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Cooling And Heating

How Much Water In A Radiator?

Written by Dale Richardson - Updated: June 23, 2023

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Today's topic is nice and quick. We're going to be discussing, you guessed it, how much water is in a radiator! This is super exciting stuff that requires your utmost attention, so be sure to stay tuned!

This is a bit of a complicated question that has a variety of answers, depending on the home. Radiators don't generally "have water in them," they rather have water pass through them. While it can stay inside, the amount depends on the size of your radiator.

Now let's get right into it, shall we?

Read Next: Hot water upstairs but not downstairs? Here's why.

Radiators: How Do They Work?

First things first - how do radiators work? This is important knowledge to have, so just stay with me a bit longer.

Radiators have water passed through them from your central heating system. Your boiler brings water up to temp and then directs it to the radiator, which in turn holds some of it, losing some to steam, and recirculating the rest back to the heater. As steam reaches a cold radiator, it condenses and turns from steam, back into water. This will result in liquid sitting in your radiator.

If you've ever bled a radiator (releasing built-up air), then you know that a lot of moisture can come out. Depending on the size and brand of your radiator, it can hold as much as 10 litres, if not more - that's a lot of water.

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But Seriously, How Much Water is In There?



Now that you know how radiators work (in a short SparkNotes type of way), it is likely clear that this isn't a super easy question to answer. I could say "at most, 15 litres," and be right most of the time... but if you have a big or old radiator, that answer could vary greatly!

It all boils (ha! water heating joke) down to your particular radiator. The best way to find this out, if you're really determined, is to bleed all of the water out of it and measure! I know, that's not a great solution, but it's really the only surefire way to get a concrete answer.

Let's take a look at the vast range of radiators and heating systems and their capacities really quick, shall we? Hopefully, that'll show you why I can't give a concrete answer here:

  • Cast iron radiators, on average, require about 14 litres of water per kW of heat output.
  • Fan coil radiators generally need about 8 litres of water per kW of heat output.
  • Heating convectors and baseboards (not radiators) need roughly 6 litres of hot water per kW of heat output.
  • Radiant heat flooring (also not a radiator)uses roughly 23 litres of water per kW of heat output - that's a lot more than the rest!
  • Steel panel radiators usually need about 11 litres of hot water per kW of heat output.

Final Thoughts

As you can see, the question of how much water radiators hold is a bit of a complicated one. There are a lot of radiators out there, both old and new, and they each have different requirements. While a steel panel radiator only needs 11 litres of hot water, a cast iron radiator needs more and a fan coil heater needs much less. The best, surefire way to find out the answer is to drain your radiator.

Please, if you choose to do this, turn off the heat first and allow it to cool. I know it seems like common sense, but I kinda have to say it. Realistically, in most modern homes, you've got a device that measures and controls all of this for you. That means that you generally won't ever need to know this - but if you're curious, I can't stop you. I'm a writer, not a cop.

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