My furnace blower runs but never produces heat. What is happening?

The furnace is starting its sequence and stopping before gas is admitted. The most frequent reasons are that the blower is not reaching the speed needed to close the airflow-proving switch — very often a weak battery — or that the intake or exhaust is restricted so the proof can never be satisfied. Both are safety interlocks behaving correctly.

Why does my furnace work on shore power but not on battery?

Because on shore power the converter is holding the DC system up, and on battery it is not. That is a strong indication the battery bank or its connections cannot support the blower, rather than that the furnace has a fault. It is one of the clearest diagnostic splits in the whole coach.

Can I clear a wasp nest out of the furnace vent myself?

Looking at the vent from outside is reasonable and useful. Clearing it is not an owner job: the intake and exhaust are part of a sealed combustion path, and disturbing or damaging that path is precisely how carbon monoxide gets into a coach. Report what you can see and let a technician clear and verify it.

It lights and then shuts off after a minute or two. Is that dangerous?

It is the safety system working, which is the opposite of dangerous — but it does mean something is genuinely wrong, most often flame sensing or an airflow restriction. What matters is not repeatedly cycling it. Every retry admits gas to a chamber that has already failed to run properly.

Repair guide

RV furnace repair

Where in the ignition sequence it is stopping — which is what identifies the fault.

The start sequence as a diagnostic ladder, plus the battery-voltage requirement. An RV furnace proves airflow before it will admit gas, so a weak battery that cannot spin the blower fast enough produces a no-heat fault that has nothing to do with gas at all.

Stop-work system

A furnace burns propane inside the coach. A fault in the burner, exhaust, or sail switch is a carbon-monoxide risk. Stop and call a technician.

Nothing on this page is a repair instruction for this system. What follows is what to observe and record so a technician can work efficiently — the work itself is theirs.

What brings owners here

The blower runs continuously but the air is never warm
It clicks or sparks repeatedly and then gives up
It lights, runs briefly, then shuts down and retries
Nothing at all happens when the thermostat calls for heat
It works on shore power but not on battery

What to observe and write down

These are observations, not repairs. Each one narrows the problem before anybody charges you for time.

01
Treat any gas smell or soot as a stop-work condition
An RV furnace burns propane inside the coach and vents combustion products outside through a sealed path. A propane smell, soot around the exterior vent, or a carbon-monoxide alarm means shut the propane off at the tank, ventilate, get out, and call a technician. Do not keep cycling it to see if it clears.
02
Measure battery voltage with the furnace trying to start
The furnace blower must reach a set speed before the control will allow gas to flow. On a weak battery the blower turns too slowly, that proof never happens, and the furnace locks out — with no gas fault present at all. Note the voltage during the attempt, not at rest.
03
Note where in the sequence it stops
Blower only; blower then a clicking igniter; blower, ignition, brief flame, then shutdown. Those three describe three different faults, and reporting which one you have is more useful than any other single observation.
04
Look at the exterior vent from outside for obstruction
The combustion air intake and exhaust are usually a combined exterior assembly. Insect nests — mud daubers and wasps in particular — are a very common blockage, and they build in exactly the season the furnace is not used. Looking is fine; clearing a combustion path is technician work.
05
Confirm propane is actually available
Check that the tank valve is open and that other propane appliances light. A furnace is often the first appliance used after a long gap and is regularly blamed for an empty tank or a closed valve.

How a technician works through it

Knowing the order matters: it tells you whether a quote skipped a step, and it tells you what a technician still has to establish before anyone can say what is wrong.

01
Verify DC voltage under blower load
The airflow proof is the gate the whole sequence passes through, and it is voltage-dependent. Confirming supply first prevents an unnecessary gas-side diagnosis.
02
Confirm the airflow proof and the return path
The safety switch that proves airflow will not close on a blocked intake, a blocked exhaust, or a restricted return. This is a safety interlock doing its job, and the correct repair is clearing the restriction, never bypassing the switch.
03
Check the gas supply, regulator and valve in sequence
Pressure is confirmed at the appliance rather than assumed from a full tank. Regulator faults present as appliances that work individually but not together, or that fail in cold weather.
04
Assess ignition and flame sensing
A furnace that lights and then drops out is usually failing to prove flame rather than failing to make it. That is a specific and repairable fault, and it is distinct from a no-ignition condition.

What drives the cost

We publish no dollar figures anywhere on this site, because real prices vary by market, season, and technician, and a number we invented would look authoritative and help nobody. What is stable is what moves the number.

Whether the fault is DC supply (often the battery, not the furnace at all) versus a furnace component
Access — many furnaces are built into cabinetry or a compartment and must be partly withdrawn
Whether the sealed combustion chamber is compromised, which is a replacement rather than a repair
Duct and return-air work if the fault is airflow restriction rather than the appliance
Whether a carbon-monoxide inspection is warranted alongside the repair

Before you weigh any of that, settle who is paying. Warranty, insurance, or you works through the order that protects a claim — because authorising a repair before you have the answer is what turns a covered failure into one you funded.

Mobile or shop?

Well suited to mobile service — diagnosis, control boards, igniters, sail and limit switches, gas valves, blower motors and blockage clearance are all standard on-site work. A furnace with a compromised combustion chamber is replaced rather than repaired, which is still usually a mobile job.

Where this plays out differently

A handful of markets change the advice on this system rather than merely hosting it.

Denver Front Range, CO
Whether a furnace failing at altitude is an altitude problem, a battery problem, or a genuine fault.

Common questions

My furnace blower runs but never produces heat. What is happening?: The furnace is starting its sequence and stopping before gas is admitted. The most frequent reasons are that the blower is not reaching the speed needed to close the airflow-proving switch — very often a weak battery — or that the intake or exhaust is restricted so the proof can never be satisfied. Both are safety interlocks behaving correctly.
Why does my furnace work on shore power but not on battery?: Because on shore power the converter is holding the DC system up, and on battery it is not. That is a strong indication the battery bank or its connections cannot support the blower, rather than that the furnace has a fault. It is one of the clearest diagnostic splits in the whole coach.
Can I clear a wasp nest out of the furnace vent myself?: Looking at the vent from outside is reasonable and useful. Clearing it is not an owner job: the intake and exhaust are part of a sealed combustion path, and disturbing or damaging that path is precisely how carbon monoxide gets into a coach. Report what you can see and let a technician clear and verify it.
It lights and then shuts off after a minute or two. Is that dangerous?: It is the safety system working, which is the opposite of dangerous — but it does mean something is genuinely wrong, most often flame sensing or an airflow restriction. What matters is not repeatedly cycling it. Every retry admits gas to a chamber that has already failed to run properly.

Sources for the claims on this page

RV forced-air furnaces use an airflow-proving safety switch that must close before the gas valve is permitted to open, so inadequate blower speed prevents ignition entirely.
Industry consensusCross-manufacturer industry consensus for RV forced-air furnaces
Insect nesting in combustion air intakes and exhaust vents is a widely documented cause of seasonal furnace failure in RVs.
Industry consensusIndustry-wide maintenance consensus
A furnace burning propane inside the living space with a fault in the burner, exhaust, or airflow proof presents a carbon-monoxide risk and is a stop-work condition for owner troubleshooting.
Industry consensusIndustry-wide safety consensus; mirrored in this site’s safety gate for furnaces

Related systems

Next step

Work through structured triage for this system and you will end up with a written service brief: what you observed, in what order, and what has not been established yet. It is the document that lets a technician quote properly instead of guessing on arrival.

Being straight about what this is. No technician is signed to any territory yet — not one. We do not match you, dispatch anyone, or promise a call. What you get is the brief, and a request form that copies it to us so we know the demand is real. If you need someone today, take the brief and send it to a local technician directly; it works just as well that way.

Start triage Request a technician