First time with a cat temp probe

[Ashful]

So, I finally got a cat temp probe installed on one of my cat stoves. I find I can burn an hour or two in bypass mode with 300*F to 350*F stove top, without the cat ever getting much above 100*F. Of course, after I engage the cat, it goes right to 800*F, but I'm having trouble understanding all the folks who claim they get their cat to 500*F before engaging.

This is a downdraft cat stove.

 

[clemsonfor]

Its not hard for me to get mine to 400-600F before i close the bypass? I guess its the design of your stove? MIne from a cold start i can get my small kindling started then feed some small splits then a few bigger ones on top to get a good coal bed going for the first real reload as i call it after a few hours. But while i am burning this load is when i will close the cat usually sometimes not depending upon how much wood i loaded. If i forget to close the damper i have looked and seen an easy 600-700F on my probe that sits above my cats, which means they are even hotter.

My stove is designed with the cats in the front of the stove. They sit inside a metal housing that hangs down about 2"s in the front of the fire box. Maybe its the area there located in the stove and how recessed they are?

I dont think my probe even registers below 100 or is it 200?

 

[coltfever]

with my stove I have no problem getting up to cat temp of 500 in about 20 to 25 mins.

 

[Ashful]

Well, for full disclosure, I was using wood I carried in from my covered porch last night, in the midst of a hurricane. It was Walnut cut and split last February, and likely could be seasoned a bit longer. It was also coming in from very damp, albeit covered, conditions outside last night.

However, stovetop was still at 330*F for more than an hour, and cat was at less than 100*F.

My cat probe is an Omega high temperature K-type themocouple probe. It reads -328*F to +2282*F, with an accuracy of better than 0.75% in the range where I'm using it.

 

[mellow]

Must be due to being a downdraft stove and not getting the exhaust heat up top. Mine gets up to 500 easy before I close the bypass, but again like others it is in the top front.

 

[jharkin]

IF your stove top is only 300 after 2 hours, the fire probably inst burning hot enough to get the cat hot. Do you have the air wide open? I have a downdraft stove also and I can get the cat to 500 in an hour on bypass easily (usually about 45min from a cold start)- but I need to get the stove top up to 450-500 to do it.

The only time I hear of getting cat light-off on temps that low are for soapstone stoves (Woodstock) where there is a much bigger temperature differential between the firebox and the stovetop.

 

[jharkin]

Thinking about this more, if 350 stovetop is the best you can reach after 2 hours wide open burning- Id say the wood is VERY suspect. My stove would probably melt before I got to 2 hours wide open.

 

[Ashful]

Perhaps, but don't forget I only had 4 medium/small splits in a 3 cu ft stove. Could also be the new insulated liner, which terminates closer than I would like to the flagstone chimney top. Could also be slowed (but not an hour) by the fact that the wood was coming in damp from outside.

One thing that was bothering me last night is that the fire was burning very low on max air in bypass mode, but would roar to life any time I opened the stove door. Almost seemed like my primary air path is clogged, although I can't see how. Figured it might be due to the 50-70 mph wids howling outside.

 

[clemsonfor]

If thats the case you need to leave your door cracked to get the stove rolling!! I have a 3.5cuft stove and with 4 medium spits i can easy burn them up in 2 hours wide open

 

[EatenByLimestone]

Oh, this is a post about woodstoves...

When I read, "First time with a cat temp probe" I started hoping for a video.

I have nothing to add here.


Matt

 

[Ashful]

So, more experimenting today. Came home at 4:30pm, loaded the stove up on the coals remaining from this morning (only 3 medium splits went all day!), and headed out to cut down a tree. Told the wife to close the bypass damper as soon as she got going good, and then lower primary air to 50%. Came back in to 450 stove top and 925 cat probe, so she can crank heat okay.

Tried opening the bypass and the cat quickly fell back down to 225*F. Closed bypass, back up to 825*F. It does seem my cat just does not run hot in bypass, likely because it's mounted low in the rear, and the bypass door is high in the rear. When the bypass is open, I suspect zero smoke is running thru that cat.

Experimented with engaging early tonight, and found that if I engage just a minute or two too early, the cat will stall somewhere around 400*F. If I open the bypass again, wait another minute or two to get the fire going stronger, it will really take off when I close the bypass, even though it started from the same low temperature both times. The good and bad of it is that there seems to be no magic cat probe or stove top temperature at which to engage the cat. It's more when the fire is going strong enough to have sufficient momentum to carry it thru the minute or two of stall when engaging the cat, and waiting for it to get up to "light off" temperature.

Currently cruising with stove top 275*F and cat probe 860*F. With the new insulated liner, I can crank air down to the stop and there's no sign of back-puffing. I'm getting 8 hours out of some pretty small loads. I think the unusually low temps running wide open yesterday were a combo of weird draft conditions (70 mph wind) and rain soaked wood.

 

[Ashful]

Final check before bed, and 1120*F on the cat with still 270*F on stove top at min air setting. Wife and dogs all comatose in front of stove.

 

[Ashful]

On three medium splits with min air setting, stovetop held 250-275 and room temp held 70-72 all night. Still at 225 stovetop and 70 room temp.

Cat probe peaked at 1250, some time during the night. Max recommended is 1800.

Will try a bigger load tonight.

 

[jharkin]

For a downdraft cat stove the behavior you see with the bypass is completely normal. Opening the bypass diverts the smoke away from the cat and it should cool off.

Don't be afraid to leave the air wide open and get the stove top to 500-600 before engaging cat. I almost never start turning the air control down from full until I'm on cat.

 

[Ashful]

Thanks. Yep, I keep air full open until cat is engaged and up to temp, typically above 700*F. I prefer not to get the stove top up to 500-600, because this is a big stove, and running it that hot will make my room uncomfortably warm. The happy place, at least in our current weather (and all of last winter... very mild), is right around 250*F on the stove top. Maybe I'll run it hotter when the weather gets colder.

 

[jharkin]

Thanks. Yep, I keep air full open until cat is engaged and up to temp, typically above 700*F. I prefer not to get the stove top up to 500-600, because this is a big stove, and running it that hot will make my room uncomfortably warm. The happy place, at least in our current weather (and all of last winter... very mild), is right around 250*F on the stove top. Maybe I'll run it hotter when the weather gets colder.

Yup, you ran into the same reason why I don't even light my stove typically till after Thanksgiving each year. It throws too much heat on mild days and when I try to burn super low I have trouble getting a clean burn.

 

[Ashful]

I don't think I'm having much trouble getting a clean burn, here. As long as that cat stays lit, I'm burning clean. Chimney sweep was just here last week, and said I did good last year.

My primary problem last year was I had marginal draft, and would get back-puffing any time I tried to run extremely low. So, like the non-cat guys, I had to settle for smaller loads of wood to keep the stove temp down at medium draft. Now it seems I can stuff her fuller and run on minimum draft setting.

 

[Ashful]

Well, tonight was full of new experiences, and that new cat probe came in mighty handy. Being already tired of the two-load process to get the stove up to temp and then loaded for an overnight run, and getting home to start the stove much later than usual, I tried building my first top-down fire. Bottom up, I had four big splits (6" x 6" x 20"), fire starter, kindling, and then two small splits. I should have had three small splits on the top, but more on that later.

I got things warmed up and the cat engaged, before heading up to put the kid to bed. I set the primary air to 50%, leaving the wife with instructions to go to 25% then 0% over the next 15 minutes. Apparently she forgot, and when I came down to check things 30 minutes later, the stove was a very warm 575*F with the cat at 1170*F. Not crazy hot, by any means, but the big load was just starting to really take off, more than I'd like to see. I figured another 20 minutes in that mode, and we could have had a glowing stove.

I started to throttle down from 50% air to 0%, and quickly became aware of how useful a cat probe is. I was able to play the game between lowering the air and watching the cat probe temp shoot up with each change, keeping the cat probe temp below 1400*F while still slowing the fire as quickly as possible.

We're now cruising at 0% primary air control, stove top at 500*F and falling, and cat holding roughly 1000*F. Room temp came up from 67*F to 72*F very quickly with this monster stove at 575*F. Now we're holding steady.

Lessons learned:

1. Put more small splits on top of the kindling when starting a top-down fire. While my stove top got quickly up to 450*F, because the fire was started literally right under the top, I did not have enough small ammo loaded to get things going to the point where I could engage my cat. Throwing in a third, medium-size split, got things going better.

2. When your stove is going way stronger than you'd like, perhaps on the verge of a run-away, resist the urge to shut down the air too quickly, lest your secondary system go completely nuclear. Work it down in very small increments. (Actually, I already knew that, but this reinforced the concept)

3. If you're not running with a cat temp probe, for goodness sake... get one! If nothing else, it will make you understand how your stove works so much better.

 

[clemsonfor]

When i load my stove all the way up, my CAT probe will cruise around 1500-1700F.

These shoulder fires i have had the last 2 days with say 3 medium to large pine splits and a piece or 2 of smaller oak, i can run 12 hours on them keep about 1300sqft @ 66F at the lowest area, and be cruising at 1200F max. The more wood i put in the higher the CAT temp goes it seems even when set at the same air setting. Which only makes more sense, there is more wood to offgas.

My cats seem to be toward the end of their life. I noticed when cleaning them this season there were many hair line cracks in them. Also at the end of last season and the smoke over the last few days from the chimney has been noticable with the cats engaged. Its way reducing it but i still get (under CAts engaged) maybe 5-15% of the smoke that i would get running on bypass. Not sure what it will do if im at 1500F as i have not been that hot yet this season?But at these lower CAT temps i get slight smoke from the chimney.

 

[Ashful]

When i load my stove all the way up, my CAT probe will cruise around 1500-1700F.

I'm no expert, but that sounds awful hot, by everything I've read. My manual puts the operating temp for the Jotul cat's at 500 - 1400*F typical, with a damage threshold of 1800*F. I've seen similar numbers reported elsewhere.

 

[jharkin]

I'm no expert, but that sounds awful hot, by everything I've read. My manual puts the operating temp for the Jotul cat's at 500 - 1400*F typical, with a damage threshold of 1800*F. I've seen similar numbers reported elsewhere.

On full loads I will those high temps often as well. The VC manual doesn't mention temps but Condar lists 1000-1600 as typical operating range and 1700 as max. I have gone over 1700 by accident with full loads of splits taht are a bit too small. When it hapens I go on bypass to let it cool off. Usually when the cat temp drops to 800 thats a sign the load is down to coals and its reload time.

 

[jharkin]

Alrighty then - first fire of the year for me!!

Ok, Joful, I just did my first cold start and timed things to give you some comparison numbers if it helps.. One other point I forgot that's different on my end is the VC is a thermostat controlled stove - so even with the air wide open it will dial back the air somewhat as it heats up.

The wood here is VERY dry. What I'm burning now is leftovers of last season so its all CSS 2-3 full years. A lot of oak. Its also been inside in my woodbox since before the storm.

I have the condar probe that doesn't read below 500 so I cant really tell how fast the cat chamber heats.

Bypass open, air 100%
11:05 am - build a modified top down fire, 2 med splits with paper, 3 decks of crisscrossed kindling, then one med split. Leave door cracked and light paper.
11:10 - Fire established, close door
11:30 - Griddle temp hit 500F and stabilized, cat cool
11:40 - Kicked down the fire and add one more split, cat still reads cool
11:50 - New load is going, griddle at 475F, cat still reads cool
I figure this is long enough so close bypass, air still at full
11:55 - after3-4 minutes cat prob reads 500 and starts climbing fast
Turn down air to 80%
12:00 - Cat probe 1100
Turn down air to 50%
12:10 - cat probe 1250
Turn down air to 25%

20 minutes later I'm cruising at a nice low simmer on 4 splits, air is about 20%, griddle 400F, cat probe 1000F. Its 49F outside and 72F inside.

If I had put this much wood on an established coal bed it would run a bit hotter.

 

[Ashful]

Thanks for posting! Yep, those are similar to what I've seen in the last few fires. The first was started in a hurricane, with water-sprayed wood and unusual draft conditions. Last night, top down fire (so kindling was closer to stove top), I had stove top at 500*F within a few minutes of starting. Like you, cat stayed cold (100*F) until engaged, also close to the 45 minute mark.

 

[clemsonfor]

On full loads I will those high temps often as well. The VC manual doesn't mention temps but Condar lists 1000-1600 as typical operating range and 1700 as max. I have gone over 1700 by accident with full loads of splits taht are a bit too small. When it hapens I go on bypass to let it cool off. Usually when the cat temp drops to 800 thats a sign the load is down to coals and its reload time.

What i have read to. Operating range up to say 1500 with damage occuring over 1700, which may explain my cracks.

There is no way to keep my cat below 1400 while engaged??

 

[mellow]

Mine climbs to 1700 as well on a full load. I usually close the bypass at 600-800 according to the cat prob and turn down the air in 25% increments.