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How Does Boiler Frost Protection Heating Work?


One of the selling points of most modern boiler models is the built-in frost protection feature that prevents the water in the boiler from freezing during the coldest of winters. And you wouldn’t want your boiler freezing as it can wreak havoc in the boiler system, resulting in blockages and burst pipes, among others. But what exactly is boiler frost protection, why do you need it, and how does it work?

What is boiler frost protection?

Boiler frost protection is a built-in safety feature that regulates the temperature of the water in the boiler system to ensure it doesn’t drop low enough for the water to freeze. So when frost protection is active, you can rest well knowing that your boiler won’t freeze or stop working due to ice blockages.

How does boiler frost protection work?

The boiler frost protection feature usually monitors the temperature of the water in the unit and turns on the boiler once the water drops below a certain temperature. And once the water has reached a safe temperature, the boiler shuts down. You should, therefore, not be surprised to hear your boiler firing up for a few minutes before shutting down on a cold day.

It is recommended that you set your boiler’s frost protection feature to kick in when the water temperature in the boiler unit is around five degrees Celsius. Going lower than five degrees is still possible; however, there is the risk of the water freezing before it can effectively warm up. Additionally, if you set it at a higher temperature, say, 8 degrees Celsius, the boiler will be firing up more frequently, resulting in higher energy bills.

During frost protection, the boiler heats the water to 10 degrees Celsius before automatically shutting down. The water in the boiler unit begins to cool down due to heat loss, and the boiler turns itself back on when the temperature is about five degrees Celsius. This cycling on and off will continue until external temperatures are high enough such that there is no risk of freezing.

Here is a step-by-step summary of how the boiler frost protection feature works when activated:

  1. The temperature of the water in the boiler unit drops to a temperature of five degrees due to freezing weather
  2. The boiler automatically fires up and starts to heat the water in the unit
  3. The temperature of the water in the system rises to ten degrees Celsius, and the boiler powers down
  4. The water in the boiler cools down, and the process repeats itself until the weather conditions improve or the boiler is turned on for central heating and domestic hot water demand.

Is it necessary that my boiler should have frost protection?

Absolutely not. The boiler frost protection feature is only helpful if you live in a particularly cold region prone to sub-zero temperatures. However, frost protection won’t be necessary if your boiler is installed in a warm room such as the kitchen or utility room. A boiler installed in a warm room in your home is less likely to freeze than one installed in a garage or a loft.

Will boiler frost protection turn my central heating on?

The ability of the boiler to turn itself on during frost protection might get you worried about your central heating coming on even when you are not home. However, there is no need to panic as the boiler won’t turn your central heating on when it is firing up for frost protection. The boiler will heat the water within the system during frost protection but won’t circulate it around your central heating.

As such, if you find your central heating coming on during frost protection, you need to call in a certified heating engineer as your boiler is likely to have a faulty diverter valve.

Will boiler frost protection increase my energy bills?

When the boiler fires up for frost protection, it will use energy to heat the water in the unit. This energy will reflect on your bill, meaning it would be higher than usual.  However, paying an extra couple of pounds a year for frost protection is worth it. This is because the cost of frost protection is a small fraction of the cost of repairing the damage freezing can inflict on a boiler.

It is good to note that frost protection won’t significantly inflate your energy bill unless you have set the temperatures way high. Setting the frost protection temperatures to around five degrees Celsius means the boiler will only ignite on freezing days. Doing this will keep the frost protection low and ensure your boiler continues to heat your home without fault.

What if my boiler doesn’t have frost protection?

Not all boilers have a built-in frost protection feature. And if your boiler doesn’t have one, you shouldn’t panic as there are alternative ways to protect your boiler and central heating system from frost protection. Some of these alternative frost protection features include:

  1. Boiler frost protection thermostats

Frost protection thermostats are special thermostats that can be fitted to your existing central heating system. These thermostats will keep the water in your heating system from dropping below a certain temperature. They are usually as effective as built-in frost protection and cost between £20 and £30, excluding installation.

  1. Smart thermostats

These work similar to boiler frost protection thermostats but have extra benefits such as internet connectivity and adaptability to your heating habits. Smart thermostats such as Nest and Hive cost you between £200 and £500 and will prevent your pipes from freezing.

  1. Pipe insulation jackets

Pipe insulation jackets will keep your hot water pipes from freezing and also ensure the hot water in those pipes will stay warmer for longer. Insulation jackets are usually inexpensive and cost between £5 and £7 per metre.

What are the signs of frozen central heating?

It is usually hard to tell when you have a frozen central heating system. However, if you suspect your system has frozen, check the condensate pipe that usually exits your home through an external wall close to the boiler. If your hunch is correct, there will be ice on the end of the pipe.

There are also other common signs of a frozen central heating system which include:

  • There is no running water in your hot pipes
  • The boiler shuts down immediately after starting
  • The central heating system won’t come on
  • The boiler is making bubbling and gurgling sounds
  • There are burst pipes in your central heating system, especially ones near the boiler location
  • There are signs of water damage to walls or floors
  • There is a blockage in your central heating system
  • The boiler control panel displays a fault code associated with a frozen boiler

What would happen if my boiler froze?

A frozen boiler will disrupt the operation of your central heating system, resulting in substantial repair costs and uncomfortable temperatures in your home.

Some of the problems associated with frozen central heating pipes and boilers include:

  • Damaged heat exchangers as water tend to expand when it becomes ice, causing the heat exchanger to rupture.
  • Freezing can cause irreparable damage to the boiler, resulting in you needing to invest in a replacement boiler.
  • Water damage to floors, walls, and furniture due to burst pipes
  • Blockages in the central heating system, meaning the hot water won’t be able to circulate and warm your radiators
  • Your boiler won’t fire up or will shut down immediately after ignition

What should I do if I find my central heating frozen?

Although frost protection is supposed to prevent your boiler, and in extension, your central heating system from freezing, sometimes the unexpected happens and your system is frozen. Frozen central heating systems are common if your boiler doesn’t have frost protection or you forgot to turn it on. It is also uncommon to find your central heating frozen despite your boiler’s frost protection being on, especially if the boiler has some issues.

Here is a summary of what you should do if you find your central heating pipes frozen.

  • Wrap any exposed outdoor pipes (especially the condensate pipe) with a warm towel dipped in warm water to help thaw the frozen water. Alternatively, you can pour warm water on the pipes, but never hot water, as the sudden temperature changes might cause the pipes to crack.
  • If external temperatures are lower than two degrees Celsius, pouring warm water on the frozen condensate pipe would be useless as the water will freeze again quickly. Instead, fill a hot water bottle with boiling water and secure it to the frozen pipe until the ice thaws.
  • Warm the interior of your home using portable heaters, as the warmth will help defrost any internal pipework.
  • Turn off the water supply to the boiler using the stop cock. This will help avoid any excessive pressure, which might cause some of the pipes to burst.
  • Once the ice has thawed, inspect your pipes for any bursts and leakages before turning the water back on.
  • If you’re your central heating won’t come on even after the frozen water has thawed, you should call a heating engineer as they would know what to do.
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