Controlling cat temp

[arnermd]

Hello all and Happy Fall....

Most of you are familiar with my years long struggle to control cat temps. I may have found the issue.

• For as long as I can remember I have noticed that my air flapper seems to not consistently close all the way, with the lowest air setting sometimes I could reach behind the stove, pull the wire and I would feel and hear it snap closed. Just a light metal to metal click, ever so slight movement.
• I decided to investigate further this summer and managed to get the air flapper assy out without tearing the whole stove apart. It was not easy and took some patience and finagling but I got it out. The location makes it hard to get to and uncomfortable.
• I took pictures of the wire routing through the lever to make sure I could put it back the same way it came out.
• I then set it up on my bench at the same angle it would be in the stove and started working the lever open and closed. I was able to get it to hang up occasionally if I very gently set it back to full closed. 2 pics below show it hung up slightly open and forced fully closed. I have video but I don't see a way to post it directly....
• I found the interference was one corner of the flapper was touching the frame before it went fully closed. Why? The holes on one side of the hinge are oversized and not drilled in the right spot allowing the flapper to hang too low on one side and thus make contact with the frame. Sorry I forgot to take good pics of this....
• My fix was to bush the oversized hole with some aluminum shim I formed and then file the spot where it was hitting. I then discovered the flapper door was not sitting flat against the frame all the way around. So a lot more filing and lapping was needed. I got it good enough so I could just see a tiny sliver of light through the seat with the flapper fully closed.
• Reinstallation was difficult, the lever has to be in just the right orientation and there is a casting web that prevents just sliding it back in. With the help of a borescope I was able to get the lever past the web (without bending it) and set everything back in place. Also installed a new gasket around the frame.
• Threading the wire back in was a challenge as it needs to go in from the inside and then make a 90 degree turn under the cap screw that pinches it in place.
• Pretty sure it was not designed to be removed from the outside, which seems silly to me, they could have made it a lot easier with some minor mods to the castings. Designing this thinking that you need to tear down the whole stove to replace the air flapper seems like an oversight to me.

Bottom line: In my opinion this was a mfg defect that should have been caught at the factory, it has been this way since I bought it new which might explain why I have had this problem all along....

So.... stove is fully assembled now and ready to go. Fingers crossed this will solve my issues.....

Flapper hung up partially open

Flapper fully closed

Flapper assy removed

Cable routing

 

[AsylumResident]

@arnermd Before you are able to completely put this to practice, if we think this out, will this adjustment solve the problem?

I know you have experimented with blocking off the 'epa' hole(s) and other areas that draw in secondary air. I know that gave inconsistent results and also increases potential of creosote build up. I know we are not able to accurately calculate the volume of air being supplied to the combustion during the various alterations. But would blocking the primary air more securely give us better control of cat temperature? Would it have much of an effect to stop the cat from running away on us since this method doesn't address the uncontrolled secondary air intake?

If this were to solve the problem, by what mechanism would it solve the problem? I'm just asking the questions. Believe me, I want this to solve the problem. I hope this does fix our issues. I know we discussed a potential of primary air intake somehow leaking additional air into the secondary causing the runaways. If that is the case, this could reduce excess air in the secondary but I would think plugging the 'epa' hole(s) would give a similar result since that would be reducing the volume of air being draw in.

 

[arnermd]

@arnermd Before you are able to completely put this to practice, if we think this out, will this adjustment solve the problem?

I know you have experimented with blocking off the 'epa' hole(s) and other areas that draw in secondary air. I know that gave inconsistent results and also increases potential of creosote build up. I know we are not able to accurately calculate the volume of air being supplied to the combustion during the various alterations. But would blocking the primary air more securely give us better control of cat temperature? Would it have much of an effect to stop the cat from running away on us since this method doesn't address the uncontrolled secondary air intake?

If this were to solve the problem, by what mechanism would it solve the problem? I'm just asking the questions. Believe me, I want this to solve the problem. I hope this does fix our issues. I know we discussed a potential of primary air intake somehow leaking additional air into the secondary causing the runaways. If that is the case, this could reduce excess air in the secondary but I would think plugging the 'epa' hole(s) would give a similar result since that would be reducing the volume of air being draw in.

Amazing to me how much critical thinking goes on in this forum. I can always count on this community to ask the hard questions and challenge all assumptions.....

Let me start by saying I do not know if this will fix the issue, it may be Jan till I know for sure, as I generally do not have this problem until we get into the 24x7 burn modes.... (Maybe colder outside temps with increased draft increases the leakage to a point where it over-temps?)

I hear what you are saying and that was my thought for years, if I block the air somewhere else I should be able to get the right net flow and compensate for the leakage through the air flapper. I think that is still true, but I am wondering if the leakage path through my flapper may have been bigger than the EPA holes. The perimeter of the flapper is pretty big.... roughly 2" x 4" so maybe the leak path was just too big?

With regards to secondary flow... I believe the design intent was to run the cat lean (excess air). Without secondary air control, the way the design "regulates" cat temp is by reducing the amount of fuel (smoke) the cats gets by controlling the primary burn rate via primary air control. Hope that makes sense.... Its also possible that the hotter the secondary burn gets, the more secondary air it can pump to the cat, which will also reduce temps, assuming the cat is running lean. But I suspect this is a second order effect.

Other thoughts:

• This leaky air flapper is the first thing I have found that was clearly "not right" and I believe it has been this way since new. So I am hopeful this solves the problem.... but time will tell.
• You may recall last year I experimented with magnets on the flapper and it seemed to help, maybe not 100% effective but better for sure. While I had the flapper on the bench I installed the magnets as they were last year and it did in fact exert enough force to pull that flapper full closed. Of course it works both ways, so it was also harder to lift the flapper off the seat to increase the air flow. That hysteresis would negatively affect the controllability of the temp.

We shall see, I am cautiously optimistic. But I have been here before....

 

[vaft]

I've followed yours (and others) frustrations with their VC stoves over the years while making note of how similar they are to the difficulties I've had with mine. I ended up taking a drastic solution -- I disabled the air control entirely.

The fact that the air control is attached to a bi-metallic coil was a selling point for me. How nice that it will automatically adjust itself depending on the intensity of the stove's heat output! Fantastic! In retrospect, it was always a source of doubt and uncertainty. Is it opening when I want it closed? Is it closing when I want it open? The mechanism itself also seemed very janky to me: A braided steel wire rubbing across multiple 90 degree turns can easily get snagged or pinched.. The flapper itself only closes due to gravity, (and it's not very heavy) and even a little bit of tension in the wire will prevent it from closing. Out of desperation, I removed the bi-metallic asssembly, wire and all. I 'borrowed' a metal nail file from my wife and used it as a thin shim to just barely keep the flapper slightly open.

My intention was to fabricate my own external automatic air control, but I've never gotten around to it because I've never needed to. Leaving it detached and barely open has worked perfectly **FOR ME**. Nobody else should do it, leave your VC the way it was intended, but in my case, I consider it problem solved. I never need to adjust it, and it dang sure never needs to be open more than a hair -- i cannot imagine the resulting inferno if the flapper were open to it's maximum.

Lastly, and I know this has been mentioned before -- the ash box on these stoves is leaky. It's plain to see how much the flames dance around wildly on an empty ash box, but everything is smooth and stable when it's packed full of ash. I don't even bother emptying it anymore, just shovel the firebox out. Actually, everything about my stove was leaky. The doors, the glass, the stovetop, baffles, there are gaskets all over the place, some are well hidden. I had to replace all of mine before I could get the stove working even remotely correct.

Good luck, you practically have to build your own woodstove in order to get a VC working correctly, but it IS possible.

 

[AsylumResident]

I have thought about the coil. Having the issues I have experienced since I have had the stove, I have thought about doing without it to give me exact control over the primary air, not the illusion of it. Interesting to hear that someone as done just that.

 

[AsylumResident]

@arnermd do you still have the bimetal coil on your primary air installed? Or did you alter that when you went through this process with the flapper?

 

[Woodsplitter67]

Yo.. supp man.. another good read here.. keep up the good work

 

[arnermd]

@arnermd do you still have the bimetal coil on your primary air installed? Or did you alter that when you went through this process with the flapper?

Yes I still have my bimetal installed.

 

[arnermd]

I've followed yours (and others) frustrations with their VC stoves over the years while making note of how similar they are to the difficulties I've had with mine. I ended up taking a drastic solution -- I disabled the air control entirely.

The fact that the air control is attached to a bi-metallic coil was a selling point for me. How nice that it will automatically adjust itself depending on the intensity of the stove's heat output! Fantastic! In retrospect, it was always a source of doubt and uncertainty. Is it opening when I want it closed? Is it closing when I want it open? The mechanism itself also seemed very janky to me: A braided steel wire rubbing across multiple 90 degree turns can easily get snagged or pinched.. The flapper itself only closes due to gravity, (and it's not very heavy) and even a little bit of tension in the wire will prevent it from closing. Out of desperation, I removed the bi-metallic asssembly, wire and all. I 'borrowed' a metal nail file from my wife and used it as a thin shim to just barely keep the flapper slightly open.

My intention was to fabricate my own external automatic air control, but I've never gotten around to it because I've never needed to. Leaving it detached and barely open has worked perfectly **FOR ME**. Nobody else should do it, leave your VC the way it was intended, but in my case, I consider it problem solved. I never need to adjust it, and it dang sure never needs to be open more than a hair -- i cannot imagine the resulting inferno if the flapper were open to it's maximum.

Lastly, and I know this has been mentioned before -- the ash box on these stoves is leaky. It's plain to see how much the flames dance around wildly on an empty ash box, but everything is smooth and stable when it's packed full of ash. I don't even bother emptying it anymore, just shovel the firebox out. Actually, everything about my stove was leaky. The doors, the glass, the stovetop, baffles, there are gaskets all over the place, some are well hidden. I had to replace all of mine before I could get the stove working even remotely correct.

Good luck, you practically have to build your own woodstove in order to get a VC working correctly, but it IS possible.

Interesting to me that a fixed air flapper position is working well for you. Will be curious to see how well it works when it gets cold out and you have increased draft.

One minor correction though... there is no rubbing on the steel cable. It is a fixed length and always under some tension, yes only gravity keeps it in tension. As long as it is in tension there is no relative motion between the flapper arm and the cable. There should also be no rubbing of the cable on any cast iron parts, if there is it is routed wrong. It is hard to visualize how it all works, I had to take my stove apart a couple times to really understand it. Once I figured it out, I think it is really quite clever.

I have many times thought about using an electronic actuator linear actuator to electronically control the air flapper. It should not be necessary but in my case it would provide more force to overcome the friction in my flapper and really seat it closed. Or I could just replace my flapper....

 

[arnermd]

Update on my flapper....

I have been burning off and on the last few weeks and had a few runaway conditions where the stove went to 1450+. But... unlike before when I closed the air down the cat temps eventually came down. Sometimes it took 30 minutes or so....

I also found my flapper is still not going full closed, I was able to tug on the cable and feel it move just a hair, better than it was before, but still not quite perfect. At some point I may replace the whole assembly, hopefully I get a good one when I buy the replacement.

I think there are a couple things going on here that are confounding things....

• Up until this weekend I have been dealing with low draft due to warm out side temps. I noticed during reloads I was getting some smoke in the house, that is a very rare occurrence for me. Since it has gotten colder this problem has gone away.
• I have been running my old ceramic cat and it was a struggle to get it up to 1000F. I suspect this cat is worn out and with the low draft it just needed very high air settings for long periods of time. I believe the increase air lead to an intense primary burn and once I engaged the cat it created a lot of smoke from the residual heat, resulting in the high cat temps.
• I have noticed since I improved the damper, I have much better control over the firing rate with the cat damper open. In the past I could close the air all the way down and the primary burn would be pretty intense. Once I engaged the cat it would calm down. No I notice when I close the air with the damper open I can see the reduction in flame intensity, immediately.
• To help my damper close I put my bar magnets back on the flapper frame and that seems to be effective.
• 2 days ago I decide to give my old steel cat a vinegar bath and throw it in. 3-4 full loads now and it has been burning like a dream. Loading on a nice bed of coals, the cat climbs to 1000 in 20 minutes or so and I can throttle the air back to about halfway. It spikes to 1300F about 1 hour into the burn and then just hangs in there for hours at 1000 - 1200. If this continues I will be very happy.
• I am also not burning ash right now, I am into some very seasoned red oak.

I continue to be hopeful, but still to early to claim victory.

 

[Woodsplitter67]

Dude.. nice job... super happy