Hi, welcome to the forum, it can be variant of Cholinergic urticaria wherein itching (no rash or hives) is brought on by a hypersensitive reaction to body heat. Symptoms follow any stimulus to sweat such as exercise (sometimes called exercise-induced urticaria), heat from the sun (which could also indicate solar urticaria), saunas, hot showers (reaction to water can also indicate water urticaria), spicy foods which may cause an increase in body temperature or even stress due to blushing or anger. Some people only have symptoms during the winter months where their body temperature rises when it is unacclimatized to heat.
The itching typically develops within a few minutes of a rise in body temperature. Cholinergic urticaria can be very difficult to treat. Most treatment plans for cholinergic urticaria involve being aware of one's triggers.
Drug treatment is typically in the form of antihistamines, H2-receptor antagonists. As the disease may be physiological in origin, psychological treatments such as stress management can sometimes lessen severity and occurrence. I suggest you to consult allergy specialist. Take care and regards.
Yes it is possible that you are allergic to the Mercaptan. I am!
http://allergy-book.blogspot.com/2007/11/is-your-house-gassing-you.html
It could be the dust that is getting stirred up by the furnace. The dust settles all summer in the ducts, furnace gets turned on --> all dust is airborne now.