See no evil, hear no evil, smell no evil. Sometimes your senses will tell you that your heating system is feeling under the weather, even if you're staying comfortable. There are some weird signs a heating system may exhibit when it needs attention right away. Symptom 1) Signs of leakage around the furnace. You should never see any drainage, or anything wet around your furnace. Sometimes the culprit is an overhead pipe or drain that's leaking.