Heat Pumps That Don’t Stop in the Cold

Low ambient heat output performance degradation is the single largest reason for customer dissatisfaction with air-to-air heat pumps. As the outdoor ambient decreases, the ability for many brands of heat pumps to produce useful heat declines.  This is due to many factors directly related  to heat pump design as well as the poor standards of insulation in older housing stock. The lower the outdoor ambient temperature, the greater the differential between indoor and outdoor temperature across the building fabric. This causes higher rates of heat leakage and effectively increases the heat load on the heat pump in low ambient conditions.

Outdoor unit coil size is critical to a heat pump’s ability to absorb useful heat. Size really does matter in heat pump coil design. The larger the coil effective surface area , the easier it is for the coil to absorb heat at a given airflow rate and temperature. Many brands  use small outdoor coils and rely on higher compressor frequencies to increase performance at low ambient. In New Zealand conditions where we are prone to high relative humidity at low temperatures — especially 5°C and lower — the outdoor coil will operate below zero degrees.

This will cause a build up of ice on the outdoor coil. The smaller the coil the faster the ice will build up and the more often the unit will have to go into defrost cycle to clear this ice. While a unit is defrosting it is not providing the customer with useful heat. This is most common on units that are designed primarily for cooling performance and for overseas conditions.

Software is also a critical factor in heat pump low ambient performance. The ability to defrost on demand only using intuitive software logic greatly increases useful performance. Delivery air temperature or off coil temperature is also a critical performance factor. The higher the air off temperature that can be maintained during low outdoor conditions, the quicker the controlled space will be up to temperature and the quicker the outdoor unit compressor frequency can ramp down and thus run more energy efficiently. Many brands and models of heat pump struggle with this key performance requirement.

When selecting heat pumps in areas of New Zealand where low ambient conditions exist, one should consider quality brands with superior low ambient performance characteristics  and premium performance technology over price. Technology such as HyperCore® from Mitsubishi Electric that are able to deliver high air off conditions and maximum heat output down to sub zero conditions should be considered. These units have been installed in several difficult locations such as high in the Kaimai ranges, Timaru and in and around Taupo where perma frost, sub zero day time conditions and early morning fog and mist are prevalent. In these locations the premium HyperCore® units have exceeded customer expectations and performed where the originally installed brands were not able to perform.