Heat Pump Glycol vs Water UK 2026

Most UK air-source heat pumps in 2026 are monoblock units - the refrigerant cycle is fully contained within the outdoor unit. Only water flows between the outdoor unit + indoor (hydronic) system via insulated pipes.

The risk: if the water in the outdoor unit's heat exchanger freezes during a power cut + cold snap, the heat exchanger can rupture. To prevent this, modern heat pumps include:

• Frost protection mode: when outdoor temperature drops below ~+3°C + the heat pump is off-cycle, the controller runs the circulator pump continuously to keep water moving (moving water doesn't freeze readily).
• Backup electric heater: can fire briefly to keep water above freezing in extreme conditions.
• Auto-drain on power loss (some premium models): valves open + drain the heat exchanger water on prolonged power-loss + low temperature.

Result: monoblock heat pumps with frost-protection mode active don't need glycol in normal UK conditions. The exception is install configurations that defeat frost protection (isolated outdoor sections, off-grid backup absence).

1. Ground source heat pumps (ALWAYS):

• The ground loop sits at 0-5°C year-round + circulates through 30-100m of buried pipe.
• Even with circulation, the loop water can freeze in cold ground.
• Standard: 25-30% propylene glycol in the ground loop. Sometimes ethylene glycol but less common (toxic in case of leak).
• The internal heat exchanger side of GSHP may use plain water (separated by the ground loop's plate heat exchanger).

2. Air-source heat pumps with isolated outdoor pipework:

• Some split-system air-source installs run hydronic pipe outdoors between heat pump + indoor cylinder.
• If outdoor pipework can freeze independently of the heat pump's frost-protection, glycol is required.
• Standard: 15-25% propylene glycol for typical UK install.

3. Off-grid properties with unreliable power:

• Power cuts disable frost-protection mode + auto-drain.
• For properties prone to extended power cuts in winter, glycol provides cold-weather insurance.
• Standard: 20-30% propylene glycol depending on minimum expected outdoor temperature.

Glycol-water mixes (vs treated water):

• Heat transfer rate: drops 3-10% depending on concentration (5% at 15% glycol, 10% at 30% glycol). Translates directly to lower SCOP.
• Viscosity: increases substantially - the circulator pump uses 10-15% more electricity to maintain flow rate.
• System wear: glycol degrades over time (5-7 year service life typical), needing periodic top-up + eventual full system flush + refill. Untreated, breakdown products become corrosive.
• Cost: initial glycol fill GBP 100-300 depending on system volume; annual top-ups GBP 30-50; full flush + refill GBP 200-400 every 5-7 years.

For installs that genuinely need glycol, these costs are justified. For installs that don't, using glycol defensively costs efficiency + money without benefit.

For typical UK air-source heat pump installs (monoblock with frost-protection mode + reliable mains power):

• Treated water with corrosion inhibitor (Fernox F1, Sentinel X100): typical 1-2% concentration in the hydronic circuit.
• No glycol needed: heat pump's own frost-protection handles freeze risk under all normal UK conditions.
• Annual service: check water quality + top up inhibitor as needed. Much simpler than glycol management.
• Cost: inhibitor + initial fill ~GBP 30-60; annual top-up ~GBP 10-20.

This is the standard UK monoblock air-source install configuration. Reputable installers default to this; some installers push glycol as a defensive measure that's typically unnecessary + slightly reduces SCOP for the install's lifetime.

1. Does this install need glycol, and if yes, why? Installer should be able to point at a specific condition (GSHP / split-system / off-grid / coastal) justifying glycol. 'Just for safety' isn't sufficient.
2. What concentration + which glycol type? Propylene glycol is standard (less toxic than ethylene); concentration should match expected minimum outdoor temperature with reasonable margin (typically 20-25% for UK installs).
3. What's the service schedule + cost over 10 years? Annual top-ups (GBP 30-50/year) + flush + refill every 5-7 years (GBP 200-400 each). Factor this into total cost of ownership.