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Fact Sheet – Heat Pumps For Radiators
Heat pumps work best in well-insulated buildings with “wet” underfloor heating systems fitted
Heat pumps can heat buildings using radiators, providing that the building needs no more than
about 50oC flow temperature into the radiators to meet target temperatures – which are usually
18oC throughout, 21oC in the living room, at the DOT (Design Outdoor Temperature – usually -
1oC). If 50oC is insufficient, then more insulation needs to be added, along with larger radiators.
Please see the separate fact sheet on “Radiators For Heat Pumps” for more information on these.
The higher the water output temperature from the heat pump, the lower the efficiency – and the
more it costs to run the heat pump. Therefore the “payback” period against a fossil fuel boiler will
The better insulated a building, the lower the flow temperature required in the radiators or
underfloor heating system.
A “wet” underfloor heating distribution system, which should ideally consist of pipes buried in
deep concrete screed on the ground floor, without insulative coverings, in a new building, should
not need a flow temperature from the heat pump of more than about 35oC.
Retrofitting insulation under floors, and adding underfloor heating concurrent with this work, is
highly desirable, and practically possible in many buildings, although it can cause disruption. This
option should always be the first consideration when considering fitting a heat pump to any
If radiators are to be used then there are two further main considerations.
A “wet” underfloor heating system will absorb the full output of the heat pump, so the heat pump
will run continuously until the building is up to temperature. This means that there will usually
only be one compressor start per heating period, and this will coincide with the start of any cheap
“banded” tariff. Radiators are a “demand” based system, which means that the heat pump will
“cycle” on and off several times per hour. Each time a heat pump starts, like many other
appliances such as computers and washing machines, some disruption is caused on the electricity
network, which usually consists of “flicker” on tungsten filament bulbs. If a heat pump is to be
connected to radiators it is therefore essential to consider the robustness of the power supply to
Where the building has either: -
1. A three-phase power supply, and a three-phase heat pump is fitted or
2. Its own individual transformer serving just that building
- then the chance of any disruption being experienced in any neighbouring properties is
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However, where a heat pump is connected to a power supply from a single-phase transformer
shared with another property then great care must be taken. This can be mitigated by fitting a
device to the compressors that reduces the current drawn at start up. Commonly called a “soft
start”, this type of device does not work with scroll compressors, and should not be fitted. A new
type of device has been developed specifically for scroll compressors, and this is fitted as standard
inside all single-phase Kensa Compact Heat Pumps For Radiators – please see separate FactSheet
on Special Features Of Kensa Compact Heat Pumps For Radiators.
The second consideration is that of tariff. It doesn’t really matter when you run a heat pump
connected to a “wet” underfloor heating system – as it takes a very long time to heat up a
building, and a long time for it to cool down. As radiators are a “demand” based system, the
building will “call” for heat at any time of the day, whether a cheaper banded tariff is available or
not. As off-peak electricity is roughly one-third of the cost of peak electricity, the electricity cost
for a heat pump connected to radiators could be three times that compared to a heat pump
connected to underfloor heating. There are some tariffs that will help mitigate this, such as
Scottish & Southern’s “Economy 10” (E10) – please see the separate fact sheet on this subject.
The E10 tariff gives ten hours of off-peak electricity split into three-bands, including during the
day. Other energy companies have similar tariffs for heat pumps, and the best idea to contact
your local energy provider.