What’s Inside a Heat Pump? A Complete Guide to Refrigerants and Their Impact

Behind the quiet hum of a heat pump lies a powerful secret: refrigerants. These specialist liquids (or gases, depending on their temperature and pressure) are the unsung heroes responsible for transferring heat from outside into your home. But not all refrigerants are created equal — and understanding the differences can help you choose a heat pump that’s efficient, safe, and future-proof.

What Does a Refrigerant Actually Do?

At its core, a heat pump works by moving heat from a colder place (outside) to a warmer place (inside). To achieve this, it relies on a refrigerant — a substance that can easily switch between liquid and gas at low temperatures.

Here’s how it works:

1. The refrigerant absorbs heat from the outdoor air, ground, or water.

2. It boils into a gas at low temperature.

3. The gas is compressed, which makes it much hotter.

4. The hot gas transfers heat to your home’s water system (for radiators, underfloor heating, or hot water).

5. The refrigerant cools, condenses back into a liquid, and the cycle repeats.

This process is sealed inside the heat pump — the refrigerant never escapes unless there’s a fault.

Why the Right Refrigerant Matters

Not all refrigerants behave the same. Choosing the right one affects:

• Efficiency (how much heat you get per unit of energy)

• Operating temperature range (vital in cold UK winters)

• Environmental impact (measured by GWP and ODP)

• Safety (some refrigerants are flammable or toxic)

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Key Properties of Heat Pump Refrigerants

Property	Description
Boiling Point	Needs to boil at very low temperatures (e.g. -20°C) to work in cold air.
GWP (Global Warming Potential)	A measure of its impact if released into the atmosphere. CO₂ = 1. Some refrigerants exceed 2,000.
ODP (Ozone Depletion Potential)	Older refrigerants damaged the ozone layer. Modern refrigerants aim for ODP = 0.
Flammability	Propane-based refrigerants are flammable but efficient and low GWP.
Toxicity	Some refrigerants (e.g., ammonia) are toxic and require careful handling.

Common Refrigerants Used in Heat Pumps

R-410A (Hydrofluorocarbon Blend)

• Used in older domestic systems

• Non-ozone depleting

• High efficiency

• Very high GWP (~2088)

• Being phased out under environmental regulations

  
  

R-134a

• Used in medium and large systems

• High GWP, not as efficient

• Being phased out

  
  

R-32

• Improved efficiency over R-410A

• Lower GWP (~675), but still significant

• Mildly flammable

  
  

R-290 (Propane) — The Emerging Favourite

• Very low GWP (~3)

• Natural refrigerant with excellent thermodynamic properties

• Highly flammable, so systems have strict charge size and installation regulations

• Ideal for high-flow temperature systems (e.g. older UK homes with radiators that need 70°C water)

• Becoming the go-to choice for many modern domestic heat pumps

Fun fact: R-290 (propane) is the same compound used in camping gas — and even in some aerosol sprays.

Is It Safe to Use Propane in a Heat Pump?

Yes, when properly installed. Although propane is flammable, domestic heat pumps use very small quantities — often less than a kilogram. Units are designed with strict safety standards, and the risk is no greater than using natural gas in your home.

Why it’s used despite flammability:

• Excellent efficiency at high temperatures

• Extremely low GWP (environmentally friendly)

• Compatible with UK heating needs

  
  

What About CO₂ and Ammonia?

These refrigerants are used in large commercial heat pump systems, especially where efficiency and environmental performance are top priorities.

• CO₂ (R-744): Zero GWP, excellent efficiency, not common in domestic systems yet

• Ammonia (NH₃): Zero GWP and ODP, but toxic and flammable; best reserved for industrial settings

  
  

Do Homeowners Need to Worry About Refrigerants?

Generally, no. If you’re choosing a heat pump, your installer will match the refrigerant to your heating needs and local building regulations. However, you might want to:

• Ask which refrigerant your heat pump uses

• Choose R-290 if you want maximum efficiency and minimal environmental impact

• Understand that refrigerants with high GWP (like R-410A) are being phased out, so consider future availability and serviceability

Summary: Choosing the Right Refrigerant

Refrigerant	GWP	Flammability	Notes
R-410A	~2088	No	Being phased out
R-32	~675	Mild	More efficient, lower GWP than R-410A
R-290 (Propane)	~3	Yes	Low GWP, high performance, strict safety rules
R-744 (CO₂)	1	No	Excellent, but rare in homes
NH₃ (Ammonia)	0	Yes, Toxic	Commercial use only