branchburner said:
RenovationGeorge said:
pgmr said:
My supposition is that it is the reduced volume of air leaving the cat stove that allows it to cool off more quickly. But, I've been wrong before and welcome other ideas.
That's my guess too. I'm guessing that less air flow is required for a cat burn than for thermal secondaries, and that increases the exhaust gas's dwell time in the stove, and lets it give up more of its heat and cool more before going up the chimney.
This makes a lot of sense. I would imagine to meet EPA numbers it is common for a non-cat stove to be designed with a much greater volume of non-adjustable incoming secondary air. You can't dial it down like a cat stove. Since the EPA goal is lower emissions, not lower heat loss, some efficiency is certainly sacrificed by design in the (overly?) leaky non-cats.
A non cat stove accomplishes the reburn solely by keeping the firebox temperatures above the natural unaided ignition temperature of smoke (Ive seen 1200F quoted). To maintain high heat requires burning up the fuel faster. Burning fuel faster means consuming more air. By EPA regs non-cat stoves are supposed to be designed so that you are not able to burn them low enough to kill off the secondaries completely so they have to allow a lot of air.
The catalyst allows secondary combustion to occur at much lower ignition temps (500F). Lower teams mean less fuel burned so you need less air. So the designers can get away with a lower minimum air input.
The result of this is that for practical purposes cat stoves can remain burning - and burning cleanly - at lower temperatures than non-cat. This is what makes cat stoves so useful in the shoulder season and in mild areas. When burning at high heat this advantage goes away and non-cat stoves are as efficient or maybe even more if designed right.
As an illustration I found an old thread someplace that linked to a pdf presentation on catalytic combustion made by the folks at blaze king. If I can remember where it was I will post the link... One part of it was a study of two 40k BTU stoves, on cat, the other non cat. They tested them at 4 burn rates. The result:
At low and medium burn, the cat stove produces as little as 1/4 the particulates of the non-cat and was much more efficient.
At high the stoves were equal.
At max burn the cat stove actually made slightly more particulates.
Graphed out it shows that the higher published efficiencies of cat stoves are achieved at low/medium burn rates. At max burn most EPA stoves are fairly close in efficiency regardless of technology.