RV Furnace Woes

Going old school brings heat and happiness

Words and Photos by Perry Mack

A common occurrence with RV furnaces is a popping sound that seems to appear suddenly and for no good reason. By that, I mean we didn’t drop it, kick it or run over it – all valid reasons for an appliance to stop working properly. After fiddling with the furnace in our new-to-us 2014 Outdoors RV Black Rock travel trailer – turning it off, on and making sure the tanks were full – it stopped working completely. However the next day it worked perfectly.

The most challenging repair to diagnose is the one with intermittent failure. When the furnace failed altogether, the fan would start and then we could hear the furnace ignition system fire three times but not start the burn.

Your car won’t run better after you wash it, but your furnace might. Our tech cleaned the furnace out using compressed air and we vacuumed the ductwork. A dirty furnace burner won’t ignite, promptly causing a build-up of gas. When it finally does ignite, there is a loud pop. In between the popping noises, our furnace was running but without producing the proper amount of heat.

Unfortunately, this didn’t solve our problem.

Our tech explained there are a couple parts that typically cause furnaces to fail, the igniter and the motherboard. Replacing these is beyond the expertise of most RVers. Under the guidance of an honest and qualified mobile RV tech, we began searching for probable and less expensive solutions. Without the need for specialized diagnostic equipment.

Like a combustion engine, the furnace needs two things to start – fuel and a spark. Since we could hear a spark, we decided to check the fuel system. The first potential problem were the fittings on the pigtails that connect the tanks to the auto-switching regulator. Red Acme nuts indicate high flow lines (400,000 BTU/Hr), which is not what this system needs. They should be light green Acme nuts indicating 200,000 BTU/Hr.

The colour of the nut is important since inside the Acme nut is a spring-loaded excess flow check valve, which is designed to shut off the propane if the flow ever exceeds significantly more than your rig would use if all your propane appliances were turned on at once. If the rating is too high (as in our case), the valve may not close if there is a rupture down the line. Since all our appliances together can only consume 80,000 BTU/hr the high flow (red) Acme nut is not effective.

One of our propane techs also suggested that these check-valves are prone to failure. If it gets stuck in a partially closed position, it will prevent sufficient propane from reaching the appliances. Your furnace requires the most flow of any RV system. As a result, an impairment can allow most appliances to function normally (like your cooktop or fridge) except for the furnace. Since the check-valve resets when you disconnect and reconnect your tanks, it’s a good candidate for intermittent failure.

If you’re old enough, you may recall hoses that connected to propane tanks with a left-hand (reverse) thread fitting, called POL (or Put On Left). And they still can. Old school connections thread onto the inside of the tank valve and don’t have a check-valve to give you grief. After closing the valves on our tanks, we switched out both pigtails for ones equipped with ‘old-school’ POL fittings.

But there was still no joy for the furnace. Next was the regulator.

Many regulators auto-switch from one tank to another and some provide you with an indication of how much fuel remains. They drain one tank, which you can then remove and refill with the ability to still operate with the remaining tank. These regulators can also fail. Old school regulators have no gauge and don’t auto-switch. They allow propane to flow from the tank with the most pressure, draining both tanks equally until you have no fuel in either tank. This means keeping a close eye on the propane level. The upside is that these ‘old-school’ regulators have fewer parts to wear out or get gummed up by contaminants like oil in your liquid propane (LP).

After switching out the regulator, the furnace fired up to full blast in perfect working order and has continued to work perfectly. A satisfying end to a frustrating issue.

The two morals to this story are that before you spend big money switching out expensive parts within your furnace, look for the simple, less expensive solutions. Sometimes old school beats new school.

Note: Propane is a flammable, asphyxiating gas. Always consult a qualified tech.