Trillium Furnaces - ITT RV18AA02

[David Tilston]

The things I do for.... fun?

I have an unusual interest in taking things apart. Putting them back together, less so.

I have stared at the gas valve that is installed in almost every Trillium trailer that I have seen. Trillium was fond of Duo-Therm who seems very fond of the ITT RV18AA series of gas valves.

Related threads:
http://www.fiberglassrv.com/forums/f...lve-85951.html
http://www.fiberglassrv.com/forums/f...rol-86142.html

The photos are from the dissection of the defective valve that was part of the transaction in the first related thread. I have no idea what was wrong with it. There was no obvious flaw. I may have been able to fix it, but I have now sent out parts from it. So it is now just parts, let me know if you need something other then the diaphragm from the control valve.

The first picture is actually a process flow drawing that I did. It may not be correct, but it is my best guess.

I still can't quite figure out the operation of the main control valve. I get how it closes, the thermostat valve closes and line pressure slowly flows through the orifice to pressurize the back of the diaphragm at the same pressure as the front of the diaphragm. This allows the spring that is behind the diaphragm to push the flat surface of the rubber diaphragm to press against the valve seat. The opening is similar. The thermostat valve opens, and then vents the space behind the diaphragm to the discharge port of the valve. When the valve is closed, the discharge port should be at atmospheric pressure. The orifice prevents the flow of gas that would re-pressurize this space. The resulting pressure differential then pushes the diaphragm off the valve seat. What I am having trouble with is, how is this pressure difference maintained, when the control valve is open, and the pressure on either side should be the same? Maybe the valve acts a bit like a regulator. It only opens enough to allow the gas to flow to the burner, but at a lower pressure then the supply pressure.

I hope someone can comment on this.

The control sequence kind of goes like this:
- The operator turns the three way, plug style, selector valve to the pilot position. This lines up a very small slot on plug with the pilot port, but the main port remains closed.
- when the push button, (selector knob) is pressed, it opens the pilot override valve, allowing gas to flow to the pilot port, through a needle valve adjustment.

- When the pilot is lit, it heats up a thermocouple, (K type?). This generates enough power to allow an electrical coil to hold the spring loaded, pilot override valve in the open position. The drawing should show some sort of mechanical interlock between the selector valve, and the pilot override, but I don't know how to show that.
- The push button is released, but the pilot remains lit, and the selector valve can now be turned to the ON position. This allows gas to flow to the control valve chamber, and now the valve is in automatic operation.

If the pilot light goes out, then there is no power to the magnet, and the pilot override valve closes, the gas is thus completely shut off. The power that holds open this valve is millivolts, at milliamps, likely, milliwatts.

[David Tilston]

The electrical part of the valve.

[David Tilston]

You can see the orifice that feeds the thermostat valve. It is the very small hole, surrounded by circle, inside the bottom of the circle that the diaphragm sits in. The gas passage to the back of the diaphragm is the larger hole just below the orifice. The gas passage to the back of the diaphragm is the small slot cut in the diaphragm cover, near the bottom, as seen in the third picture. The pilot ports are just above the large circle that the diaphragm sits in. The larger port comes from the pilot slot in the plug valve. The smaller port above it goes out the needle valve then to the pilot connection on the top of the valve.

[David Tilston]

This should be split into two subjects. The thermostat valve and the selector valve. Oops.

The fifth and sixth pictures are meant to show the thermostat valve in the closed and open positions.

[David Tilston]

The rest of the knobs set.

[David Tilston]

Last picture. Every part in the valve.

[Borrego Dave]

Thanks for the last chuckle of my day Dave. I do the same thing but I tell my wife I'm just putsing in the garage.

[Gullygossner]

Thanks for doing this teardown, it is immensely beneficial to see the workings of these.

[Steve Hague]

Thanks so much David. Your tear-down pictures gave me courage and understanding that allowed me to dig in and repair my stuck valve. The black valve seemed to be stuck with grime and age, but it let loose after I pushed it out from the front. I cleaned it enough to get it moving, but I did not lubricate it. Do you know what type of grease would be appropriate? Anyway, I put it back together, reinstalled it, and bench-tested the furnace. Seems to work well now, so I will move on to reinstalling it in my 1978 Trillium 1300 project trailer.

[David Tilston]

Originally Posted by Steve Hague

Thanks so much David. Your tear-down pictures gave me courage and understanding that allowed me to dig in and repair my stuck valve. The black valve seemed to be stuck with grime and age, but it let loose after I pushed it out from the front. I cleaned it enough to get it moving, but I did not lubricate it. Do you know what type of grease would be appropriate? Anyway, I put it back together, reinstalled it, and bench-tested the furnace. Seems to work well now, so I will move on to reinstalling it in my 1978 Trillium 1300 project trailer.

Steve, I really have no idea. I would go with something thin, and not likely to evaporate, if there is such a thing. But that is just a guess. Sorry.

[Steve Hague]

Originally Posted by David Tilston

Steve, I really have no idea. I would go with something thin, and not likely to evaporate, if there is such a thing. But that is just a guess. Sorry.

Thanks Dave, just another rv maintenance topic to put on my list. It seems that I never run out of search topics for YouTube and Google that keeps me busy and entertained during the hard, cold midwestern winter nights to come.
Cheers!
Steve