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Gas Units- Just the Facts: Part I- Millivolt Systems

Millivolt Systems (Standing Pilot)

This system is the oldest type of gas operating system. The HPBA is working on legislation to phase these types of units out, but that doesn’t mean that we won’t still run in to them from time to time. Until then, there are still some manufacturers who will continue to make gas units with these systems until they are told to stop. Here is a short breakdown of how the systems work, as well as some Frequently Asked Questions and Answers.

diagramTo the left is a typical millivolt (standing pilot) system. At the top is the pilot assembly; consisting of a pilot tube / orifice/ hood, a thermocouple, a thermopile and a sparker electrode. These are all connected to the valve in different ways, except for the sparker; it is attached to an ignitor.

The sparker does what its name implies, it sparks as an ignition source at the pilot hood so that the spark combined with gas or propane will light the pilot light. This is in place so you don’t have to light the pilot with a match.

The pilot light hits the thermocouple and the thermopile simultaneously. The thermocouple’s job is to keep the pilot lit. As long as the thermocouple is putting out enough voltage, created by contact with the pilot, it allows the gas valve to continue to send gas to the pilot tube and keep the pilot lit.

When the unit calls for heat (either by thermostat or simply being turned ON), the thermopile puts out enough voltage to open the electromagnet in the gas valve that sends fuel to the burner, turning on your fireplace!

Frequently Asked Questions

My pilot won’t stay lit, is it the thermocouple?

Not necessarily. The thermocouple is the most common part to replace on these systems, but sometimes other issues are to blame, such as low fuel or a dirty or clogged pilot hood. You should have preventative maintenance performed annually on your unit to make sure things like this don’t happen.

My pilot went out, is gas leaking into my fireplace?

Assuming your valve is fine, no. Remember, the thermocouple’s job is to let the valve know that it is ok to send fuel to the burner because the pilot light is lit. If the pilot light goes out, the valve closes completely and does not allow gas through to the burner or pilot assembly until the pilot is lit again.

I need a new thermocouple, they’re all the same right?

In short, not really. All thermocouples put out the same range of voltage, but they vary in length, the size of the tip that the pilot needs to hit, they even screw in to different pilot assemblies in different ways.  Each manufacturer uses one of several different companies to manufacture its pilot assemblies and valves. For your stove to work properly, the correct parts need to be installed by a qualified technician.

My pilot is lit, but my burner won’t turn on, does my valve need to be replaced?

Probably not. Remember, the thermopile and thermocouple work together to let the valve know that it’s ok to release gas to the burner. If there are any issues along this path of communication, your valve will not open. We service thousands of gas units every year and it is very rare indeed that the valve needs to be replaced. Be wary of any repair person who says the valve needs to be replaced but cannot explain why. Many times an unqualified person will not be able to figure out why the valve will not open and will then decide the valve needs to be replaced. The issue reoccurs because the valve was never the problem.

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