[JRTJH]

Bob,

A propane system that is "functioning correctly" can't or won't selectively operate one appliance but not the other. You're missing the point of the "safety cutoff system"... When it is activated, the propane system is NOT functioning correctly any longer.

I think you're missing the pressure/volume concept. You can turn on your water hose faucet just a "smidgen", say 1/8th of a turn and if your water pressure is 60 PSI, if the hose nozzle on the "other end of the hose" is closed, in a minute or two, you'll have 60 PSI in the water hose. BUT, if you open the nozzle, the pressure built up in the hose will "shoot" water out at 60 PSI for a few seconds, then the water flow will drop to a "dribble". That's because the "pressure is available" but the "flow is not sustainable with the small amount of volume being supplied. As the VOLUME drops, so does the PRESSURE drop....

Just as the water hose will lose pressure and start to "dribble" (volume) and not shoot at full force, in your propane system, when there is no demand from an appliance, the propane will build up to the regulator's pressure of 11" WC, but when the "nozzle" is opened (stove burner), the volume is not being supplied by the safety cutoff to "sustain the volume" and the "pressure will drop.

To illustrate to yourself, take a 20 lb propane tank out into an open area, connect a pigtail to it with the other end of the pigtail open to the atmosphere. Turn on the tank valve and see what happens. The "high pressure flow" will stop, but there will be a very small "hiss" of propane still flowing from the open end of the pigtail. If you TEE a manometer on the end of the pigtail and close the other end of the TEE , it will build up to 11" WC, but if you open the other end of the TEE, the pressure will drop because there is not sufficient volume of propane flowing to maintain the pressure. As the pigtail empties the propane, the pressure falls. That "small hiss" of propane is sufficient to operate a 2000 BTU burner, but not a 20,000 BTU burner.

Now, if there was "sufficient volume" flowing through the pigtail, (normal operation) the manometer would continue to read 11" WC.

Pressure is directly dependent on volume. Without adequate "resupplying" of propane, the pressure MUST fall, it's simply "emptying the chamber faster than you're filling it"......

In the gas control valve on the furnace, the input pressure is 11" WC. The gas control valve drops that pressure to 10.6" WC. If the propane control valve can not maintain 10.6" WC, the safety valve in the furnace disables propane flow. There is no "safety valve on the stovetop burner to disable flow, so the "propane hiss" that you hear will continue to provide the "greatly reduced volume" of propane to that burner as long as the "propane hiss" at the pigtail safety cutoff continues to flow a small amount of propane. But, if you have a manometer on that line, it will not be 11" WC, so the furnace gas control valve will not open to allow any of the "propane hiss" to operate that appliance.

The reason there is a "propane hiss" in the pigtail is also a safety device. It allows for a "low pressure bleed" to prevent the tank from overpressurizing (should it be in a fire) so the tank will "bleed slowly rather than explode"(from overpressurizing). In other words, that "hiss" allows the propane tank to "cook off" rather than explode.