The affix ñya comes in the sense of 'joined with' after the word gṛhapati in the Instrumental -3rd case in construction.,
Thus गृहपतिना संयुक्तः = गार्हपत्यः 'the Garhapatya Fire': i. e. a Fire particularly consecrated by the Householder. The word संज्ञायां of the last sutra should be read into it, thus गार्हपत्यः means a particular Fire, and not every thing relating to a गृहपति ॥ The Fire in which husband (grihapati), together with or joined with his wife, performs sacrifice is called Garhapatya. That sacrifice cannot be performed in Dakshinagni. Or that fire in which Grihapati hymns are recited is called Garhapatya.,