Winter isn’t as far off as it seems (cue the collective sigh), which means it’s time to start thinking about heating systems again. If your home or commercial building was built prior to the 1950s, it’s very likely that your heating system consists of a boiler and steam radiators. Although these are rarely put into place these days, steam radiators are an efficient and reliable heating source. But you might hear some hissing coming from yours, especially if you haven’t had your system serviced recently. Here’s what’s probably going on:

How Steam Radiators Work (Roughly)

In order to understand the hissing sound your radiators make, you’ll need a basic understanding of how steam radiators work. In non-technical terms, water is heated by your boiler. When it gets to the right temperature, it enters the pipes as steam. When it gets to your radiator, it releases its heat on the radiator’s fins and condenses back into cold water. The cycle then starts again, with ideally very little water loss.

Steam Vents/Traps on Radiators

One part that a radiator needs to work effectively is a steam vent or steam trap. Essentially, steam traps are mechanical devices containing valves that can open or close. There are several types of valves, but the steam trap valves used for radiators are thermostatic valves, meaning they work based on temperature. Steam radiators need ventilation, and that’s what these mechanisms provide. When the air is cold, at the beginning of the hating cycle, the valve is open, allowing that air to escape — and producing the hissing sound associated with steam traps on radiators. When the steam causes the radiator to get hotter, however, the valve closes, retaining the steam so that the radiator stays hot and can perform its intended purpose of heating a space.

When Hissing Indicates a Problem

Hissing at the beginning of the heating cycle is normal (since that’s just cold air escaping from the steam vent). But if you continue to hear that sound when the radiator is hot, it’s time to replace the valve and/or call in a technician. That indicates that steam is actually being let out, which will lower the efficiency of your overall heating system.

Do you have any other questions about steam vents or radiator valves? Join the discussion in the comments.