Why Is My Reverse Osmosis Water Hot?

Why Is My Reverse Osmosis Water Hot?

When it comes to water filtration, nothing works as effectively or efficiently as Reverse Osmosis (RO) systems. These units are designed to eliminate 99% of all contaminants commonly found in domestic water. As such, RO systems are becoming increasingly popular, even for residential applications, as they offer the safest and cleanest home water solution. However, should you be getting RO water that is hot?

Under no circumstances should your reverse osmosis water be hot. Reverse osmosis units have no mechanical or electrical parts that can heat up water and are always connected to a cold water line. If the water comes out hot from the faucet, something is wrong with the RO filter. The most likely causes of hot reverse osmosis water are a clogged filter, a plumbing problem allowing hot water to enter the cold water line, or the RO water tank is adjacent to a heater.

Hot water can easily damage your RO membrane, which is arguably the most important and expensive component of a reverse osmosis purification system. As such, if your RO water is hot, it is important that you address this problem immediately before it can cause more damage. To learn more about possible scenarios and solutions to RO water being hot, read on.

Read my comprehensive article about reverse osmosis.

Hot Reverse Osmosis Water – Possible Scenarios & Solutions

Most reverse osmosis units run on cold water and there is a very good reason for this. RO units depend on a semi-permeable membrane to remove most of the microscopic contaminants such as chemicals, metals, total dissolved solids (TDS), and more from water.

When hot water passes through a reverse osmosis membrane it can damage its extremely fine filters causing contaminants to pass through and compromising water quality. This is why all RO manufacturers recommend only allowing cold water or room temperature water to pass through the RO filtration unit.

If you are experiencing hot water coming out of your reverse osmosis unit this isn’t normal. Warm water is OK, but it should never be hot!

Below, we have outlined a few possible scenarios that could result in your reverse osmosis water being hot.

1. Hot water entering the unit

It could be that a plumbing malfunction or a broken valve is allowing hot water to enter the cold water line which is raising the water temperature.

If the water in the piping is hot this means that hot water is getting mixed in with your cold water line and you will likely need to get a plumber to get the issue resolved.

However, if the water entering the unit is cold and yet hot water is flowing out through the faucet something is definitely not right.

The best thing would be to contact a water treatment RO specialist to come and take a look at the unit.

2. Clogged RO filter

If you haven’t replaced your RO filters in the past year, it is highly likely that your RO filters are clogged. This makes it difficult for the unit to produce pure water effectively or efficiently.

Although highly unlikely, clogged RO filters can cause water to heat up. This is due to the increased pressure of water being unable to pass through the clogged filters, especially, if the RO unit has a booster pump installed. This can cause both the unit temperature and water temperature to rise.

However, this should still not lead to your water being hot. At best it would make the water slightly warmer than it would be otherwise. Still, it’s best to replace the filters to ensure the unit continues to run smoothly and efficiently.

Read my article about the importance of maintaining your reverse osmosis system.

3. Stored water inside the UV reactor

UV filters are highly effective at eliminating all types of microorganisms, including algae, and other organic matter that is commonly found in domestic water.

Most reverse osmosis systems come with a UV filter. The UV filter is made up of UV reactor vessels that slowly heat up from the light and heat produced by the UV bulb.

If you turn on the faucet and for the first second or so the water is a tad hot, it is likely that this is the water that has been sitting inside the UV reactor. As a result, it has a higher temperature than the rest of the water stored or flowing inside the unit.

UV bulbs or UV reactor vessels cannot heat up flowing water as the heat dispensed is minimal. So, if the hot water issue is persistent it can’t be because of this.

4. Heating vent or hot water pipes

Most residential RO units are usually installed in a cabinet underneath the kitchen sink. If there is a heating vent close to or inside the cabinet housing the RO unit this could be increasing the water temperature resulting in hot RO water.

If the piping for a dishwasher or washing machine is also running inside the cabinet where the reverse osmosis unit is stored, the heat dispensed from the pipes can cause the inside temperature of the cabinet to get really warm. This heats up the water inside the filter housings of your reverse osmosis unit resulting in hot or warm water.

Ensure that no heating vents or hot water pipes are anywhere close to where an RO unit is installed so this issue doesn’t arise.


Q) What are the dangers of having hot water flowing through your RO unit?

Hot water (over 100 °F) can damage the pre-filters as well as the semi-permeable membrane of your RO unit. This is because heated water usually has high levels of silt and other sediments that can burn the DI resin present in the membrane quickly – deteriorating the longevity of the filters.

Q) Can warmer water result in faster reverse osmosis water production?

Yes, warm water can in fact result in faster RO production. This is because the warmer the water the more spaced out the molecules are likely to be. This allows water molecules to pass through the membrane easily resulting in faster RO production. However, water that is too warm can also easily damage the filters so it could prove to be an expensive way to boost production.

Final Thoughts

If the purified water coming out of your RO unit is hot, and it is not because of any of the possible scenarios above, you should immediately contact the unit manufacturer for answers or call in an expert to investigate the problem. Although highly unlikely, hot water could be a result of a chemical reaction occurring inside your RO unit. If this is the case it is best to leave it in the hands of experts.

Chief Guru

Richard Boch is a chemical engineer responsible for designing water filtration systems for industrial and residential customers. He has more than 20 years of experience with ion exchange, activated carbon, and reverse osmosis. Richard's expertise has made him a go-to source for municipalities and businesses looking to improve their water quality. When he's not working, Richard enjoys spending time with his wife and two young children. You can also follow him on LinkedIn, Twitter and Facebook.

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