In this study, the effect of carbon nanotubes (CNTs) on nucleate boiling heat transfer is investigated. Two halocarbon refrigerants of RI 23 and R 134a for building chillers were used as working fluids and 1.0 vol.% of CNTs was added to them to examine the heat transfer enhancement with CNTs. The experimental apparatus was composed of a stainless steel vessel and a 152.0 mm long plain horizontal tube of 19.0 turn outside diameter heated by a cartridge heater. All data were obtained at the pool temperature of 7 degrees C in the heat flux range of 10-80 kW m(-2). Test results showed that CNTs increase nucleate boiling heat transfer coefficients for these refrigerants. Especially, large enhancement up to 36.6% was observed at low heat fluxes of less than 30 kW m-2. With increasing heat flux, however, the enhancement was suppressed due to vigorous bubble generation. Fouling on the heat transfer surface was not observed during the course of this study. Optimum dispersion of CNTs should be examined for their commercial application to enhance nucleate boiling heat transfer in building air-conditioning applications. (c) 2006 Elsevier B.V. All rights reserved.