Hindu marriage is conducted with offerings made in the fire. Aupasana, which must be performed every day, is commenced in this fire and it must be preserved throughout one's life. The seven pakayajnas (see: Shrauta), rites including upanayana and shraaddha, must be conducted in the aupasana fire. The son lights his aupasana fire during his marriage from his father's aupasana fire. The son's fire, like his father's, must be maintained throughout his life. Thus, without any break, the sacred fire is kept burning in the family generation after generation.
All rites in which the aupasana fire is used and pertain to an individual and his family are "Grhyakarmas". The seven pakayajnas also belong to this category. They are related exclusively to the family and are not very elaborate. Even so they are conducive to the good of the world outside also. Grhyasutras deal with such rites. They belong to the Smritis and are called "Smarta-karmas".
औपासन ॥ Aupaasana
During vivaha samskara the one is initiated to enter the grihasta ashram by setting up the आवसथ्याग्निः ॥ Avasatyaagni to the north of the vedi. (Paaraskara Grhya Sutra Adhyaya 1 Kandika 2 Mantra 1)
आवसथ्याधानं दारकाले १ दायाद्यकाल एकेषाम् २ ॥ āvasathyādhānaṁ dārakālē 1 dāyādyakāla ēkēṣām 2 (Paara.Grhy.Sutra. 1.2.1).
Meaning : The setting up of Aavasatya agni is (performed) at the time of wedding. At the time of inheritance according to some.
He receives additional procedures to perform nityakarma. A few features include
- The son lights his aupasana fire during his marriage from his father's aupasana fire. The son's aupasana fire, like his father's must be maintained throughout his life. Thus, without any break, the sacred fire is kept burning in the family generation after generation.
- All the daily ahutis and cooking involve the aupasana fire and pertain exclusively to the individual and his family. Thus, "Grhyakarmas" including the seven pakayajnas involve the aupasana agni or Grhyagni or smarthagani. They are related exclusively to the family and are not very elaborate.
- The seven pakayajnas, samskara rituals like namakarana, upanayana and sraddha must be conducted in the aupasana fire.
- Starting from marriage, a grihasta (along with his wife) offers aahuti twice daily at twilight time (dawn and dusk) in the Gaarhapatya agni, this is also called as Aupasana.
- Aupasana is to be performed by the grihasthas of the three varnas. Valmiki Ramayana in Ayodhya kanda mentions that Sri Ramachandra, a kshatriya, enquires Bharata about tending the Agni in the house.
कच्चिद् अग्निषु ते युक्तो विधिज्नो मतिमान् ऋजुः | हुतम् च होष्यमाणम् च काले वेदयते सदा || २-१००-१२ (Valm. Rama. 2.100.12)
Meaning : "I hope that a brahmin who is versed in the traditions, who is intelligent and just, employed in your sacred fires, always informs you in time, about a sacrificial fire having been or going to be fed with oblations."
- Aahutis must be offered to Surya in the morning and Agni in the evening.
- Every grihastha has to maintain the agni until he is prevented by old age or illness or death. In case where he cannot perform it on behalf of him his son, brother or son-in-law.  According to Satapatha Brahmana
एतद्वै जरामर्यं सत्रं यदग्निहोत्रं जरया वा ह्येवास्मान्मुच्यते मृत्युना वा (Sata. Brah. 22.214.171.124)
ētadvai jarāmaryaṁ satraṁ yadagnihōtraṁ jarayā vā hyēvāsmānmucyatē mr̥tyunā vā (Sata. Brah. 126.96.36.199)
Summary: this agnihotram is 'jaraamarya satra' because he can get relief from this only if he dies or becomes old.
- Performance of nitya agnihotra is mandated to the grihasta men of all three varnas. If due to any reason, a grihasta has not conducted agnihotra rituals, he can do so after performing the praayaschitta vidhis as given in the Srauta sastras.
- As a nityakriya agnihotra, Anuditahomi's perform homa before sunrise and some Uditahomi's after sunset.
- Mantras for conducting Agnihotra are covered in Krishna Yajurveda's Taittiriya Samhita in Kanda 1(the 5th Prapathaka contains the mantras for पुनराधेयम् and अग्न्युपस्थानम् )
- Morning aahutis are made to the deities Surya and Prajapati, while the evening aahutis are made to Agni and Prajapati.
- Nitya agnihotra is not to be performed by an uninitiated bachelor or widows.