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Water Heater Primer

Water Heaters - a Primer

by Gary Brinck

Most modern RV water heaters (WH) have two ways to heat water, operating in either Electric or LP Gas mode. However in entry level RVs the WH may be LP gas only. And in some RVs there will be a third heating method called Motoraid which uses waste heat from the engine to heat the water while you drive. Finally, some high end RVs may have an entirely different system in which the water heater is part of a diesel & electric central heating system usually called Aquahot or Hydrohot. This sytem is an entirely different animal and will not be discussed here.

There will be separate On/Off switches for the Electric and LP Gas Modes. The heater can be operated in either mode or both can be used to speed up the heating process. Both are controlled by a thermostat, so the heater only runs as needed. In LP gas mode, a heater control circuit board insures safe operation, so 12v power is needed for it to operate.

Most WH have separate thermostats for Electric and LP Gas modes and the two modes are pretty much independent of each other. If one does not work for some reason, usually the other will still be functional. The standard thermostats are fixed temperature - you cannot adjust them.

The standard water heater sizes in RVs are 6 gallon or 10 gallon, much smaller than residential heater, so you may not be able to enjoy quite as long a hot shower as at home. A few models are advertised as being 16 gallon heaters, but these are actually 10 gallon heaters that superheat the water and then mix cold water at the outlet, making them roughly equivalent to having a 16 gallon heater.

Electric Mode

When you switch on the Electric mode, the Electric Mode thermostat controls whether 120vac power flows to the electric heater element or not. The heater element cycles on & off as needed, heating the water, just like your home water heater. The WH uses a fairly substantial amount of electricity, about 8 amps (1000 watts), so you may not want to operate it in Electric Mode if power is limited or if you want to run other high power appliances like an air conditioner or microwave oven. You can switch it off temporarily (the water will stay hot for some time) or change over to LP Gas mode.

LP Gas Mode

Basic model gas WH have a pilot light which must be lit manually with a match or sparking device. There is a manual valve that must be set to the Pilot position to light the pilot and then you move the valve to a Run position for continued operation. Manual pilot WH were common in older RVs but are used only in the most basic models these days. Once the pilot light is lit and the heater control circuit board detects a flame at the flame sensor, the gas mode thermostat starts and stops the flow of LP gas to the burner where it is ignited by the pilot flame. If the pilot goes out, no flame is detected and no gas is alowed to flow, preventing any danger from leaking LP gas.

Most modern WH are the DSI (Direct Spark Ignition) type, which has no pilot light. When you switch on the water heater in LP Gas mode, the gas mode thermostat will call for it to be heated as needed. When heating is called for, the heater control circuit board will turn on the igniter, which makes a spark at the front of the heater's gas burner tube. Then the control board opens the gas valve, allowing gas to flow into the burner, where the spark [hopefully] ignites it. At the same time, the control board starts sensing for a flame [the igniter doubles as a flame sensor]. When the burner flame is sensed, then the igniter is switched off. If no flame is detected after a preset time (about 15 seconds), the gas valve is closed, the igniter turned off, and the system “locks out,” meaning it won’t allow another attempt at restarting until the system is reset by turning it off and on again.

While you can run a DSI WH in LP Gas Mode while driving, this is generally not necessary since it heats up within 20-30 minutes after it is turned on anyway. A Manual Pilot type of WH usually does not work while driving - the wind blows the pilot light out. Few RVers leave their water heater on while traveling - it's just a waste of fuel.