Water

In our current building stock energy used to generate domestic hot water (DHW) remains small in comparison to the energy required for space heating. However, as the buildings lower their energy consumption by adopting more efficient building fabric standards and taking other steps to achieve ultra-low energy performance, the energy used by the hot water systems take on a more prominent role in their operational energy breakdown. This trend is particularly pronounced in residential buildings, but also true in commercial buildings. DHW energy use is still significant and should be minimised as much as possible.

Water has the capacity to absorb a lot of heat for every degree of temperature rise. It has a high “specific heat capacity” so heating it will always consume a large amount of heat energy.

The three approaches which can be taken to reduce hot water energy consumption are focused on:

• Reducing consumption, including minimising wastage
• Generating the heat efficiently or renewably
• Recovering heat from the wastewater or recycling the hot water itself

Some of the hidden energy losses arising due to water consumption in buildings are not so obvious and are to do with drainage design. Soil and vent pipes (SVPs) penetrating a roof can form thermal bridges. In existing buildings and even those designed to modern Building Regulations the heat loss can be proportionally insignificant, but as we target ultra-low energy performance thermal bridging through SVPs can become significant.

Heat is also lost in the water that drains away. In most DHW and drainage designs, all the energy that was put into heating the water gets wasted by literally flushing it down the drains. Where there’s high DHW use the heat from what gets flushed down the drains could be recovered to preheat the mains water before it is heated up. Small packaged units to be fitted beneath shower trays are available on a small scale but they are currently a costly way to save carbon – between £350-£500/teCO₂. While still quite rare, to deliver the most efficient buildings performance these systems may need to be considered.

The first step is to consider reducing the hot water demand through water efficient sanitary ware. This is usually cost neutral; it has the added benefit of reducing water consumption and thus the burden on the water authorities. Subsequently carbon emissions will be reduced up and downstream of the building.

Next simplify the distribution system and consider heat loss through fabric penetrations. Carefully installed insulation on pipework and fittings at every point in the system and well implemented commissioning are also critical.

Through efficient heat sources such as air source heat pumps (ASHP) the energy used in DHW systems can be reduced by at least 50% in comparison to direct electric or gas boilers. Gas boilers and direct electric heating will be around 100% efficient; ASHP technology will range from 250%-300% efficient in comparison. There may also be opportunities to make use of renewable heat from waste heat sources nearby to the building

Barriers to best practice and low energy performance:

• Efficiencies can suffer when building layouts do not stack or consolidate wet room locations which is often difficult to achieve for architectural reasons.
• The lifecycle financial costs of electric systems is still usually higher than less efficient gas. Natural gas is currently very cheap.
• The capital cost of heat pumps can present a barrier to clients, but these have lower running costs than direct electric systems. The good news is the cost of heat pumps is dropping and the efficiencies are rising.

The top practical measures to introduce now to enable net zero carbon:

• Prioritise water efficient fittings carefully checking specification at each stage.
• In large commercial buildings point of use instantaneous water heaters can avoid large system distribution heat losses.
• Minimise distribution losses through reducing pipe lengths and minimising lateral distribution.
• Supply central water systems with efficient heat sources such as heat pumps
• For ultra-low energy buildings minimise soil and vent pipe penetrations through the fabric through considering the use of air admittance valves (AAVs) where the building maintenance regime allows.
• Minimise pumping energy for wastewater and rainwater systems by prioritising gravity fed systems.
• Commission systems properly

It’s always worth coming back to the principles of good environmental design. Prioritising the simplest measures such as water efficient fittings, efficient distribution and excellent commissioning is cost effective and overall will have the best chance of success in operation.