I have gas fireplace that wont light when i turn it to on i replaced the thermocoupler and it is in the flame and turns cherry red but when i turn it to on the safety valve shuts of ---why

There should be a small electric motor that turns the wheel that looks like flames. That little motor may not be working making it look like the flames don't work. May have to replace the motor.

If it has a pilot light, (website says it does) I suspect its the thermocouple. These are easy to change yourself and cheap, you can buy a universal thermocouple and it will be cheaper than a Napolean one. The end of the thermocouple is a fat copper pin that sits poking into a small pilot flame. The heat of the flame on the copper tells the system that the gas is burning ok so it allows more gas through. But after a few years the copper end gets covered in black charring and this slightly insulates it from the heat, so it signals to turn off the gas.

One, ensure that the flame is entirely one color blue, no light blue in the center, no orange or yellow. If you replaced the thermocouple yourself, ensure you connected it to the gas valve and/or ensure that the connection to the gas valve is tight. If it is a new thermocouple you may have a gas valve issue, this is uncommon and rare. However, the valve may not be receiving the signal from the thermocouple. If the color of the flame is incorrect, turn fireplace off, and clean out the pilot tube and then relight. If none of this works, call your local trained professional. Remember you are dealing with gas, being cheap can put you in danger.

I would start by replacing the thermocouple. It is suppose to keep the gas valve open when in operation. If the mercury in the thermocouple dries out it won't keep the gas valve open and thus the flame goes out. If that does not work then you have a gas valve problem.

sounds as though you need to get the thermocouple(copper wire that sits in the pilot flame) replaced, check first to make sure it is tight at the gas valve end, as they can loosen over time due to expansion and contraction.

Stephanie, I 'm not sure how it was determined, that your direct-vent gas fireplace needs a Thermocouple? As a dirty pilot light will also mimic a faulty thermocouple. In addition, if the fire hasn't been used since Winter of last tear, and the gas line to the unit has been turned OFF since then, the air in the gas line must be purge, before the Pilot will light and hold.

If all of the above can safely be ruled out, then the thermocouple should be replaced. The facing of the fireplace will have to be removed, to access the Thermocouple.and it's connections to the gas control valve. You didn't mention which Rinnai Gas Fireplace you have, but most if their units, use Thermocouple part # 90193475. If you're going to replace it, it would be a good idea to also replace the Thermocouple Retainer, part # 9377.

I hope this helps you to further troubleshoot and solve the problem. Please let me know. Thanks.

check pilot light to make sure properly in place with thermocouple check gas feed for proper pressure and new doesnt mean free from defects -bad valve,thermocouple,temp sensor

The steps involved can vary slightly depending on your furnace model and pilot valve type, so if possible try and find the instructions inside the furnace door cover or in the instruction manual. This is for a gas fired furnace, but applies to any gas-fueled appliance including water heater. Two of the most common types of pilot valve body assemblies either have a red reset button and a gas valve or no reset button and a valve knob that can be depressed. But whichever type you've got, if you have an older model furnace with a standing pilot (flame is lit all the time) then this is the basic procedure.

• Turn you thermostat to 80 degrees or to a setting that will demand heat. Make sure the thermostat is in "Heat" mode.
• Go to the furnace and find the pilot valve. The pilot valve body is a box shaped device into which the main gas line will run and is located near the gas burners. It will usually have a gas **** or valve knob that reads "On, "Pilot" and "Off." Locate this gas **** or knob.
• Turn the knob or gas **** to the "Off" position and wait about 3 minutes for any residual gas to clear away.
• Find the pilot. It's located near the gas burner tube assembly in the furnace.
• Get your match or lighter ready. Sometimes the pilot is hard to reach. If possible, try and use a long fireplace match when you light it. If you don't have a fireplace match then a butane BBQ grill lighter works well too. If you don't have that, then you can fasten a match to the end of a stick when you light the pilot

• If you have a pilot with a red Reset button, turn the gas valve from "Off," to "Pilot."
• If you have a pilot with no red button, turn the knob from "Off" to "Pilot."
• Place the lit match or ignited lighter tip at the pilot and depress and hold the button or depress the knob as appropriate. Depress the button or knob for about 30 seconds. This maintains gas flow to the pilot until the thermocouple gets hot enough to open the main gas valve.
• Once the pilot stays lit, slowly release the button or knob and turn the gas **** or knob from the "Pilot" position to the "On" position. This will ignite the burners and keep the flow of gas supplied as required for the burners as called for by the thermostat.
• If the burners fail to ignite then it may be because the thermocouple did not get hot enough to open the gas valve. Wait a few minutes and then repeat the above procedure. This time hold the red reset button or depress the knob for about 45-60 seconds.
• Once the main furnace burners ignite, adjust the thermostat to the desired setting.
• If you've followed the previous steps on lighting the pilot and it still does not light after, or will not stay lit, then you probably have a problem with the thermocouple or an adjustment needs to be made to the pilot. You'll need a furnace service technician to replace or repair these parts. You may find that the pilot lights, but it is an anemic looking flame. That's a different problem we'll look at in the next section.
  
• If the pilot lights but the flame is a weak yellow flame, it will not get hot enough to heat the thermocouple to its set point allowing the gas valve to open.
  A natural gas flame should be a bright blue with the tip of the flame having just a tinge of yellow. A propane flame should have a bluish green flame with a tinge of yellow at the tip. The flame should be strong enough to hit the thermocouple tip about 1/2 inch from the tip end. If the flame is weak or shaky looking, check to see that a breeze or draft is not blowing on it. Adjusting the Flame
  There is usually a small screw on the pilot valve body that will adjust the flame. You may have to refer to the manufacturer's instructions to find the screw. Turn the screw as needed to adjust the flame throw. Yellow Flame
  A yellow flame is caused by lack of air and incomplete combustion. It can be caused by a dirty pilot tube tip. Split Flame
  This is caused by dirt in the pilot tube. Take a needle or small nail and gently clean the tube. Flickering or Wavering Flame
  A flame that flickers is usually caused by a draft.
  

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The fireplace probably has a thermocoupler. It produces current when engulfed by the pilot light flame. The small current produced (you can check for output with a votmeter) is enought to allow the gas valve to open when you turn on the switch to start the flame. The solid metal tube may not be properly connecting with the flame - repostion the pilot light and make sure surrounding air flow does not distort the pilot light pattern too much. OR the surface of the device may be slightly oxidized - very gently sand or wire brush it. Again check for small voltage output. If no volts - REPLACE THERMOCOUPLER.

Also: read this link;
http://answers.yahoo.com/question/index?qid=20081101104024AA43AJF

CB

Gas apliances have safety features built into them so you don't blow yourself up. The pilot heats a thermocoupler which tells the gas valve there is a flame, open the main gas to the burner. If you can see the flame area of the pilot look for a round post about 3/16 inch in diamiter. This should be directly in the flame, if not bend the bracket so it is in one side of the pilot flame. The thermocouple is attached to a mounting bracket. This will be held in place by a 1/4 inch hex screw. remove the screw and the capilary tube leading to the gas control valve. Before you do anything, close the main gas supply to the fireplace and disconnect the power to the fan. You may need to semi dismantle the fireplace to get the thermocouple. The gas valve end is a brass 1/4 pipe thread plug. Turn the outer nut and remove without twisting the tube. Reverse this order to install a new one. Besure not to twist the capilary tube, tighten brass nut 1/4 turn past hand tight. Your better heating supply house will have these in stock. Restarting the pilot will take a few tries because the gas was off. Your gas suplier would charge about $75 to fix this. if you don't have the proper tools and it dosen't look pretty stright forward don't do it. You could get dead.