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I have a Thermador Pro Harmony range (PRG305WH/04) that's just about one year old. The oven takes an extraordinarily long time to preheat and exhibits some other odd behavior as well. The worst of the symptoms is propane gas burning inside of the air shutter of the burner tube, preventing gas from reaching the igniter and burning inside the Venturi tube as it should. Here's a link to a YouTube video I took where you can hear the wind tunnel noise the gas makes as it's burning inside of the air shutter: https://www.youtube.com/watch?v=2J1gDDVUPTE

As mentioned above, this range has been converted to liquid propane (LP) from natural gas (NG). The repair tech initially thought it was being caused by low gas pressure reaching the appliance. I had the propane company come to the house to check. He found that the inches of water column (WC) was about 11 so he adjusted the second-stage regulator to about 12.5 instead. This has had no noticeable effect on the functioning of the range.

All burners operate correctly with a clear blue flame and no noticeable orange/yellow tips. We're running out of obvious things to check, so I've made a list of what I believe to be all the involved components:

  • Propane supply line to the kitchen
    • The propane company installed a temporary T where the propane line enters the kitchen and we ran various features of the range. With all features off, we measured 12.5 WC. With all burners operating we measured as low as 11.8 WC. With only the oven operating we measured 12.3 WC. I believe all of these readings should be sufficient to operate the range, even if we had everything running at once. If not, what else can we try?
  • Propane supply line from the kitchen to the appliance
    • The propane company examined the line and didn't have any comment on whether this could affect performance of the range. Are there any supply lines to avoid, or ones that we could be using instead just to rule this out?
  • Range internal gas regulator
    • We verified that the regulator is correctly adjusted for LP, where the "LP" text is pointing up instead of down (refer to LP kit instructions, pgs. 10 and 18).
  • Gas valve for bake orifice
    • We can hear the gas valve open and gas start flowing about 6-8 seconds after the igniter comes on. I am told that it's very unlikely for the gas valve to fail or not open entirely even after many years. How can I verify the gas valve is operating correctly?
  • Bake orifice in burner tube
    • We verified that the NG orifice was replaced with the correct LP orifice measuring 1.34mm (refer to instructions, pg. 6).
  • Air shutter in burner tube
    • We verified this is full open (refer to instructions, pg. 18).
  • Oven Venturi tube
    • The repair tech replaced the Venturi tube, being sure to also swap the NG orifice on the new assembly for the LP orifice. This had no effect on performance.
  • Oven igniter for Venturi tube
    • This comes on quickly (2-3 seconds) and I believe it stays on whenever gas is flowing. Should the igniter always be on whenever the oven gas valve is open?
    • Gas ignites within 6-8 seconds of the gas valve opening. Should the gas be igniting more quickly?

The oven takes close to 25 minutes to reach 350 F. I can reproduce the sound in the video by letting the oven heat for a while (~ 10-15 minutes), opening the door to interrupt preheating, closing the door, letting igniter come back on and gas valve open again. After this, the gas ignites inside the burner tube right by orifice inside the air shutter, never getting gas/flame into the oven Venturi tube again. If I let it operate this way (we did once because we thought it was simply a fan running) it eventually produces a horrible smell that made my wife's eyes water and gave me a pounding headache. I'm guessing this was the smell of brass or the insulating material heating and burning off.

We don't use the oven very often but do need it for the fall and winter holidays. Since the manufacturer warranty period is near to expiring, I really need to find a resolution. Any ideas that I haven't tried yet?

Edit: this is the conversion kit that was used (https://www.ajmadison.com/cgi-bin/ajmadison/PALPKITGW5.html). Updated this question with page references from that guide (https://assets.ajmadison.com/ajmadison/itemdocs/8001148006_A.pdf).

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  • Burning in the wrong location seems definitely warranty territory., Unless they have a published specification for preheat time, or admit that that's too long a time, that might just be how long this oven takes to preheat. Your pressure readings indicate that the oven is not using nearly as much gas as the top burners do, which does not seem normal in my gas range experiences (the oven burner is generally relatively huge.)
    – Ecnerwal
    Commented Sep 13, 2022 at 12:15
  • @Ecnerwal from appliancesconnection.com/thermador-prg305wh.html, all 5 burners would account for 59000 BTU while the oven burner should be operating at 20500 BTU. But I agree I would expect more gas flow to the oven and therefore a noticeable drop in WC from the propane source.
    – Yuck
    Commented Sep 13, 2022 at 12:22
  • The BTU ratio of "All Top Burners" to "Rated Oven Burner" is 2.87. The ratio of pressure drop from static you report for the same conditions is 3.5, though that may be down to rounding (would expect 12.25 if same as BTU ratio.) Or your all burners operating reading includes the oven? In that case it's about as expected (12.32)
    – Ecnerwal
    Commented Sep 13, 2022 at 12:33
  • "Less than one year old" = Contact manufacturer for warranty repair NOW before the warranty expires!
    – FreeMan
    Commented Sep 13, 2022 at 13:19

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Gas supply pressure seems suitable but it's the gas pressure right at the orifice during flow that determines how the unit performs.

Look for a kink in the supply tubing that runs from the gas valve to the orifice. Kinks can sometimes occur at the factory, and this could reduce flow and pressure enough to let flame flash back to the venturi tube.

If there are no kinks and it's possible to remove said supply tubing, remove it and blow compressed air through it (with orifice removed) to be sure there are no obstructions. I found a crumb of brass in one such tube right near the orifice.

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  • There was a metal burr inside the brass bake orifice itself. We took that out, replaced it with a new bake orifice, and now have a strong, blue flame.
    – Yuck
    Commented Oct 11, 2022 at 13:43

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