Furnace on the Fritz?

by Bill Gehr | Jun 10, 2024 | Pending Approval

Furnace model being inspected and service by man in blue shirt, brown shorts, and brown visor.

Furnace models with an outside access door are much easier to inspect and service. When it comes to blowing out debris, compressed air is your friend.

Photos by author and Bill Gehr

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Understanding the top 10 reasons why furnaces fail — and how to rectify them — will help keep your interior toasty warm during colder weather

Winter can be a special season to travel in an RV — and when the temperature drops, modern comfort heating systems will keep the interior cozy. Granted, there are a number of ways to warm up the interior, but most RVers rely on their LP-gas furnaces. Everything is copesetic when they work, but when something goes awry, it can be a challenge to keep occupants comfortable inside the RV. That’s why it’s important to have at least a basic understanding of what can go wrong and how to mitigate the situation — even when on the road, far from service centers.

It’s not possible to have an effective HVAC system without a source of heat. Murphy’s Law suggests that a furnace will only fail when it’s needed the most. Keeping an eye on internal components and knowing what to look for when a furnace fails — or becomes erratic — can make a big difference in whether you’ll be shivering inside a blanket or enjoying the interior space in comfort. The following applies to typical furnaces found in RVs. (Hydronic and Truma models are not covered here.)

1. LP-gas Pressure

Inadequate gas pressure will cause short cycling or a faulty ignition. Test the system for proper LP-gas pressure using a manometer (most people use a dial manometer), which should be 11 inches water column with all the appliances off. While testing the gas pressure, start the furnace ignition procedure to verify that the pressure does not completely drop out, which will indicate that the pressure regulator is defective and needs to be replaced. It’s a good idea to replace the regulator every five years, regardless of whether or not it’s working.

If you find yourself far from a service center and do not have access to a manometer, a quick way of checking for a drop in gas pressure is to ignite the burners on your stove and turn the furnace on; if the burner flames drop considerably in size or completely go out, the pressure regulator is likely defective.

Inadequate LP-gas pressure is one of the more common maladies affecting furnace performance. Pressure should be checked if you suspect a problem with ignition; a dial manometer is the most convenient tool for checking LP-gas pressure — and regulator integrity.

hand pointing to tap at the bottom of the valve

This is a typical gas valve used in most older furnaces, especially Suburban models. There is a tap at the bottom of the valve allowing you to diagnose low pressure problems with a manometer.

2. Home Sweet Home — for Bugs

Mud daubers love the smell of propane and will nest in the intake or discharge pipes, creating a blockage. (Of course, debris can also cause a similar problem.) Using a bright flashlight, look into both pipes to detect any blockage. If the blockage is deep into the system — normally a problem caused by a mud dauber nest attached a blower wheel — you will not be able to see an obstruction from the outside. If you suspect there is a mud dauber nest inside the blower wheel, it will be necessary to remove the furnace and dismantle the blower assembly.

Another possible blockage could be traced to bug screens attached to the outside of the furnace at the intake and discharge ports. Manufacturers do not recommend this as it can impede air intake and discharge if the screens are allowed to plug up. If you insist on using these screens, make it a point to inspect the surface for blockage on a regular basis.

exterior panel on this Suburban furnace being removed

The exterior panel on this Suburban furnace can be removed easily, providing access to the components.

Carefully pull the access panel away from the sidewall, exposing the intake and exhaust tubes. Mud daubers love the smell of LP-gas and will hang out in the intake and exhaust tubes, restricting airflow, which leads to performance issues and possible furnace failure.

Use a bright flashlight to inspect the intake and exhaust tubes for insects and other debris. These obstructions can be cleared with a wire hanger after bending a makeshift hook in the end.

A flashlight can also be used to inspect a portion of the fan, which can be bombarded by mud daubers and other debris.

air gun with a long nozzle clearing debris and dust

An air gun with a long nozzle can make short work of clearing debris and dust from most of the furnace components, including the circuit board. Cleaning is more difficult in furnaces without an outside access panel.

3. Oh No, the Furnace Won’t Come On

The obvious first check is for a blown fuse. If it’s good, make sure you have adequate voltage at the fuse and the furnace. Voltage can be verified using a multimeter and probes. There should be no less than 10.8 to 11 volts DC for the furnace to operate, but keep in mind that if voltage is that low, don’t expect the furnace to run very long. Nominal voltage with a fully charged battery should be more than 12 volts DC. At this point, you’ll need to evaluate whether the charging system — be it via the converter, inverter-charger or solar panels — is functioning properly. Most furnaces will have an on/off switch, which must have power across it. This switch is accessible via an outside door, but those furnaces without such access will have to be removed to check for power. If there is power at and through the switch, the next check is at the time delay relay, if so equipped. Replace as necessary after testing. While checking these components, be aware that a loose connection can also cause a no-start problem.

voltage to the circuit board is verified by using an accurate multimeter

Incoming 12-volt DC power is checked with a simple test light. At this point, you are only checking for the presence of power, not exact voltage. Voltage to the circuit board is verified by using an accurate multimeter. A furnace will actually work when voltage dips to 10.8 to 11, but don’t expect it to run for long. If voltage is low, suspect poor battery charging, or even bad batteries and/or converter.

On/off switches are provided in most furnaces. While it may seem over simplified to suggest that the switch must be in the “On” position to operate, this step is overlooked more often than expected.

If the furnace fails to start, the first step is to check power at the fuse panel.

4. Where’s the Air?

If the blower motor fails or is functioning erratically, the furnace will not ignite. The first step is to determine if there is adequate voltage to the furnace (see No. 3). Does the motor sound like it’s running too slowly? If so, the brushes may be worn and not making proper contact on the motor windings. A mud dauber nest can also prevent the blower wheels from turning at top speed. Some furnaces may give you an audible hint that the motor is about to fail: If you hear squealing from inside the furnace upon start up, the chances are the motor is toast. If the blower motor tries to start and then stops, check the voltage at the blower motor. The furnace board determines that the fan is running and providing adequate air flow by using a sail switch, which closes when enough air hits the sail to activate the switch. These fail frequently; some RVers will carry extras in their toolbox, just in case. (See No. 7 for more.)

This image shows the typical configuration of an intake blower in a Suburban furnace. Not all intake blowers are accessible without removing the cover. Nests or other debris can create an improper air-fuel mixture.

5. Keeping the Air Flowing

Blocking the return air vent and/or heat registers is a no-no. For example, it’s not uncommon for towels or other items to fall out of drawers and/or cabinets and block the return air vent. Manufacturers require a certain amount of space (in square inches) around the return air vent and a certain number of heat registers to properly evacuate heat from the furnace.

Floor registers can collect an unbelievable amount of dirt over time — including pet hair, which can cause a serious problem by blocking the air flow through the ducting. If you have pets that shed and are concerned about the integrity of the floor registers, remove them and apply a coarse screen material that will not impede the air flow, but will keep pet hair and debris out of the ducting — and inspect/clean frequently to make sure air flow is not impeded. Some fifth wheels have a return air vent in the basement, which can be blocked unintentionally by nearby stored items. Become familiar with the return air configuration and take measures to keep the vent clear at all times. If the return air vent is located in the galley, remove it and check periodically for anything that could have been dropped out of a drawer and possibly impede air flow.

intake and exhaust ports covered with screens

Although furnace manufactures suggest otherwise, many owners cover the intake and exhaust ports with screens available in RV supply stores. These screens must be inspected frequently and cleaned of any obstructions to prevent furnace disfunction and overheating.

Floor registers are susceptible to blockage caused by dog air and other debris. While these registers can be covered with screen material, air flow must not be impeded. Frequent inspection and cleaning are in order.

Shown is a typical direct discharge type of furnace common in small trailers. As you can see, simple access makes it easy to clean and inspect for wayward materials (think towels, etc.) that may block air flow.

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