Thermal oil heaters can be considered as an alternative to steam boilers for process heating use. Instead of boiling water, thermal oil heaters use heat transfer oils with high boiling points which allows operation at low pressures. To increase thermal oil heater efficiency, a nanofluid consisting of a common heat transfer oil, the synthetic TH66, and copper nanoparticles has been proposed. Based on existing correlations for nanofluids, a figure of merit (FOM) was created to evaluate heat transfer performance while factoring in pumping power increases. A maximum FOM increase of 13% was found for a thermal oil heater using the nanofluid when compared to one that uses the base fluid oil.