Achieving lift-off in the downdraft cat

[Ashful]

So, I've been up late the last few evenings, trying to get that evening load to light-off. I've been starting from a cold stove most evenings, and probably rushing things a bit. However, I can have the stove top at 550F and the outside of the single wall pipe at 600F, close bypass damper, and watch the cat go to only 550F. If I start closing the air down (incrementally, over however long a period I desire), the cat temp will fall back into the 400's.

It seems I must get the cat up above 700F, to get it to stay at a reasonable temp, when I lower the air control. I can eventually do this, but it means either running the stove in bypass an hour with air closed down pretty far to prevent overheating the flue, or engaging the cat and just running with the air wide open until the cat temps finally start to climb out of the 500's.

Frustrating, but I'm guessing my wood supply is the issue. I'm going to have to resplit and check a few pieces this weekend. I'm burning Sandy walnut and ash, split last Thanksgiving.

 

[begreen]

metal or ceramic cat?

 

[Ashful]

Condar steel cats in both stoves. Both new at the beginning of last season.

 

[Ashful]

I had the stove with the taller chimney stalled around 500F, with the air wide open for a long time, when I decided to go back to bypass. I came back 4 minutes later, and stovetop had shot up to 600+ with outside of single wall at 800+. Snapped the bypass shut and cat shot right to 1100F.

Then the trouble was the cat was shooting into the 1700's five minutes later, but that's common with walnut, it seems.

 

[begreen]

Condar steel cats in both stoves. Both new at the beginning of last season.

Given your unique dual, F12 setup it would be interesting to have a ceramic cat in one of these stoves for comparison. I suspect it would light off faster and at a lower temp but could be wrong.

 

[Ashful]

I'm also probably rushing my cold starts. I've been outside working until 10pm every night, and I get up at 6:30am, so I've been basically firing the stove cold each evening when I get in, and trying to get her cruising by 11pm (failed again!). Rather than running a small load thru, to get her heated up, I've been building a fire with small stuff on bottom, and the big stuff right on that, to get a one-shot take-off. I think it should work with dry wood, but maybe it just can't be done.

 

[Ashful]

Given your unique dual, F12 setup it would be interesting to have a ceramic cat in one of these stoves for comparison. I suspect it would light off faster and at a lower temp but could be wrong.

I had ceramic cat's in both of them, but both were very old. Comparison is tough, though, with the difference in chimney heights and vintage. The newer stove on the taller chimney usually drafts so much better than the old one on the short chimney.

 

[Ashful]

11:46pm, and both stoves are finally cruising steady. The overtemp cat is trending back down. I've got a 4 year old that wakes me once each night, and the alarm clock at 6:30am... better get to bed! Half hour earlier than the last two nights. Good night, begreen!

 

[begreen]

Pleasant dreams.

 

[fire_man]

I've noticed it's much easier to get lightoff when you have a really good hot coal base. Bypassing even when the stovetop is hot does not lightoff at reasonable temps unless there are a ton of nice juicy hot coals.

 

[jharkin]

You are not alone Joful and its not just the Jotul (say that 3 times fast ), I think its downdraft stoves in general - I have the same issue with My Enocore/Condar steelcat. A small load will lightoff nice and easy and on the first load if its only a few splits I can get the cat to light as soon as I have the griddle up to 500 even if the cat probe is not reading hot yet.

If I pack it full OTOH I need to get the griddle up over 600 sometimes and let it burn in bypass longer before closing down to get the cat lit. I think there is just a lot more surface area burning off moisture that overwhelms the cat if you try to shut down early on a big load.

And this is all with 2-3 year wood that's mostly 15-18%. I rarely try a full load without first burning a small 3 split cycle to build a coal base.

 

[mellow]

Hot coal bed seems to be the trick with the Harmons as well.

Just as a thought, could you rig up some type of insulated container to hold the leftover hot coals in the morning (like you have time to sift through coals). I was thinking a super insulated ceramic pot.

You take those coals out at night and reuse them to have a good starter coal bed?

 

[Todd]

Sounds frustrating. How do you load your splits? Maybe a more cross hatching load would work better for these shoulder season fires? I can have some problems as well this time of year if I pack the stove too tightly.

 

[Ashful]

I'm glad to hear the experience of you guys agrees with my own. If I load just a few splits, it lights off pretty easy, definitely around 500F. It's usually the big loads that cause the most trouble.

I can keep enough of a coal bed for an easy restart for about 24 hours, but that's a weak coal bed, probably not hot enough to be a big factor in cat light-off. Unfortunately, I'm only burning 1 load per night in this weather, so that's just what I'm dealing with. If I were spending more time in the house, I'd run a small load at 8:30pm, in preparation for stuffing the stove at 10pm. But, I'm out in the shop (or on a ladder working windows) each night until 10pm, and trying to get showered and into bed by 11pm, leaving little time to dicker around with stoves!

I guess I'm just going to have to either run smaller loads, or find some way to pre-heat with a small load (the way the manual instructs), getting a nice coal bed prior to stuffing the stove full.

 

[mellow]

Ok, this is out there, but I know you are handy. What if you built an over sized charcoal chimney and burned some wood in it before going inside, it wouldn't take that long to get going with some starters under it, that way you can have some hot coals to dump in the wood stove when you go inside.

 

[jharkin]

Take it one step further and he could keep a bag of hardwood lump charcoal by the hearth and just toss some in before ligthing up. Instant takeoff!


/JK

Joful, you are not alone in your frustrations.... Nice to know I m not alone in mine

 

[pgmr]

I don't have a downdraft stove, but I do use a technique to help get the cat to temp a little faster. Once the wood has started to burn well, I close the bypass damper about 1/2 to 3/4 of the way. Stove still has a good draw, but the partial restriction causes more of the exhaust (and heat) to go through the cat. Once the probe reaches 500F I shut the damper all the way.

Might be worth a shot to see if you can force some of the gases through the cat to get it up to the light off temp quicker.

 

[Ashful]

I like that idea! Mine is designed to be fully shut and locked with a cam, or it falls fully open, so that might take some figuring.

Lit both stoves with smaller loads around 6pm today, and both cat's took off without much issue, despite warm temps (mid-50's) and weak draft. Jeremy must be onto something with regard to load size.

I reloaded both when I came back in at 10pm, with a medium load (three medium splits and 4-5 smaller splits), but my coal beds were weak (first load was too small for four hours). Took both stoves up to 500-550F stovetop, engaged the cat, and both peaked around 550 - 600 on the cat probe. As soon as I started to lower primary air (after 5 minutes, small increments), the cat temps began to fall.

I went back to bypass on both, let each build up some fire for about 3 minutes, re-closed the bypass, and both took off. I'm now cruising with cat's at 800 - 1000F, and stovetops at 500F and falling. It seems that's a typical routine here:

1. Heat stove to 550F.
2. Engage cat, and watch it stall at 550F.
3. Try lowering air control, and watch cat fall below 500F.
4. Go back to bypass for a few minutes, get stove above 550F.
5. Engage cat a second time, and watch it take off.
6. Lower air control, and watch cat do it's thing.

 

[pgmr]

I know that magnets can be demagnetized by high heat, but since your stove is not that hot when first starting up, you could try a large magnet to hold the damper control in a partially open position. The one that first comes to my mind is a welding magnet like this:

 

[Backwoods Savage]

Given your unique dual, F12 setup it would be interesting to have a ceramic cat in one of these stoves for comparison. I suspect it would light off faster and at a lower temp but could be wrong.

In our situation we found the steel cats will light off at a lower temperature than the ceramic.

 

[Ashful]

Okay... I had a rare night off from major renovation work tonight (new contractor started today... the push to get everything prepared for him is done), so I had the rare chance to experiment. I pulled out my trusty old Firelight 12 manual (new concept... RTFM), and followed it to the "T". I'm happy to say that both cat's lit off in zero time flat, after loading the final large load of fuel. However, I'm sad to say that the entire warm-up and load procedure for two stoves (in parallel, other than the actual loading) took 1.5 hours.

So, what was I missing? One very simple step: engage the catalyst on a smaller load first, to get it pre-heated and lit-off, prior to loading the big fuel.

The Jotul published procedure:

1. Load kindling, and get 'er lit. [9:30pm]
2. Load 3 - 4 small/medium splits, with ample space between splits. Allow to burn several minutes before closing and latching doors.
3. Allow this small load to burn vigorously (5 - 10 minutes), then close bypass damper. [I did this when stove top hit 500F]
4. Leave air control in full-open position, and allow this load to burn down to a nice charcoal bed.
5. Load the big fuel. [10:30pm]
6. Allow stove to operate at WOT for 5 - 15 minutes [I let it go until stove top hit 550F, under 10 minutes]
7. Close bypass damper. It will take 5 to 10 additional minutes for the catalyst to reach a good operating temperature [minute took about 30 seconds... SteelCat!]
8. Set air control for desired heat output level.

Phew...

 

[Seasoned Oak]

Holy heat loss batman,260 million BTUs a year?. You need a nuclear reactor.

 

[Todd]

How much kindling do you use at start up? Maybe a 1/2 to 3/4 filled fire box of 1-2" diameter kindling would do the trick and take less time? I like to use a lot of kindling and burn it hot to get that chimney warmed up, then pack the fire box full of splits and usually have her dialed in in about 30-45 min.

 

[Ashful]

Yeah, Randy... big old stone house, with more than 50 windows and 8 exterior doors. Of those windows, 26 are original to 1773, and the walls are plaster on stone (no insulation). The house is also arranged in a way that is very inefficient, a "U" shape floor plan, with a lot of exterior surface area per volume. The heat loss calc was actually based on my oil usage x boiler efficiency, and I'm sure I'm losing more now, as we keep the house much warmer with wood than we ever did on oil.

Todd, I use a good handful of kindling, but not much more. What I was really missing is the step immediately following the kindling, when I put on a few smaller (2" x 3" type size) splits... I wasn't engaging the catalyst at this point, to get that assembly warmed, prior to loading the big fuel. This is surely why I had already found the engage / disengage / re-engage behavior worked.

Now, this morning, I rushed things perhaps too much on the stove with the taller chimney. I'm able to burn that stove slower (no back-puffing), and came down to a very lush coal bed on that stove. I threw five big splits in, and it took off like hell. I was back up to 550F stovetop in less than 10 minutes, so I engaged the cat, and she took off immediately to 1100F. I worked the air control down to 1/2 after just a few minutes, then 1/4, and then closed, on 5 minute intervals. The cat was running maybe 1300F, and life was good... until I went back and checked the cat probe a half hour later, and found it had dropped to 432F! Clearly, I rushed things. I was due to leave for work, so I just had to leave it that way.

 

[jharkin]

The cat was running maybe 1300F, and life was good... until I went back and checked the cat probe a half hour later, and found it had dropped to 432F!

Thats really wierd... Of all the strange behavior Ive experienced Ive never had mine stall out once it had already lit off to 1000+