I have a Harman Advance Pellet Stove that I've been using to heat my cape style home for 2 yrs. The only time I use my furnace/oil system is when I leave the house for more than a 3 day period. I clean it out every week. I use the Room Temp /auto mode set to 75 and the feed set to 3.5-4, depending on the temp outside. It's worked perfectly until 6 weeks ago, when I noticed a few issues going on: the temp in the house drops and it doesn't consistently go on automatically, the flame is much larger now-reaching up to the top elements there, the glass used to take the full week to get blackened around the edges-but now it gets that way within a couple hours. I had my son check this out and he thought it wasn't venting properly, so went outside and dismantled the pipes. He found that soot had almost entirely closed the vent pipe at the top of the elbow. Cleared this out and the stove has been a little better now. Still doesn't operate automatically and the glass still accumulates more grey film and black than it did before having this issue. My question is this: What caused the soot to collect in the venting pipes to begin with? The installation was done to Harman recommendation. My set up looks like this: I have a straight pipe coming out with one elbow (down low). This goes through the Harman part that goes through the house. One elbow and straight up above roofline outside. Worried that this issue will repeat itself if I don't find out what caused it. Thanks!