Why Does a Propane Furnace Quit in Deep Cold or Heavy Snow?

A propane furnace that ran perfectly in November and locks out on the coldest night of January has usually not broken. It has been starved. Everything between the liquid in the tank and the burner has to work at temperatures the equipment only sees a few days a year, and in this climate those days come with wind and drifting snow attached. Knowing which failures belong to the fuel system and which belong to the furnace saves a lot of time, and it is most of what a propane and LP furnace service call sorts out.

Why does deep cold reduce how much propane a tank can deliver?

Propane leaves a tank as vapor, but it is stored as liquid, and it has to boil to make that vapor. The heat for boiling comes from the surroundings through the tank wall, so the vaporization rate depends on how cold it is outside and on how much of the tank's inner surface is in contact with liquid. Draw vapor fast enough and the liquid cools itself further, which slows the process down more.

Two consequences follow, and they arrive together on the worst night of the winter. Propane's vapor pressure falls steeply as the temperature drops. And a tank that is low on liquid has far less wetted surface area to boil from. A tank that carried the house at a quarter full in October may not carry it at a quarter full in a deep cold snap, when the furnace, the water heater and a fireplace all want gas at once.

What does a propane regulator do, and why must its vent stay clear?

A propane regulator drops tank pressure, which varies with temperature, down to the steady low pressure appliances are designed to burn. Most residential installations use two stages: a first-stage regulator at the tank and a second-stage regulator at the building. Each one has a vent — a small opening, usually under a protective cap, that lets the diaphragm inside move.

That vent has to stay open and pointed down. Blocked with ice, packed with snow, or turned upward so it collects water that later freezes, the diaphragm cannot move correctly and the regulator stops controlling pressure properly. The symptom at the appliance is low or erratic gas pressure: pilots that will not stay lit, burners that light and drop out, a furnace that locks out on flame failure.

Three ways snow and ice stop the gas

Snow reaches a propane system in more than one way, and the three below account for most winter supply calls in the townships around Duluth and Superior.

• Burial. Drifting snow covers the tank dome and the regulator entirely. Melt and refreeze then turns that cover into ice around the vent.
• Freezing rain and drizzle. A glaze over the regulator cap seals the vent without burying anything, so the installation looks fine from a distance.
• Roof avalanche. Snow and ice releasing off a steel roof onto a tank, a regulator or a copper service line below can bend, shear or crack them. This one is a leak hazard, not just a supply problem.

Moisture inside the system is a fourth, less visible cause. Water that has found its way into the gas can freeze at the regulator orifice and block it, which produces the same starved-appliance symptoms. That one belongs to the propane supplier — they have a remedy for it and will apply it if they find it. It is not something a homeowner treats.

What to clear after a storm, and in what order

This is homeowner work, and doing it after every significant storm prevents most winter no-heat calls in rural service areas. Everything on this list is outside the equipment; none of it involves opening anything.

1. Brush snow off the tank dome and the regulator by hand or with a soft broom. Do not chip or strike ice off a regulator or its piping.
2. Clear a path to the tank wide enough for a delivery hose, and shovel out the fill connection.
3. Clear snow and ice away from the furnace and water heater vent terminations on the outside wall, and from any fresh air intake beside them.
4. Look up. If the tank or the service line sits under an eave where snow releases, that is a problem to solve before it happens, not after.
5. Check the tank gauge while you are out there and write the number down, so you know how fast you are using fuel in cold weather.
6. Re-mark the tank and the line route with tall stakes if the plow has taken the markers out.

Telling a fuel problem from a furnace problem

Supply problems and furnace problems produce overlapping symptoms, but the pattern across appliances usually separates them. If every gas appliance in the building is unhappy at once, the fuel system is the suspect. If only the furnace is affected while the range and water heater behave normally, look at the furnace.

Reading the symptom: fuel supply or the appliance itself

What you observe	More likely	First move
Furnace, water heater and range all misbehaving	Fuel supply or regulator	Check tank gauge, clear regulator, call supplier
Furnace lights then drops out repeatedly in a cold snap	Low supply pressure	Check tank level and regulator for ice
Only the furnace affected, other appliances fine	The furnace	Book a furnace diagnostic
Nothing works and the gauge reads very low	Out of fuel	Call the supplier; the system needs a leak check on restart
Yellow, lazy flames on the range	Pressure or combustion air	Stop using it and call
Tank or line struck by falling snow	Physical damage	Stay clear and call the supplier immediately

Running a tank completely empty is a separate situation with its own procedure, because the system has to be leak-checked and every appliance relit by someone qualified before service resumes — that sequence is covered in what happens when a propane furnace runs out of fuel. A pilot that will not stay lit on a water heater during a cold stretch often has the same underlying supply cause, and the other possibilities are in why a water heater pilot keeps going out.

Tank placement is the fix you only get to make once

Where the tank and the service line sit decides how much of this you deal with every winter. A tank set clear of roof drip lines and away from the path snow takes off a steel roof avoids the avalanche problem entirely. A tank on the windward side of a building collects drifts; the lee side usually collects less. A service line routed where the plow swings gets found by the plow eventually.

Township properties in Wrenshall, Thomson and across the line in Maple are where these details earn their keep, because a long driveway and a distant supplier turn a small supply problem into a long cold evening. If the tank is already in a bad spot, the supplier owns the tank and its placement, so that conversation is with them.

Who to call, and when

Call the propane supplier for anything upstream of the building: tank level, the regulator, the service line, a delivery, a suspected leak outside, or physical damage from snow or a vehicle. They own that equipment and they are the right people to touch it. Call an HVAC technician for anything from the building's gas piping inward — a furnace that will not light with good supply pressure, a lockout code, an inducer or pressure switch fault, or propane furnace service and combustion adjustment for an appliance that has never quite run right on LP. And if you smell gas at any point, neither of those calls happens from inside the building.