eli5, how does a city hot water system work? How does hot water remain heated while traveling through pipes to our faucets?


Hello everyone, recently my city has been doing some repairs to the city’s hot water system and i was without hot water for about 3 weeks.

That got me wondering, how exactly does this work? I read something about a heating agent but i don’t know what that means. And how does the water stay hot while travelling from the place it gets heated to our homes?


In: 25

It may not be the same everywhere but there are a few ways you can do this.

You can circulate water in a loop, not a straight pipe, so it comes back to be re-heated if it isn’t used immediately. This is how tall apartment buildings and hotels keep hot water immediately available on top floors.

You could also put some sort of heater in distant locations. This could be electricity though that can be expensive/inefficient. More likely it would be gas, another way you could do it is sending steam out from the same plant that originally heated the water, but this doesn’t necessarily make things any cheaper or easier vs running a loop and re-heating the water as necessary.

The pipes would also be insulated of course. Water actually holds an enormous amount of heat energy so if there is no easy path for a lot of heat to quickly escape, the water is going to stay hot for a while.

Where do you live that hot water is centrally provided? Where I live (USA), we only get cold water supplied to our homes and have in-home water heaters.

My understanding is that municipal hot water systems are REALLY rare, according to wikipedia there’s something like 20 cities in the US that do this.

In the vast majority of the country only cold water is supplied by the city. Every home (or building) has a method of heating the water on site and it’s only at that point that it breaks into 2 pipes (1 hot and 1 cold). There’s a few methods for heating the water but the most common is a hot water tank (gas or electric), other homes have an “on demand” hot water heater that’s attached to the home’s furnace.

To answer your question, the pipes are insulated so the water stays hot while traveling.

I mean, the hot water heater is physically inside your house. Hot water doesn’t have to travel all that far in order to get to your sink or shower.

A large city by me in the Midwest US has hot and chilled water for heating and cooling buildings down town. This is recirculated back to the district energy buildings with many giant boilers for hot water and cooling towers outside for the chilled water.

These are supplied to buildings for HVAC use. Most of the large buildings downtown are on domestic instead of having their own boilers and cooling equipment.

Cold potable (drinking water) is supplied and run through a heat exchanger where the domestic hot loop heats it up to make hot water for showers etc. The now colder domestic hot goes back to the district boilers to be reheated.

Domestic hot water supply wouldn’t be consumed at least here in the states it is used in coils that make hot or cold air and heat exchanges to make potable hot water.