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Ask Dave: Water heater works on 120-volt but not on LP. What to check?

Dear Dave,
I have a Suburban water heater with dual 120-volt/LP. It works fine on electric. However, when I try to use the LP, the pilot light lights and stays on. But when I try to turn the valve to “On” for the burner to light, the pilot light goes out. What should I check out? Thanks. —Virgil

Dear Virgil,
I assume you have a Suburban SW6PE or SW10PE, which are 6- or 10-gallon models with LP gas and electric. Does this happen when you are plugged into shoreline power or dry camping? The reason for asking this is the module board needs 12-volt power from the house batteries. If they are sulfated, the voltage can look good but drop like a rock when anything tries to get power. If you are plugged into shoreline power, the converter will typically provide a maintenance charge of 13.2 volts when it is not charging. Make sure it is plugged in and getting sufficient power.

What to check

If you are getting a steady flame for the pilot, it’s probably not the thermocouple. According to the service manual, you should turn the gas knob past the white line and then to the “on” position. If you are operating it in this manner, then you need to check the burner assembly for restrictions in the orifice, burner tube, flue, and exhaust. I would suggest using an air gun similar to the one here used in cleaning out the burner assembly of a refrigerator. You may even need to replace the orifice if it cannot be cleaned and is restricting LP flow.

Do other LP appliances work?

Do all the other LP appliances work as designed? If you have a weak or faulty LP regulator and not getting proper pressure, you would notice it in other appliances. A quick test is to light one of the burners on the stove top and look for a steady blue flame, then light another one, and then the third. This will show what is happening when more than one source is drawing LP and if your regulator is doing it’s job. If the flame goes down and stays down, the regulator is not keeping up.

If all these check out OK, then it’s time to get a certified technician to perform a water column or pressure test at the water heater to determine the airflow from the gas valve and/or orifice, as it would appear that not enough LP is getting to the burner assembly.

Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club.

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Steve Hericks
6 months ago

I beg to differ on your discounting the thermocouple. It sounds EXACTLY like a thermocouple problem. If the pilot is not heating the thermocouple, there will be no electricity to open the main gas valve so when turning the control from pilot to on, there will be no gas. To diagnose, remove the thermocouple connector at the control and check the voltage from the end terminal to the sheath tube. It should read 25-30mV with the pilot on and warmed up.

Thomas D
6 months ago

Daves go to theory, bad or sulfated batteries. I knew he mention it. How about the thermostats..? Gas has one electric has one and i don know how its wired but there is an ECO or high temp cut out. The gas valve coil my be bad you could have NO 12 volt at the heater. My camper has a switch in the pantry to turn off heater. Any number of unmentioned proplems that are easily solved by use of a DVM. ( electric meter) get one and learn to use it.
Oh and dont forget to check for sulfated batteries.

Bob p
6 months ago

It could also be a spider built web inside the tube as they seem to love the smell of propane.

Jesse Crouse
6 months ago

The important thing is get a TRAINED, CERTIFIED and INSURED tech to do the test. LP is nothing to fool around with.

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