Hot Water System Design
The local water supply is often too cold to be used by building occupants. For this reason, water heaters are fundamental in plumbing installations. There are many types available, each with advantages and disadvantages. Hot water systems can be classified by energy source and by water delivery method:
- Water heating can be accomplished with combustion, using fuels like natural gas or propane, or with electricity. Combustion heating normally has a lower cost than resistance heating, but electric heat pumps can compete with fuel-based heating.
- Storage heaters have a hot water tank, while tankless heaters provide fast heating as water flows through them. Tankless heaters normally have a higher efficiency, since storage heaters have standby losses when holding hot water.
Combustion heaters must be properly vented to remove flue gases, but this is not an issue with electric heaters. However, the operating cost of a resistance heater can be very high, especially in places with expensive electricity like NYC. If you are planning a building that does not burn any fossil fuels locally, a heat pump water heater is recommended.
Regardless of the type of water heater used, the plumbing design must prevent the accumulation of stagnant warm water. This condition creates an ideal environment for Legionella bacteria, which causes a severe respiratory illness. Some common sites where Legionella can grow are cooling towers and dead legs in piping.
When hot water is needed in a specific fixture, it must first travel from the water heater to the point of use. However, there are some applications in which hot water is required immediately, such as hotel rooms. There are two solutions in this case:
- The plumbing fixture that requires immediate hot water can be equipped with a dedicated tankless heater. This is an effective solution when only one or a few fixtures require instant hot water, but it becomes very expensive when many fixtures need it.
- Another option is installing a recirculation loop for all the fixtures that need immediate hot water. A single water heater can be connected to the loop, instead of many smaller units.
The energy efficiency of water heaters is described by an energy factor, which is a ratio of heating output to energy input. Only heat pumps can achieve an energy factor above 1, since they use an inverse refrigeration cycle to capture ambient heat. In all other hot water systems, the heat input comes directly from the energy source used.
When comparing water heaters, the operating cost depends on both the energy factor and the heat source. Electric resistance heaters have an EF of nearly 1.0, while gas heaters are typically below 0.8 EF. However, resistance heaters cost more to operate, since one kilowatt-hour of electricity is much more expensive than the equivalent 3,412 BTU of natural gas.
