@tabner Thanks again for your input. And
@mdocod thank you kindly for jumping in with such detail and knowledge learned from your experience! This is a ton of knowledge that I love to know, and answered questions I hadn't even had yet.
I added a blower to my stove a month ago to pull more heat from it, and afterwards noticed doing so does rob from the cat temps. I've been careful to keep it in the active range so as not to damage the cat, but it became a battle as my chimney cap was slowly choking itself. I certainly made the issue worse the last 10 or so burns where I ended up smoking out my house. I didn't realize the exhaust issue and was trying to activate the cat hoping that additional burn would add the heat needed to get the stove operating properly. This was pushing smoke from what may be high moisture pine through the cat repeatedly as the fire always smoldered out. Do you know how likely this may have damaged the cat?
I have been making sure to get my cat temps up past halfway towards to 'too hot' range to intentionally help burn off any soot, but this was 10+ days back before the issues in my last paragraph. Now that I replaced the cap yesterday it's working great and I made sure to give it a long hot burn. I've never been able to get it past 3/4 the way to 'too hot', maybe because of the pine.
The forgot to add an spark arrestor screen to my cap, since you also live in Colorado do you feel these are crucial to prevent fires? How often do you need to clean yours? By the way I have family that also live near the Black Forest.
You and Tabner both educated me that I'm not supposed to be feeding these stoves one or two logs at a time. I've been doing that the past 2 months since first using it. To bad it wasn't explained to me sooner but I'm glad I know now. Would the dirtiest part of the burn be at the beginning when the moisture is burning out? I haven't yet reloaded with a full load because I've been slow feeding it, but the times I've started a full load when cold it seems to create an awful amount of smoke. Because my wood may be too wet I'm imagine I should leave the bypass open during this phase to not cause damage to the cat. Now that the cap is replaced I'm able to crack the door for better draft like you mentioned. But with a new full load do you think the cat can handle the dirty part of the burn so long as it's hot enough and I've 'cleared the throat' like you mention?
On the Green Mountain the cat is in the back past the bypass door so I'm never able to see if flames are ever licking it, but when roaring it's certainly enough to get up through the bypass.
I don't have a flue gas probe, all I have is the cat thermometer that came with the stove. If you feel I should get one do I drill through the double walled pipe to install it?
Blower:
I would not run a blower on your stove until you have a burn strategy figured out that is hot and clean enough not to plug your chimney cap. Don't rob heat from the system until you have more heat than the system can handle or more than it needs to run without the issues you're having. I have one of those thermoelectric fans, a couple of enameled cast iron steamers, and a long section of single wall stove pipe all acting as heat robbers to my stove system, but I'm burning ~45lb loads of 9% Ponderosa in a Mansfield. That's a whole bunch of dry soft fuel that wants to burn hot and fast. The heat robbers in my application are warranted, as they keep the system from over-heating while putting more heat in the house. If I started burning a wetter, harder wood, I would likely have to make adjustments to the thermal impedance or burn rates of the system.
Built-in Temp Probe :
I think the temp probe on the side of these catalytic Hearthstones can be misleading. If the stove is hot but settled into the coaling stage of the burn, the probe will read "active" despite there not actually being any combustion taking place in the cat. If the blower is cooling the dial face of the probe, it will read lower temps than if not, which may create discrepancies/confusion. Just because the probe is in the "active" range somewhere doesn't mean that we can throw a heap of fresh wood gases at it and expect it to be immediately combusting efficiently. A fresh load of wood will produce relatively cool gases. For me, a full load of wood from a cool start (stove body under 125F), will usually drive that temp probe to about 80-90% of the way to the "too hot" transition line. Subsequent fuel loads in a warm stove on hot coals will usually flirt with and ride right on the "active/too hot" transition on the probe for several hours of the burn cycle. I suspect that the bigger firebox plays a role here, but I would still expect a load of properly seasoned fast burning pine to spike temps closer to the max on that probe, especially with brand new cats, which are known to be a bit more "active."
Flue Temp Probe:
A wood stove is sort of like a 5 speed transmission.
1st gear: light fire / get moving.
2nd gear: door open burn / onramp to highway hard acceleration to 65MPH.
3rd gear: door closed full air burn / continue to accelerate to 80MPH to get around that silly cement mixer and merge into traffic.
4th gear: engage cat / stabilize speed into the flow of traffic.
5th gear: cruise control / cruise control.
Reverse: plugged up chimney cap smoke billowing into house / In-law removal tool.
You can drive a car without a tach and you can operate a wood stove without a temp probe on the exhaust, but you probably won't know exactly where redline is in each gear this way, and also don't have a good visual cue for rev matching or when to shift in general. A temp probe will make it easier to develop a reliable repeatable startup and reload procedure, and well as dial in a healthy cruise setting, however, when driving a car, watching the tac all the time will eventually lead to a crash. Don't forget to watch where you're going! What's happening in the firebox is still the most important indicator of how the burn is going. Temp probes lag behind what is going on.
Double wall requires a "probe" style thermometer, and yes, that means drilling the stove pipe, Install in accordance with the thermometer install instruction. Most call for 18" above the surface of the stove.
Spark Arrestor / Cap:
In dry, fire prone areas like this, I prefer to see a spark arrestor on the chimney cap. We have a standard chimney cap with integrated spark arrestor here. There are lots of factors at play for fire risk from a stove chimney that can add up to very little or significant risk depending on how they all align. I don't think the large-hole screen on most chimney caps is going to stop every possible spark or ember, but nor do I think it's likely to have sparks or embers make their way that high up the pipe except when stove door is open (throat clearing operation). Personally I only burn when temps are below freezing and relative humidity is over ~50%, or some combination of temp/humidity that achieves an equivalent or lower fire danger. I think that, and keeping a cleared, defensible space of minimal surface fuel around your home and on your roof are more important factors than the spark arrestor. This time of year we often have snow cover on the ground, and that puts my mind at ease for burning.
I have not had to do anything special to keep the chimney cap / arrestor clear here. I sweep the chimney about twice per burning season. Otherwise, my plan is to make sure I burn hot enough and clean enough and do routine "throat clearing" exercises of the system to keep the cap clear. I do not like ladders
The stove manual for your Green Mountain 60 actually recommends operating the stove for 35-45 minutes a day at the high-burn-rate (door closed, air control wide open, cat engaged) to help keep the chimney clean. If it's not burning 24/7, then that high-fire burn rate should be done as part of the startup process each time it is fired up from cold. That little 2 cubic foot stove weighs 500 lbs
That is a LOT of thermal mass to heat up from a small-medium size firebox. I would be careful not to let the stove putter while the stove is still relatively cold. Let it rip full throttle long enough to warm up the stove pretty good! This will also give the stove a chance to work most of the moisture out of the wood with plenty of active combustion to drive it out the chimney hot enough not to condense.
Cat Damage:
Your cats are probably fine but if your chimney/cap is that plugged up there's a chance your cats are also plugged up. I would inspect them and make sure they are clear, not packed with soot. If they are plugged up use an ash vac (for indoor use) or a shop vac or compressed air (outside!) to clean them out. If they are like mine, they can be lifted out pretty easily for cleaning, check the stove manual for specific instructions on that. At minimum the cats and the various surfaces above the baffle in the stove should be vacuumed every time the chimney is swept.
I think you should engage the cat after a fuel reload or fresh start as soon as you have reached 500-600F EGT's with the door closed and air control wide open. You'll need a flue temp probe to know when this is. If the wood is appropriately dry, and you have the fire properly excited before closing the door, then you should be getting very strong secondary combustion in the stove, and this should be handling 85-95% of the combustion process in the box before anything even gets to the cat. If that's not happening when you close the door, bypass cat and crack the door back open to induce a rocket-stove effect until things are hot enough to roll flames from the secondaries when the door is shut. These stoves run best when operated as hybrids. Steady active primary/secondary flaming combustion through most of the off-gassing part of the burn cycle (~2-3 hours for pine), then after the flames drop out with the wood gas still slowly coming out of the heap of shrunken transitioning logs, that's when the cat should wake up and take over the combustion of wood gases. At this later stage of the burn, the release of wood gases is too slow to sustain active flames in the box, but is still plenty of wood gas to keep the cats going. You'll often see the temp probe above the cat spike the highest during this part of the burn (around the 3-5 hour mark with pine). After that everything should settle down to coals.
If the door is shut I wouldn't worry about flames damaging the cat when its engaged. The biggest risk of damage there is when a hot flame reaches back and is sucked through a cold cat. When the door is shut, the flames and the gases are all steadily moving like a steady river of hot gases and plasma. The design of space above the baffle on these stoves creates a choke point that forces that river of gases to come down and through a slot between the baffle and the bypass assembly before entering the space where the bottom of the cats are exposed. As long as the door is shut, I would expect the slow river of flames to stay up near the roof of the stove. They aren't going to want to naturally traverse down into the choke point.
Other thoughts:
I don't think that 300-500F EGT's is an appropriate goal for soft-wood burning. That sounds like a great range to be on on a steady burn from pitch-free hardwoods, but softwood is full of pitch. If you run your stove with only 300-500F EGT's on pitch-laden softwoods during the off-gassing, resin-soot-release phase of the burn, especially with wetter than ideal woods, you can expect a repeat of that chimney cap situation or plugged up cats. I learned from folks on this forum that the soot needs at least 750F to burn off, and in my experience if there isn't enough extra heat and extra air coming into the system to burn it off, it can drive the cats to an over-rich condition, snuff them out, and plug them up. Not only do soft woods want to burn faster and hotter, they should be allowed to burn faster and hotter for a clean burn that doesn't plug up the chimney or cats
The GM 60 manual says that the stove generally produces 800-1300F EGT's just above the cats.
The popular FlueGard probe for double wall EGT measurements includes the following general purpose instructions to consider:
- 100℉ to 400℉: Temperature too low. Incomplete combustion, causing smoke, soot and hazardous creosote. Open draft and/or add dry fuel.
- 400℉ to 900℉: Safe operating temperature. Complete combustion and best efficiency.
- 900℉ to 1200℉: Wasting energy, possibly overheating. While high temperatures are often reached on initial firing, should not be maintained for normal operation. Reduce draft.
