Heat recovery from waste hot water

Achieving CO2 savings in new-build dwellings has, to date, focussed on the reduction of space heating loads and application of low and zero carbon technologies.

As fabric specifications are pushed to the limits, particularly on sites with limited space for increasing wall thicknesses, turning attention to reducing domestic hot water (DHW) demand can provide another route to achieve challenging CO2 reduction targets.

In a series of test dwellings computer modelled to achieve compliance with the proposed Building Regulations Part L1A 2013 CO2 targets and fabric energy efficiency standards (FEES), we calculated that DHW accounted for between 49% (for a large detached property) and 77% (for a small mid floor flat) of total heat loads.

Clearly any savings that can be achieved on such a significant load will have a major impact on overall CO2 reductions from a new development.

 

Waste water heat recovery

Now recognised in Appendix G of SAP 2009, a relatively new way of achieving a reduction in DHW load and CO2 emissions is the use of waste water heat recovery.

This technology recovers heat from shower or bath water by running the waste water through a heat exchanger.  This waste heat is then transferred to the cold feed to combi-boiler/cylinder and the cold feed to the shower.

Waste water heat recovery provides the only route to reducing overall DHW load in SAP 2009 other than basic compliance with Part G requirements (which provides a 5% reduction in water heating load for dwellings with a potential consumption of water not exceeding 125l/person/day).

 

Performance of Waste Water Heat Recovery

There are currently 70 products available which provide these direct benefits in SAP 2009 calculations.

A typical installation involves a heat exchanger connected to the waste water pipe from the shower.  The cold feed to the combi-boiler (or cylinder) as well as the shower is run through this heat exchanger, pre-warming this water.  Many systems require a vertical drop of 2m – 2.5m below the shower, but there are products which are suitable for use with showers in flats or on ground floors.

The following table outlines the benefits we have calculated in four different dwelling types designed to a high standard of energy efficiency with water heating provided by gas boilers.

 

 

The significant benefits of between 5% and 8% saving on the dwelling emission rate (DER) are akin to changing from a centralised mechanical extract ventilation system to an efficient mechanical ventilation with heat recovery system.

 

Key considerations

  • In order to maximise the potential benefits of waste water heat recovery, heat should be recovered from all showers in the dwelling.  It is feasible to connect more than one shower to a single heat recovery system.
  • The most effective systems feed pre-warmed water to both the shower and the boiler or cylinder.
  • Systems with high heat exchange efficiency should be selected for maximum benefits.
  • A typical system requires a cold water pressure of at least 1.5 bar.
  • Early consideration of pipe runs and layouts is required to ensure systems are practicable.
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