Extending the Standard Model by adding a scalar field transforming as a septet under $SU(2)_L$ preserves the $ρ$ parameter at tree level and can satisfy experimental constraints on the electroweak parameters $S$ and $T$. This work presents the first fully general phenomenological study of such an extension. We examine constraints on the septet model couplings based on electroweak and Higgs observables, and use LHC searches for new physics to bound the mass of the septet to be above $∼ 400$ GeV at a $95\%$ CL. ; Comment: pdfLateX, 17 pages, 6 figures, reference added. Version published in JHEP