Drain water heat recovery is an uncommon measure in multi-unit residential buildings. There is
technology available for the purpose but the knowing and experience of the heat recovery systems is
little. The purpose of this thesis is to evaluate the future potential of drain water heat recovery in
multi-unit residential buildings.
A major part of the multi-unit residential buildings in Sweden were built during modernismen and
1975). Many of the buildings have worn out drain and water supply systems and
many are in need of a general refurbishment. If drain water heat recovery is considered for a building
it is suitable to install the recovery system at the same time as the refurbishment to minimize the
Existing heat recovery systems consist of heat exchangers, heat pumps or a combination of both.
There are passive heat exchangers that are placed close to the shower, on the vertical drain pipe and
on the horizontal drain pipe. Recovery systems comprising heat pumps are more complicated and
space demanding. They are rarely suited for multi-unit residential buildings but have the potential of
recovering more heat from the drain water.
In some buildings, where heat recovery systems have been installed, the performance of the systems
has been measured. The measurements show that passive heat exchangers can recover about 10
15 % of the hot water energy consumption, not including standstill losses and hot water circulation.
Calculations indicate that the energy savings for passive heat exchangers could be 20-25 %. If several
kinds of heat exchangers are combined the energy savings could be almost 40 %. Recovery systems
with heat pumps could generate even larger energy savings, between 50
70 %. However, with heat
pumps the electricity consumption increases and that has to be considered in profitability
calculations. When developing new heat recovery systems the emphasis should be on energy storage
and reactivity since most taps are short and unpredictable.
In a technology procurement there should be demands on energy effectiveness. Based on experience
from installed heat recovery systems and calculations the following is suggested:
The buildings heating and hot water energy consumption must decrease with at least 15 %
the current hot water energy consumption.
The buildings heating and hot water energy consumption should decrease with at least
of the current hot water energy consumption.
Furthermore there should be demands on profitability. The present value of future energy savings
ought to exceed the investment cost and the present value of future costs of the system within a
period of time. The following is suggested:
The condition must be fulfilled within
The condition should be fulfilled within