Understanding the Role of Anti-Freeze Valves vs. Glycol in Heat Pump Systems

In cold climates, protecting heat pump systems from freezing is essential to ensure efficient operation and prevent damage. Traditionally, glycol has been used to lower the freezing point of the fluid within these systems. However, an alternative method involves the use of anti-freeze valves, which automatically drain the system when temperatures drop below a certain threshold.

This article explores whether anti-freeze valves can serve as a replacement for glycol in heat pump systems, examining the advantages and disadvantages of both approaches to help you determine the most suitable freeze protection strategy for your needs.

The Drawbacks of Using Glycol in Heat Pump Systems

Most heat pump installers will be familiar with glycol, and if you've worked with us through our MCS Umbrella Scheme or attended our heat pump training, we’ve likely recommended glycol as part of your system setup. While glycol is commonly used, it's important to be aware of a few downsides:

Cost: Glycol can be expensive, and typically, multiple containers are required to fill a heat pump system adequately. A 20-liter container of glycol costs around £200, meaning that using several tubs can significantly increase the overall cost of the system.
Maintenance: Glycol needs regular testing and replacement to ensure it continues to function effectively. If you need to drain the system, the glycol must be replaced, which adds ongoing maintenance costs. This is something we've covered in our heat pump training, as it’s crucial to factor in maintenance when considering system longevity and performance.
Performance: Glycol is a viscous liquid, and even when diluted with water, it can reduce the efficiency of a heat pump. This increased viscosity means the system has to work harder to circulate the fluid, which can impact overall energy efficiency.
4. Toxicity: Glycol is toxic if ingested, so great care must be taken when draining the system to avoid contamination.

What at are the Benefits of Anti-Freeze Valves?

Anti-freeze valves are designed to drain down a heat pump system if the temperature of the heating circuit (not the outdoor temperature) drops below 3°C degrees. Each valve contains a small sensor that triggers when the heating circuit falls below this temperature. The system would have to lose power (and the pumps stop running) or we would have to enter a new ice age for this to happen. Similarly to glycol, this is done to prevent the heating system from being damaged in cold conditions. Here are the benefits to using anti-freeze valves:

Cost: For a pair of anti-freeze valves, the cost is comparable to that of a 20l tub of glycol (£200) however, only one pair is needed.
Maintenance: The valves are a fit and forget product, requiring no maintenance and should still be working in 7-8 years.
Performance: Anti-freeze valves serve as an alternative to glycol which means your heat pump system will work more efficiently due to the fluidity of the water.

To find out more, we’ve put together a short video to explain the benefits:

What about the heat pump warranty?

Many installers will be concerned about the possibility of invalidating a heat pump warranty by using anti-freeze valves. Thankfully, this is not the case. Most manufacturers state, ‘there must be anti-freeze protection’ and do not list glycol as the be all, end all. In fact, Bosch specifically state NOT to use glycol.

In the case of Mitsubishi, here’s their stance on anti-freeze valves:

“Anti-freeze valves can be used, but you must ensure the freeze stat function on the FTC6 is enabled.”

The Mitsubishi Ecodan has a function whereby when the outdoor temperature gets below 5 degrees, it runs the pumps to ensure water doesn't freeze. So anti-freeze valves are supported by Mitsubishi when that function is used (which it always is).

Can you use Glycol and anti freeze valves?

Some installers have asked about the possibility of using both anti-freeze valves and glycol for extra protection. This is not something we recommend. Mechanically, the system would work without a hitch, however, it would be a complete waste of money for your customer and wouldn’t add any value.

Adding glycol to the system would reduce the freezing point of the water in the system, however, it would not change the temperature at which the anti-freeze valves open. Therefore, if the valves were triggered, the system would be drained meaning you would waste all the glycol.

Stock Available

We now hold stock of anti-freeze valves and are recommending them to our installers. To find out more, get in touch or download the spec sheet below:

MCS Umbrella Scheme

If you want to install MCS accredited heat pumps and help your customers access funding like the Boiler Upgrade Scheme, our Alto Assured MCS Umbrella Scheme is the perfect solution. We’ll handle all the MCS paperwork and compliance for you, and our expert team will also design, supply, and commission the system, so you can focus entirely on the installation.

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Can Anti-Freeze Valves Replace Glycol in a Heat Pump System?