Heat Pump Freeze Protection: Are We Overreacting?

In the world of heat pumps, manufacturers and suppliers often warn us about the risk of units freezing up, which understandably causes a lot of concern. But is this panic really justified?

At its core, a heat pump is just a big box in your garden, connected to your home with water pipes that, if installed correctly, should be well insulated. The concern stems from the fact that water freezes at 0°C, and when water freezes, it expands with enough force to split pipes or damage components within the heat pump. Naturally, no one wants to deal with that.

But here’s the good news: there are a few ways to prevent this from happening, and the solutions are simpler than they might seem.

Add Antifreeze

One common solution is to add glycol (antifreeze) to the water in the system, similar to what you’d do with a car radiator. Glycol lowers the freezing point of water, often down to -15°C, depending on the mix.

There are 37.3 million cars in the UK, most with glycol in their radiators. When was the last time you heard of someone’s car springing a water leak due to freezing? Glycol works, but it does come with some downsides: it’s thicker than water, making it harder to pump through your system. It’s not cheap, and it reduces the water’s ability to absorb heat, meaning the system has to work a little harder. Plus, if it leaks, it can stain carpets (it comes in a variety of colours, but none you’d want on your floor). And perhaps most frustrating, it’s a pain to pump into the system.

Install Antifreeze Valves

Another option is to fit antifreeze valves. These are designed to release water from the system if it gets close to freezing. Sounds great, right? Well, in theory, yes. But in practice, it’s not always ideal. When these valves open, they drain the system of water, meaning you’ll need to refill it. And refilling isn’t as simple as just turning a tap — you’ll have to add more inhibitor and purge any air from the system, which can be tricky.

So, it’s freezing cold, the valves open, and now you’re without heating or hot water until a plumber can come by. And guess what? It’s winter, and they’re busy. So, while antifreeze valves are a good idea in theory, they can be a bit of a hassle in reality.

Let the Heat Pump Protect Itself

Most modern heat pumps come with built-in freeze protection. Here’s how it works.

When it’s cold outside, the heat pump is usually running, so the pipes stay warm and there’s no risk of freezing. But if the system is off (say, because you’re away on holiday), and the water inside the unit drops below 10°C, the heat pump and circulation pump will automatically start to keep things warm. This is stage one of freeze protection.

Stage two kicks in when the ambient temperature drops below -2°C. At this point, the circulation pump runs constantly to keep the water moving, because moving water is harder to freeze. As long as the power stays on, the system can look after itself.

But what happens if there’s a power cut? If temperatures drop below freezing and the power is off, the heat pump can’t protect itself. This is where antifreeze or antifreeze valves come in handy.

But Here’s the Thing…

Think about outdoor oil boilers — they’re also big boxes in the garden, connected by water pipes, and at risk of freezing if the temperature drops. Yet, no one uses glycol or antifreeze valves on oil boilers.

If there’s a power cut, the oil boiler stops. It can’t protect itself either. Sound familiar?

Yet, how often have you heard of an oil boiler freezing up? It’s about as common as people being attacked by swans (a rare but entertaining urban myth). And if you do meet someone whose oil boiler froze, they probably just called their insurance company and claimed for a new one, just like you’d do if your pipes froze in the loft.

The Real Risk

In my 15 years working with heat pumps, I’ve sold over 10,000 units. How many froze up? Two.

In one case, the homeowner switched off the power in January and went on holiday for two weeks. In the other, the customer was experimenting with the unit and forced it into defrost mode, just to see what would happen. Neither system had antifreeze or antifreeze valves.

So, while the risk of freezing is real, the likelihood of it happening is incredibly rare. If you follow good installation practices and make use of your heat pump’s built-in protections, freezing shouldn’t be something that keeps you up at night.

In the end, it’s worth considering your options, but don’t let the fear of freezing send you into a panic. Chances are, your heat pump will be just fine.