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My Rennai Cosyglow 650 does not stay on when ignition button is released. I suspect a dirty thermocouple (as per fault finding chart) where is and how can I rectify this problem??

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Yes if the unit does not stay on after the ignition control button is released then this the dirty or faulty thermocouple.
The thermocouple is a gas furnace component located near the pilot light burner. It is a safety device that shuts off the gas if the pilot light goes out or the electric igniter fails.
If the pilot light won't stay lit, the thermocouple may be faulty and should be adjusted or replaced. To adjust the thermocouple, you must tighten the thermocouple nut with a wrench. Take care not to apply too much pressure to the nut -- just tighten it slightly. Then try lighting the pilot. If the pilot won't stay lit, replace the thermocouple with a new one of the same type. Here's how to replace a thermocouple:

Step 1: Unscrew copper lead and connection nut inside threaded connection to gas line. Under mounting bracket at thermocouple tube, unscrew bracket nut that holds tube in place.
Step 2: Insert new thermocouple into hole in bracket. Be sure steel tube is up and copper lead is down. Under bracket, screw bracket nut over tube. Push connection nut to threaded connection where copper lead connects to gas line. Make sure connection is clean and dry.
Step 3: Tightly screw nut into place, but do not over-tighten. Both bracket nut and connection nut should be only a little tighter than if hand-tightened.

The thermocouple is installed next to the pilot light. A bracket holds
it in place, steel tube up and copper lead down.

The thermocouple is the pilot's, well, co-pilot! It is the electronic device that senses if the pilot flame is hot enough to sustain burning the gas fuel from the burner. If the thermocouple thinks it's safe, then it keeps open the main gas valve located in the pilot assembly. If the thermocouple does not sense enough heat from the pilot flame (such as when the pilot is out), then the thermocouple shuts off the gas valve to the burners.
How the Thermocouple Works
So what is this thing and how does it work? Well the thermocouple (technically called a thermocouple junction) is a device that contains two metal wires welded at the ends and placed inside a protective metal case. The thermocouple sensor is found at the business end of the pilot flame and is designed to be placed in the hottest part of the flame. The other end is connected to the pilot valve body. As the thermocouple heats up, it produces a small amount of electricity and when it gets hot enough from the pilot, send a signal to open the gas valve by using a solenoid operated by a 24 volt transformer. The thermocouple calls the shots, and by converting heat to an electrical signal, it allows the gas valve to open or close.Once the gas valve is open, gas is then constantly supplied to the pilot and as required for the gas burners (as called for by the thermostat). If the pilot goes out, then the thermocouple gets cold and produces no electric signal to open the gas valve's solenoid and the gas valve shuts off the gas supply to the pilot and burners.
Thanks. Keep updated for any more query. You can rate this solution and show your appreciation.

Posted on Aug 23, 2010

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Have 85000 but all-pro propane heater. starts fine good flame, release the gas valve button, heater shuts down no gas


If you have a thermocouple and standing pilot as described the gas cannot flow full to the burners with the gas valve button depressed because the position of the gas valve to allow the button to be depressed is not in the 'on' position....its in the 'pilot' position.

Are you sure you cleaned the 'thermocouple' and not just a 'spark ignition electrode' ?? One with spark ignition will not have a gas valve that has a pilot position.... Kinda confusing and conflicting data to sort through ..... Hmmmm....

If you have the spark ignition style, the problem COULD be in the 'flame rectification' circuit. IF it has the ignition electrode, the other side of the furnace will contain the 'flame sensor' that senses the flame and completes the ground circuit of the electronic module through the flame. Its not doing it, so either it is oxidized up inhibiting its ability to conduct electricity through the flame or there is residual oxidation within some of the terminal connections of the ground circuit.

IF what you have is the 'standing pilot', the first suggestion is to replace the thermocouple. A more remote possibility is the gas valve has a defective safety coil that drops out when you release the pilot button.

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If pilot stay lit, then thermocouple is working.
If pilot will light, then you have gas. If pilot goes out when pilot button is released, then thermocouple is next suspect.

Thermocouple tip must sit in pilot flame.
The pilot light heat applied to two different metals inside thermocouple causes small current.
The current travels back to gas control valve through copper line.
If the gas valve receive this current, then a magnetic switch stays open and allows gas into the gas control valve.
The magnetic switch is on the end of the red button you push down when lighting pilot. Essentially the red button is an override to the magnetic switch.
If the gas control valve does not receive the current, then the magnetic switch does not stay open when you release the red button.
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