Catalysis of the reaction: NADP+ + 4,4-dimethyl-5-alpha-cholesta-8,24-dien-3-beta-ol = NADPH + H+ + 4,4-dimethyl-5-alpha-cholesta-8,14,24-trien-3-beta-ol.
4,4-dimethyl-5alpha-cholesta-8,24-dien-3beta-ol:NADP+ delta14-oxidoreductase activity