This morning I just received a feedback from other buyer of burner.
There must not be technical problem of burner.
In your case I guess that it is due to lack of lining on top of burning element, which creates a by-pass of air between wall of burner and burning element
Just for the purpose of your confidence on my burner,
From: Nick Enslinger <firstname.lastname@example.org>
To: zhaohui wang <email@example.com>
Sent: Monday, July 30, 2012 5:44 AM
Subject: Wolf controller
I have been using the pellet burner and Wolf controller you made for me. It works very well , but I do have 1 question. On the Wolf controller, is there a way to set the minimum fan speed? The problem I am seeing is that on manual the lowest few settings the fan doesn't spin. The fan doesn't start turning until I move it up a few settings. This affects the auto function as well since the temp has to drop several degrees before the fan kicks on. Thanks!
( comment by manufacturer: leave the controller work its own way, do not disturb, after 20 minutes, temperature will be stable)