You Techies: Secondary Air Question

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logbutcher

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Big payday on this: the VC cat Encore stove uses a thermostat-type coil to control a secondary air flap. The secondary air adds air to the primary air for more efficient reburning. I'm doing some major maintenance on the beast.
Now:
1. Should the secondary air flap be closed when the burn is hot, then opened when the primary air is shut low for a longer, cooler burn ?
or
2. The reverse: secondary air more open in hot burns,and closed in cooler, less primary air fires?

My take: the coil expands when hot, dropping the secondary air flap= less air. Do I win the bottle ?:monkey: Or............:confused:
 
I've not seen a design where the secondary air adds to the primary air. Every design I've seen the secondary air enters the firebox via secondary air tubes.

What you are describing sounds like a system set up so when the stove is started and gets hot that primary air is automatically reduced with a bi-metal coil in an attempt to keep the stove a more constant temperature and from running away as a result of having it's primary air wide open.

Just random musing as I'm not familiar with your stove. It does seem ashamed that more stoves don't use bi-metal coils to regulate primary air input after they get up and running. My 30 year old Riteway Model 37 did and it worked well.

I think it would also be nice if wood stoves could incorperate some type of system where after the the stove had burnt most of it's load.....that primary air would automatically be reduced so that hours later that enough coals would be left to start up another fire easily. I've never seen this on a stove and of course most air controls are manual so the air stays at a high setting and burns out the coals. A bi-metal coil like is on my Riteway opens the air up so the coals burn out even faster in an attempt to keep the stove temperature up. I'd call it the 'coal saver/next fire starter' design.

That's it TreeCo, the original Vermont Casting engineering patented a bi-metallic 'thermostatic coil for primary air along with a "flap" for regulating secondary air way back in the glorious 70's of the wood stove revolution. Their later cat stoves also used the bi-metallic coil for regulating secondary air in slower burns to add O2 to the gases. It did work to even out the spikes in the burn cycle. We've had many of the VC stoves since the 70's...they were cult-like with yearly parties in Vermont. Excellent marketing and quality builds....then, until the CFM buy-out around '95.

In the rebuilding of this 10 year Encore my brain lost it.:angry: The secondary air coil closes down when cold.

The VC cat stoves are super machines ( and SWMBO needs the aesthetics of red porcelain :heart:) when everything is running correctly. If you look at a schematic of the Encore or Defiant the complexity is well, complex. Then, cast iron deforms over time when the stove is used 24/7 from November through March making any repairs or part replacement exciting: grinding, heating, re-tapping; choice semantics and good Laphroaig
assist the process. :blob2:

Now if only there were nice looking, classic all-steel cat stoves. The noncats are really mommie jobs controlling the burn by not allowing the primary air to be shut way down, or up high. That's the other stove burning: Jotul Oslo.
 
Got a 2550 cat new last year. The secondary air is separate from the primary air. After the combustor is engaged and burning, the secondary bi-metal coil attempts to CLOSE the secondary intake the hotter it gets in the refractory box. The shutter will only close so much though, because a limiter peg keeps the pivoting shutter open a minimum amount. If it were opposite that(IE, the shutter OPENS the hotter the refractory gets), you'd get positive feedback and thermal runaway would cause overheating.
The secondary intake blows fresh air forward under the refractory box, up the backside of the lower fireback, under the hood and into the cat with the smoke.
Then the new air and smoke curve downward into the combustor, move downward through the cat and burn in the lower 2/3 of the refractory box.
Next time you have the lower fireback removed, shine a flashlight into the secondary intake to see this route.
We're still trying to locate where this 2550's neurons are. It sometimes has a mind of its own.
The primary and secondary bi-metal coils are not mechanically connected. Only the temperature in the refractory box adjusts the opening of the secondary shutter.
 
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Got a 2550 cat new last year. The secondary air is separate from the primary air. After the combustor is engaged and burning, the secondary bi-metal coil attempts to CLOSE the secondary intake the hotter it gets in the refractory box. The shutter will only close so much though, because a limiter peg keeps the pivoting shutter open a minimum amount. If it were opposite that(IE, the shutter OPENS the hotter the refractory gets), you'd get positive feedback and thermal runaway would cause overheating.
The secondary intake blows fresh air forward under the refractory box, up the backside of the lower fireback, under the hood and into the cat with the smoke.
Then the new air and smoke curve downward into the combustor, move downward through the cat and burn in the lower 2/3 of the refractory box.
Next time you have the lower fireback removed, shine a flashlight into the secondary intake to see this route.
We're still trying to locate where this 2550's neurons are. It sometimes has a mind of its own.
The primary and secondary bi-metal coils are not mechanically connected. Only the temperature in the refractory box adjusts the opening of the secondary shutter.

Pretty much what I thot before re-reading the tech Service Manual (SM). BTW we're on our 3rd cat Encore since it was first made by the real Vermont Castings in '89.
In the SM for dealers is says to install the secondary air in the closed position when cold--with the 'limiting peg' leaving a slight opening. It didn't make sense: more secondary air when hotter, less air when cold. So, for once, I followed the engineer's instructions. We'll see when I get the whole beast back together.

Sud Chemie OEM's the combustors for Condor, Rutland, and others they say. I got the newer all-steel cat this time since the average cat lasts ~12,000 hours of real burn time. The steel replaces the ceramic honeycomb . It's supposed to "light off" at a lower temp, and not mechanically deteriorate like ceramic.
We shall see. :dizzy:
 
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