Domestic Hot Water
The green aspects of a well designed hot water system are:
- As much of the heat as possible should be from a renewable source such as solar or be reclaimed heat
- Heat from boilers should be produced as efficiently as possible.
- Hot water storage should be very well insulated. Although cylinders usually come with a 50mm or so layer of polyurethane insulation bonded to them this may still leak more than 2kWh of heat per 24 hrs. This hardly matters in winter but out of the heating season this is over 360 kWh. In cost alone this is maybe £20 p.a. and would account for 2% of the total prime energy budget in a medium sized Passivhaus, so it is worth adding extra insulation if space permits.
- Controls should ensure that water is not heated to a higher temperature than necessary. However domestic hot water should always be raised to a temperature greater than 60ºC. before use so that Legionella bacteria are killed. This means that if water is first heated by a solar collector then it will usually need further heating by either a boiler or an immersion heater to reach 60 - 65ºC.
- Pipe runs should be as short as possible to not only save heat but also water. See Service ducts
- Pipes should be well insulated.
- Appliances should be economical with hot water
Occasionally these conditions conflict with each other such as when a hot water tap in a remote part of a building would be better having its own less efficient electric POU (point of use) heater rather than relying on long pipe runs.
