What might cause a DFG-100 LP Oven to have a hard relight (like gas not purging) after working flawlessly?

[fixbear]

Not knowing if it is under a hood or piped draft i’ll answer as if under a hood.  Check your draft diverter on the top rear to be clean.  Check your flame mixture to be right.  to lean will do this. Make sure your burners are clean and have no crud around the holes.  Especially near the ignition point. How is the pilot flame during ignition?  If out of position or low it will allow a buildup before ignition.

 

They used several different ignition systems on these, including a hot surface prior to 1986.

[albumen]

It is under a hood and oven man date is 2004.  I will check the diverter.  I removed the burners and cleaned/vac just a month ago plus cleaned the burners with wire brush and checked for flow using compressed air.  Pilot Flame looked good and strong but I can recheck it.  I will look into the lean issue and report back.  Thank you.

[ectofix]

The serial number you gave (030904ra0118) makes it a March 2004 model.  The “8” you gave us as the last digit is probably a “B” to indicate the oven was sold as the bottom oven in a stacked set.

 

So given the year, it uses spark ignition to light the pilot.  A separate flame rod proves flame.  That old Johnson Control module has NO pre-purge.  From what I just read in an old service manual, the ignition module for LP gas is single trial and must prove flame within 25-45 seconds.

 

Regarding that flue draft diverter, I suggest making sure there’s nothing sitting on top of the oven that could be blocking it.  That’s just the first thing I always check whenever there are ignition issues.

 

For testing the module, there’s really not much you can test if it’s working – other than observing its sequence of operation.  For control of the burners, once it’s trialed for ignition and proves flame (at the pilot), it simply goes to its next step by sending 24vac to the main burner valve.  The module has no way of monitoring the MAIN burner’s functional integrity.  So it wouldn’t have a response for instances such as the burner flashback that you’re observing.

 

I have four things that I’d look for in your case – in this order:

– With the oven cold and off:

1. Check for proper ventilation

* LP-fired equipment is bad about building up soot.  Shine a flashlight in there at the burners and check for any black, bumpy, pillow-like formations.

* Also take a peek down inside the flue to check for the same.  If you see any, it needs cleaned out.

2. Check condition of the burners

* Again…check anything that could be blocking air/fuel flow through them and the stamped burner ports.

* Remove the burners and clean them if you see anything.

– With the oven ON and calling for heat:

3. Ensure proper gas pressure

* 11″ to 13″WC inlet pressure – with this oven and ALL other gas appliances at a full burn.

* 10″WC manifold pressure (to the main burners).

4. Ensure proper primary air adjustment

* This is done with those shutters at the feed-ends of the burners.  They control the amount of air mixing with the gas.

* Open or close the air shutters to achieve optimum flame characteristics.

> If the burner flames are soft and unstable or show yellow tipping, increase the amount of air by opening the air shutter.  Be aware that yellow flames cause soot.  However,some grades of LP gas will cause yellow tipping no matter what you do.

> If the burner flames are LIFTING off the burner and sorta dancing, then there’s too much primary air.  Close the air shutters some until that stops.

 

After all that is squared away, if you still have the same problem popping up after the 1-2 hours that you said it happens – I’d be suspicious of a possible intermittent gas valve issue.

[albumen]

ectofix: completed #1 there was some but minimal.  Started to remove the burners to recheck for blockage, being careful, I removed the pilot burner and gas supply. As I started to remove the supply tube from the pilot burner to clean the pilot area of slight buildup, the igniter wire in the ceramic appeared loose.  Checking it…it popped out with little effort.  So I have a new pilot kit on order.  Will test and see if problem is resolved due to loose/bad igniter wire and ground.

Prior to this I checked/watched the oven heat.  The burners did not have a yellow tip and did not dance for what I could see…they looked even and smooth for the length of the burner on either side.  I did feel they should of been cranking harder than they were since I just turned it on.  Once I replace the pilot, I will pull the shield to get a better look and watch it relight.  Parts should be here Wed Apr 5 / Thurs Apr 6 

[fixbear]

I doubt that the wire connection is you primary problem.  The condition you described a delay in the burner ignition.  Being a pilot ignition that has to prove before the burner valve opens means that the delay is in the burner, pilot relationship. Fuel mix is accumulating in the burner area prior to burner ignition.. So either the ignition is in the wrong position or the burner has some form of restriction prior to the end where the pilot flame is. Or the mix is slow to get to the ignition area. 

 

With the gas being heavier than air it requires the higher pressure to mix well vs. NG.  That’s why the NG pressure is 5 inches and large nozzle verses 11 inches and a smaller nozzle. Propane also requires a higher percentage of oxygen so they want more velocity in the carburetor venturi for mixing. 

 

. Propane is a lot different than Natural gas.  It weigh’s a little more than 1.5 to air (heavier).  NG is like .6. (lighter).  Being that you can run several hours before the problem occurs, I has to have something to do with heat saturation.  Think that Gas pressure monitoring may help here. Especially manifold.  It could be the gas valve/pressure regulator is slow in opening once hot.. Low pressure will cause a delay in a ignitable mix from reaching the  end of the burner at the pilot.  Wether it be from mixture percentage at beginning of flow,  or speed of getting there. ie.; lean early and then normal causing a delayed ignition at the pilot but proper mix in the middle of the burner.  

Make sure to check the valve vent to be clear.

[albumen]

Fixbear: thank you…I did not think so 100% either on the igniter wire but sometimes little things add up.  I agree the valve might be slow to open when hot which is what it seems like.  I also found one of the internal Axial coolant fans behind the gas valve was no longer working allowing heat to build up there.  No idea if or when it quit working as I have not been in that compartment before.  Got one of those coming too.  I will check for build up on the ignition site of the burners today and valve vent.  

[albumen]

The pilot assembly has been replaced.  The gas monitoring has been completed.  The oven ran for 5 hrs without fault except for this morning.  It failed to relight(no sparking no gas) as the gas valve did not open per the manometer did not  detect any gas.  The blue box was clicking to activate the valve but it did not open.  I unplugged the power and tried again…lite no problem.   Here are the LP pressures during the cycle.

Valve entrance: 11.06 with two other units running.  did not wafer.

Manifold: 4.22 – 4.96

I think when this unit was converted they must not of changed the orifice in the valve.  Not sure if this is causing the flashback but it explains why we struggle with this oven on a consistent basis.  I am considering just replacing the entire valve or buying a conversion kit.  It appears the ignition module must change also so perhaps the kit.  Thoughts?

[ectofix]

albumen wrote:

 

Manifold: 4.22 – 4.96

I think when this unit was converted they must not of changed the orifice in the valve.  Not sure if this is causing the flashback but it explains why we struggle with this oven on a consistent basis.  I am considering just replacing the entire valve or buying a conversion kit.  It appears the ignition module must change also so perhaps the kit.  Thoughts?

Well, the manifold pressure you read falls well below the adjustable range of a combination valve set up for LP (see picture below).

 

So this oven was a conversion instead of being LP from the factory?   What does the oven’s data plate say?

 

FYI:  There’s no orifice in the valve to replace during a conversion.  The regulator’s spring is changed out, then appropriately color-coded adjuster screw and adjuster cap are installed.

 

Take a close look at the combination valve’s sticker.  The sticker on it will state what pressure range it was manufactured for.  Also, if the valve was manufactured or converted to LP, the pressure regulator adjuster cap will be black instead of silver.  If the valve was converted, whoever converted should’ve applied an additional (yellow) sticker stating it was converted:

(credit to PartsTOWN for the photos…which I modified a little for instructional purposes)

 

If all indications on the valve show it as being a LP valve…or one converted to LP, then I’d venture in considering the valve as faulty.

 

However, if conversion  indications aren’t present, then a conversion kit would be the next step.  IF you’re not a technician (as your profile states), then I strongly suggest you hire one to do the conversion.

[albumen]

correct this was originally a NG unit I bought used and I hired the same company to convert it.  The company I bought them from is no longer in existence. Orifice: yeah I see that now in the conversion kit.  They  claimed 7 yrs ago they did the conversion on both ovens per the invoice.  They did not mark the data plate as being converted on either of the units I bought.  The oven in question has a silver valve spring cover.  They did not mark the valve as being converted as it still has the NG name sticker on the side.   However, it has ran like this for seven years without any real issues but flashback.   I would think if they had not done it, the oven would have serious issues with large flames on the burners due to the differing pressures in gas.  It appears if was done it is very much suspect.  Correct I am not a certified tech but I do have mechanical ability but not interested in blowing myself up.  I have to make sure what I tell the tech is the likely situation due to where my kitchen is located is geographically remote.  So I have to do some simple things/basic troubleshooting – gas conversion not one of them but I think I could of come closer than the people I paid .  It is $135 for a tech to just show up and $85 per hour + parts then any return visit is another $135 after that as I said I have already paid for this once.  So what should I look for in a tech to know they are legitimate besides their company name?  

[fixbear]

Your manifold pressure tell it all. You have the wrong regulator on the unit.  Manifold pressure should be 10 inches with propane.  Understand that the gas valve is a 3 piece valve in that it does 3 functions.  Yes it turns on the pilot flow, and the main flow,  But it also has a pressure regulator built into it to maintain the proper pressure and also has a metering valve to adjust the pilot. (some are external).  This fully explains the delayed ignition as per my previous post on mixing and pressures.  What really surprises me is that it worked correctly at all.  Obviously who ever converted it was not qualified to do so and they are lucky no one got hurt or the kitchen was damaged.  What Ectofix posted is correct,  Everything involved with a correct conversion now has to be checked to have been done right..