Three reasons the furnace quits, and only one needs a technician

Twenty degrees outside, the furnace stops some time around midnight, and you wake at six to a rig at forty-one and no idea how long it has been like that. The question at that moment is not whether the heat stopped. It is whether you are driving to a propane fill, plugging into something, or telephoning somebody who fixes furnaces.

An RV furnace stops for three reasons and they need three different mornings. It ran out of propane. The house battery sagged below the control board’s cutoff and it refused to light. Or it faulted, which is the only one of the three that is actually the furnace.

The interior temperature tells you the heat stopped. It cannot tell you which of the three it was — and every one of the readings that can is a reading you very likely already have.

Knowing that it stopped

One temperature sensor in the living area, on two thresholds. Warn below 55°F and go critical below 45 — or better, set the warning about ten degrees under your thermostat setpoint, so a normal furnace cycle never touches it and anything that does touch it means something. Most of these report every 60 seconds, some every 120, and neither number is the constraint here.

The gap between the two rungs does more work than it looks. It is a rate-of-fall reading: a well-insulated rig on a twenty-degree night sheds roughly two to four degrees an hour with no heat, a thin-walled one about twice that. Watch how long it takes to get from the warning to the critical and you know, with no other instrument, how much time you have and whether the furnace is off or merely losing.

Placement decides whether any of this works. A sensor by the entry door or a slide seal trips on the draft; one in a cabinet above a duct never trips at all; and a long rig with the bedroom door shut is honestly two different temperatures. Freeze protection is the longer argument about where these go, and it is worth reading before you set a threshold rather than after.

Knowing why

a. The propane, which runs out sooner than it reads

Warn at 25 per cent and go critical at 10 — and note that the warning is deliberately higher than you would set it in summer. A cold cylinder cannot vaporize fast enough to keep a 30,000 BTU furnace lit, so on a hard night the furnace starves at twenty per cent while the tank still reads twenty per cent. The gauge is not wrong. It is answering a question about liquid, and the furnace is asking one about vapor.

A Mopeka reads every 300 seconds, which is ample for something that empties over hours, and it broadcasts its own reading quality — alert on that too, or a poor echo off a painted cylinder base gives you a phantom empty. Liquid propane also contracts in the cold, so a cylinder that read 30 per cent on a mild afternoon reads a little lower at ten degrees without anybody using any. If you have a SeeLevel 709-BTP7 it carries propane as one of its channels and costs you nothing; the BTP3 does not carry propane at all. The full treatment is in how much propane is left.

b. The battery, which stops it before it is dead

Most furnace control boards refuse to ignite somewhere around 10.5 to 11 volts, which is well above what you would call a dead bank. So the app’s own defaults are the right ones here: warn at 12.0 volts, critical at 11.6. That critical lands before the furnace quits rather than after, which is the entire point of putting it there.

Read the trend rather than one sample. Voltage sags while the blower runs and recovers when it stops, so on a tired lead-acid bank a 12.0 volt warning can trip on every single cycle, and a cold battery reads lower than a warm one at the same charge. A SmartShunt gives you a counted percentage as well, which is the cleaner number on lithium, where voltage barely moves until the bank is nearly empty. A Lancol gives you voltage and nothing else, which is enough for this one job.

c. The blower, on the current chart

This is the reading no other device gives you, and it is the reason a shunt is worth its money in winter. An RV furnace blower pulls roughly 7 to 12 amps while it runs, so a cycling furnace draws a repeating step on the current chart — on for ten or twenty minutes, off for twenty or forty. You can see the furnace working. On a freezing night, current that has not stepped up in 90 minutes is a furnace that is not running.

Two things blur it. A shunt reads the whole 12 volt system, so it tells you a ten-amp load stopped rather than which one, and lights, a fridge on DC and a tank pad all add current that can be mistaken for a blower. And on shore power the converter’s charge current masks a 7 amp load entirely unless you read the sign. The SmartShunt needs its Instant Readout key pasted into the hub before any of this decodes, and it has to be a battery monitor: Victron’s solar chargers and DC-DC converters are recognized but their layouts are not decoded.

d. The pedestal, if your winter heat is electric

Space heaters and heat pumps stop the instant the pedestal does; a propane furnace keeps running, if the thermostat is set to gas. A Power Watchdog going silent is the fastest signal in this whole family at 30 seconds — the pedestal powers it, so losing the pedestal costs you the device rather than giving you a tidy zero — and cold nights are exactly when campground loops trip, because every rig on the loop has two heaters running. That is a real outage rather than a false alarm, and it usually ends when somebody resets a breaker.

One thing that is not yet a method: the EasyTouch RV thermostat connects and we have a driver, but the field mapping is heuristic pending a live capture, so nothing decodes today. It would be the one reading that says furnace mode is on and the setpoint is not being met, which is why it is annoying that it is not finished. Do not buy one for this on our word.

The combinations

This is the family where the cookbook pattern pays best, because the three causes have three different mornings attached.

• Interior falling, propane under 10 per cent — it ran out of gas. Swap the cylinder or open the second one, and if the rig recovers within the hour the furnace itself is fine.
• Interior falling, voltage under 11.6 — the board hit its cutoff. Plug in or start the generator and it will relight on its own. Expect the wet bay to freeze before the interior does.
• Interior falling, propane fine, battery fine, no current step in 90 minutes — the furnace has both its inputs and is not running. Sail switch, limit switch, igniter or a lockout. This is the one that needs a person, and nothing remote will fix it.
• Interior falling and the Watchdog gone quiet — your electric heat stopped with the campground. The propane furnace is not the problem unless the thermostat was set to electric only.
• Blower still cycling and the rig getting colder anyway — the furnace is running and losing. Undersized for the night, a duct off, a slide seal open, or short-cycling on a dirty filter. Cycles getting progressively shorter on the chart is the limit switch.
• Every sensor stale at the same moment — the hub lost power, not the furnace. That is a different article, and the fix is to move the hub to 12 volts.

What none of this tells you

Why it faulted. The readings narrow it to which input is missing, and when none of them is missing they narrow it to “the furnace”, which is where instrumentation stops and somebody opens the access panel. Nothing here relights a pilot, restarts a furnace or switches a thermostat, and we would not build that if we could.

The interior sensor also lags by an hour or more, because the mass of the rig hides the first part of the drop. That matters most for the thing it is not watching: the wet bay freezes before the living area gets cold enough to warn you. If the rig is unattended in real cold, plan for that rather than hoping the interior threshold catches it.

What to do about it

Start with one interior sensor and a warning ten degrees under your setpoint. It is cheap, you probably own it already, and it catches all three versions of this. Then add whichever reading answers the question you actually face: propane if you run on cylinders, a shunt if you spend cold nights on the battery.

If you are choosing between those two, the shunt is the more informative, because its current trace is the only place in the catalog where you can watch the furnace itself work. Everything else in this article tells you about the furnace’s inputs. That one tells you about the furnace.

Where to get one

In the order above: something to read the room, then the input you are most likely to run out of. The compatibility database lists every sensor that does the first job, and the sensor comparison is the argument about which.