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Gas valve withthermopile only 

GAS SMART Explanation of millivolt systems.

Service and trouble shooting guide.

Millivolts systemMany gas appliances are fitted with millivolt systemsto protect and/or control the burner operation.

The millivolt system is powered by a thermopile thatoperates a millivolt valve. Before explaining anyfurther let’s have a look at a thermopile.

ThermopileThermopile or Thermo-electric generators, converts heatdirectly into electricity, using the voltage generated at an in-series connected array of thermocouples known as a“thermopile”. The history begins in 1821 when ThomasJohann Seebeck found that an electrical current would flowin a circuit made from two dissimilar metals, with the junctions at different temperatures.

The Thermopile was initially used for measurements of temperature and infra-red radiation, but was also rapidly put

to use as a stable supply of electricity for other physicsexperimentation.

This Markus' thermopilewon a prize in 1864/5from the Vienna Societyfor the Promotion of Science and generated55 mV.

Today’s thermopiles are muchsmaller and generate about750 mV- enough to actuate asmall solenoid that keeps thegas valve open.

Millivolt Gas ValvesMany manufacturers producing millivolt gas valvesfor the gas appliance industry. It was recognisedthat with a millivolt system, gas appliances could becontrolled and protected. Several valves are used intoday’s gas appliances, some are;

Gas valve with thermo-coupleand thermopile

This valve is similar to the thermocouple only valve,however it has a pilot which heats up BOTH a thermocoupleand a thermopile. The thermocouple still acts to prove that

the pilot flame is on and allows this flame to continue after start-up. The thermopile is used to power a second circuitwhich is used to open the main valve. This second circuit ispowerful enough (300-600 millivolts) to allow the use of athermostat, wall switch or control switch to operate the mainvalve. Control of the valve is obtained simply by hooking apair of wires to two terminals located on the valve. 

This valve has the thermo-couple circuit connected to thethermopile. When heated thethermocouple acts to provethat the pilot flame is on andallows this flame to continueafter start-up. At the same time

the thermopile is used topower a second circuit whichis used to open the main valveand allow the use of athermostat, wall switch or control switch to operate the

main valve. You can recognise this type by having a pilotwith only a thermopile. 

Gas Pilots

This pilot is oftenused in gas fryersand gas heaters with

a high gas input. Itreacts quickly whenthe gas pilot has

gone out and shutoff 

the gas. (B) See

wire diagram 

This pilot is used in gasappliances were the

safety valve and thecontrol valve areelectrically connectedtogether. This systemuses 600-700mV forproving the pilot flame.

(A) See wire diagram

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Basic Wire diagram

Trouble shooting

Millivolt systems with both a thermocouple and thermopile(B), it is important first to isolate where the potential problemmay be. If the pilot can be lit and stays on after the knob isreleased, then the problem is probably with the thermopile

side of the valve.

Millivolt systems where the safety valve is operated by thethermopile (A), check where the potential problem is bylighting the pilot and if it stays on with the knob released,then the problem is probably with the control side of thevalve; e.g. loose connections, switches or thermostats.

Here are the most common problems and solutions;

1. Common problem is loose or poor connections or circuits to your appliance switch or thermostats. Thiscan be isolated by simply using a small piece of wire to  jump the "TP" and "TH" terminals located on thesevalves. If the appliance turns on when these terminalsare jumped, then you can be sure that your problem is

not in the appliance itself, but further down the switchcircuit. Make certain you have used the suggestedgauge of wire and that the length for your control runsdoes not exceed the specs given by the manufacturer.

2. Check the milli-voltage between the connections“TP/TH” and “TP” on the gas valve with the switch andthermostat in the “ON” position and the pilot burning.The voltage should be approximately 190mV DC. Mainburner may not light if readings are less than 145mVDC.

3. The milli-voltage between the connectors “TP/HP” and“TP” on the gas valve with the switch in the “OFF”position should be around 500 mV DC. If readings arelow check the pilot burner is clean and operates as per manufacture’s specification.

4. With a clean pilot and a low milli-voltage reading, checkthe gas connection to the heater. The inlet pressureshould be in the range as specified by themanufacturer.

5. If all the above fails renew the thermopile and check thevoltage.

Please Note

Never short out or remove a safety device in acontrol system. That can lead to catastrophicproblems.

Never replace a thermocouple in a millivolt systemwith a wire from the thermopile.Appliances are designed for safety reasons with;

•  a millivolt system with thermocouple or;

•  a millivolt system without a thermocouple.

Altering systems could affect the appliance safetyand therefore should not be carried out withoutauthorisation of the manufacturer.

If not sure ask help from the manufacturer orsuitable qualified service agent.

On-Off 

Switch

Thermostat

Gas Valve

Gas Valve

Thermopile

Pilot flame

Gas Burner 

A

B

Thermocouple