For the heat test, I still remembered that NYU taught us to use a torch to heat a gutta-percha. It works, but it's intimidating to see a torched gutta-percha immediately place into someone's mouth. Luckily, I decided that our office must have a water dispenser that can dispense hot drinking water, and the new water dispenser arrived less than a month ago. For that reason, I was able to perform the heat test by soaking a cotton pellet in hot water and then pressing it against the tooth #27. Worked like a charm.
This is a typical case of necrotic pulp. The rationale is that dead issue creates gas pressure, and heat increases the inner pressure (causing pain) while cold decreases the inner pressure (relieves pain).
So, really, the best option is Root Canal Therapy. I found that Dentsply's Protaper Gold and GuttaCore work well, cleanly and efficiently in simple endo cases such as this one. The x-ray indicated a well-done root canal therapy.
After the root canal therapy, patient had soreness on chewing which resolved in several days but had no symptom on hot drinks anymore.