Vermont Castings Defiant Cat issues

[xanadu1797]

Hi everyone,
I’ve been trying to figure out the cat on a Vermont Castings Defiant. It behaves like this-
With the cat damper closed (mechanism is broken) the thing will fire but it doesn’t level off and climbs past 1600 where I have the alarm set.
With the cat damper open the cat doesn’t fire at all. Or alternately if it’s ripping up around 1500 and I open the cat damper it usually descends to 600.
I’m guessing an air leak is most likely as I refitted the stove but the griddle temps are reasonable- usually between 300-600 but most often 400-500 during a run.
How do I tame this cat?

 

[begreen]

Which model Defiant?
(mechanism is broken) - What is broken and how is it being operated?
How old is the cat?

 

[xanadu1797]

I think it’s a 1945. It has the cat chamber that’s made out of that fiber material as a box- not ceramic brick.
There is a mechanism that is meant to open and close draft directly to the cat which operates like the stick on thermometers do- a little coil of metal is attached to a wire that opens and closes the door. Closed when hot, open when cold.
The cat is new-ish. Hasn’t been in operation for but a season or two.

 

[begreen]

The age of the cat rules out new cat hyperactivity. What is broken?

Has the condition of the refractory package (the engine) been checked.

 

[xanadu1797]

Yeah I replaced all those parts. The box the cat sits in is new.
The damper mechanism for the cat doesn’t work. I didn’t replace that because I saw multiple members here recommended closing it with a magnet and being done with it.

 

[begreen]

Do you mean the thermostatic air control? I thought the magnet was to stop it from tapping when in the neutral position and to ensure solid closure, but not to disable it. I that is closed, then it sounds like excess air is getting into the fire. Check the ashpan and door gaskets first.