Thermal shock on Cat Stoves

[mpaul]

I’m using a wood stove with a catalytic converter, and I understand the risks of thermal shock if the cat is engaged too early. However, my concern is slightly different: during a hot reload, when the cat is already running at 900+ degrees, opening the stove door exposes it to 70-degree indoor air.

In my stove, the catalytic converter is positioned right in front of the firebox, so I imagine it gets a direct blast of cooler air when the door is opened. Wouldn’t this sudden exposure cause thermal shock or stress the cat material?

For those of you with cat stoves, do you take specific steps to avoid this? For example, do you open the bypass sooner to try and direct the flow away from the cat, minimize door opening time, or make airflow adjustments beforehand? I’d love to hear how others handle this situation.

 

[begreen]

During a reload, as long as the bypass is open before opening the door, the cooler air should be directed behind the cat and up the bypass.

 

[gthomas785]

Yes, the cold air can cause thermal shock. In my opinion this is a bigger risk than the other way around. It just depends on your stove design. There are some stoves I would not open if the cat is at 900 degrees, you'd just have to wait for it to burn down more before reloading. On others it may be fine.

If you get a digital temperature probe for the catalyst you can see in real time how fast the temperature drops when opening various doors or dampers and that'll give you an idea of what's going on.

 

[mpaul]

Thank you both! Yes, I’m running an FPX 42 Apex, equipped with the digital probe for quick and easy temperature checks. The LED readout, conveniently located at eye level within the fan switch, allows me to monitor the temperature closely.

Before opening the door, I always open the bypass. Recently, for hot reloads, I’ve started opening the bypass about 10–15 seconds beforehand to help ensure that cooler air is directed behind the catalyst (cat). Not certain if waiting that 10 - 15 seconds makes a difference or not?

My catalyst is about two years old, and I burn roughly five cords of wood annually. I follow a three-year seasoning plan, meaning I store my wood for at least three years before burning it, keeping about 15+ cords on hand. Today, while shoveling ash and vacuuming the cat, I noticed signs of thermal shock. Since the cat is only two years old, I suspect the damage might be caused either by opening the door during hot reloads or engaging the cat too quickly afterward.

When performing a hot reload, I typically wait for the cat probe to stabilize before reengaging the cat. Often, the freshly loaded wood ignites right away, and I reengage the cat within 3–5 minutes. Providing the cat temps climb after reengaging I leave the bypass pushed in. If I see temps begin to drop, I pull the bypass out right away.

I’m troubleshooting to determine whether the thermal shock is occurring from opening the door during a hot reload or from reengaging the cat too soon.

 

[stoveliker]

My stove manual says to do what begreen mentioned: open the bypass before you open the door.
That avoids (most, not all) of the draft sucking air through the cat.
It's important to open the bypass a few minutes before you open the door. That allows the cat to cool down; because most of its high temp is from heat that it generates itself, not from what is coming up through the firebox. Opening the bypass makes most hot gases flow through the bypass and avoids the cat generating more heat. It'll cool down. It also creates a bit more draft that helps keep any ashes you might stir up when rearranging the coals in the firebox/up the flue.

But it's tough to see this on an analog cat probe (if you have one) as they are very slow to respond.

I am not nervous about closing the bypass too soon on a cold start - at least not in terms of cat shocking. Everything is warming up, and the gases coming into the cat initially are also not that hot yet. It'll heat up the cat faster. I am avoiding closing the bypass too soon if I think the cat won't very quickly get up to active temperature, as I don't want to coat the cat with creosote - that would make it inactive.

Finally, upon a hot reload, where the wood is cold (room temp, for me, not outside temps!), I do let it burn a minute or two with the bypass open. That will push the relatively colder initial gases from a starting new cycle away from the cat; don't want to cool it down more. And it likely heats up the flue more, creating more draft (which might have been low at a final lower heat output coaling stage that prompted the reload).

 

[begreen]

How old is the fireplace? If the cat is not showing deterioration after 2 yrs, it's being treated correctly.

 

[mpaul]

I installed the fireplace in 2019, but those first two years had a very steep learning curve. Based on some bad burning practices those first two years, I replaced the cat for the 2022 - 2023 burning season. The thermal shock I'm seeing today is with that new cat using 3 year old wood. Thought I was doing things right, but now I'm not so sure. Here are some pictures of the cat taken today.

 

[begreen]

I see the reason for the concern. One thing to avoid is placing cold, icy splits in the fireplace. Do the flames sometimes reach the cat face directly?
Roughly how many hours do you think are on this cat by now? They are typically good for about 10-12,000 hrs.

 

[stoveliker]

is there a flame shield in front of the cat? (BKs have that - presumably to avoid what is seen in those pics).
If not, could you make one?

Second, how tall is your flue? With a lot of draft one can suck flames into the cat more easily.

 

[mpaul]

Not sure how many hours are on the cat. Running 24/7 in the northeast for 1 1/2 seasons now. Maybe 3,000 hours (assuming cats last 12k hours putting the average yearly usage at 2,400 hrs / year).

I do not have a flame shield in front of the cat. This is something I've seen spoken a lot about on this forum regarding the design of the FPX units. The cat is right in front easily allowing flames to come into contact with it. Does damage from flames mimic thermal shock?

Flue is roughly 30' with the majority of it being all interior. Fireplace is in the center of the home with two story cathedral ceiling. Based on this I do tend to get a very decent draft.

Based on what you are all saying, my issue could be a combination of things. I'm thinking I need to open the bypass for a longer period of time before opening the door on a hot reload. Extend the time from 10 - 15 seconds to a few minutes. Second is the flame shield. I've seen some posts about this, but I'm a bit confused by them. I always thought any type of metal in a cat stove will cause significant issues. What material is a flame shield made out of and how would I construct it/keep it in place?

Anything else I may be missing?

 

[stoveliker]

I think your tall flue is contributing. My manual says (screenshot below).
My manual notes that the coating can on the ceramic substrate can peel off when flame impinges - not the same as the cracking/crumbling you see. Thermal shock seems to have a different "expression" - at least according to BK.
See the last pic in the screenshot below.

The flame shield should be the same metal as the stove. Your stove inside is metal too.

Would you be able to add a key damper? Tough in an insert, but not impossible. (Begreen will most likely bring up a great thread of another user for this.)

My feeling is that a key damper and a minute or 2 waiting after opening the bypass will help extend the life of your cat signficantly.

 

[begreen]

Yes, a 30 ft chimney is going to suck like a Hoover.

 

[mpaul]

Unfortunately this is an insulated fireplace, not an insert. A key damper isn’t an option here.
The damage I’m seeing on the cat resembles those in the first image of the BK manual.
I hope I can get another year out of this cat. I’ll at least wait several minutes after pulling the bypass before opening the door. I’ll see if I can find a thread here for a flame shield too.
Thanks for everyone’s help here!!