Cat--how does it work

[Joe Bruno]

I know the Cat thing has been dealt with here, so my apologies if this is redundant, but yes, I have Cat questions and doubts and concerns. Here's a brief CV on my wood burning experience as it may help people more current than I understand where I am coming from.

I am a city guy, Bklyn NY, who went to the woods. I studied rock and adobe masonry out West and learned to build Rumford Fireplaces that actually worked. Back in upper NY State I moved into a pre-Civil War farm house--10+ rooms. Wood heat only. Cut and split hardwood from my 40 acre lot. We had an old cast iron cook stove (1903 Kern, still have it) and, in keeping with the state of the art of the day, I bought 2 Ashley sheet-metal stoves with cast iron tops and bottoms and automatic air controls. They kept us all alive. Then the Arab oil embargo came down and crushed us. All the farmers wanted to convert back to wood stoves. There was work for me fixing up the old chimneys, installing new ones, and new-fangled stoves started to arrive from Scandinavia. Jotul was maybe the first. Eventually, went back to NY City. Left the cows behind (pardon the pun).

Today I have a Hearthstone Heritage soapstone stove, made in 2022. I assume it is the current model. I installed it to replace the fireplace as the clay tile was cracked and I did not fancy taking it down to the ground and rebuilding through my fully renovated house. Besides, I felt it was more proper to get the most out of the wood, and Rumford, good as he was, only went so far.

So here's my current situation. I use the stove to heat the great room, the kitchen and, as far as possible, a bedroom and a bath down a hallway. I've read here and elsewhere as well as the manual but I'm mystified by a lot of the tech and that people here seem like NASA stove engineers. I've read that some have trouble keeping their Cat stoves from overheating. Not here. Only once have I seen it even close; most of the time it runs much cooler. If the CatActive range is 0 to 100, mine runs around 10 to 30. While I am happy with the heat--it is even, soft and seldom gets too warm if a bit slow on the warm up--I wonder if it isn't running too cold. If it runs low on fuel and goes below Cat range, sometimes loading it up and waiting around for it to come back instead of going to bed is a hardship, so I am tempted to let it run without the cat. But then I worry what that might do. In the pre-Cat word there were no such worries. Finally, I wonder what all this Cat-concern is giving me. Do I actually get more heat out of my hard earned fuel with the Cat? Or does it just make Greta Thunberg happy? The thing is mounted at the top rear of the stove. It seems that whatever heat a burn at that point generates, it will mostly go up the flue and heat the world, but not my house. Finally, I seldom get to see the flames; the glass is always black. I clean it but by the end of the day it's black again. By now, in January, like last year, I just leave it stay black. Does anyone have any insights to share.

By the way, the set-up is back vented. The SS nipple on the back of the stove Tee's into the 6" SS corrugated liner that runs (almost) straight to the roof. The bottom of the Tee allows for a removable cap for a cleanout. The run from the back of the stove to the roof is 13 or 14 feet. Typically, the best way to start a fire, or to add a lot of new wood, is to prop the side door open to fan it up. The air intake control is usually not enough, and/or it will take forever to get up to speed. Draft, I guess, is a bit low at the outset, but the run speed is perfect for the space and the CT climate.

Thank you for your time and for any suggestions you may have.

 

[begreen]

Might be the wood. Can you get some different, known dry fuel to add to the current fuel? Try adding a few dry 2x4 cut offs to the next load and see how the cat performs then.
How tall is the flue system on the stove from stove top to chimney cap?

 

[Garbanzo62]

1) get a moisture meter and check your wood. Fresh split at room temp is best. You are looking for < 20%.
2) do not cut down the damper too soon. When I cut the air too much after a reload, that is when I start to get buildup on the glass. It should be a good 20 - 30 minutes before you cut down the air fully

Also the cat only works when it is hot enough to catalyze. You cannot engage it too soon. Make sure you at minimum 500 degrees (600 is better) before you engage the cat.

 

[Ashful]

Finally, I wonder what all this Cat-concern is giving me. Do I actually get more heat out of my hard earned fuel with the Cat? Or does it just make Greta Thunberg happy? The thing is mounted at the top rear of the stove. It seems that whatever heat a burn at that point generates, it will mostly go up the flue and heat the world, but not my house. Finally, I seldom get to see the flames; the glass is always black. I clean it but by the end of the day it's black again. By now, in January, like last year, I just leave it stay black. Does anyone have any insights to share.

lol... love the phrasing of the question. I'm no fan of the Greta Thunberg's of the world, but I am a fan of cat stoves, so let's talk about that.

A simple look at the efficiencies posted for pre-EPA stoves or EPA-exempt stoves, versus cat stoves, will tell you that yes... you are getting more heat out of the cat. The dirt put up the chimney from an older stove lacking a secondary burn system is not just useless dirt, it's unburned fuel! Catalytic and non-cat EPA stoves are called "secondary burn" stoves, because they have a secondary burn process that consumes this fuel, generating added heat in the process. The actual amount of heat generated by the secondary burn depends on the burn rate and technology, but it's very noticeable at lower burn rates... less so at higher burn rates.

It's not only keeping your chimney cleaner at low burn rates, but also squeaking extra BTU's out of your hard-earned split and dried wood. If it makes Greta happy in the process, well... I guess I'm okay with that, too.

 

[Joe Bruno]

lol... love the phrasing of the question. I'm no fan of the Greta Thunberg's of the world, but I am a fan of cat stoves, so let's talk about that.

A simple look at the efficiencies posted for pre-EPA stoves or EPA-exempt stoves, versus cat stoves, will tell you that yes... you are getting more heat out of the cat. The dirt put up the chimney from an older stove lacking a secondary burn system is not just useless dirt, it's unburned fuel! Catalytic and non-cat EPA stoves are called "secondary burn" stoves, because they have a secondary burn process that consumes this fuel, generating added heat in the process. The actual amount of heat generated by the secondary burn depends on the burn rate and technology, but it's very noticeable at lower burn rates... less so at higher burn rates.

It's not only keeping your chimney cleaner at low burn rates, but also squeaking extra BTU's out of your hard-earned split and dried wood. If it makes Greta happy in the process, well... I guess I'm okay with that, too.

Thank you. I could not resist the Greta mention; she will be flattered. Okay, so what you say helps in that the Cat has its max effect at low burn rates. I suppose at high burn rates the combustion is generally more thorough so Cat or no Cat the chimney stays cleaner and so does the air outside. But at low burn rates, when my glass gets all black, the bad stuff makes its way up the flue and either goes into the air or plugs up the chimney. Is that the correct take away from what you wrote?

 

[bholler]

Thank you. I could not resist the Greta mention; she will be flattered. Okay, so what you say helps in that the Cat has its max effect at low burn rates. I suppose at high burn rates the combustion is generally more thorough so Cat or no Cat the chimney stays cleaner and so does the air outside. But at low burn rates, when my glass gets all black, the bad stuff makes its way up the flue and either goes into the air or plugs up the chimney. Is that the correct take away from what you wrote?

If the cat is active and working properly it is burning that bad stuff up before it reaches the chimney. On well designed cat stoves the heat from the cat is transferred through the top into the home. I honestly have yet to see one of your stoves though so I can't comment.


But I agree with others check the moisture content of your wood. Also your chimney is on the short side so your draft isn't going to be great

 

[Joe Bruno]

1) get a moisture meter and check your wood. Fresh split at room temp is best. You are looking for < 20%.
2) do not cut down the damper too soon. When I cut the air too much after a reload, that is when I start to get buildup on the glass. It should be a good 20 - 30 minutes before you cut down the air fully

Also the cat only works when it is hot enough to catalyze. You cannot engage it too soon. Make sure you at minimum 500 degrees (600 is better) before you engage the cat.

okay, thank you Gargbanzo. I ordered a meter last night and a stove thermometer to lay on the soap stone. My concern with leaving the damper open has been that the stove would overheat the house and overheat the soap stone, besides wasting lots of wood. Question: when you say 500 degrees, 600 better, would that be a measurement on the surface of the soap stone. The gauge they provide with the stove has no numbers but I thought it was inserted directly in the chamber where the Cat is installed. No?

Today we cleaned the glass and I made a hotter fire. The glass is clean after one load. That's an improvement. To get up to a higher heat I had to leave the side door open a bit for over 1/2 hour. After it burned well and the temp went up to midway on the gauge that comes with the stove I damped it down to about the midway setting and kicked in the Cat. To get more time out of a load (sleep time) I will need to shut it all the way though, and that's where I think the glass gets really dirty. Also, question, can I engage the Cat for increased effeciency if the gauge is up there, even while I am stoking the fire with the extra air from the door. Or will that be bad for the Cat. (Cat is a good name for these things, as they really are like those ornery animals!).

 

[Joe Bruno]

If the cat is active and working properly it is burning that bad stuff up before it reaches the chimney. On well designed cat stoves the heat from the cat is transferred through the top into the home. I honestly have yet to see one of your stoves though so I can't comment.


But I agree with others check the moisture content of your wood. Also your chimney is on the short side so your draft isn't going to be great

thanks. my stove is the Hearthstone Heritage soap stone model, made a year ago. I think you are right about the draft being a bit spare, which is why I often need to open the side door to get a good fire to catch. I hope that is not having a bad effect on things.

 

[Joe Bruno]

Might be the wood. Can you get some different, known dry fuel to add to the current fuel? Try adding a few dry 2x4 cut offs to the next load and see how the cat performs then.
How tall is the flue system on the stove from stove top to chimney cap?

thanks. ordered a gauge for the wood. flue is only 14 feet or so from stove to rain cap. In some weather the startup is slow and needs the side door opened a bit. I asked someone else this as well: if the cat gauge is well into the range to fire it, would it be okay to do so even with the extra air coming in form the side door. or is that a no no?

 

[bholler]

thanks. my stove is the Hearthstone Heritage soap stone model, made a year ago. I think you are right about the draft being a bit spare, which is why I often need to open the side door to get a good fire to catch. I hope that is not having a bad effect on things.

Yeah I know what stove you have. I just haven't worked on any of the new model with the cat yet.

 

[Joe Bruno]

Yeah I know what stove you have. I just haven't worked on any of the new model with the cat yet.

Okay, I misunderstood when you wrote that you hadn't seen my stove(s). Now I got it. I think Cats are finicky, but I understand how they can add to the quality of the burn.

 

[bholler]

Okay, I misunderstood when you wrote that you hadn't seen my stove(s). Now I got it. I think Cats are finicky, but I understand how they can add to the quality of the burn.

Not really that finicky if it's a good design using good wood with proper draft

 

[begreen]

thanks. ordered a gauge for the wood. flue is only 14 feet or so from stove to rain cap.

That's a bit shy and may be part of the issue. This stove wants at least 16' flue. It seems to prefer stronger draft than some.
Try the 2x4 cut offs mixed in and let us know how that works out.

 

[Pinesmoke]

thanks. my stove is the Hearthstone Heritage soap stone model, made a year ago. I think you are right about the draft being a bit spare, which is why I often need to open the side door to get a good fire to catch. I hope that is not having a bad effect on things.

The guy I bought my Hearthstone from is the same guy I've bought from before and sent at least 10 ppl to him. When I was learning this stove it didn't cooperate with starts. I called him and he said "every Hearthstone I've had was cold bloodedand burn with the door cracked until you have ember. Reload and close it". Worked like he said.

ETA
When the cat is in operating range + 50-100 deg I run the cats. At reload I'll run the air open for 5 minutes more to clear the ashes I stirred up then go back on the cats. Drop the air into the burn range. When it's a full load for overnight I'll stage the air drop across 15-20 minutes before I hit closed.

 

[Ashful]

Okay, so what you say helps in that the Cat has its max effect at low burn rates.

Yes. Unburned fuel coming off the primary burn is maximized at low burn rates, so that's where the cat (or secondary burn tubes) see maximum fuel input, and do maximum work. As inlet air is increased, primary burn leaves less unburned fuel, and the secondary system (cat or burn tubes) has less work to do.

But what most new burners miss is that the stove is not the most important component in the system, even if it is the most expensive. The chimney is the engine that makes that stove run, and the wood is the fuel that makes it go. If either aren't perfect, then you'll see it most when you push the system toward the edge of its limits:

1. Low limit: If your chimney is a bit short, it may draft fine when hot at higher burn rates, but may stall the stove when turned down. This happens because less fuel is being consumed, and so less heat is going up the flue, and draft is related to temperature difference between flue inlet and outside air. If your wood is not very dry, this compounds the issue, as converting any water trapped in the wood to steam consumes a lot of BTU's (phase change). Also, that steam can re-condense on the way up, depositing the creosote molecules to which the water vapor was attached into the chimney.

2. High limit: If your chimney is too tall, the stove gets a bit unwieldy and hard to control, especially if your stove has a bypass damper (traditional non-hybrid cat stoves). Also, the added pull of a too-tall chimney can stir up a lot of fly ash in the stove, which can clog catalytic combustors. Not a huge issue, but an annoyance. This can usually all be resolved with use of a key damper.

In your case, short chimney and possibly non-ideal wood, I'd try burning at higher burn rates. I think you mentioned that this makes your home too hot, but a higher burn rate with a smaller load can resolve that, letting things cool between shorter loads. Not ideal, but a good temporary solution to a too-short chimney or too-wet wood, and probably not a bad experiment to help us debug what's going on.