Shoulder season heating with a cat stove

[begreen]

What are some things a new cat stove owner should be aware of when heating in mild weather? Can it be run too low and slow? What is the lowest temp for the cat to avoid snuffing it out? What procedures are best to avoid tarring up the chimney?

 

[Highbeam]

After running both cat and noncat stoves for many years the obvious answer is, same as any stove. Don’t run any stove so cold that flue temps fall below the condensation point.

I fail to see why there is such a divide in running these two technologies.

It is especially important in the shoulder season to properly char every fresh load to drive the moisture out of the fuel and dry the chimney before turning down and flirting with that condensation point. Multiple cold starts make more chimney accumulations than steady heat but in the shoulder seasons this is another risk.

 

[lsucet]

I am with you in in this one. Regardless, a hot initial burn is important including that the house can overheat a little thru the process.
I think driving the moist at first and a good charr takes care the creosote issue, at least minimize it a lot.
At that point I think that low flue temp is all about efficiency. Another detail I noticed is, with a good char the stove runs hotter and so is the flue temp. The cat lights up more often when tstat opens.

At the end the key is a good charr and knowing your lower setting where the cat still active is also important.

 

[begreen]

It's easy to forget all the details when one has been burning for years. The first season (typically in milder fall) for a new cat owner is going to be filled with questions. For example: What is a good char? When do you know it is ready and about how long is this? Is the char before or after the bypass is closed?

 

[lsucet]

A good char is after the cat is at temp and bypass closed. As for my stove manual 20 to 30 minutes on high is the recommendation but I really think that depends on type/species of wood and MC. Can be less or more depending on those variables. Maybe other manufacturers recommend different. Only practice can give you and let you know.

 

[Dataman]

I would open the Windows and Slider part ways. Lots better than stove at low temp.

 

[Woody Stover]

I burn in the load (char it) with the bypass open, bringing the stove up to temp so that the cat will burn right away when I close the bypass. I shoot for maybe 500 or so on the surface flue meter, which sits on the tee snout about 6" behind the rear-vent flue exit. I hold it there for maybe ten minutes or more, depending on what kind of stove top temp I'm seeing above the cat area. The manual says not to close the bypass until STT is 250 but I think that's a worst-case scenario, in case an inexperienced burner is using damp wood. I can usually close the bypass and get an immediate light-off with the stove top around 125-150. If I try to go when the stove top is, say 100, I keep flame going in the box for longer to pump flame heat to the cat and eat some of the smoke until the cat gets burning well. But as I've said, I like to see that cat light more quickly so I usually wait for at least 125 SST. I can then cut the air in a couple steps, and run the stove low where the stove top over the cat won't get much hotter than 300-350, with an air setting of .5 or less on a scale of 4, with a clean plume. The temp on the flue snout meter, which is higher I'm sure than the temp would be on a top-vent at 12-18", might be 250-300.
There are a couple ways I could do it, but I don't yet have a cat probe that is within 1/2-3/4" of the cat face so I don't have that data point, which would help. The stove top above the cat is a bit slower in indicating what the cat is doing, but not too bad.

 

[begreen]

That's a quick light-off. Is this with a steel cat?

 

[Woody Stover]

That's a quick light-off. Is this with a steel cat?

Yeah, but it's not much slower when I run the ceramic.

 

[Woody Stover]

Top-down start heats the upper firebox and cat quicker, and you can put more direct flame heat to the cat when you close the bypass, but dry wood is needed so you don't need to char the load much.

 

[begreen]

Top-down start heats the upper firebox and cat quicker, and you can put more direct flame heat to the cat when you close the bypass, but dry wood is needed so you don't need to char the load much.

I thought that top down starts would be better for this. Thanks for confirming.

 

[Woody Stover]

In the end, between seasons I run the cat pretty much like you do your secondary...I burn a small load at night, and just let the stove go out until higher room temp is desired sometime in the future.

 

[lsucet]

For me top down is the best way. The cat probe gets to the active zone quick when flue temp still real low. Once the bypass is closed the flue temp start climbing so is the cat probe.

 

[begreen]

Condensation starts occurring when the gases in the chimney drop below about 250ºF. A flue thermometer is a big help.

 

[lsucet]

Condensation starts occurring when the gases in the chimney drop below about 250ºF. A flue thermometer is a big help.

At what part of the burn cycle this applies?

 

[Highbeam]

Condensation starts occurring when the gases in the chimney drop below about 250ºF. A flue thermometer is a big help.

Also, this advice fails to differentiate between surface flue temperatures and internal flue temperatures. If the skin of the flue drops below 250 then you are at risk for condensation. That means just 500 internal flue temperatures. Most reputable thermometer manufacturers have printed labels on their thermometers to help you stay in the safe zone.

Yes, a flue thermometer is a big help and we also should recognize that the average user has pretty wet wood and pretty poor habits.

 

[begreen]

At what part of the burn cycle this applies?

A cat stove's burn cycle is different. In this case I would think when burning low and slow it is for most of the burn cycle or as long as the wood is smoldering and the cat is eating smoke, but I am not a cat stove owner, and different cat stoves makes don't burn identically. This is the point of the thread, to get cat users to share their observations.

 

[begreen]

Also, this advice fails to differentiate between surface flue temperatures and internal flue temperatures. If the skin of the flue drops below 250 then you are at risk for condensation

I said specifically - Condensation starts occurring when the gases in the chimney drop below about 250ºF. This is well past the stove pipe temp. Stove pipe temps are just a guide. How much of a temperature drop to the top of the chimney is going to vary with a lot of different factors. 250º surface stove pipe temps might be fine for a short stove pipe going to an insulated, mostly interior chimney at 45º outside and not ok for a long single wall stove pipe going to a tall outside chimney at 20º outside. The focus of this posting however is shoulder season temps.

 

[Highbeam]

If you will note I said specifically - Condensation starts occurring when the gases in the chimney drop below about 250ºF. This is well past the stove pipe temp.

I read what you had written and it is not clear. Gasses in the chimney at 250 mean 125 skin temperature which is way way below the condensation point. Maybe I'm missing something?

 

[Highbeam]

Here is the guidance. Clear as day. Don't fall into the too cool zone. Probe for double wall and surface for single wall. If you have an IR gun, refer to the surface meter temperature zones.

Worth noting that these meters have been modernized to reflect slightly lower "safe" temperatures than the older versions. Maybe to reflect cleaner burning equipment, or higher efficiencies, or just experience.

 

[begreen]

I read what you had written and it is not clear. Gasses in the chimney at 250 mean 125 skin temperature which is way way below the condensation point. Maybe I'm missing something?

Not at all. Temps read at the stove pipe, probe or surface, are not the same as what happens later in chimney as flue gas temps cool down. One is trying to compensate for the cooling of flue gases as they reach the exterior portions of the chimney. How much one compensates will depend on the installation.

Chimney insulation is much more robust so surface temps there mean little. One could have 500º flue gases and only read 100º skin temp on an outdoor chimney pipe. Thus chimney pipe skin temps don't mean much. And what if it is a masonry chimney? Maybe I am missing something but this is the first time I have heard that suggested. Neither a surface or a probe thermometer is used on an exterior chimney.

A 250º surface temp = ~ 500º flue gas temp in the stove pipe. It is unlikely that there will be a 250º temperature drop at the top of a shorter, well insulated, interior flue system when it is 45º outside. When in doubt, go by how the chimney top section looks. On inspection of cleaning, if it is mostly a thin layer of soot then there isn't evidence of creosote accumulation. If it is thick with black sote then try burning with a bit hotter flue temps. If it is tarry or glassy solids then burn drier wood and at hotter flue temps.

 

[Highbeam]

Got it, but I don't know anybody that measures temperatures at their chimney cap. That should be the coldest place and most likely to see condensation.

 

[begreen]

Got it, but I don't know anybody that measures temperatures at their chimney cap. That should be the coldest place and most likely to see condensation.

Exactly, that is why one is trying to anticipate and compensate for the cooling at that location. Chimney examination helps as does observing whether there is visible smoke when burning. The three major causes that encourage creosote buildup are a restricted air supply, unseasoned wood and cooler than normal chimney temperatures.

 

[heavy hammer]

I do not run my cat stove very low, even in shoulder season and towards the end of the burn season. I have found it is better for me and my setup to have smaller shorter fires then to try and dial the stove to low. I try to keep the flue temps up since my Kuma vents into a masonry chimney. My insert has a insulated liner and both chimney's stay pretty clean. The Masonry chimney usually has more buildup but I figure it is from everything from being a longer run 23 feet compared to 17. It being a masonry uninsulated 9 inch flue compared to a insulated 6 inch liner. I usually clean the cap and check the flue's once or twice during the burning season to just keep an eye on things and keep the stoves running properly. I was more concerned this year since I have heard members talking about their wood having more moisture than usual since the past year was such a wet year. For me running the stove at a very low idle doesn't work. I would rather have to crack a window or just let the fire die than run the stove low. This is also the case for my pacific energy summit insert. Sometimes I just start a small fire in it and never turn the blower on I just let it run its course to help keep the chill out. This time a year I use a lot of matches and kindling because I am always having a quick fire then let it die then a day or so later restart it if necessary.

 

[mar13]

Got it, but I don't know anybody that measures temperatures at their chimney cap. That should be the coldest place and most likely to see condensation.

I bet it won't be too many years before somebody is selling a remote thermometer to do such measurements. Even light bulbs are becoming Wi-Fi capable....