Work on fixed water and heating installations needs an autoriseret VVS-installatør (authorised plumbing contractor), or a firm that brings one. Ask which it is, and ask for that on the invoice (faktura). Authorisations are looked up at Sikkerhedsstyrelsen, sik.dk.
Aarhus mixes apartments in the centre with houses in Højbjerg, Tilst and Viby J. The drive across Ringvejen, and access in older stairwells, is what usually separates two quotes. The same 4 businesses as the Danish page: Aut. VVS-Installatør Lars Lykkegaard ApS, FOLDBY VVS OG SMEDEFORRETNING ApS, Mó VVS ApS, VVS-Team II ApS.