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Radiator Keeps Overflowing? Top 8 Causes (+ How to Fix)

Radiators use high water pressure and narrow tubes to create an efficient temperature regulation system. However, the high pressure can work against the radiator, causing it to overflow with water and coolant. Failure to treat this common issue can have severe consequences, including structural damage.

A radiator keeps overflowing because of these causes:

  • Water expansion
  • Too much water
  • Not enough water
  • Clogged pressure relief valves
  • Leaky air bubbles
  • Harsh chemical additives
  • High pipe temperatures
  • Low pipe temperatures

In this post, we’ll show you everything that can make your radiator overflow. We’ll also explain how you can handle each scenario, how common they are, and whether or not you can prevent them from recurring.

Hot Water Expansion

Water expands as it gets hotter and contracts as it gets colder. Since radiators use high temperatures to increase the internal pressure, water expansion plays a crucial role. However, too much expansion can cause all sorts of unwanted side effects.

For example, expanding water can cause leaks or burst pipes. Another common sign of excessive water expansion is when the water keeps flowing out of the overflow pipe. The expanding water shoots through the pipes, maximizes the radiator’s pressure and pushes out of the relief valve. Without the valves, there would be a massive pressure burst.

While hot water expansion is expected, it can be managed with a couple of tips we’ll provide below.

How to Fix

Although water expansion is normal for a radiator, it’s important to handle it correctly. Home Inspection Insider suggests opening the pressure relief valve whenever there’s too much air inside. The steam and air come from increased water temperature, so opening the valve will lower the pressure.

Opening the overflow valve is another excellent option. However, most radiators have open or automatic overflow pipes. These pipes pour excess water out of the radiator, preventing it from boiling or steaming through the unit. It’s important to ensure your overflow pipe is accessible and unclogged every time you use the radiator.

Too Much Water in the Radiator

When there’s too much water in the radiator, the pressure goes through the roof. Heated water creates air bubbles that pressurize the pipes and valves. If there’s too much pressure from the water and bubbles, the radiator will overheat. You’ll notice high-pitched sounds, loud rumbling, and water leaking from the overflow pipe.

It’s important to know how much water your radiator needs. Topping it off can cause severe pressure issues. Fortunately, all manufacturers list how much water and coolant your make and model requires. It’s better to stick within the recommended parameters than to overfill it and cause plumbing problems.

How to Fix

Here’s a list of suggestions to make sure there’s not too much water in the radiator:

  • Always follow the manufacturer’s water filling instructions.
  • Look for an overflow line for cold water and always pour in cold water instead of hot water.
  • Only use coolant or antifreeze if the company recommends it.
  • Let the radiator cool down before opening the cap, adjusting the plumbing, or adding more water to the system.
  • Consider hiring an expert to top off the radiator to get a warranty and much-needed peace of mind.
  • Check the radiator’s water level monthly (or as recommended by the brand) to ensure it’s never too high.

Not Enough Water in the Radiator

Not putting enough water in the radiator causes similar problems to an overfill. Lack of water or coolant increases the radiator’s heat. The small amount of water inside the radiator turns into steam, drastically raising the internal pressure. This process can cause plumbing cracks, broken gaskets, pipe bursts, and more.

One of the easiest ways to know if there’s not enough coolant or water in the radiator is if it’s extremely hot. Radiators radiate heat (hence the name), but too much heat becomes uncomfortable. Your radiator shouldn’t be hissing, screeching, steaming, smoking, or emitting too much heat from the pipes.

How to Fix

If there’s not enough water in the radiator, consider the following:

  • Add cold water to the radiator up to the fill line when the radiator is at room temperature.
  • Look for leaks throughout the radiator and patch, replace, or seal them.
  • Add new gaskets wherever they’re missing or damaged.
  • Put silicone lubricant on each O-ring and gasket to ensure they’re sealed.
  • Replace the radiator’s cap if it’s overheating or it keeps losing water and steam.
  • Don’t forget to add coolant to the radiator if the company calls for it.

A lack of water can be a sign of leaks, but water needs to be randomly added to a radiator due to loss of steam. Every time you use the pressure relief valve or overflow pipe, the radiator loses water and coolant. Staying on top of the water situation prevents much more severe scenarios.

Clogged Temperature Pressure Relief Valves

Beyer Plumbing claims temperature-pressure relief valves can clog and overflow. When too much pressure builds in the radiator, the excess water, steam, and air go to the relief valve. You should be able to open the valve to relieve the pressure, though some radiators have automatic relief valves. If the valve is clogged, it can’t open and prevent overflowing.

The pressure relief valve is the first line of defense between the radiator and excessive pressure. If the valve can relieve enough hot air, it doesn’t have to send the water to the drain or overflow valve. However, a clogged valve is practically useless. Your radiator will constantly overflow, causing leaks, high pressure, and more.

How to Fix

Radiators run smoothly without clogs, but you should try this method if they have clogged pressure valves:

  1. Turn off the radiator, power, and all inlet and outlet valves.
  2. Open the drain valve to remove all of the water from the radiator.
  3. Use an adjustable wrench to loosen and remove the pressure relief valve.
  4. Locate and remove the clog or replace the valve with a like-for-like relief valve (recommended by the manufacturer).
  5. Tighten the new or cleaned valve, close the drain, open the inlets and outlets, and turn everything back on after adding the water and coolant.

Test your work to ensure there’s no clog in the pressure relief valve. You should be able to open the pressure valve to release excess air until some water flows out, then close the valve.

Review Garden Fork’s YouTube guide for more details:

Leaks Cause Rising Air Bubbles

Air bubbles get trapped in a radiator from numerous leaks and heated air. While high-temperature air bubbles are normal, leaks can worsen the conditions. We explained how too much pressure could cause your system to overflow above, which is why it’s essential to seal the leaks as soon as possible.

Networx explains the most common radiator leaks happen from rust-driven corrosion, water acidity, worn gaskets, and cracks on the unions. Plumbing leaks push the water out of the pipes and invite air into them. As the radiator gets hotter, the air bubbles rise and push the water through the overflow pipe.

How to Fix

If you suspect there are leaks, try these tips:

  • Replace all plumbing issues. Radiators use unions such as couplings, elbows, 45-degrees, and more. We recommend hiring a professional plumber to remove and replace the pipes since they often require soldering.
  • Use silicone sealant and plumber’s tape on each of the threaded parts. Almost every valve will have a gasket or O-ring. If they don’t, the threaded portion can be wrapped with a plumber’s tape to provide a strong seal.
  • Test the water to ensure it’s not too acidic. Many coolants are acidic, too. A low-pH liquid can corrode the pipes, causing leaks. Even if you replace the pipes, you’ll have to do it again in a few years.
  • Steer clear of epoxies since most of them don’t work for high-PSI radiators. They’ll seal the leak, but the pressure will burst through the epoxy after a few weeks or months. This process can potentially cause worse damage than replacing the pipes.

Toxic Antifreeze Chemicals

Many people use coolant in their radiators, but using the wrong kind can cause an array of problems. Antifreeze is almost always dangerous because it contains toxic chemicals. As they heat up, the toxic steam corrodes the radiator’s pipes. Using polypropylene glycol antifreeze prevents this rare occurrence.

Note: You typically can’t use the same coolant in an in-home radiator as you can in an automobile. The concentrations, chemicals, chemical ratios, and many other factors are vastly different. Always use the coolant or antifreeze recommended by the radiator’s manufacturer with the appropriate water-to-coolant ratio.

How to Fix

Firstly, we suggest only using what the manufacturer suggests. However, if you want to use antifreeze or coolant without asking the company, it’s important to stay away from toxic chemicals.

Sanco Industries’ Propylene Glycol Antifreeze comes in a one-gallon container. This non-toxic formula is food-grade, so you don’t have to worry about sending toxic fumes throughout the house. It’s also not acidic enough to corrode the pipes, preserving your radiator. The non-alcoholic formula uses soft, reliable antifreeze that won’t burn or damage the unit.

Keep in mind that many types of antifreeze aren’t diluted. They need to be mixed with water to prevent coagulation. Since some formulas are unconcentrated, it’s a good idea to check the label to know how much your radiator needs. Adding too little or too much coolant has similar consequences to the same amount of water (as mentioned earlier in the article).

High-Temperature Settings

Radiators are made to regulate the temperature, but some of them contain high-heat settings. Using these settings is similar to maxing out the speed on your vehicle; It’s incredibly dangerous and extremely inadvisable.

It might be tempting to crank up the radiator to its highest setting, especially when it’s freezing outside. However, these adjustments quickly cause three crucial problems:

  1. Higher temperatures rapidly increase the internal pressure, which floods the system with steam.
  2. Quick and massive temperature changes make the water expand and damage the pipes.
  3. Maxing the radiator can overheat the plumbing, coils, and other components.

How to Fix

Here’s a list of our recommendations:

  • Keep your radiator within reasonable limits. There’s no reason to turn the house’s temperature over 80 degrees Fahrenheit (~27 Celsius). Check the thermostat’s limit to ensure you don’t go too high.
  • Set an automatic timer on your thermostat to prevent the radiator from getting too hot. We’ve all woken up too cold in the morning, making it hard to get out of bed. If you set a timer, you don’t have to worry about overcompensating for the freezing temperature.
  • If you think your thermostat is broken, have it inspected. Thermostats sometimes go wonky, which makes them increase or reduce the temperature randomly. Furthermore, it can make the radiator do nothing at all. Make sure the thermostat is communicating with the radiator to prevent it from overheating.

Frozen Radiator Pipes

Frozen radiator pipes cause an uncommon and often overlooked problem: Condensation. As the pipes get hotter, the condensation melts and increases the volume of water inside the radiator. Excessive amounts of water increase the interior pressure, leading to overheating, overflowing, and a potential pipe burst.

The good news is that most modern radiators are insulated to prevent condensation from becoming a threat. It’s also rare for the coils or pipes to freeze since most radiators are being used when they’re cold (people naturally run their heaters when it’s cold inside). That being said, it’s not impossible, and it happens occasionally.

How to Fix

So, what can you do to prevent the radiator from freezing and causing condensation?

Try these suggestions:

  • If permitted, insulate the pipes around the radiator with manufacturer-recommended insulation tape or foam.
  • Keep the radiator above 32 degrees Fahrenheit (0 Celsius) to prevent the pipes from freezing by setting the thermostat accordingly.
  • If you see frost on the pipes, it’s time to turn on the radiator and wipe the plumbing with a soft microfiber cloth.
  • Use antifreeze in the radiator to prevent it from freezing (this is often the most popular option, though not all radiators can use it).


  • Steve Rajeckas

    Steve Rajeckas is an HVAC hobbyist with an avid interest in learning innovative ways to keep rooms, buildings, and everything else at the optimal temperature. When he's not working on new posts for Temperature Master, he can be found reading books or exploring the outdoors.

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