Agnyadhana (Samskrit: अग्न्याधानम्) also called as Agnyadheya (अग्न्याधेयम्) is technically the process by which the Agni or sacred fire is newly established in the kundas along with the chanting of mantras as prescribed by Shrauta sutras. It is belongs to the 'isthi' class of yajnas, which means 'a yajna performed by a yajamana and his wife with the help of four priests. Agnyadheya occupies two days, the first day called Upavastha being taken up by the preliminary matters and the second with the main rites. Agnyadheya means the placing of burning coals for the generation of the garhapatya and other fires at a particular time (विशिष्टकाले) and place by a particular person to the accompaniment of certain mantras.
Agnyadheya comprehends the several acts from the bringing of the aranis (two wooden logs) to the offering of purnahuti. When this last takes place the person who engages in the rites attains the position of an Ahitagni (one who consecrated the sacred vedic fires). 
The mantras and the procedural aspects are generally similar across all vedic Shrauta sutras, with a few variations. It is classified as the first of the Haviryajnas according to Gautama Dharmasutras. However, this topic is dealt here as a prerequisite to the Agnihotra yajna. The requisite materials and procedures of Agnyadhana according to different texts may be referred to as given below.
- Kanvashatapatha Brahmana (Ekapat Kanda)
- Madhyandina Shatapatha Brahmana (Ekapat Kanda)
- Katyayana Shrauta Sutras (Adhyaya 4)
- Ashvalayana Shrauta Sutras (Adhyaya 2)
- Apastamba Shrauta Sutras (Prasna 5)
- Taittiriya Brahmana (Kanda 1)
Brief summary of various aspects of Agnyadhana are as given below
संभराः ॥ Sambharas
He who is to perform the installation of the sacred Agnis gathers the requisite materials. He collects water, gold, salts, earth (from a mole hill), pebbles. Each of these have their significance which is explained therein. The place where the fires are to be installed are cleaned and sprinkled with water. Water is verily food, as where water is present it flourishes with food. Gold makes the fire grow with splendour, for gold is indeed the seed of fire. Salt is considered the essence of Dyau and Prithvi, and when offered it makes the fire prosper with their essence. Where there is saltish ground they declare it fit for cattle. The earth collected from a mole hill enriches the fire with the essence of the earth. The earth is made firm and steady by the fixing of pebbles just as one would spread out a hide with pins. In this manner, the yajamana keeps away his inimical rivals by making this ground firm and steady and by installing the fires on it.
तस्मादपः सम्भरति - २.१.१. tasmādapaḥ sambharati - 2.1.1. hence he procures water
तस्माद्धिरण्यं सम्भरति - २.१.१. tasmāddhiraṇyaṁ sambharati - 2.1.1. hence he procures gold
तद्द्यावापृथिव्यो रसेन समर्धयति तस्मादूषान्त्सम्भरति - २.१.१. taddyāvāpr̥thivyo rasena samardhayati tasmādūṣāntsambharati - 2.1.1. it makes this (fire) prosper with the essence of these two - dyava and prthivi
तस्मादाखुकरीषं सम्भरति - २.१.१. tasmādākhukarīṣaṁ sambharati - 2.1.1. hence he secures the mole-hill.
नक्षत्राणि ॥ Nakshatras
This ceremony is to be performed when chandra (moon) is situated in certain nakshatras. Krittika, Rohini, Mrgasira, Phalguni (2 stars Purva and Uttara), Vishakha and Hasta, Chitta stars are preferred according to different Shrauta sutras.
कृत्तिकासु रोहिण्यां मृगशिरसि फल्गुनीषु विशाखयोरुत्तरयोः प्रोष्ठपदयोः ॥ १० (Asva. Srau. Sutr. 2.1.10)
kr̥ttikāsu rōhiṇyāṁ mr̥gaśirasi phalgunīṣu viśākhayōruttarayōḥ prōṣṭhapadayōḥ ॥ 10 (Asva. Srau. Sutr. 2.1.10)
कृत्तिकासु ब्राह्मण आदधीत मुख्यो ब्रह्मवर्चसी भवति ॥ ३ गृहांस्तस्याग्निर्दाहुको भवति ॥ ४ (Apas. Srau. Sutr. 5.3.3)
kr̥ttikāsu brāhmaṇa ādadhīta mukhyō brahmavarcasī bhavati ॥ 3 gr̥hāṁstasyāgnirdāhukō bhavati ॥ 4 (Apas. Srau. Sutr. 5.3.3 and 4)
Kanvashatapatha brahmana (Ekapatkanda Adhyaya 1 Brahmana 2) instructs that the installation of Agni under the constellation of Krittika is preferred as Krittika is verily the star of Agni.
कृत्तिकास्वग्नी आदधीत । एत्याहुरेतध्द वा अग्नेर्नक्षत्रं...
kr̥ttikāsvagnī ādadhīta । etyāhuretadhda vā agnernakṣatraṁ...
Establishing Agni in different stars gives different benefits to the yajamana. Setting of fires in
- Krittika and Rohini - one attains progeny and cattle
- Mrigasira - one attains excellence in wealth and community
- Phalgunis - one attains superiority
- Hasta - one attains material things
- Chitra - one overpowers his enemies and rivals (hence preferred for Kshatriyas).
ऋतु: ॥ Seasons
The people of different sections are advised to establish fire in different seasons, however, they may establish in any season as performing homa is more important than waiting for the appropriate time because a person may die at any time and instead of waiting for right time it is better to establish the fires.
वसन्तो ग्रीष्मो वर्षाः । ते देवा ऋतवः शरद्धेमन्तः शिशिरस्ते पितरो य
vasanto grīṣmo varṣāḥ । te devā r̥tavaḥ śaraddhemantaḥ śiśiraste pitaro ya
Meaning : Vasanta (Spring) Grishma (Summer) and Varsha (Rain or Monsoons) - these are the seasons of devatas. Sharad (Autumn), Hemanta (Prewinter) and Shishira (Winter) are the seasons for pitrus (forefathers).
Setting up the Agnis when Sun travels through the northern region (in the seasons of the devatas) is preferred over the time when he goes to southern region (in the seasons of the pitrus). According to Satapatha Brahmana and Asvalayana Shrautasutras, Agnyadhana is to be performed by
- ब्राह्मणो वसन्त आदधीत | brāhmaṇo vasanta ādadhīta - Brahmanas in Vasanta or Spring to be bestowed by brahmavarcas (ब्रह्मवर्चस् | lustre)
- क्षत्रियो ग्रीष्म आदधीत | kṣatriyo grīṣma ādadhīta - Kshatriya in Grishma or Summer to obtain kṣatraṁ śriyā yaśa (क्षत्रं, श्रिया, यश | splendor, glory and fame)
- वर्षास्वादधीत विड्ढि | varṣāsvādadhīta viḍḍhi - Vaishya in Varshakala or Rainy season to attain bahurhaiva prajayā paśubhirbhavati (बहुर्हैव प्रजया पशुभिर्भवति | abundance of progeny and cattle)
उपवस्था ॥ Upavastha
On the Upavastha day (one day before the actual Agnyadhana), the yajamana prepares himself by taking the purificatory bath, achamana (sipping of water), and Punyahavachana. He then takes his seat along with his wife on the darbha grass and pronounces his will to set up the sacred agnis, formally announces his rtviks and honours them by offering Madhuparka (mixture of curds, honey and ghee). He fasts through the day and remains awake the whole night tending to the fire by adding samidhas.
रात्रिं जागरणधारणे ११ शकलैर्वा १२ (Katy. Srau. Sutr. 4.8.11 and 12)
rātriṁ jāgaraṇadhāraṇe 11 śakalairvā 12
अग्निमन्थनम् ॥ Agni Manthana
When the sun rises, the rite of Agnyadhana is initiated. In the morning the Adhvaryu heats the two arani sticks and hands them over to the yajamana invoking the prescribed mantras. Agni manthana is the process by which the fire is generated by the manthana (literally churning) or friction of two sami sticks. Rig veda mentions the process of kindling of fire from Arani (fire sticks) made from Sami and Asvattha trees. This is also called Agni Samaropa.
The stick on top called the Uttara-arani is called Pururavas (Indra) and the lower arani called as Aadhara-arani is called Urvasi (Apsara). The rope used for manthana is called rasanaa. The process of generating fire is also described in various Shrauta sutra texts apart from Taittiriya Brahmana in the pauroḍāśika kāṇḍa (पौरोडाशिक-काण्ड:) under Agnyadhana prapathaka .
शमीगर्भादग्निं मन्थति । एषा वा अग्नेर्यज्ञिया तनूः । (Tait. Brah. 220.127.116.11)
śamīgarbhādagniṁ manthati । ēṣā vā agnēryajñiyā tanūḥ । (Tait. Brah. 18.104.22.168)
Meaning : From the hearth of sami (sticks) Agni is churned. This is the yajna body of Agni.
अग्न्याधानम् ॥ Agnyadhana
While chanting the mantras, Adhvaryu establishes the Garhapatya Agni on the dried cow dung cakes and places some darbha around it. (Apas. Srau. Sutr. 5.12.2). According to Kanva Shatapatha Brahmana
स भूर्भुव इत्येतैस्त्रिभिरक्षरैर्गार्हपत्यमादधाति द्वे परिशिनष्ट्ययातयामतायै... (Kanv. Sata. Brah. 22.214.171.124)
Summary : He (yajamana) installs the Garhapatya with the (uttering) these three syllables Bhurbhuvah.
He lights some samidhas from the Garhapatya Agni and formally sets up the Aahvaniya Agni (Apas. Srau. Sutr. 5.13.7). The Dakshinagni is established by taking the fire from the house of the richest person of the village or it may be produced by manthan. This is followed by Purnahuti and yajamana distributes dakshina to the priests according to his economic conditions. 
Thus the process starts with the collection of Arani and ends with Purnahuti.
यो अश्वत्थः शमीगर्भ आरुरोह त्वेसचा । तं त्वाहरामि ब्रह्मणा यज्ञियैः केतुभिः सहेति पूर्णाहुत्यन्तमग्न्याधेयं १७ (Asva. Srau. Sutr. 2.1.17)
sa bhūrbhuva ityetaistribhirakṣarairgārhapatyamādadhāti dve pariśinaṣṭyayātayāmatāyai...
After purnahuti of Agnayadhana the yajamana can be considered as an Ahitagni and he should abide by all the rules of a ahitagni yajamana. 
पुनराधानम् ॥ Purnaradhana
In the event forbidden materials come into contact with Agni, the Pavitra samidhas or the homa kunda, there would be agni nasha or destruction of agni called "Agnyopaghaata". In such case, Punaradhana also called as Punaradheya (पुनराधेयम् | or revival of Agni) is to be performed with sterner measures.
Taittiriya Samhita outlines the Punaradhanavidhana (पुनराधानविधानम्) which is similar to Agnyadhana process with a few modifications. The description of Punaradhana is also found along with the Agnyadhana procedure in a few sources like
According to Katyayana Shrauta Sutras  पुनराधेयम् is described as follows
पुनराधेयमाधानाऽप्रतिज्ञातस्य १ राज्ययशस्कामस्य वा २ त्रिरात्रावरमग्नीनुत्सृज्य ३ सद्यो वा ४ पुनर्वस्वोः ५ वर्षासु मध्यन्दिने वा ६ कुशैराधानम् ७ व्रीह्यपूपमर्कपलाशयोः पक्वं गार्हपत्यस्थाने निदधात्येवमाहमनीयस्य यावं सद्यश्चेत् ८... (Katy. Srau. Sutr. 4.11)
punarādheyamādhānā'pratijñātasya 1 rājyayaśaskāmasya vā 2 trirātrāvaramagnīnutsr̥jya 3 sadyo vā 4 punarvasvoḥ 5 varṣāsu madhyandine vā 6 kuśairādhānam 7 vrīhyapūpamarkapalāśayoḥ pakvaṁ gārhapatyasthāne nidadhātyevamāhamanīyasya yāvaṁ sadyaścet 8...
Summary : The re-establishment of Agni takes place in Punarvasu nakshatra of the rainy season during mid-day. Kusha grass is to be placed (in Agni). Ahuti made by cooking Vreehi (a type of grain) along with other items is to be placed in Garhapatya agni, yava grains to be offered in Aahvaniya.
Offerings of astakapala purodasa to Agni, ekadasakapala purodasa to Agni and Vishnu and panchakapala purodasa to Agni and Soma are offered in addition to the other ahutis.
Either the karta or his wife should be present near the Agnihotram at the time of sunrise and sunset. If both of them leave the house and cross the boundaries of the village or city without tending to the Agni and they become extinguished then on their return Agni Punaraadhana (Agni Sparsa) has to be done. Thus travel for karta at the time of homa or during the Parvadinas (Amavasya, Poornima tithis) is forbidden and Punaradhana of Agni is a must.
During the time when Asoucham occurs in the family - Rajodosha (menses time for wife), Sutakam (when there is a death or birth in the family) re-instatement of Agni by Punaradhana process has to be done. Many prayaschitta vidhis are to be performed if the Agni in the homa kunda is spent, lost due to travel, or destroyed. If for 12 days at a stretch homa is not performed (Homa lopa due to any reason) then prescribed Prayaschittas are to be performed by doing samidha samskara (purification of homa dravyas) and additional Aahutis. This observance of prayaschitta varies according to different sutras. For example : Hiranyakeshi Sutras prescribe prayaschitta after homa lopa for 12 days, Aapasthamba Sutras advise prayaschitta after homa lopa for 3 days.
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