The main 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 heater rather than relying on long pipe runs.
Traditionally, domestic hot water has simply been a choice between a combi boiler or a vented system (with the odd Point of Use water heater thrown in).
With water saving design, and zero carbon housing which will possibly become the standard in 2016, this all changes slightly for several reasons -
- with Passivhaus standards or zero carbon standards there is no need for a central heating boiler
- solar hot water collectors, which can easily supply half the hot water needed for a house, work better if they have a thermal store (because sunshine is irregular in the UK). This can be a separate store or part of a hot water cylinder which acts as a pre-heater for the boiler
- biomass heating, such as wood burning can be difficult to control and regulate accurately (though wood chip and wood pellet are easy to regulate) so again a thermal store may be needed
- when saving water becomes an issue then the length of pipe runs becomes much more important and also the diameters of pipes.
- if Combined Heat and Power (CHP) takes off then heat storage again becomes an issue
- cheaper rate night time electricity can be an argument for more thermal storage of hot water
As can be seen from the above points there are three main changes likely to happen
- with new houses there needs to be a water heater which is not based on a traditional central heating boiler more +/-»
- there is likely to be a much greater use of thermal water stores for both old and new houses more +/-»
- pipework and fittings will become more efficient in terms of not wasting heat and water more +/-»
there are several possibilities including gas and electric instantaneous water heaters (both central and Point of Use), solar water heaters and heat pump water heaters (both ground-to-air and air-to-air)
the optimum size of thermal stores is tricky to calculate because it is a trade-off between the savings in energy and the cost of the store. A small store will cost less and take up less room but it may not be able to store enough hot water between one sunny day and the next (or between one log burning and the next). A larger hot water store will always save more energy but will cost more to buy and more importantly may take up more valuable floor space. It may also cause a problem of overloading the structure.