Demand Gas Water Heaters

The search is on for a way to prevent the corrosion of copper heat exchangers used in tank-less gas water heaters.

Water heating typically is the second largest energy user in the home. Tank-less gas water heaters are combustion gas appliances that heat water only at the time it is required, as opposed to storage water heaters that store pre-heated water in an insulated tank. This, in turn, reduces the energy consumed in heating water.

There is a wide variation in the energy efficiency of tank-less gas hot water heaters with efficiencies greater than 96% possible using gas heating equipment that condenses water vapor from combustion gas and captures the latent heat of condensation.

Copper is the preferred material for transferring heat from combustion into water because of its ease of fabrication and high thermal conductivity. However, copper heat exchangers used in the highest efficiency condensing water heaters have a susceptibility to corrosion.

Finding an effective anti-corrosion coating will increase copper use in high-efficiency heating equipment and give it a leading edge over alternative materials. It also will build the foundation for the potential use of copper condensing heat exchangers in combustion air-conditioners and commercial gas boilers

A range of protective coatings were identified and tested, and new heat exchanger designs were explored. Preliminary results with chemically grafted coatings are promising, with several likely candidates identified. The best-performing coating solutions currently are undergoing further evaluation with manufacturing firms to understand their performance in end-use environments.