Heat Pump

Defining the main unit

Introduction

Selecting the right heat pump is a critical step in ensuring efficient and effective heating. The heat loss and airflow calculations performed earlier provide the foundation for this decision. By determining how much heat each room loses and how much airflow is required to maintain comfort, the heat pump can be accurately sized to meet the needs of the entire property.

Heat Loss as the Foundation

The heat loss calculation tells us how much heat the system needs to generate to maintain a comfortable temperature in each room. For example, if a room loses 1.5 kW of heat, the heat pump must supply at least 1.5 kW to offset this loss. By summing the heat loss for all rooms, the total heating demand of the property is determined. This ensures the heat pump is neither undersized (struggling to meet demand) nor oversized (wasting energy and cycling frequently).

In my case, the total heat loss across all rooms was 14.604 kW. This figure became the baseline for selecting a heat pump with sufficient heating capacity.

Airflow Requirements

The airflow calculation complements the heat loss data by determining how much warm air needs to be distributed to each room. Since air-to-air heat pumps rely on moving air to deliver heat, the system’s fan must be capable of meeting the total airflow demand. For example, if a room requires 0.2 m³/s of airflow to maintain its target temperature, the heat pump’s fan must be able to deliver this volume without strain.

The total airflow requirement for my property was 0.571 m³/s. This ensured the heat pump’s fan capacity aligned with the needs of the system.

A quick note on CoP vs SCoP and EER vs SEER

Co-efficient of Performance (CoP) measures the instantaneous efficiency of a heat pump at a specific operating condition. It is the ratio of useful heating or cooling output to the electrical energy input. Seasonal Co-efficient of Performance (SCoP), on the other hand, measures the average efficiency of a heat pump over an entire heating season, accounting for variations in outdoor temperature and operating conditions.

This is analogous to the difference between a car’s fuel efficiency measured at a constant speed versus its efficiency measured over a varied driving cycle. Both provide a more realistic representation of performance in real-world use over time.

EER and SEER are to cooling what CoP and SCop are to heating, and will be largely ignored in my assessment since I only intend using the system for heat.

Selecting the Heat Pump

With the total heat loss and airflow requirements in hand, I could confidently select a heat pump that met these specifications. Key factors included:

Key considerations for heat pump selection:

  1. Nominal Heating Capacity: Typically, it could be stated definitively that the heat pump must provide at least 14.604 kW of heating output. However, since the approaching winter necessitated the installation of the heat pump while some insulation and air-tightening improvements remained incomplete, this value represents an absolute upper bound for heat loss.
  2. Minimum Heating Capacity: This is much more important than you may initially think. When outdoor temps are higher, meaning less heat output is required from the system, the heat pump must be able to modulate down to a low output to prevent cycling. This is facilitated by an inverter.
  3. Minimum Working Temperature: -4°C
  4. Fan Capacity: The unit’s fan must be capable of delivering 0.571 m³/s of airflow to distribute heat effectively.
  5. Efficiency: A high SCoP ensures the heating system operates efficiently, reducing energy consumption and costs.
  6. Refrigerant: R32 has lower environmental impact, higher energy efficiency, and aligns with global phase-down goals, making it sustainable and cost-effective.

Final Choice

Based on these criteria, I selected the Samsung AC140RXADKG outdoor unit and the AC140RNMDKG indoor unit, which use R32 refrigerant. With a nominal heating capacity of 15.5kW, the ability to modulate down to 3.5kW, and a fan capacity of 550 m³/s, they are suitable for the requirements derived from the heat loss and airflow calculations. Additionally, it has a working temperature range down to -20°C and a SCoP of 4.