilot flame on all the time makes sure that the burners get fire again.
The other thing that makes me think that the flame should stay on is that Hot2000 (a program used to asses the heat and cooling needs for a house) takes into consideration the heat generated by the pilot light itself (if the pilot light was on just for the couple of seconds when the burners get fired then the consumption of the pilot light should not matter)
 
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Old 12-15-14, 08:02 PM
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When you enter data into any energy modelling program, intermittent pilot appliances are treated the same as anything else with electronic ignition.

Standing pilot refers to an appliance with a continuously burning pilot - old standard efficiency furnace, water heaters, gas fireplaces, etc.

Intermittent pilot, lit by a spark is no longer used in residential furnaces. Most have hot surface igniters.

Clarification:

Standing/continuously burning pilot: A pilot that stays on when the appliance is idling. Lit manually by a match and stays lit.

Spark ignited pilots were common probably from the 1980s to the mid 90s. Even some low efficient (70% efficient or less) furnaces had intermittent pilot.
 

Last edited by user 10; 12-15-14 at 09:42 PM.
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Old 12-15-14, 08:21 PM
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There is no monitoring of the main flame. It is assumed that if the pilot is lit that any gas coming from the main burner will also be lit. If the pilot goes out the main and pilot valves (actually a combination valve) closes and there is an attempt to re-ignite the pilot.
 
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Old 12-15-14, 08:27 PM
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Hi Muggle

I happen to have one of the old (very first) high efficiency furnaces made in Canada
Apparently my pilot is on during the entire heat call (I mean till the temperature reaches the desired level)
When you say continuously burning pilot are you referring to a pilot that continues to be lit after the set temperature was reached and the furnace's power completely cut by the thermostat ? If that is the case then the modelling program might indeed not refer to my type of pilot, although I am not sure if in my furnace's case the heat generated by the pilot should be ignored
 
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Old 12-15-14, 08:48 PM
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There is no monitoring of the main flame. It is assumed that if the pilot is lit that any gas coming from the main burner will also be lit. If the pilot goes out the main and pilot valves (actually a combination valve) closes and there is an attempt to re-ignite the pilot.
Yes that is kind of in line with what I assumed in my previous post
 
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