Tappvattenvärmning med värmepump. Krav och erfarenheter.
This licentiate thesis deals with system solutions for heat pump hot water heating in
Sweden. Hot water heating requires higher temperatures than space heating and therefore
lowers the annual performance of the heat pump system. In existing single family houses,
space heating represents the majority of heat pump delivered energy. The demand for
space heating decreases in new buildings meanwhile the demand for hot water heating
remains, concurrently efficient hot water heating becomes more important.
The choice of system solution affects the possibility to save energy. The prerequisites for
system development in small houses are examined in this work. It is done partly by a
summary of today’s heat pump systems, i.e. the heat pump unit and storage tank, based
on different principles for hot water heating. The summary includes pros and cons with
these system solutions. The work is also done partly by a survey of demands,
requirements, recommendations and testing methods for hot water that can influence the
system design. In addition, analysis of data from field measurements on three domestic
heat pump systems shows demands, energy relationships and heat pump operation in
existing single family houses.
Heat pump hot water heating requires some kind of storing of energy since the
instantaneous power requirement for hot water heating is huge and the heat pump power
is designed for the space heating demand. Typically, tap water is stored in a tank and
natural convection is used for the hot water heating. It is important to maintain the
temperature stratification in order to maximize the energy extraction from the storage
tank. Systems depending on natural convection for water heating, risk unwanted mixing
when tapping or recharging occurs. In order to avoid this, the design of the system is
important. Storing of water in a closed water loop system decreases the risk of Legionella
growth and makes it possible to use a simpler and cheaper tank, compared to fresh water
storage as in conventional systems. Hot water heating in an external heat exchanger
lowers the risk of mixing in the tank. The heat exchanger dimensions and power can be
chosen more freely since it is not limited by the space in the tank as with internal heat
Few measurements on tap water usage are available from single family houses while
many exist for multifamily buildings. Standard estimate energy usage for hot water
heating is typically between 4000-5000 kWh/year in single family houses. The results
from a questionnaire survey made for single family houses with new heat pump
installations, shows that the users are well satisfied with the amount and temperature of
Testing methods for hot water heaters tend to move towards detailed tapping patterns that
should represent the hot water demand during an actual day instead of a few and large
tappings. Even energy efficiency is dealt with in the testing methods by requirements on
system efficiency for hot water heaters according to the Ecodesign Directive. Demands
on hot water have low impact on the heat pump dimensioning, however tapping patterns
can be of importance for the heat pump control system design.
The operation of three heat pump systems in single family houses is shown through
analysis of data from field measurements. Two of the systems are equipped with a
desuperheater. The analysis shows annual demand, monthly variations of energy use and
detailed heat pump operation during both a summer and winter week.
Simplicity and manufacturing costs are prioritized when designing heat pump systems
and performance of hot water heating has not been prioritized in existing system
solutions. System design can and has been affected by testing methods when connected to
a performance requirement. Requirements in the national building code such as limited
maximum power for space heating have been important. Potential for system
improvements exist with capacity control and better insulation.