Engineering practice · Heat pumps
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‘Operates down to −25 °C’ — why this is not enough to choose a heat pump
The first page of the catalogue says the heat pump operates down to −25 °C. To a client, this often sounds like an answer to the main question: there will be enough heat in winter.
I would break this statement down into three questions. Is operation permitted at that temperature? What heating capacity does the unit deliver? How much electricity does it need to do so? The lower operating limit, capacity and efficiency are different characteristics. Manufacturers list them separately. (Panasonic — Aquarea T-CAP M Series: technical specifications)
Cold-weather capability does not mean heat losses are covered
We start with the building: what temperature needs to be maintained, how much heat is lost through the building envelope, and what contribution ventilation makes. Floor area alone does not answer these questions. SEAI also links heat pump assessment to heat losses, heat emitters and controls. (SEAI — Heat pumps: system assessment and design inputs)
Suppose an illustrative building has a design heat demand of 12 kW at −20 °C. For the selected unit under the same conditions, at the required water temperature, the assumed available capacity is 8,5 kW.
Shortfall:
The heat pump may be working correctly, but the building still needs another 3,5 kW. This is not a hidden fault. It is the result of the selected sizing. The shortfall can be deliberately covered by a backup heat source, but it cannot be ignored.
All figures in this example are illustrative. Actual selection uses the specifications of the particular model.
Water temperature changes the picture
The designation A−7/W35 describes operation with outdoor air at −7 °C and leaving water at +35 °C. This is a different operating point from A−7/W55.
Panasonic’s published table for the 12 kW version of the Aquarea T-CAP M Series gives a COP of 5,06 at A+7/W35, 3,00 at A−7/W35 and 2,17 at A−7/W55. Heating output is stated as 12 kW at these points. (Panasonic — Aquarea T-CAP M Series: technical specifications)
The calculated electrical input is therefore:
The same heating output, but different power consumption. These are the specifications of the stated version under rated conditions, not average values for all heat pumps.
Defrosting is part of winter operation
When frost forms on the outdoor heat exchanger, the system runs a defrost cycle. Air temperature is not the only factor: humidity and the unit’s specific control algorithm also matter. When assessing sustained operation, the entire cycle must be considered, rather than selecting the best minute between defrosts. The effect of these algorithms on overall efficiency is also studied in field trials. (Energies — field study of heat pump defrost algorithms)
There is no need to deduct an arbitrary ‘15% for defrosting’ again if it is already accounted for in the performance data used. First establish the manufacturer’s methodology, then apply any necessary corrections.
What equipment selection should include
I would require a comparison of the building’s demand with the heat pump’s capabilities at several outdoor temperatures. For each point, this should show the required water temperature, available heating output, electrical load and contribution from supplementary heating.
Operation during the shoulder seasons is a separate issue. A large unit selected solely for the coldest day must also be checked at low load.
Do not simply choose a heat pump that keeps running in freezing weather. Choose a system that delivers the required heat with known consumption and a clear backup heating strategy.