Whilst I am a firm believer in the potential economics of heat pumps, I am concerned by the number of calls for assistance we received from heat pump installers during the period of sub-zero temperatures experienced at the beginning of December ‘12.
The main issue raised was in some way expected as defrost water from an air source heat pump accumulates on the ground below, and sometimes even builds up inside the heat pump casing itself.
The commercial refrigeration industry has always needed to combat problems associated with defrost water drainage. In a freezer-room, a defrost water drip- tray would be fitted with trace heaters to ensure water could not freeze on contact. The drip tray would also be fitted with a single dedicated water drainage point, feeding into plumbed pipe work, again equipped with a trace heater tape and covered with heavy thermal insulation.
None of the air source heat pumps we have examined possess any form of tray heat at all, and drip-trays often have multiple drain holes, making a single dedicated drain off point impossible.
When defrost water cannot easily drain from the drip-tray, each subsequent defrost cycle progressively makes the situation worse.
When ice build-up commences, the situation can quickly accelerate with each defrost until excessive ice build-up eventually encroaches into the path of the fan, and leads to failure of the heat pump.
What’s the solution?
Make sure that defrost water can leave the heat pump effectively by ensuring the heat pump is elevated above ground level by at least 300mm, and a trace heater is fitted to the drip-tray.
Trace heaters can be purchased from various sources and should be installed only by qualified personnel.
Heaters chosen should be low on energy requirement, and therefore need to be insulated on the external surface of the drip tray to ensure that heat is not lost to the atmosphere.
Trace heaters should only be energised when absolutely necessary i.e. when sub-zero temperatures prevail.
Manufacturers need to be consulted to ensure any alterations or additions do not invalidate the heat pump guarantee.
Bivalent energy
As outdoor temperatures plummet, so does the output of the heat pump, and of course, the all-important COP.
Recently, the Department of Energy and Climate Change legislated for the use of bivalent energy. In short, the heat pump must singularly deliver the total heat requirement for a given property, in ambient temperatures down to approximately -3°C.
The problem is that the energy deficit, created by fluctuating ambient conditions, is difficult to quantify and equally difficult to supplement with any degree of accuracy.
To maintain the highest COP possible, the energy deficit cannot be over-catered as this would waste energy, and if under-catered, will result in a cold customer.
Currently, many manufacturers of heat pumps incorporate a “climatic compensation heater” embedded within the heat pump unit.
This is NOT the ideal location to add bivalent energy, and can adversely impact on the heat pump’s running economics.
Accurate measurement and proportional control of bivalent energy is the only way to ensure the heating demand is met, but above all, that energy is not being wasted and the COP is not being compromised by over-compensation.
I will discuss how to accomplish these both economically and effectively next month.