Page 79 - Build 152

100% 6 90%
All types of heating Heat pumps only
80% 70% 60%
5
4
50% 3 40%
30% 20% 10%
2
1
00
Morning Day Evening Night 24 hours
Figure 1: General household heating.
can be misleading however, as these values are typically determined when the outdoor temperature is 7°C, much warmer than a cold winter’s day. When the temperature outside drops, the e ciency of the heat pump is also reduced.
It is often when the outdoor temperatures are lowest that heat is required from heat pumps. A typical case may be after an evening of heating, the occupants all go to bed and turn o the heat pump. The cold overnight temperatures then drive the indoor temperatures down to their lowest point just before people start getting up. The heat pump is then turned on and operates at its hardest to get the room back up to a comfortable temperature.
The outdoor conditions at this time may also cause frosting problems with the outdoor unit. There are a variety of defrosting approaches. One approach is to reverse the heat pump cycle so the outdoor unit is defrosted by pumping heat from the inside of the house. This requires a blast of cold air from the indoor unit. This defrosting approach also reduces the energy performance of the heat pump.
Typically 2 times the heat output as energy input
An often quoted performance factor for heat pumps is that they provide 3 times the heat output as the energy input going into them. This gure tends to align with the laboratory test method of assessing performance.
Measuring the heat output of a heat pump in actual use in a real house is di cult. A method to estimate the performance in actual
1.0 1.2
1.4 1.6
1.8 2.0 2.2
2.4 2.6 2.8 3.0 3.2 3.4
Measured heating performance factor
Figure 2: Heat pump performance values from study.
practice was examined for a small subsample of the heat pumps in the heat pump study.
While these performance values are somewhat appropriate, on average, the performance of the heat pumps in actual use was to provide 2 times the heat output for the energy input into the system. This is somewhat less than the commonly used 3 times. The range of performance values using this method is shown in Figure 2 and was mostly between 1.5 and 3 times the energy input.
Reasons for poorer performance
In addition to operating at times when the outdoor conditions are cooler, other factors that may reduce heat pump performance in actual use are:
● non-ideal air ows from the indoor unit
● poor location and placement of the outdoor unit
● operating at elevated temperature or using incorrect settings ● on-o operation of the heat pump.
Getting better all the time
Heat pumps have become a well established technology, and occu- pants tend to readily recommend them to others. Their performance in actual use tends to be lower than the declared value. However, manufacturers are improving heat pump technologies all the time, and newer models generally perform better than older ones.
For more This work is reported in BRANZ Study Report 329 Heat pumps in New Zealand, which can be freely downloaded from www.branz.co.nz/shop.
Build 152 — February/March 2016 — 77
Proportion of households
Number of heat pumps