Air Con Water Heating

Air Conditioning, Waste Heat and Hot Water: Rio de Janeiro

Waste heat from air conditioning systems can be arecovered and used to produce “free” domestic hot water – a process which improves cooling efficiency and extends compressor life. Photo by John’K’ licensed under creative commons.

Air conditioning is common in Rio due to the hot and humid conditions. Air conditioning systems remove heat from living spaces and expel this to the outside air. This waste heat can be recovered and used to heat water.

This has particular significance in Rio de Janeiro, where electric showers and air conditioning are responsible for a large proportion of domestic electricity usage. Recovering waste heat and using it to provide hot water helps improve energy efficiency and reduce greenhouse gas emissions [1].

Air conditioning units can be modified so that waste heat is used to provide domestic hot water [2], [3] – a modification which improves the efficiency of the air conditioner. Under normal circumstances, waste heat is rejected from an air conditioner’s condenser to the atmosphere.

It is relatively simple to adapt the air conditioning system so that this waste heat is recovered and used to heat water, and there are several products on the market which do this. This has a number of benefits:

  • Air conditioner efficiency is improved
  • Compressor life is extended
  • The cooling effect of the air conditioner occurs more rapidly
  • The air conditioning system generates “free” hot water when in operation
  • The extra costs associated with water heating are relatively small

In one experimental test, 60 litres of water were heated from 30°C to 75°C after 8 hours of operation [3]. Such systems have the potential to substantially reduce water-heating costs, as well as to improve the efficiency of air conditioning.

Hot Water System

Heat recovery systems as outlined above require hot water to be piped and stored separately. This contrasts with the widespread practice in Rio de Janeiro of a simple cold water supply with an electrically heated shower head. Creating a separate hot water system, though a relatively simple task, would entail extra costs and the payback time for such systems should be evaluated.

  1. Carbon Trust, “Heat recovery: A guide to key systems and applications,” 2011. [Online]. Available:
  2. J. Ji, T. Chow, G. Pei, J. Dong, and W. He, “Domestic air-conditioner and integrated water heater for subtropical climate,” Applied Thermal Engineering, Apr-2003. [Online]. Available:
  3. M. M. Rahman, C. W. Meng, and A. Ng, “Air Conditioning and Water Heating- An Environmental Friendly and Cost Effective Way of Waste Heat Recovery,” 2007. [Online]. Available: