FPX Apex 42 fire unexpectedly going out

[mpaul]

Fourth year burning in a 42 Apex Cat stove. The issue I describe below just started a few weeks ago and is only happening when I’m burning from my pile of Ash. The Ash is two years old and is measuring 15% - 18% MC on a fresh split.

So I’ll get my fire going. Basic bottom up cold start fire. Will dial everything in with my Cat cruising anywhere between 800 - 1,000 degrees. Air at about 10 - 20%. Fire will be cruising for about 30 -60 minutes with no issues. Then the flames will go out unexpectedly and the Cat temps spike to 1,200 + degrees (inside stove glows red from Cat being so hot). Cat temps will continue to rapidly climb until I get the flames going again.

I open the air to 100% and sometimes open the bypass to reignite the flames and cool the Cat. Usually get significant blow back when the fire reignites and then I reengage the Cat and dial the temps back down.

What would be causing these flames to go out like this?

 

[RockyMtnGriz]

Have you tried turning the air down instead of up? I'm thinking that less air and mostly running on the cat might be the solution.

Edit - Another thought is air entering above the fire, where it would support mostly secondary combustion in the cat, but not primary combustion in the fuel. Could you have a gasket leak that has developed?

This is a similar situation to the problems I've been posting about lately with trying to burn extremely dry wood with smoke resulting. It sounds like in your situation you have enough secondary air to burn the gasses you are producing but want to reduce the cat temperature. In my mind, there's two ways to do that. Decrease the gasses to the cat by burning them in the primary fire through more primary air, or decrease the gasses to the cat by restricting the primary air.

I wouldn't consider the flames going out in a cat stove to be unusual, but since you say this has just started happening, apparently, it's unusual for your stove. Maybe with this wood, at this MC, the trick would be to expect the flames to go out and turn down the primary air earlier and perhaps more. That might help limit the cat temp spike and help the stove settle into cat mode a little less dramatically.

 

[begreen]

Is this an FPX Apex 42 fireplace? If the cat temps are going up, then it is burning the wood gases coming off the splits. As long as it stays below about 1500º that's ok. It's not uncommon for there to be no flames in a pure cat fireplace or stove when the air is set low. For more flame try burning smaller, intense fires with a bit more air.

If this is a new cat it will be a bit hyperactive. They settle down after burning for a few weeks.

 

[mpaul]

Thank you both for the help and advice.

This is the FPX 42 Apex. My fourth year burning with it. First year was a learning experience and ended up putting a new Cat and new door gasket in at the start of this season. I now burn about 5 cords a year heating my 2,000 sq ft home solely with this fireplace.

I totally get the low/slow Cat mentality and I do tend to dial it down and push for the 9-10 hr burn times during the colder months.

This just seems a bit different though. With it cruising at ~15% air and just a small flame for a while and then the flame just goes out and the Cat temp shoot’s up rapidly. Roughly 100 degrees per minute and keeps climbing until I get the wood to reignite.

If it happens again I may try dialing it down further or letting it go just to see if the Cat temp stabilizes on its own. I do feel that eventually the wood would reignite on its own when looking at the situation. I fear the blow back could be significant though if it were to happen.

 

[stoveliker]

The cat temp shooting up is a *consequence* of the flame going out; less fuel is burnt in the primary combustion, so the cat will have to eat more.

Why the flame goes out is anyone's guess. Could be you're near the bottom of a stable primary burn with the air setting, i.e. any lower and you'd have no flame. Being near that transition, the stove could easily extinguish the primary burn if a wind gust decreases draft momentarily. Or of your wood falls/collapses/rearranges. And it can pick up flame again easily as well.

As long as the cat stays below 1400-1500, you're fine and no need to change air or bypass.

 

[begreen]

When was the chimney system and the chimney cap last cleaned?

 

[mpaul]

The cat temp shooting up is a *consequence* of the flame going out; less fuel is burnt in the primary combustion, so the cat will have to eat more.

Why the flame goes out is anyone's guess. Could be you're near the bottom of a stable primary burn with the air setting, i.e. any lower and you'd have no flame. Being near that transition, the stove could easily extinguish the primary burn if a wind gust decreases draft momentarily. Or of your wood falls/collapses/rearranges. And it can pick up flame again easily as well.

As long as the cat stays below 1400-1500, you're fine and no need to change air or bypass.

This makes a lot of sense. I always knew the Cat temp climbed because it had more to consume with no flames. I was just more consumed with why the flames went out.

Not sure if I’ll be burning anymore of that wood this season. With everything warming up I’m switching over to my pine stash more and more now. I will keep all this in mind if/when i encounter this scenario again.

Thanks again. Really love all the help here!

 

[mpaul]

When was the chimney system and the chimney cap last cleaned?

Cleaned annually last time was right at the beginning of this burning season believe September.

Want to emphasize it only happens with the Ash. I have a load of pine in now without any issues

 

[stoveliker]

Off gassing rates vary per wood species. A change in the system (e.g. chimney issues) can therefore sometimes show up in one fuel species.

This is not common imo, but possible.

 

[begreen]

Off gassing rates vary per wood species. A change in the system (e.g. chimney issues) can therefore sometimes show up in one fuel species.

This is not common imo, but possible.

Yes, that was my thought. It's a process of elimination. Pine has high oil content and does flame more.

 

[mpaul]

So could all be relatively normal which would ease my mind.
I’ll try riding it out next time it happens. If the Cat starts to approach that 1500 mark I’ll pull the bypass.