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....