Gas doesnt stay on.

[sctclimbs]

I have a natural gas Lochinvar, power vented water heater. It is about 8 yr. old. It has an electric, glow-type, igniter that has what looks like a flame sensor or thermocouple right next to it combined in one unit. Last night there was no hot water. Pulled the igniter/flame sensor out and cleaned them off with steel wool and replaced. It would then work but after just a few minutes the gas would go off again.
Problem with the flame sensor/thermocouple? Thermostat?

Just for added information. The vent fan was constantly running even with the gas off. From a previous thread it seemd that if the fan was running it would mean that the themostat was OK. That correct?

[jadnashua]

Assuming this is a closed combustion device, they almost all do a post-purge after a fail to fire event. Could be 5-minutes or more to ensure there's no gas buildup before it tries again.

If it starts up, lights the burner, then turns off before it heats the water to temp, it could be the flame sensor is not working, or it could be the circuit is bad on the motherboard. Check the connections for corrosion and any loose or broken connections. A less likely thing would be if the aquastat isn't giving reliable readings, and thinks the water actually then was up to temp. Your user manual is the best source of troubleshooting info, and the installation manual usually has more complete info, if you are are mechanically inclined. There may be a test procedure for the heat sensor.

[sctclimbs]

Unfortunately the manual left by the previous home owner is not helpful at all and cant find anything better at the Lochinvar website.

This does have a 5 minute lockout before trying to refire but it is not refiring every 5 minutes. It fires for 5-10 minutes and then stops but the vent fan stays running indefinitely. If I unplug it and wait 5 minutes it will refire.

Took the stat off the side of the heater so it wasnt feeling any change in temperature, didnt change anything though.

Dont see any corroded wires.. Dont see any sort motherboard anywhere though. Is this internal?

[jadnashua]

The flame sensor often needs some sort of a circuit to operate. The rest of the system needs some sort of a controller. It's usually the main controller - it may be a sealed box. It probably has led indicators on it showing the status.

Have you tried shutting the power off for a few minutes, then reapplying?

These systems have numerous interlocks...usually there's a vacuum switch on the power vent. If that is bad, the hose has a leak in it, or has come off, that would keep it from firing. Often a hose leak is caused by a split at the very end, or a kink. Are any lights flashing, or displaying an error code?

[sctclimbs]

No lights of any kind on the control box. It says on the box, do not to try and open it.

I will check for the hose leak when i get home tonight.

If I unplug the power cord and wait 5 minutes and plug it back in, it will fire up again for another 5-10 minutes. I'm thinking if it were the flame sensor it would not stay on that long.

[Thatguy]

Post a schematic and parts list, if you can find one somewhere. Just knowing what parts you have and the spec's on the parts may be enough. A parts supplier may even have a schematic.
Otherwise, it's troubleshooting by parts replacement.

[Redwood]

What model # do you have?

[jadnashua]

Make sure you don't have something like a bird's nest, bird, mouse, rat, or something blocking the vent outlet.

[sctclimbs]

Lochinvar Model# DVN051
The control valve is a Honeywell SV9501.
Igniter/flame senor is Honeywell q3400a 1008

Nothing blocking the vent, no leak in the hose either.

[sctclimbs]

Here is a diagram and parts list:
http://www.lochinvar.com/_linefiles/DVN03-05.pdf
(My model serial number starts with XC, so it is the first model shown)

[sctclimbs]

here is the full user manual with wiring diagrams:
http://www.lochinvar.com/_linefiles/RPD-i&s.pdf

[Thatguy]

Last night there was no hot water.1
Pulled the igniter/flame sensor out and cleaned them off with steel wool and replaced. It would then work but after just a few minutes the gas would go off again. 2
Problem with the flame sensor/thermocouple? 3
Thermostat?4

The vent fan was constantly running even with the gas off. 5
From a previous thread it seemd that if the fan was running it would mean that the themostat was OK. That correct?6

1 bummer
2 then dirt is not the problem.
If the blue and yel don't have 24v present it's your 'former.
It looks like if the red wire doesn't have 24v present then one of those 3 switches is correctly or incorrectly telling the logic that there is a safety problem. I'd call this an "inhibit" signal. From the diagram, 24v present may mean "inhibit" because the EXH PRES SWITCH looks to be normally open.
I don't know what the correct voltages should be on the blue,blue and black wires, but I'd say this is a likely cause. If it's ~28 millivolts you may have a hard time reading this with a conventional DVM. Some thermcouples put out 750 mV.
3 see 2
4 see 5&6
5&6 According to your diagram anytime the 'stat calls for heat your 120v blower motor will run. It "doesn't know" if the gas is on or not.

On second thought, if the gas comes on at all the 'former has to be good.

[Redwood]

I'd give the flame sensor a good look...
It's the kind that has a metal rod through a creamic insulator with 1 wire off it to the control. Right?

Look for carbon tracking on the ceramic insulator to ground and clean it with a non-conductive pad like scotchbright.

This is sending a low millivolt signal to indicate combustion and is easily shorted.


I would also look at the blower and make sure the blades are not packed with dirt lowering its flow but I don't think that is the problem because they usually close the proving circuit before the burner will even light.

[Thatguy]

I'd give the flame sensor a good look...
It's the kind that has a metal rod through a creamic insulator with 1 wire off it to the control. Right?

Look for carbon tracking on the ceramic insulator to ground and clean it with a non-conductive pad like scotchbright.

This is sending a low millivolt signal to indicate combustion and is easily shorted.


I would also look at the blower and make sure the blades are not packed with dirt lowering its flow but I don't think that is the problem because they usually close the proving circuit before the burner will even light.

Today I learned something
http://www.johnwoodwaterheaters.com/pdfs/TB114.pdf

[sctclimbs]

Well she is back up and running. Not exactly sure why. I took off the cover around the switches by the blower and resat the connections and then gave everything a good blast from my air compressor. A nice cloud of dust came out of the blower motor. And we are back in business! At least for the time being. Started reading the threads on tank vs. tankless in case it craps out again here in the near future but i wont open up that can worms here .
Much obliged gentleman for your help!