Jerry’s Tech Talk: Replacing Gas Safety Valves
June 10, 2009 by: Jerry Green
There are many different types of safety valves. The most common are:
1. FMDA
2. BASO
3. TS
4. Combination Type
- An FMDA Gas Safety Valve
1. The FMDA safety valve is the only type with the thermocouple permanently attached to it. This means the thermocouple cannot be replaced; the entire safety valve must be replaced if the thermocouple fails. The easiest way to identify an FMDA type safety is a ½” diameter red button on the bottom of the valve. You must know the gas pipe size and if the pilot tube is an “in and out” or an “out only.” An “in and out” safety valve has two threaded holes at the top of the part, one for gas for the pilot to come in and one for gas to go out. An “out only” safety valve has just one threaded hole to connect gas for the pilot to.
- A BASO Gas Safety Valve
2. The BASO safety valve can vary in design depending on the piece of equipment it is on, so it is important to know the brand name, model and serial number of the piece of equipment to get the correct safety valve the first time. The easiest way to identify a BASO valve is by the 15/16” diameter red pilot button. The thermocouple is separate from the safety.
- A TS Gas Safety Valve
3. The TS type safety valve is the only one that can be rebuilt. It is similar to the FMDA and BASO types in that it has “in and out” or “out only” pilot tubing, so you must know what is in your equipment. A rebuilt kit is available in both and it is not necessary to replace the body unless it is damaged. The body has no moving parts in it. The easiest way to identify the TS safety is by the 5/8” diameter red button. The thermocouple is also separate from this safety, similar to the BASO.
- A Millivolt Type Combination Safety Valve
4. Combination safety valves come in three different styles:
A combination valve is a gas valve with the safety built into it. Most combination valves are found in fryers.
How to determine which type you have:
- If there are two wire leads coming out of the valve then it would be the 120 volt.
- If the wire leads are screwed to the top terminal block, and two tubes are coming out of the top of the valve, it is the tubing type combination safety valve.
- If the wires screw into a terminal block it would be a mulitvolt type.If you are not sure, just provide the brand name, model and serial numbers.
Every combination valve uses either a thermopile or thermocouple. The most common is a thermopile, and there are two different thermopiles:
The screw-in type screws directly into the body and the two-lead type has a terminal block on the combo valve to directly screw into.
The FMDA Gas Safety Valve does not have a thermocouple permanently attached to it. What you are mistaking for a thermcouple is a capalary tube. The capalary tube is permanently fixed to the gas valve because it is filled with mercury. When the end of the tube which is heated by the pilot flame and gets hot enough the mercury travels towards the gas valve completing the connnection to allow for the magnatie to hold. In turn with the maganite holding gas flow is allowed when requisted by the thermostat. The action of a thermocoulpe / thermopile and a capalary tube are diffrent but accomplish the same task.
Andrew, FMDA valves do not use mercury.