Furnace and Hydronic Heating System Repair for Coaches
Written by RV Systems Lead, RV Systems and Electrical Lead, OCRV Center. Reviewed by Claims Coordinator
In shortA furnace where the blower runs and no flame arrives is usually a sail switch or a voltage problem. Heating work runs $350 to $8,500+ at $260 per hour.
What does RV furnace and heating system repair cover?
Heating repair covers forced air furnaces such as the Suburban NT series, combination units like the Truma Combi, and diesel fired hydronic systems including AquaHot. Rancho Santa Margarita owners bring coaches to Yorba Linda, where the furnace, its 12 volt supply and its LP supply are tested together.
- Blower runs but no ignition usually means the sail switch never closed
- Weak batteries cause furnace faults far more often than owners expect
- Repeated limit switch trips point at blocked ducting
- Hydronic systems are a separate skill set from forced air furnaces
- Price range
- $350 to $8,500+2 to 35 labor hours at $260 per hour
- Labor rate
- $260 per hourMechanical and electrical department
- Typical turnaround
- 2 to 10 shop daysHydronic system work drives the long end
- Written estimate
- $150Credited in full against an authorized repair
- Systems serviced
- Suburban NT, Dometic and Atwood forced air, Truma Combi, AquaHot, Webasto, Espar
- Hazmat and disposal
- $45 flatApplies on jobs that open the LP or diesel fuel system
Last verified
Heat in a coach comes from one of three architectures and they have almost nothing in common. A forced air LP furnace pushes combustion heat through ducting. A Truma Combi makes space heat and domestic hot water from one compact unit. A hydronic system such as AquaHot burns diesel or runs an electric element to heat a coolant loop, then distributes that heat through exchangers in zones around the coach. Diagnosing one of these with habits learned on another wastes days.
What they share is a dependence on the systems around them. A furnace that will not light is often a 12 volt problem rather than a gas problem, because the blower has to reach full speed before the sequence continues. A hydronic system that will not fire is often a fuel pickup or a burner screen loaded with carbon. We test the supporting systems as part of the diagnosis rather than after the parts fail to fix it.
Three heating architectures, three different repairs
Forced air LP furnaces are the most common by a wide margin. A Suburban NT or an equivalent Dometic unit pulls combustion air from outside, burns propane in a sealed heat exchanger, and pushes cabin air across that exchanger into ducting. The cabin air and the combustion air never mix, which is the whole safety premise, and it is why a cracked heat exchanger is a serious finding rather than an efficiency note.
A Truma Combi does space heat and domestic hot water in one unit and it reports faults through a control panel with codes, which changes diagnosis into a mix of code reading and physical testing. A hydronic system such as AquaHot is closer to a small boiler: a diesel burner or an electric element heats coolant, pumps circulate it, and heat exchangers with fans deliver it in zones. When a hydronic coach loses heat in one bedroom only, the burner is almost certainly fine and a pump is not.
- Forced air LP: Suburban NT, Dometic, Atwood, duct distributed
- Combination: Truma Combi, space heat and hot water in one unit
- Hydronic: AquaHot and Oasis, coolant loop with zoned exchangers
- Diesel burners: Webasto and Espar, glow pin and burner screen service
The ignition sequence and where it stops
A forced air furnace does not simply light when the thermostat calls. It runs a sequence: the thermostat closes, the blower starts, the blower reaches enough speed to push a small vane called the sail switch closed, the sail switch tells the board that air is actually moving, the board opens the gas valve and sparks the electrode, and the flame sense circuit confirms combustion. Each of those is a permission and the furnace stops at the first one it cannot get.
This is why the complaint owners report most often, a blower that runs with no heat, is so informative. The blower running means the thermostat and the motor are fine. No ignition attempt at all means the board never got past the sail switch. Three attempts and a lockout means the board got past the sail switch and never saw flame, which moves the investigation to the electrode, the valve and the gas supply.
- Thermostat closes and calls for heat
- Blower motor starts and comes up to speed
- Sail switch closes on proven airflow
- Board opens the gas valve and energizes the electrode
- Flame sense confirms combustion and the cycle continues
Why a weak battery bank looks exactly like a broken furnace
Furnace blowers draw meaningfully on the 12 volt system, and a bank that reads fine at rest can sag hard the moment the motor spins up. When it does, the blower turns slower, the sail switch does not quite close, and the sequence stops. From inside the coach that is indistinguishable from a failed switch or a dead board, and it is why we measure voltage at the furnace terminals with the blower running rather than at the battery posts with everything off.
Aging gel and AGM banks are the usual context. They hold surface voltage well and collapse under load, which makes them very good at fooling a resting voltmeter. Coaches that spend months in a storage lot between trips are the classic case: the bank has been slowly cycling down for a season, the first cold night of the trip is when the furnace is finally asked to work, and the owner concludes the furnace died in storage.
Hydronic systems and what they actually need
A diesel fired hydronic system carries maintenance obligations that a forced air furnace does not. The burner screen accumulates carbon and needs periodic service. The glow pin is a consumable. The fuel metering pump has a service life. Coolant needs to be the right chemistry and needs to be changed. Owners who bought a used diesel coach with an AquaHot rarely receive any of that history, and the first symptom of years of skipped service is a burner that lights hard, smokes, or refuses altogether.
Zone distribution is the other half. Each area of the coach has a heat exchanger with a fan, fed by circulating pumps. A single seized pump or an air lock in the loop takes out a section of the coach while everything else works normally. That pattern, hot water available and one zone cold, is diagnostic on its own and it saves an enormous amount of time when the owner reports it accurately.
Before you proceed
Hydronic burner service intervals are not suggestions. A carbon loaded burner screen left in place accelerates its own failure and takes more expensive components with it.
What heating system repair costs
Work bills at $260 per hour from the mechanical and electrical department, across a range of $350 to $8,500 and higher on large hydronic overhauls. A sail switch, a blower clean and a multi cycle verification sits at the bottom. Board, valve and electrode work with duct correction sits in the middle. A full hydronic burner and zone system service on a forty five foot diesel coach reaches the top and can pass it.
Parts carry standard markup, sales tax applies to parts and materials, and labor is not taxed. A $45 hazmat and disposal charge applies to jobs that open the LP or diesel fuel system. Jobs over $2,000 take a 50 percent deposit at authorization. The $150 written estimate is credited in full against an authorized repair.
The coaches that need this most
Southern California owners heat less than owners elsewhere, and that is precisely the problem. A furnace that runs three weekends a year sits idle long enough for the blower wheel to load with dust and for something to build a nest in the exhaust. The failures we see are storage failures far more often than wear failures, and they surface on the first genuinely cold night at elevation rather than at home.
Full time owners and anyone chasing winter into the Sierra or the high desert run at the opposite extreme. Those coaches see real duty cycles, real duct wear and, on the diesel pushers, real hydronic maintenance intervals coming due on schedule rather than by surprise.
Heat exchangers, exhaust and carbon monoxide
The sealed heat exchanger is what keeps combustion products out of the cabin, and it is the one finding on a furnace that ends the conversation about repair. A cracked exchanger means the unit is replaced. There is no patch, no weld and no acceptable monitoring plan. We inspect the exchanger on every furnace that comes apart and we document what we find either way.
Exhaust and combustion air routing matters just as much. A blocked exhaust tube, a nest in the intake, or a previous repair that left the concentric vent misassembled all push combustion products where they do not belong. If your coach has a carbon monoxide alarm that has gone off even once, treat that as the priority complaint and say so at intake rather than mentioning it in passing.
What is included
- 12 volt supply testing under load
- Voltage is measured at the furnace with the blower running, not at the battery at rest. This one measurement resolves a large share of no heat complaints.
- Sail and limit switch verification
- Both switches are tested in circuit through a real heat cycle rather than bench checked and assumed good.
- Burner, electrode and combustion path service
- Electrode gap set to specification, burner cleaned, and the combustion air and exhaust path checked for nesting and carbon.
- LP pressure verification
- Manometer reading at the manifold under load, so a drifted regulator is never mistaken for a furnace fault.
- Duct and register balancingwhere applicable
- Runs traced end to end for crushed sections and disconnected boots, which is where the cold bedroom complaint usually lives.
- Hydronic burner and zone servicewhere applicable
- Burner screen, glow pin and fuel metering checked on diesel fired systems, with each zone pump and exchanger verified separately.
How the work runs
Cold start reproduction
1 to 2 hours
The system is run from cold with the coach on battery and again on shore power. Heating faults are strongly correlated with supply conditions, and reproducing the failure on both sources tells us within an hour whether we are chasing a heater or chasing a battery bank.
Electrical sequence trace
1 to 3 hours
Voltage at the unit under blower load, sail switch closure, limit switch state and board outputs are recorded in sequence. A furnace ignition cycle is a chain of permissions and finding which link never granted its permission is the entire diagnosis.
Gas or fuel supply verification
1 to 2 hours
LP supply is measured with a manometer at the manifold under load. On diesel fired systems, fuel pickup, metering pump operation and burner screen condition are inspected instead. Supply problems mimic component failures convincingly and cost owners real money when skipped.
Repair or component replacement
2 to 20 hours
Failed switches, boards, motors, electrodes, zone pumps or burner components are replaced. On hydronic systems this frequently means removing panels and working in a compartment designed by someone who never had to service it.
Multi cycle verification and register survey
1 to 4 hours
A minimum of five consecutive ignition cycles are run, and heat output is measured at each register or zone individually. A furnace that lights once for a technician has not proven anything about a cold night in the desert.
What this costs
$350 to $8,500+
2 to 35 labor hours at $260 per hour, mechanical and electrical.
- Hazmat and disposal $45 flat when chemicals, refrigerant or LP is involved.
- Parts at $100 or less carry 100 percent markup. Parts over $100 carry 35 percent. Special order parts require a full non refundable deposit at order.
- Sales tax of 7.75 percent applies to parts and materials. Labor is not taxed.
- A 50 percent deposit is taken at authorization on any job over $2,000, with an additional 25 percent when parts arrive on any job over $10,000. Final balance is due at pickup.
- A sail switch, blower clean and multi cycle verification sits at the $350 end.
- AquaHot burner and zone system overhaul on a large diesel coach runs to the top of the range and beyond.
- Duct repair inside finished cabinetry is quoted separately because access drives the hours, not the part.
How estimating is billed
- Written repair estimate: $150, credited in full against an authorized repair.
- In depth diagnostic, scan and programming: $285 per hour, one hour minimum, credited in full against an authorized repair.
- In shop pre purchase inspection: $400 to $1,200 depending on unit size and systems count.
- Insurance walk ins are accepted during posted hours. All inspection and estimating work is performed in shop at the Yorba Linda facility.
What you will notice first
- The blower spins up and runs, but no flame ever arrives and it eventually shuts down
- Three ignition attempts, three clicks, then a lockout that needs the thermostat cycled to reset
- It lights, heats for a few minutes, then shuts off and repeats that pattern all night
- The front register is warm while the bedroom register at the back is producing nothing
- A smell of unburned propane at the exterior exhaust after a failed start
- The furnace works fine on shore power and refuses to light on battery alone
- On a hydronic coach, hot water is available but the zones never warm up
- A rhythmic ticking from the burner compartment with no heat output at all
How this fails, and what we do about it
| Failure mode | How we diagnose it | The fix |
|---|---|---|
| Blower runs, no ignition attempt | The sail switch is not closing. Confirm by checking continuity across it with the blower at speed, then measure voltage at the furnace during that same run. A wheel packed with dust, a tired motor and a weak battery all produce identical behavior. | Clean the wheel, correct the 12 volt supply, and replace the switch only when it fails with proven airflow and proven voltage behind it. |
| Short cycling on the limit switch | The unit lights, heats, then shuts down early and repeats. That is the limit switch doing its job because the heat exchanger is running hotter than it should, which nearly always means air is not leaving fast enough. | Trace the ducting end to end for crushed runs, disconnected boots and closed registers. Correct the airflow restriction first and confirm the limit switch stops tripping before replacing it. |
| Lockout after three ignition attempts | The board asked for flame three times and never confirmed it. Check electrode gap and condition, then verify supply pressure with a manometer. A sooted electrode and low LP pressure both produce a textbook three try lockout. | Set the electrode gap to specification or replace the assembly, correct the supply, then run a minimum of five consecutive successful ignitions before calling the repair complete. |
| Hydronic system produces hot water but no cabin heat | The burner and the boiler loop are working, so the fault sits in distribution. Check zone pump operation, air locks in the loop, and heat exchanger fans in each zone. A single seized zone pump takes out an entire section of the coach. | Replace the failed zone pump or exchanger fan, purge air from the loop properly, then verify heat delivery at every register individually rather than at the thermostat. |
Why it happened
- The sail switch failing to close because the blower wheel is dirty, the motor is tired or 12 volt supply is low, which stops the ignition sequence before it starts
- Limit switch trips caused by blocked or collapsed ducting, which lets the heat exchanger overheat and shuts the unit down as designed
- Battery voltage sag under blower load, which is the single most underestimated cause of furnace complaints in coaches with aging gel or AGM banks
- Electrode gap out of specification or a sooted electrode, which prevents reliable spark at the burner
- LP supply pressure out of range, verified with a manometer at the manifold rather than assumed from another appliance
- On diesel fired hydronic systems, a carbon loaded burner screen or a failing glow pin, both of which are scheduled maintenance items that get skipped
Parts and brands we work with
- Suburban NT series forced air furnaces including the NT and SF families
- Dometic and Atwood forced air furnaces with direct spark ignition boards
- Truma Combi combination space and water heating units
- AquaHot and Oasis diesel fired hydronic systems with zone pumps and heat exchangers
- Webasto and Espar diesel burners, glow pins, burner screens and fuel metering pumps
- Sail switches, limit switches, DSI boards, blower motors, gas valves and thermostats
What not to try yourself
- Jumping the sail switch to make the furnace light. That switch exists to confirm airflow before gas flows, and defeating it puts combustion into a heat exchanger with no air moving through it.
- Closing registers to force more heat to the back of the coach. Restricting the duct system is exactly what trips the limit switch and short cycles the furnace.
- Running a hydronic burner with a known carbon loaded screen through one more winter. Carbon buildup accelerates and the parts that fail next are considerably more expensive than a screen.
- Storing a coach with the furnace exhaust unsealed against nesting. Mud daubers and rodents both find those tubes, and the discovery usually happens on the first cold night of a trip.
Which vehicles we do this on
Vehicles we perform Furnace and Heat on
Diesel PusherDiesel pusher repair means working on a rear engine coach built on a welded Freightliner XC or Spartan rail, with a full body painted fiberglass shell and large basement baysLuxury Class ALuxury Class A repair covers Spartan K3 tag axle chassis coaches from Newmar, Entegra and Tiffin, with multi stage paint, porcelain tile floors, stone counters and Aqua-Hot hydronic systemsClass CClass C motorhome repair centers on the cabover bunk, the joint where the coach body meets the factory van cab, and the stick built or laminated house behind itStandardStandard fifth wheel repair concentrates on the pin box and upper deck welds, the floor at the step up transition, the front cap above the pin and the roof front radius seamTravel TrailerTravel trailer repair covers A frame tongue welds, the front cap that takes road debris, the roof front radius seam, sidewall delamination starting at a window corner, and equalizer and axle wearCamper Van ConversionCamper van conversion repair covers aftermarket build outs in bare cargo vans: Sprinter, Transit and ProMaster shells finished by a shop or an ownerHalf the furnaces I see are not broken, they are hungry. Somebody has a five year old battery bank, the blower pulls it down under load, the sail switch never quite closes and the board never asks for gas. Owner hears the fan and assumes gas. I put a meter on the furnace terminals with the blower spinning and the number tells the story right there. On the hydronic coaches it is a completely different world, and honestly the biggest thing I wish owners knew is that the burner screen is a maintenance item with an interval, not a part you wait to fail.
Appendix: questions and answers
- Why does my RV furnace blow air but never produce heat?
- The ignition sequence stopped at the sail switch. The blower must move enough air to close that switch before the board will open the gas valve. A dust loaded blower wheel, a tired motor or a battery bank sagging under load all prevent that. We measure voltage at the furnace with the blower running to separate the three.
- What makes an RV furnace light and shut off every few minutes?
- The limit switch is protecting a heat exchanger that is running too hot, which almost always means air is not leaving fast enough. Crushed ducting, a disconnected duct boot behind cabinetry, or too many closed registers are the usual causes. Fixing the airflow stops the cycling. Replacing the switch alone does not.
- Why is one room in my coach always cold when the furnace runs?
- On a forced air coach it is normally a duct that has come apart or been crushed behind cabinetry, so the heat is being delivered into a wall cavity instead of the room. On a hydronic coach the same symptom usually means a seized zone pump or an air lock in that section of the loop.
- How often does an AquaHot system need service?
- The diesel burner side carries real maintenance intervals: burner screen cleaning, glow pin inspection, fuel filter and coolant service. Owners who buy a used coach almost never receive that history. We inspect the burner assembly and coolant condition at diagnosis so the actual state of the system is documented rather than guessed at.
- Can a cracked RV furnace heat exchanger be repaired?
- No. The sealed exchanger is the barrier between combustion products and the air you breathe in the coach, and a crack in it means the furnace is replaced rather than repaired. There is no weld, patch or sealant that is acceptable there. We inspect the exchanger whenever a furnace is opened and document the finding either way.
Open a file on this repair
Tell us the vehicle, what happened and whether a claim is open. We will tell you what the scope looks like and what it takes to get you back on the road.
Or call (949) 799-3387