So when you do this burning cardboard do you bypass the control or turn it up or wait until the water is quit cool?
Mark, When I do my Heatmaster 10KE I pick a warmer day and let the fire die down or crank the heat in the house with little to no wood in the box to cool the water load and let the water cool to like 100deg or so and then load it up with cardboard, as much as I can get my hands on, and then let it rip with the door open. Sometimes I open the bottom ashpan drawer after its going good, and after Ive scraped and emptied it out in advance. Be aware the flames WILL be rolling out the door and upwards a few feet as well as flying pieces of paper if the cardboard is small. This intense fire rolling out the door will burn off the pretty little sticker / placard at the top of the door on the heatmasters so be aware. Your insulated chimney extension if so equipped will also most likely catch fire on the inside and burn intense shooting a small flame out the top also- This is a good thing, meaning its burning up the creosote

Dont just light the thing and walk away if its close to any combustibles etc. Be smart and watch it. I've never had an issue but its good practice to be prepared. Once the water reaches your setpoint (180deg +- ) it'll shut the fan down and the fire will die down if it hasn't already, at which time you can load it full again with wood and close everything back up (ash pan included)
FYI, If you try to do this with the door closed it wont be able to get enough oxygen to the fire and you will a slower burn which will not be intense or hot enough to accomplish the task at hand. The blower just isnt strong enough to deliver the amount of air needed.
You know you got it right, when everything turns white

Yes, Im a poet -LOL
Happy Pyro !!
-Ryan
