Here is an interesting thought; if tomorrow, we all stopped using hot water for bathing and washing clothes we could probably reduce energy consumption by as much as 33%.
That, apparently, is the proportion of the global household energy bill that goes towards heating tap water. Obviously, the likelihood of everyone on the planet foregoing the pleasure of a hot bath or clean clothes is very remote. But, we can ensure that the way we heat our DHW (Domestic Hot water) is more energy efficient and less wasteful.
This is still a common scenario in many households: fill a tank with cold water and heat until the water reaches the desired temperature. Use some sort of thermal blanket to prevent the heat simply leaking away. Finally, add a thermostat to avoid accidental scalding.
It sounds and is inefficient. It is also potentially unhealthy and, in larger households, frequently results in someone having to do without because there is not enough hot water to go around. And, if the hot water is not used and starts to cool, energy is needed to re-heat it: Energy that could be used elsewhere.
Modern combi and condensing boilers do away with the storage cylinder and are an excellent solution for the sort of traditional single-family dwellings we are used to in the UK. However, land shortages are contributing to the rapid demise of traditional terraced, semi and detached houses. So, copying the continental model, we are building apartments. In Cambridge, for instance, apartments now account for 90% of all new-build homes.
In Europe, hot water, like heating, is produced in a central source and then distributed via a District or Community Heating System, to individual dwellings. It is the point at which it is transferred to each unit that determines much of the network’s overall energy efficiency.
The most efficient solution is to transfer heat from the primary hot water source to individual hot water systems using a plate heat exchanger. The heat transfer system can be designed for instantaneous or semi-instantaneous operation. In an instantaneous system, the heat from the primary hot water source is transferred instantly to the incoming cold water flow which then flows directly to the hot water taps.
In a semi-instantaneous system, hot water is produced in the same way – i.e. through transfer via the heat exchanger – but is then stored in a tank and drawn off by the individual systems as required. Both systems offer advantages and disadvantages and their selection is governed by a number of different factors.
An on-demand hot water system requires large volumes of primary hot water to be able to replenish hot water as it is being drawn off. Consequently, both boiler and heat exchanger are larger than in a semi-instantaneous system. On the other hand, with no storage tank required, the installation and space costs for the system are lower.
Eliminating the storage vessel eliminates bacterial growth in the system and along with it the risk of Legionella and other diseases. Mechanical or electronic valves are used to keep the tap water at a safe temperature (typically 50ºC at the point of delivery) while a pump is needed to circulate the hot water to each dwelling.
In a semi-instantaneous system, water heated by the compact heat exchanger is stored in an intermediate tank before being distributed to taps and other outlets. The coldest water, from the tank bottom, is used to cool the district heating return flow. Since the storage tank is sized to meet peak consumption, the actual heat exchanger can be relatively small. This, in turn, means that the district heating pipe-work – or the boiler – can also be down-sized. Another important difference in the semi-instantaneous installations is that the pump is also used to re-charge the tank as the hot water is consumed.
With two heat exchangers linked together in series, the incoming flow passes through the first heat exchanger to be heated for radiator hot water. On its way out of the exchanger the primary return water is routed through the second heat exchanger where it pre-heats the hot tap water flow. Such a two-stage arrangement is suitable for both instantaneous and semi-instantaneous hot water systems. Alternatively, using a by-pass of the first exchanger it is possible to heat a fraction of the hot tap water directly from the primary heat source. Either way, the system makes the most of the available energy.