I currently have a fireplace (HEATNGLO RSW-41) covered under UL-127. The Jotul woodstove I want to install says if I use an existing prefab chimney it has to be for type HT(2100 deg. F) and covered under UL-103. The fireplace mfr says their prefab chimney is type HT and rated to 2100. What's the difference between the two and does UL-103 have stricter guidelines than UL-127? Thanks!!