HVAC For Beginners

Steam Heating System

A Homeowner's Guide

With our steam heating system guide, we will help you to understand the basics of how your system works. These systems are most common in the Northeast Corridor of the USA in older homes.

There are three common system designs: low pressure (steam pressures less than 15 psi), vapor systems (1-16 ounces of pressure) & high pressure (greater than 15 psi). High pressure systems are more common in industrial/commercial applications. Vapor systems are rare. The most common type is the low pressure system and so that is what we will discuss here.

Basic steam heating system parts

The typical residential steam system includes a boiler to produce steam, radiators to release the heat from the steam into the room, and the piping connecting the two.

The boiler usually burns gas or fuel oil to produce heat to raise the temperature of water to the boiling point.

In order to properly control the system, three basic components are required. A thermostat tells the system when heat is required. A pressuretrol turns the burner of the boiler on and off based on the pressure in the system. This is generally set at between 1 & 3 psi. This setting impacts not only comfort but also the responsiveness, noise level, fuel economy, and life of the system. Generally speaking, the lower the pressure is the better it is for the system. In a water based system, a pump circulates the heat while in a steam system it is accomplished by the pressure of the steam. Enough pressure is required to overcome the friction of the pipes. A low water cutoff (LWCO) is the third basic control and its purpose is to keep the boiler from operating without water in it.

A pressure relief valve ("pop safety") is also added to the system to protect from overpressure due to the boiler failing to shut off. Before these were installed, a steam boiler could explode.

Basic operation of a steam heating system

When the thermostat senses that the air temperature is below the desired temperature, it closes a switch to allow the heating system to operate. The pressuretrol senses the existing pressure in the boiler and turns on the burner if it is less than the setpoint and there is sufficient water in the boiler as determined by the LWCO.

There are air vents in the system typically located at each radiator as well as in the main steam pipe. These vents open when there is no steam pressure and close when steam gets to them. When the pressure of the steam can overcome the friction of the piping, it travels through the main piping to the radiators. The steam gives off its latent heat to the room and it condenses to water which is returned to the boiler.

A one pipe steam heating system will use the same pipe that provided the steam to the radiators to return the condensate back to the boiler.

In a two pipe system, the steam will travel to the top of the radiator and the condensate will leave the bottom of the radiator and travel back to the boiler through a separate pipe. A steam trap separates the two pipes as it opens to let air pass through the radiator, closes when steam reaches it, and reopens when the condensate has cooled. When there is not sufficient height in the basement to allow for the proper slope of the pipe to return the water to the boiler, a condensate pump is used.