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While heat pumps have to be rated for efficiency under laboratory conditions at a 7°C ambient outdoor temperature (called H1), actual mid-winter temperatures in some parts of New Zealand, particularly in the central North Island and in the South Island, will give significantly different performance. For this reason, standardised testing of heat pumps can also be rated at two additional levels of specific temperature and humidity. These are called H2 and H3 and are shown in Table 3.1. Table 3.1 Outdoor ambient temperature rating conditions These ratings allow a wider range of design temperatures to be used to select heat pumps for areas where the temperature may often be below 0 ̊C and ensure that, at low ambient temperatures, the heat pump will still provide the expected performance (see 4.5 Step 3: Calculate heat load requirement). Manufacturers will often test their product at other temperatures (e.g. -5, 0, 5, 10 etc). 3.4 Impacts of defrost cycle on efficiency For optimal heat pump efficiency, a system must be correctly sized to minimise energy losses that occur during the defrost cycle. The defrost cycle is necessary to remove ice build-up on evaporator coils. Ice build-up occurs at around at 0-4 ̊C (especially in high humidity), when any water vapour in the air will start to condense and freeze onto the evaporator (outdoor heat exchanger) coils. This will disrupt the heat flow, and the coils must be de-iced for heating to be able to continue. To remove ice build-up on the coils, most heat pumps have a defrost cycle where the system switches into cooling mode (taking some heat from inside), which could effectively cool the room. Some systems have a closed loop cycle to use waste heat from the motor/compressor to defrost the coils. While this is occurring, no heat is supplied to indoors. The defrost frequency and performance are critical to heat pump efficiency. Undersized heat pumps will need to defrost frequently in low ambient temperatures, reducing the system’s ability to reach and maintain set point. If the defrost cycle operates too frequently or if it does not operate often enough, it will not provide sufficient heating, and heat pump operation will be compromised. The defrost cycle control is either: • a time-temperature defrost starting and stopping at preset times (30-, 60- or 90-minute intervals); or • on-demand defrost, which is generally more efficient because it operates only when it detects frost build-up on the outdoor coil by monitoring air and coil temperature, outdoor airflow, pressure differential across the coil and refrigerant pressure. Systems that include a dry-coil defrost cycle briefly run the outdoor fan at maximum speed before the system starts to heat again, to remove any water that may still be on the coil fins and would immediately refreeze. This operation can be seen by water vapour blowing from the outdoor unit before the heating cycle resumes. Rating Outdoor ambient temperature ( ̊C) rating conditions Dry bulb (DB) H1 7 ̊C H2 2 ̊C H3 -7 ̊C 15PDF Image | Heat pump installation Good Practice Guide
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