I should clarify first the meaning of the term "Heat". The term "Heat" has two meanings: in a broad sense, it is a general term for any pattern of Heat, external, internal, latent and including "Fire". In a narrow sense, it refers to "Heat" as opposed to "Fire".
In order to differentiate Heat and Fire, I would like to refer first to the patterns of the 6 Stages and 4 Levels. The patterns of the 6 Stages (Tai Yang, Yang Ming, Shao Yang, Tai Yin, Shao Yin and Jue Yin) were formulated by Zhang Zhong Jing in his Shang Han Lun (approximately 200 AD). The patterns of the 4 Levels were formulated by Ye Tian Shi in his Wen Bing Lun (1746 AD).
Within the 6 Stages, at the Yang Ming stage there are two separate patterns: Yang Ming Channel and Yang Ming Organ. Within the 4 Levels, at the Qi Level, there are two patterns: Stomach-Heat and Intestines Dry Heat. These patterns are basically identical and the formulae used are the same, so that Yang Ming Channel and Yang Ming Organ of the 6 Stages are the same as Stomach-Heat and Intestines Dry Heat (of the 4 Levels) respectively.
6 STAGES 4 LEVELS
Yang Ming Channel = Stomach-Heat
Yang Ming Organ = Intestines Dry Heat
Yang Ming Channel and Stomach-Heat pertain to "Heat"; Yang Ming Organ and Intestines Dry Heat pertain to "Fire".
The symptoms of Yang Ming Channel pattern (and of Stomach-Heat) are: high fever that is worse in the afternoon, feeling of heat, intense thirst, profuse sweating, red tongue with yellow coating, Overflowing-Rapid pulse.
The symptoms of Yang Ming Organ pattern (and of Intestines Dry Heat) are: high fever that is higher in the afternoon, constipation, dry stools, burning in the anus, abdominal fullness and pain, irritability, delirium, red tongue with thick-yellow-dry coating, Deep-Full-Rapid pulse.
The Stomach-Heat pattern at the Qi Level (and the Yang Ming Channel pattern) is treated by pungent-cold herbs (such as Shi Gao Gypsum) which clear Heat because the Heat is still superficial enough to be expelled outwards towards the surface with pungent herbs. The formula for this pattern is Bai Hu Tang White Tiger Decoction which relies on Shi Gao to clear Heat by expelling it outwards.
The Intestines Dry-Heat pattern of the Qi Level (and the Yang Ming Organ pattern) is treated by bitter-cold herbs (such as Da Huang Radix et Rhizoma Rhei) which drain Fire by moving downwards and stimulating the bowel movement. In the case of Fire, this is too deep in the Interior to be expelled by pungent herbs. The formula for this pattern is Tiao Wei Cheng Qi Tang Regulating the Stomach and Conducting Qi Decoction that relies on Da Huang to drain Fire downwards (through the bowel movement).
I have presented these two patterns from the 6 Stages and 4 Levels only to highlight the difference between "Heat" and "Fire": the patterns of Stomach-Heat of the 4 Levels and Yang Ming Channel of the 6 Stages are an example of "Heat"; the patterns of Intestines Dry Heat of the 4 Levels and Yang Ming Organ of the 6 Stages are an example of "Fire".
Let us now therefore further explore the difference between Heat and Fire. First of all, they obviously have many common characteristics: both Heat and Fire pertain to "Heat" in a broad sense.
The common clinical manifestations of Heat and Fire are: feeling of heat, thirst, red face, dark urine, Rapid pulse and red tongue with yellow coating. Fire differs from Heat in two important respects: it is simply more intense than Heat and it is at a deeper energetic level. That is why Heat can be expelled outwards with pungent-cold herbs but Fire cannot be: it is too deep to be expelled outwards and it can only be drained downwards with bitter-cold herbs.
Fire has four characteristics that Heat does not have and these are:
1) More intense: pronounced feeling of heat, intense thirst, red face
2) More drying: dark urine, dry stools, dry tongue
3) Affects the Shen more: agitation, mental restlessness, insomnia, manic behaviour
4) May cause bleeding: epistaxis, coughing up blood, vomiting blood, blood in urine or stools.
The tongue is an important and useful distinguishing sign between Heat and Fire. In Heat, the tongue is red with a yellow coating: in Fire, it is a deeper red and it has a very thick and dry, dark coating that can range from dark-yellow to brown to black. The two photos below illustrate this difference (the top being Heat and the bottom Fire).
Please note that, from the point of view of the 4 Levels, both Heat and Fire pertain to the Qi Level: at the Qi Level, the tongue has a coating.
Why is it important to differentiate between Heat and Fire? It is because the treatment of the two is different. As explained above, Heat can be expelled outwards with pungent-cold herbs while Fire can only be eliminated by draining downwards (through the bowel movement) with bitter-cold herbs. Mistakenly draining Fire with bitter-cold in the case of Heat is indeed a mistake and one that may make the patient worse.
Examples of pungent-cold herbs that clear Heat are: Shi Gao Gypsum, Lu Gen Rhizoma Phragmitis, Zhu Ye Folium Phyllostachys nigrae.
Examples of bitter-cold herbs that drain Fire are: Da Huang Radix et Rhizoma Rhei, Long Dan Cao Radix Gentianae, Huang Qin Radix Scutellariae, Huang Lian Radix Coptis.
Examples of formulae that clear Heat are: Bai Hu Tang White Tiger Decoction, Zhu Ye Shi Gao Tang Phyllostachys-Gypsum Decoction,
Examples of formulae that drain Fire are: Tiao Wei Cheng Qi Tang Regulating the Stomach and Conducting Qi Decoction, Long Dan Xie Gan Tang Gentiana Draining the Liver Decoction, Huang Lian Jie Du Tang Coptis Eliminating Toxin Decoction, San Huang Xie Xin Tang Three Yellows Draining the Heart Decoction.
Let us take the Stomach as an example and illustrate the difference between Stomach-Heat and Stomach-Fire.
Slight epigastric pain, thirst, feeling of heat, red tongue with yellow coating. The tongue below illustrates Stomach-Heat.
A representative formula for Stomach-Heat would be Yu Nu Jian Jade Woman Decoction which contains Shi Gao Gypsum.
Epigastric pain, excessive hunger, intense thirst, dry mouth, feeling of heat, mental restlessness, dry stools, deep-red tongue with thick-brown coating. The tongue below illustrates Stomach-Fire.
A representative formula for Stomach-Fire would be Qing Wei San Clearing the Stomach Powder which contains Huang Lian Rhizoma Coptidis.