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After three years installed, we decided to revisit a Victorian hard-to-heat house on the Sandringham estate in Norfolk and look at how efficient the single-phase Ochsner GMLW 14 air source heat pump has been since installation. 


What’s very special about this GMLW 14 heat pump is that it is a split system design connecting the outdoor and indoor unit with refrigeration lines and the Compressor which is normally in the outside unit is infact located in the indoor unit. In some respects it is a ground source heat pump connected to a stainelss steel table. The outdoor unit is a horizontal table as opposed to a vertical design allowing for 3 times the pipe spacing, the defrost cycle utilises the heat generated previously by the heat pump to defrost the pipe and with the horizontal pipes having large spacing this results an an efficient defrost cycle. On the astetics the outdoor table can be ordered in any RAL colour to blend in with your property. 


 OCHSNER GMLW 14 AIr Source Heat PumpRecap: Ling House is a Grade 2 listed, six-bedroom B&B family home on the Sandringham Estate. It has solid brick walls with no insulation, single glazed windows and no insulation on the floor or walls along with an insulated loft. The property is heated via original character radiators, and therefore the flow temperature of the heat pump is required to be 50 deg on a weather compensated heating curve. This temperature is well within the 65 deg capacity of the GMLW 14 heat pump. It is also a split system meaning the compressor is located indoor in the plantroom, and refrigeration lines connect to the outside stainless steel heat exchanger which allows for higher efficiency. Note: split systems are very different from most of the Monobloc air source heat pumps on the UK market. 


We are often asked “how efficient is an air source heat pump?” 


Well, firstly there are 2 or 3 very distinct technology types in the category of air source heat pumps and with each type, the efficiency and lifetime vary. The entry-level monoblocs are the budget option with the least efficiency and shortest design life of ~25,000 run hours or ~8 years. Then there are split system vertical fan heat pumps and split system horizontal table heat pumps (with a design life of 60,000 run hours or ~20 years. So it is worth asking what technology is in the air source heat pump. 


There are also two ways to answer the efficiency question: 


  1. people tend to refer to the manufacturer’s datasheet when the new heat pump is tested in a laboratory under ideal conditions or;
  2. show the actually measured performance of the heat pump from a UK installation. To do this we need to measure both energy in (electricty) and energy out (heat), or in other words, the electricity consumed by the heat pump using an electricity meter (C & D in the photo below) and the heat produced by extracted energy from the rain and air (A & B in the photo below) using a heat meter. 


In the pictures below, you can see the readings from these two types of meters and to calculate the efficiency of this heat pump at flow temperature 50 deg you divide one by the other. 


These two Ochsner GMLW 14 split system table heat pumps after three years are performing well with an average efficiency of 3.84 this is an excellent performance at such high flow temperatures. 


In summary, efficiency COPs in the manufacturer’s datasheets or particularly in the MCS database where the manufacturer is allowed to adjust to a specific bivalent temperature, may not be what is achieved in the field and depends on the quality of the design and quality of the installation. To answer the question, if you want to to know how efficient an air source heat pump is, first ask what technology type of air source heat pump is it (Monobloc, split system or split table system,) then ask for measurements from an installation to be sure.



Air source heat pump actial efficiency

Air source heat pump actual efficiency

Air source heat pump in Norfolk








GMLW 14 Air Source Heat Pump datasheet COP vs Measured COP

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