Freeze protection is essential for domestic heat pumps, particularly in areas where temperatures stay below 0°C for extended periods. Two main methods are used to protect outdoor units and exposed pipework:

  1. Glycol antifreeze mix in the primary circuit

  2. Freeze-protection valves with untreated water

Glycol Antifreeze

A 20-30% glycol/water mix protects the heat exchanger and pipework even during power outages.

Pros:

  • Highly reliable in cold climates and areas with frequent power cuts

Cons:

  • Higher installation cost

  • Slight reduction in heat transfer and COP

  • Increased pumping effort due to higher viscosity

  • Requires periodic checks of glycol concentration and inhibitor levels

Freeze-Protection Valves

The system uses untreated water; valves at low points open at around -3°C to drain exposed pipework.

Pros:

  • No efficiency impact

  • Lower pumping energy

  • Lower cost and simpler maintenance

Cons:

  • If activated, the system drains and heating stops until refilled

  • Depends on correct positioning and installation

MCS guidance supports either method when risks are properly managed. In southern and urban areas, where sub-zero periods are brief and external pipe runs are minimal freeze-protection valves are typically adequate. In rural or northern regions, or where power cuts are common, glycol is generally the safer option despite its small efficiency penalty.

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