Thermostat & Thermocouple

[rph_999]

Can someone pls explain difference between a Thermostat & Thermocouple on a gas boiler.

Thanks Paul

[Will]

A thermocouple is a device which generates an electrical charge when heated. It is often used on a boiler to indicate that the pilot light is on and hot, so it should be safe to allow gas to pass to the main burner for heating water, your house, etc. Should the pilot light go out, the thermocouple will sort of "tell" the boiler to shut off the gas, as it isn't safe to fire up.

A thermostat is used as a switch, which goes on and off at a certain temperature. For example, in a boiler it will tell the boiler that the water in the circuit is plenty hot enough and that it doesn't need to be heated any more, in your house it will tell your central heating that the room is warm enough and doesn't need heating any more and in your car it would say that your engine is too hot and that the radiator valve should open, and perhaps the cooling fan should turn on.

HTH,

Will

[gasman pete]

[Harry Bloomfield]

rph [e-mail] formulated on Wednesday :

A thermocouple develops a small current when heated, the current is used to power a tiny solenoid to keep open a small gas valve for the pilot light.

The thermostat simply switches on and off at a preset temperature. The temperature being measured is the water temperature in the boiler. It switches the main gas valve for the burner on and off.

[Andy Burns]

Or rather, stop telling it that it ought to be safe to supply gas.

[Newshound]

To be really fussy, the thermocouple generates a small voltage (not a current) which depends on the temperature at the tip (or more accurately, the temperature difference between there and wherever you are measuring the voltage). This voltage will generate a very small current indeed and these days it is normally amplified by some sort of microchip in order to be useful.

So, as the other guys said, the thermocouple and associated circuitry is a thermometer which measures the temperature somewhere. A thermostat decides if somewhere is too hot or too cold. Your car radiator thermostat decides whether the engine is warm enough to need to cool the water by passing it through the radiator; if not, it runs the heater first for your benefit and to help defrost or demist the screen. The traditional central heating "room thermostat" turns the boiler on or off electrically. Similarly the fridge thermostat. Thermostatic radiator valves switch off the water flow if they think the room is warm enough. Shower thermostats wind the hot water flow up or down to keep you happy.

[Dave Fawthrop]

| |rph [e-mail] wrote: |> Can someone pls explain difference between a Thermostat & Thermocouple |> on a gas boiler. |>

|> Thanks Paul | |A thermocouple is a device which generates an electrical charge when |heated. It is often used on a boiler to indicate that the pilot light |is on and hot, so it should be safe to allow gas to pass to the main |burner for heating water, your house, etc. Should the pilot light go |out, the thermocouple will sort of "tell" the boiler to shut off the |gas, as it isn't safe to fire up.

Which is why you have to press something while lighting the gas and hold it down for 10 seconds to allow the pilot light to heat up the thermocouple. | |A thermostat is used as a switch, which goes on and off at a certain |temperature. For example, in a boiler it will tell the boiler that the |water in the circuit is plenty hot enough and that it doesn't need to |be heated any more, in your house it will tell your central heating |that the room is warm enough and doesn't need heating any more and in |your car it would say that your engine is too hot and that the radiator |valve should open, and perhaps the cooling fan should turn on.

[Ed Sirett]

On Wed, 17 Jan 2007 13:26:37 -0800, rph_999 wrote:

From the low tech boiler FAQ. (not yet available)

The thermionic or pilot valve: The pilot light heats up a rod of metal called the thermocouple, in fact it is two metals joined together and they are separately connected, electrically, to a coaxial copper wire that looks and feels much like a

2mm copper capillary tube. When (the rod part) is heated to several hundred Celsius (red hot is too much) a small voltage (say 25mV) is produced. This does not sound a lot but it is quite sufficient to drive enough current through a solenoid coil to hold the pilot valve open against a spring. There is not enough power to open the valve that has to be done manually by the user. Should the pilot light go out for any reason the valve will close under spring pressure and the boiler will not fire. If you extinguish a pilot light (say by briefly interrupting the gas supply) after a delay a 'clunk' will be heard from inside the multifunction valve as this valve springs shut. The maximum delay must be no more than 60s. You will not be able to relight a pilot light until this clunk has occurred. With experience you can sometimes briefly release a little of the pressure on the button as the pilot light is warming the thermocouple up. This is useful as it can tell you whether the thermocouple is good, dead or disconnected. If there is some 'stickiness' in the valve, after you have been holding the valve in for a good 30 seconds but not enough to 'stay in' then the thermocouple is either finished or the pilot flame is not doing its job right. If there is absolutely no stickiness whatsoever then a circuit break is more likely or just possibly a failed solenoid in the valve. The solenoid can be tested for electrical continuity to confirm.