Purmo Group Ltd,
General overview of the
structure and functioning of a heating system in apartment buildings renovation
heating, particularly hot-water heating where the heat emitters used are
radiators and convectors, is the most common type of heating system in all
places where, during the cold seasons of the year, continuous heating is needed.
In Europe alone, it is estimated that there are one billion
radiators/convectors in use.
There is a
reason for their popularity: Properly designed and properly constructed
radiator heating systems work reliably, last a long time, and deliver an
excellent thermal comfort. Their reliability is enhanced by decades of user
experience of the functioning of both the components and the assembled whole.
Indeed, radiator systems have revealed to be one of the least problematic of
buildings’ different technical systems.
In terms of
the structure of their piping, radiator networks are of two basic types:
one-pipe systems and two-pipe systems (Figure 1). Two-pipe systems are by far the
most popular apartment buildings. The use of vertical one-pipe systems in
apartment buildings was widespread in Eastern Europe. To some extent,
horizontal one-pipe systems are used mainly in small buildings. Because of
their deficient in cooling and the resultingly weak energy efficiency, it is
advisable to shift from one-pipe systems to the two-pipe option.
Structure of a radiator network: a two-pipe system (left) and a one-pipe system
presentation focuses on radiator networks in apartment buildings that are
renovated. It is also of key importance to be able to renovate the heating
systems while the residents are on site. If it is possible to move the
residents to temporary housing for the duration of the renovation, this would
offer opportunities for other types of technical solutions.
Actions to be performed on
a heating network during the renovation.
renovation of building stock are guided by the statutorily defined objective
(EU EPBD requirements) for improving buildings’ energy efficiency to the level
of a nearly zero-energy building (nZEB), the renovation
actions shall ensure that the target energy efficiency is achieved, and that
the repairs help to create the conditions for making the buildings carbon
buildings, the key energy renovation focus is to reduce heat losses from the
building envelope, such as the replacement of windows and outer doors, and the
improvement of the heating insulation. Actions aimed at increasing active
energy efficiency include, e.g., shifting to carbon neutral systems for heat
generation, installation of heat recovery equipment, reduction of electrical
devices’ consumption, arrangements to reduce the consumption of tap water
(particularly domestic hot water), and implementing measurement of water and
energy consumption. Buildings’ own electricity generation systems are being
installed in ever-increasing numbers. Reduction of cooling needs and the setup
of more energy-efficient cooling systems are also an important part of
to these measures, one of the most energy-efficient and cost-effective actions
that can be taken is the enhancement of radiator networks and turning them into
low-temperature heating systems. Heating systems and their functioning are of
decisive importance to thermal comfort, energy efficiency and energy costs.
the energy efficiency of heat generation in areas such as heat pumps and
district heating, the heating network’s temperatures need to be brought to a
considerably lower level than before (Figure 2). The aim is to improve the
efficiency of heat generation and at the same time lower the costs of
generating heat energy.
Examples: The old building (left) has high flow water temperatures and a convex
heating curve. The new and deep-renovated old buildings (right) have low flow
water temperatures due to the low heat demand and a concave heating curve due
to the high influence of solar and internal heat gains.
remodeling of a building changes certain building features. The heating needs
of the rooms change, as do the ratios of heating needs between the different
rooms. This means the heating network needs to be redesigned with dimensions
adapted to the new conditions and requirements. Generally, what should be
retained from the older system are the heating network’s transmission lines and
risers. It is practical to replace the radiator connection pipes with new ones
when possible (Figure 3).
A properly dimensioned radiator will have a large heat-radiating surface. A new
radiator and its valves are easiest to install when the radiator connection
pipes from risers to radiator valves are replaced.
radiators should be dimensioned to the right size for a low-temperature system,
while ensuring that their heat-radiating surface is as large as possible,
taking into account the space available for installation. Radiator valves
should be replaced with precisely preset thermostat valves. The existing risers
should be equipped with automatic differential pressure control valves.
The heating network should be balanced using calculated values. It is also
advisable to update the temperature controller and the water circulation pump.
energy-efficient building, up to 60–80% of heating needs during a heating
period can be covered by heat gains from residents and electrical devices, and
the direct radiation of the sun. The radiator and thermostat, working together,
make it possible to utilize free quantities of heat.
practice, achieving a balanced heating network is straightforward because in
the new operating situation, the old supply risers are looser and are no longer
a source of friction losses: If one chooses a pressure difference level of,
e.g., 10 kPa, this pressure difference will be precisely preserved, even
with radiator valves. The settings values of radiator valves can therefore be
determined almost entirely on the basis of the design heat demand. A low pressure difference ensures that a radiator valve
functions precisely, without making noises, and also ensures good cooling of
Dimensioning and energy efficiency
of radiators at district heating and heat pump sites
district heating connection, a functional level for the dimensioning
temperature is 60/30/21°C (flow temperature/return temp/room temp). Heavy
cooling, i.e., low temperature of return water – improves the energy efficiency
of district heating: The network’s ground losses are smaller, lower levels of
flow and pumping power are made possible, the boiler operating efficiency
improves when the temperatures offlue gasses decrease, and increased
condensation enhances the functioning of the flue gas scrubbers reducing the
particle emissions (Figure 4). Thanks to these benefits, many
district heating providers have also been able to reduce consumer tariffs. In
energy prices, it is typical to have a €2/MWh deduction for each degree the
temperature of the return water is reduced; for example, the monthly average of
return water temperature compared to a reference temperature of 50°C. Some
district heating providers also issue fines when the temperature of return
water exceeds the reference temperature.
The condensation of output gases is significantly enhanced when the temperature
of the return water falls to under 50°C, in which case boiler efficiency can
improve by up to 10%.
condensation of flue gases, and the boiler efficiency that such condensation
affords, pertains to all types of heating boilers, such as bio-mass, gas and
Heat pump system
important to a heat pump’s efficiency to keep the heating system’s temperatures
low. When the heating need is low, radiators can also be dimensioned for very
operating efficiency of a heat pump is described with the coefficient of
performance (COP), which is the ratio of the heat generated by the heat pump
system (Q) to the work done by the electrical
energy of the compressor (W).
Q is the useful heat supplied or
removed by the considered system
W is the work required by the
The expression COPa is also used for the annual coefficient of
practice, the temperature of the supply water is of decisive importance because
a heat pump’s coefficient of performance (COP) is about 2/3 dependent on the
supply water’s temperature, and 1/3 on the temperature of the return water (Figure 5). For this reason, in the dimensioning of a heat pump, a temperature
of, e.g., 50/40°C (flow/return temperature) is better than 60/30°C, the latter
of which is suitable for district heating. For a guideline value, one can
assume that a decrease of 10°C degrees in supply water temperature will improve
COP by about 30%, which, at an annual level, means that the heat coefficient COPa rises by 12-15%, with focus on the space heating.
A heating network’s supply water temperature has about a 2/3 impact on a heat
pump’s COP, and return water has an impact of about 1/3 – compare the
coefficients of the regression equations.
of domestic hot water (above 55°C) solely with geothermal heat pumps and
outdoor air heat pumps is often not economic. For most heat pumps, 50°C can be
regarded as a reasonable temperature lift. The larger the temperature lift, the
lower the COP becomes (Figure 6). The optimal temperature increase
level depends on the COP corresponding to the threshold temperature level in
question, and the prevailing price ratio between electricity and other forms of
Typical COPa values gathered from different sources.
heat source, exhaust air heat pumps use ventilation exhaust air, which has a
high temperature (in the range of 22°C year-round). With a high initial
temperature, an exhaust air heat pump can produce warm supply water and
domestic hot water energy efficiently. But be aware that given the limited
extract airflow of a mechanical ventilation system the capacity of heat pumps using
the exhaust air as their heat source is limited.
speaking, it is recommended to use a heat pump in parallel with district
heating or a heating boiler if the capacity of the heat pump is insufficient on
its own to achieve economic heating of domestic hot water or peak efficiency of
the heating system.
one should remember that such hybrid systems always require high-quality
control and connection systems to ensure optimal functioning.