Aims and objectives: This study evaluates the short‐term (3 months), medium‐term (6 months) and long‐term (12 months) effect of family nursing therapeutic conversations added to conventional care versus conventional care on social support, family health and family functioning in outpatients with heart failure and their family members. Background: It has been emphasised that increased social support from nurses is an important resource to strengthen family health and family functioning and thus improve the psychological well‐being of patients with heart failure and their close family members. Design: A randomised multicentre trial. Methods: A randomised multicentre trial adhering to the CONSORT checklist was performed in three Danish heart failure clinics. Consecutive patients (n = 468) with family members (n = 322) were randomly assigned to either the intervention or control group. Participants were asked to fill out family functioning, family health and social support questionnaires. Data were measured ahead of first consultation and again after 3, 6 and 12 months. Results: Social support scores increased statistically significant both at short‐term (p = 0.002) medium‐term (p = 0.008) and long‐term (p = 0.018) among patients and their family members (p = <0.001; 0.007 and 0.014 respectively) in the intervention group in comparison with the control group. Both patients and their family members reported increased reinforcement, feedback, decision‐making capability and collaboration with the nurse. No significant differences between the intervention and control groups were seen in the family health and family functioning scales among patients and family members. Conclusions: Family nursing therapeutic conversations were superior to conventional care in providing social support from nurses. Relevance to clinical practice: Family nursing therapeutic conversations are suitable to improve the support from nurses among families living with heart failure.