Why Do City Sewers Hold So Much Heat?

Hot City Sewer

Something you may have noticed about New York is that it’s very steamy. The steam billows from manholes around the city and wafts up into the air – but why? As it turns out, city sewers can get pretty warm. But this raises another question: why are the city sewers so hot?

City sewers hold a lot of heat because they move heated wastewater from buildings. Wastewater is warmer than the regular water supply because it’s used for residential and commercial building utilities. Condensation contributes to sewer too. Runoff water hits hot sewer pipes and produces steam.

In the rest of this article, we’ll discuss precisely why sewers get so warm and what (if any) benefits the accumulated heat has. The answers will surprise you!

Why Are Sewers Hot?

There are two primary reasons why sewers are hot: they’re filled with heated wastewater, and there’s an abundant amount of condensation happening.

Sewers are hot because they’re filled with water that’s around 50-70°F (10-21.11°C). Water comes from buildings and stays at a constant temperature as it travels through the sewers to treatment plants. There’s also plenty of condensation from water draining into manholes from all over the city.

Wastewater is water that you use for a specific purpose and disposed of. Flushed toilet water, water from dishwashers, washing machines, showers, and sinks are all wastewater. There are two kinds of wastewater; greywater (tub, appliance, non-kitchen sink, shower water) or black water (kitchen sink and toilet water).

Most of what composes wastewater is still H20 – 99.9%, but there’s the 0.1% that can be practically anything else.

Nitrogen, oil, grease, bacteria, viruses, phosphorus, other random solid material; anything that can find its way into used water can make up the 0.1%. Used water can’t (or at least probably shouldn’t) be reused for anything else. As previously mentioned, it may contain dangerous substances.

Some arid states in the US allow greywater use for irrigation, but even then, it has to be treated before reuse.

Wastewater is warmer than the regular water supply because it’s heated inside commercial or residential plumbing. Wastewater usually averages around 50 to 70°F (10-21.11°C), which isn’t incredibly hot but is warm enough to increase the ambient temperature of the enclosed sewer.

Condensation and How It Warms City Sewers

The other reason city sewers get so hot is condensation. I’ve mentioned it various times throughout this article already, but you might be wondering what condensation has to do with sewers. Before we get into how the process heats underneath city streets, let’s explain what condensation is.

Condensation is the part of the water cycle where vapor converts back to liquid again. To specify, what happens is that the surrounding warm air turns vapor into water when it meets a cool surface. Heated water vapor can also turn back into liquid when it encounters cool air.

In the sewers, condensation occurs when cooler water (like rain or melting snow) flows into maintenance holes and comes into contact with the hot pipes. The water heats until it’s vapor and meets either a colder surface (like the ground or a wall) or colder gasses and turns back to water.

While “vapor back to water” sounds like it doesn’t explain how condensation creates sewer heat, the “trick” is that the process never stops.

There’s a constant cycle of condensation that despite vapor eventually turning back to cool water, there’s plenty of steam to keep things warm.

What’s With the Sewer Steam?

I mentioned New York and its billowing plumes of steam earlier, but where does it come from – or rather, why do some sewers cough steam out?

There are two sources, one of which we’ve already named: steam-delivering pipes and condensation. Steamy sewers aren’t just native to New York. Detroit uses steam pipes too.

Steam comes from certain city sewers because some places use steam to power their homes and businesses. Piping delivers steam through the city under the streets, which contributes to the heat. It also comes from naturally occurring condensation.

New York’s steam pipes are managed and maintained by Con Edison, a power company that provides service to NYC and Westchester. They help manage the steam by directing it with large orange and white stacks to keep it from blinding traffic and burning people.

Energy Pipeline Basics

Con Edison (or other power companies) doesn’t siphon steam from the sewer but instead send it into insulated pipes that start from their power plant. The sewers just help keep the piping out of the way. This kind of energy generation is called district heating (or teleheating) – pipelines provide power to a specific area.

District heating can use fossil fuels (such as coal) and renewable energy. Steam happens to be a source of power that provides a lot of heat, which is helpful in an area like New York City as temperatures can get very low.

These steam transference pipes can get hot, which is part of the reason city sewers retain so much heat.

Steam From Condensation

The vapor from condensation is just steam. Although some of the steam escaping the sewers is from the energy pipes, plenty of it is from the water cycle doing its job.

The exact science behind why steam rises has to do with evaporation. When water gets hot, it evaporates and expands. Once the fog hits 212°F (100°C), it quickly boils and grows faster, which creates steam.

The steam travels upward because it expands and becomes less dense when gas is hotter than the colder air surrounding it.

Harnessing Sewer Heat

Despite its seemingly gross origins, sewer heat can be helpful. We’ve already talked about how hot wastewater can be; people can mine plenty of energy from it.

In Denver, the idea is to use a gigantic pump to suck away the residual heat sitting underground from the water and convert it to power the local building interiors.

Wastewater stays 50-70°F (10-21.11°C) during the entire year so that buildings can use the pumping process for warmth in the winter and cool air during hot months. There’s no risk of exposure to raw sewage either. Citizens won’t have to worry about being subjected to foul smells.

Wastewater sludge can be converted into usable energy too. The sludge emits methane, which is captured by thermal hydrolysis and anaerobic digestion. Thermal hydrolysis uses heat to sterilize and destroy organic matter (food, manure, other biosolids) inside the muck.

Anaerobic digestion is when bacteria continues to break down organic materials. Once both processes are completed, what’s left is a biogas which is used for energy.


City sewers hold so much heat because of warm wastewater, condensation, and steam. Wastewater can reach as high as 70°F (21.11°C); this used water stays at that temperature all year.

Condensation and hot steam pipes add to the underground heat. The runoff (rain and snow) water hits the piping and creates steam. Steam also comes from the pipelines that provide renewable energy throughout specific New York and Detroit cities.

Buildings can harness the warmth that comes from city sewer water via pumps that remove the heat and use it to warm and cool residential and commercial locations.