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Gas Heater Issue, Saying lit

I have a Rudd Gas furnace that has started giving the following problem: When it starts up sometimes the burner lights then shuts off after only a few seconds. It will try 3 times to relight but does not stay burning. The blower motor comes on even though the burner is off and will run for several minutes then it too shuts off. When it does this I must turn the thermostat off and wait a few minutes for the system to reset before running it again. It always starts and runs fine after I restart it.
I am using a programmable thermostat that changes when the timer tells it to, either raising the temp or at night lowering it.

This is not something that is consistant, it never happens under the same conditons or during any particular timer cycle.
Any Ideas????

Re: Gas Heater Issue, Saying lit

Start with the thermostat---the t-stat is a SWITCH that must have solid contact in order to turn the furnace on & off.

Remove the t-stat cover & clean any contacts you see inside with a clean white envelope or piece of emery cloth.

Make sure the thermostat is level on the wall.

If the above doesn't work, & you can locate the 2 wires that control the heat, temporarily turn off the furnace switch & remove the wires from the t-stat and short them together for 10 minutes turn on the furnace & see if the problem clears up---if putting the 2 t-stat wires together & separating them solves the problem, the problem is in the t-stat; if the problem persists regardless of the t-stat wires position, the problem is elsewhere in the control components.

Other contributors will be along to provide further tips on servicing gas-fired furnaces.

Re: Gas Heater Issue, Saying lit

Two possibilities come to mind.
1. The flame sensor is dirty and not detecting the flame or the flame is not going all the way across the burners to the sensor. It would be on the opposite side of where the igniter is located.

2. If the furnace is a power vent unit and you have a small flue pipe going into and old large chimney, you may not be getting enough draft. You would need to add a chimney liner, the same size as the flue, all the way up and out the chimney. It generally happened with my furnace after we had it off for the night and tried to get get heat in the morning on cold days.


Re: Gas Heater Issue, Saying lit

The computer control board monitors all of your limit controls and other safety devices which must be closed to allow the furnace ignitor to glow and start the ignition process.

If there is a break down in the pressure switch or other limit safety controls ( interlocks ) the furnace will not proceed with the ignition sequence----for safety . Most furnaces will try this sequence for a total of three times then lock out.

On most furnaces the only way to get them out of lock out mode is to turn the emergency shutoff switch (looks like a light switch) on the side of the furnace to off and then back on again.

When your thermostat calls for heat the draft inducer motor should start which creates a draft in your vent pipe. If your vent piping is open ---- your pressure switch attached by a small plastic or rubber tube senses the negative pressure produced by the draft inducer.
If this is working properly the pressure switch will close the connection between two wires and send a signal to the control board to continue with the ignition process.

The draft inducer runs for 30 seconds to a minute before you hear an audiable click sound of the gas valve opening or a gas hissing sound.
[COLOR=#000000]The ignitor should glow and ignite the burners , a flame sensor (a small metal probe about 1/8" in diameter, with a white porcelain base) senses the flame , if lit.

If after 8 to 10 seconds the hissing sounds stops with [COLOR=#000000]no ignition of gas --- your furnace shuts down and goes into a lock out condition. [/COLOR]
[COLOR=#000000]You turn your power switch back off and on again --- then the sequence starts all over again with no ignition of the gas ---- I would suggest that you inspect your ignitor closely for cracks. [COLOR=blue]Make sure you do not touch the ignitor with your bare hands.[/COLOR][/COLOR] You need to purchase and install a new ignitor if cracks are found..

No visual crack is seen , 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.
A flame sensor is a safety device. When your furnace's main burners fire up the flame sensor picks up the heat from the furnace's main burning and sends a signal back to the main furnace control board to let the main burner stay on.
Or if the main burners do not ignite, the flame sensor does not pick up the heat and signals the main control board to shut the gas valve and main burners down.

Over the years flame sensors can build up a light coating of burned-in dust and dirt. This insulates the flame sensor so that it doesn't sense the heat of the flame well enough .You might need a new flame sensor, but most of the time they can be cleaned and will work well after cleaning.
After turning your furnace power off , you can clean the flame sensor with light sand paper, emery cloth or steel wool. Reinstall the flame sensor and you are back in business.

The above igniter and flame sensor , blocked vent ( usually with direct vented ) are the first and common places to start.

You can eliminate or confirm a thermostat issue at the furnace as well if you know your way around the inside. Otherwise you can disconnect the " W and " R " wires at the thermostat --- short those wires together to start the furnace.

There are other things like pressure switch , roll out limit switch , high limit switch that can cause issues with the furnace going into lockout.

Re: Gas Heater Issue, Saying lit

In later model furnaces there will be a blinking red light which will tell you what the problem is, if you have one of these lights make sure you count the blinks. On the inside of the access panel the codes should be listed, don't turn off the emergency switch before getting the blinking codes or you will have to go through the cycle over again. Good Luck!! :):)

Re: Gas Heater Issue, Saying lit

Sten ---- good point about the status LED.:cool:

Re: Gas Heater Issue, Staying lit, FIXED

Thanks for all the input on this issue. After replacing the thermostat, cleaning the entire unit, it was still cycling. I did some research on another site and tried the following:
I took a small wire brush and cleaned the flame sensor end. It appears that this simple action has solved the whole issue. The heater now lights and stays lit as it is supposed to.

It appears that the constant flame on the end of the sensor causes a slight build up of carbon, even if you cannot see it. The sensor for my unit costs $100 and so far cleaning it with a small brass bristle brush has solved the issue. If the proclain around the sensor is cracked, then it must be replaced.

The sensor is usually connected by a single red or orange wire and the probe end is directly in the path of the flame.

Again, thanks for all the input.

Re: Gas Heater Issue, Saying lit

Jack nailed that one :).Glad you got heat, Flame Sensors are designed to shut the furnace off after 5 seconds of not seeing flame, at $100 a pop for a flames sensor it must be made of gold. Good Luck!!!

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