Heat Pumps: An Overview

image of a heat pump outdoor unit

Heat Pump Fundamentals

Heat pumps operate on the refrigeration cycle, using the evaporation and condensation of a refrigerant to absorb heat from a low temperature source and release it at a higher temperature. The main components are the evaporator, compressor, condenser, and expansion valve.

The evaporator allows the refrigerant to evaporate, absorbing heat from the external air, ground, or water. The compressor increases the pressure and temperature of the gas refrigerant. The condenser then condenses the hot refrigerant, releasing heat to the heating or hot water system.

An expansion valve lowers the pressure after the condenser to begin the cycle again. The efficiency of a heat pump is measured by the coefficient of performance (COP), which is the ratio of heating capacity to the electrical input power. Typical COP values range from 2 to 5. Higher COPs indicate greater efficiency.

Common low temperature heat sources used are air, ground, bodies of water, and exhaust air. Heat is released to space heating and domestic hot water systems in the building.

System Types and Selection Factors

Air source heat pumps (ASHP) use external air as the heat source. In an air-to-air system, the indoor space is directly heated by air circulated over the condenser heat exchanger.

More common is air-to-water systems which heat hydronic distribution systems. ASHPs come in split and packaged configurations.

Split systems have separate outdoor compressor/evaporator units and indoor units with the condenser. This allows flexible installation locations.

Packaged systems integrate all components in a single outdoor unit. Monoblock ASHPs have non-variable compressors which cycle on/off. Inverter-driven variable capacity compressors can modulate heating output and are more efficient.

In ground source heat pumps (GSHP), buried vertical or horizontal loops circulate fluid to exchange heat with the earth. Closed loops use recirculated water/antifreeze solution.

Open loops draw and discharge groundwater. Closed loops avoid groundwater contamination but require more extensive digging or drilling.

GSHPs can maintain higher efficiencies in cold climates but have higher upfront costs. Exhaust air heat pumps (EAHP) recover heat from building ventilation airflows and are suitable for spaces with high air exchange like bathrooms. EAHPs provide ventilation air heating/cooling but may need supplementary heating.

Key selection factors for heat pumps include the required heating and cooling capacity, hot water demand, type of distribution system, operating temperatures, efficiency, operating costs, available installation space, upfront costs, and regulatory requirements.

Proper sizing of the heat pump system components is critical for optimal performance and efficiency.

Hydronic Distribution Systems

Common heat emitters used with heat pumps include in-floor radiant systems, radiators, and fan coils. Radiant floor heating, where pipes are embedded in the floor, allows the lowest operating temperatures down to 25-30°C. This maximizes heat pump efficiency.

Radiators can also work with moderate temperature heat pumps with decreased output capacity compared to high temperature systems. Fan coils use a water coil and electric fan to provide rapid heating of indoor air, but require higher water temperatures around 45-50°C.

Buffer tanks are used to store heated water and cushion the gap between heat pump output and heating demand.

They allow the heat pump to maintain steady output despite fluctuations in demand. Buffers are required if there is zone heating control, different temperature circuits, or limited piping volume.

The buffer size depends on heat pump power, minimum run time, and system volume. Buffers help prevent short cycling which reduces compressor life.

Pipes must be sized to accommodate the required water flow rates from the heat pump while limiting pressure drops. Smaller diameters lower costs but increase pumping needs and flow resistance.

Pipes below 15mm diameter may starve flow for heat pumps. Larger pipes reduce velocity and pressure losses. High temperature differentials between supply and return lines indicates insufficient flow rates. Optimal pipe sizing must balance these factors.

Electrical and Controls

Heat pumps require dedicated power supply circuits sized appropriately to handle startup surges and steady operation. In the UK, heat pump installers must notify distribution network operators to review grid capacity at the site. Control systems help improve heat pump efficiency.

Weather compensation uses outdoor temperature to adjust the water temperature setpoint, lowering it during warmer periods. The controller follows a heat curve relating the two temperatures.

Thermostats sense indoor conditions to avoid overheating. Smart controls can optimize start times and operate during off-peak electricity rates. Integrated systems link the heat pump with solar panels, hot water tanks, and building controls.

Installation and Commissioning

Heat pumps must be installed following all applicable codes and standards. Permitted use and locations vary. Installers must verify heat distribution systems were correctly implemented and adequate freeze protection exists.

Refrigerant pipes must be properly pressure tested. Ground loops require flushing to remove debris and air pockets before pressure testing. Initial water flows should be smooth without obstructions. Standing water from hydrostatic tests must be properly disposed of.

Once filled, the system should be purged of air and circulated at full flow rate, checking for leaks and vibrations.

Balance valves ensure even flow across parallel collector loops or circuits. The refrigerant charge level must be optimized for full heat transfer, without overcharging the system. System controls must be calibrated and configured, including weather compensation settings.

All sensors should read temperatures accurately. The compressor current draw should match specifications. Backup heating elements should activate at the correct setpoints.

Commissioning includes verifying correct flow rates, return and supply temperatures at all operating points, pressure reliefs, electrical conditions, and noise/vibration levels. Functional testing steps through all modes and actuators to validate performance.

Anti-freeze concentration must be confirmed with a refractometer when using ground loops. The completed system should meet handover checklists and be issued a certificate of compliance.

Maintenance and Service

Routine maintenance on heat pumps improves longevity and efficiency. Tasks should follow manufacturer instructions. Maintenance logs documenting system performance help diagnose issues. Air filters, evaporator coils, fans, and airflow paths need regular inspection and cleaning. Condensers may require brushing or hosing.

Manufacturers specify glycol inspection intervals for ground loops. Electrical connections, capacitors, contactors, and controls should be checked for tightness and corrosion. Refrigerant levels can be verified by measuring operating pressures and temperatures.

Common troubleshooting steps include checking error codes, sensor measurements, and actuators. Unusual compressor current draw, freezing of coils, or reduced heating capacity indicate refrigerant charging faults.

Other causes of reduced output are dirty filters or coils, fans and pumps not operating, airflow blockages, and valves stuck closed.

Suspected refrigerant leaks require an HVAC technician. Higher than expected run times or rapid temperature swings suggest a control problem. Noisy compressors or uneven collector loop temperatures point to potential mechanical issues.

Annual maintenance should log temperatures and pressures at key points to identify performance degradation over time. Refrigerant system repairs should only be conducted by certified technicians but installers can perform most standard maintenance procedures on the balance of system themselves with proper training.

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