Excessive soot: LPG

[JB Thompson]

Today's inspection revealed a set of gas logs in a prefab fireplace unit. There was excessive soot on the top logs and the damper and the chimney entrance.

They were using LP gas.

What is causing the excessive soot?

Bob Harper - are you out there?

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[Marc M]

Looks like poor drafting. Check to see if the termination components are all listed by the manufacture. Are they burning with the doors half open, closed, fully open? Manufacture may spec that too. Is there a combustion air vent?

[Jim Luttrall]

Improper fuel to air mixture. Might be the air shutter adjustment, log placement, or just a spider in the equipment. The more yellow the flame (for looks) the worse the fuel / air mix. Proper blue flame resulting from correct air / fuel mixture does not impress the wife. Looks like they went a little overboard though.

[JB Thompson]

That's what I'm thinking, but for a different reason. Would this be caused by the wrong orifice on the gas log set? Natural gas vs. LP

[Jim Luttrall]

That's what I'm thinking, but for a different reason. Would this be caused by the wrong orifice on the gas log set? Natural gas vs. LP

I see it all the time in Natural gas units that are not likely to have had any conversions. Did you see the flame? Bright yellow or lazy dark blue?
Let's see if I can remember the right direction on Propane vs Nat. Gas; propane gets a smaller orifice so that would yield an over rich condition if Nat. Gas orifice was left in a propane application if all other things were equal which would appear to be a dark blue lazy flame. Bob?

[Bob Harper]

Ok, looks like we have vented gas logs installed into a factory built fireplace. If ventfree logs, Houston, we have a problem....

Let me ask you: if the gas logs were removed how would you feel about having all that soot in the firebox, damper, and chimney flue?

Those logs do appear to be out of position, which in and of itself, it can cause sooting. There are a lot of other things that can cause it but this is not one to armchair quarterback on.

Bob

[Bob Harper]

LP=small orifices; NG=larger

If you left a unit set up for NG but fired it with LPG, it would Vesuvisize and possibly burn the house down. The opposite would result in a weak pilot flame that kept dropping out and a very small burner flame.

The flame color may not change as much as you'd think. The more gas you blow through an orifice into the mixer tube the more air is entrained through the air shutter( venturi). Also, LPG entrains more primary air than NG at a given volume.

Typically, high primary aeration results in short, intense blue flames; low primary air typically results in tall, lazy yellow flames. The hearth industry tries for blue bases with yellow tips.

Now, a low manifold pressure can result in short yellow flames that impinge on the logs causing sooting. High manifold pressure can result in flame impingement, too depending upon log position. High primary aeration blows more air/ fuel through the burner resulting in stronger flames. High secondary airflows such as with open hearth fireplaces result in cooling the flames thus causing some sooting.

These logs appear to be ceramic/ refractory logs, which are expected to soot up some. Ceramic fiber logs tend not to soot but can. Mfrs. generally show a basic log position plan for ceramic logs but allow some repositioning, esp. to minimize flame impingement. Ceramic fiber logs usually have some sort of indexing such as pins or flat spots or are screwed together but usually odn't soot.


These are just a few causes.
HTH,
Bob

[JB Thompson]

flame seemed to be normal looking

so, you're saying that there could be several reasons for the sooting - but that it's difficult to determine over cyberspace photos ?

[Bob Harper]

flame seemed to be normal looking

so, you're saying that there could be several reasons for the sooting - but that it's difficult to determine over cyberspace photos ?

Any pics further back showing the entire burner and logset?

[JB Thompson]

no, just those two. Sorry.

thanks for the help

[John Kogel]

Any pics further back showing the entire burner and logset?

Thanks, Bob. Now can you do an armchair analysis of this one? He's on the 5th floor there.
Is this far enough back?

[Rob Yarboro]

I'll armchair this one......sooting problem that needs to be addressed by a professional.

[David Bell]

Check gas valve for NG or LP stickers, conversion requires labeling. Also check for blockage of orifices.