Sunline 1,
OK that is what I thought, being 20F outside, running the furnace and then you went into auto switch over.
I see 2 things that may add together to create what you saw OR it is all the fact that it was cold out and running the highest LP gas appliance we have, the furnace on an almost empty tank.
Since this may be the 1st time this tank has been filled by the gallon or pound you may have noticed there is some variation in how full the tank can be filled. In the new style tanks there is an overfill device. It is a float valve that prevents the tank from being overfill beyond 80% total fill. The float valve is part of the stem that screws down into the tank below the shut off knob. If they are filling and not using the bleeder, and if the float shuts off early it cannot take any more. To prove this out, next time weigh the tank before going to get it filled and if the weight equals the tare weight then compare to after being filled.
Looks like these:
Propane OPD - Overfill Prevention Device Cylinder Valves
Those valves are screwed into a NPT taper thread in the top of the tank and they is a slight variation on where they tighten up. If screwed in 1/16" deeper it will be less fill and 1/16" less will be more fill. On a cylinder of that diameter it may end being 1# of propane. That is not all of the 5 # but "might" be a part of it.
The bigger issue could be the reduced gas vaporization effect of being cold out, the tank being close to empty lowering the wettable area for creating vapor and the furnace running pulling a high demand for gas vapor. The change over regulator senses low pressure to do the switch. If the demand for gas vapor is larger than the tank can create, the pressure drops from the furnace running as there is not enough vapor volume being generated to keep the pressure up. This creates a momentary low pressure, low enough to trip the regulator to switch the regulator.
And actually if you left the tank alone, if there is LP gas left it would continue to boil and raise the pressure back up, just it takes longer. But it's too late as the regulator already switched. During times of the year where the outside temps are higher, this problem is not seen and you coast right through this until there is no LP left.
See this chart on the lower right. It shows the vapor pressure in the tank per degrees temperature outside the tank.
http://www.winnebagoind.com/resource...20LP%20Gas.pdf
At 20F there is 40.8 psi in the tank where as at 50F for example there is 77.1 psi. I believe the switch over may happen around 8 psi. I cannot find the spec on the Marshall Brass one but found it on another brand. Since we have a small diameter tank, there "might" not be enough of the LP surface area exposed for the needed vaporization to occur at the low temps outside.
And here is one of the easier to understand writeups on how vaporization takes place. Now granted this site is talking about vaporizers for different applications but it explains the tank size, temperature, wettable area and how vaporization works.
Algas SDI: World Class Liquid Vaporizers & Gas Mixing Solutions
Here is a good example of the LP boiling effect that goes on to create this pressure to stop the boiling.
http://www.propanesafety.com/uploade...r%20Demand.pdf
And here is another article on LP systems in cold weather.
http://www.documentation.emersonproc...500147t012.pdf
If you look at a home that uses LP gas, the tank is real long and larger in diameter. Yes this is to create more volume to last longer but they created more wetted surface area to create more vapor with the long larger horizontal tank. They solved the volume problem by creating a large enough wetted surface for cold temperatures.
Since you now are filling the tank by the gallon or pound, you are not overpaying for 5 left over pounds like the flat rate places do. You are only paying for what was used. I have a local garden center that sells it by the pound and they weigh fill it and check full with the bleeder valve.
I have not yet tracked down the BTU generation of 30# cylinder at 20 F yet. I can find a 150 gallon tank, just no the 30#'er. The furnace can be pulling 30,000 BTU's and when the tank gets close to empty and the diameter of the dome on the bottom of the tanks gets smaller, the wettable area is smaller and may not be enough to generate enough vapor to feed the furnace.
This may be your explanation or may not be, but it at least adds up to what "might" be.
Hope this helps
John
Once I understand it, I can leave it alone. Until then, well curiosity gets the best of me some times. The learning never stops. Until your post, I never dug deep enough to better understand the LP boiling process.
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