23 September 2011


Heating performance of conventional air-source heat pumps dramatically drops at
outdoor temperatures below -5°C, and therefore supplementary heating sources are required.
In cold climates, if electricity is used as a back-up heating source, at outdoor temperatures
below -12°C it may coincide with the grid peak power demand. Additional energy sources,
other than electricity, could consequently help in avoiding the use of an expensive, highquality energy source for residential comfort heating. This paper presents a variable speed
heat pump with a non-electrical back-up energy source. It highlights the original design and
features, as well as the main energy performance. In the conventional air-to-air heating mode,
the heat pump balance point has moved from -5°C to -9°C. Between the new balance point
and the shutdown outdoor temperature (-12°C), the compressor operates about 70% of the
time at maximum speed, with net heating gains exceeding 2 kW, but with lower coefficients
of performance. In the combustion gas-to-air heating mode, activated below outdoor
temperatures of -12°C, a propane burner provides heat to a very compact add-on evaporator.
In this mode, fuel consumption is 76% lower and the heating power supplied 13.3% higher
compared to those of similar dual-energy heat pumps.