Cold hearth syndrome happens when the dangerous gases that are supposed to be released through the chimney are redirected back to your house. Several factors lead to this problem.   One is when the temperature of the flue is too low and placed below the neutral pressure plane of the house. The second is when the chimney flue is not insulated properly. Here are the signs to know if your fireplace has a cold hearth syndrome:   ●  Cold air from the fireplace ●  Smoke goes into the room instead of the chimney ●  Foul odor coming from your fireplace   If you suspect that your fireplace has a cold hearth syndrome, it’s best to have fireplace professionals check it before you use it and cause accidents.