Switch Terms

A while back, I had a tech who was having some trouble finding a 35 PSI (2.41 bar) make-on-fall pressure switch. One adjustable switch in the catalog said (SPDT), but he didn't quite understand what that meant. In that case, it means single-pole, double-throw, and the “double-throw” part means that the switch has terminals in both the close-on-rise and close-on-fall directions. Another common example of this sort of switch is a “three-way” light switch.


A single-pole, single-throw (SPST) switch is like a typical light switch. It only has one path (pole), and it is only closed or open.

A double-pole, single-throw (DPST) switch that is quite common is a two-pole contactor. It has two switches, but they only open and close in one direction.

A common double-pole, double-throw (DPDT) switch in HVAC/R is the 90-340 relay (and many other relays), where it has two circuits, and they alternate between closed and open terminals.






Related Tech Tips

UV Lights
A quick note about UV lights: they work like sunlight in that they prevent and kill many types of bacteria and fungi when exposed to the light on surfaces. They do not generally do a great job of killing spores suspended in the air stream. UV lights are great at killing yucky stuff on surfaces […]
Read more
How to Determine the Friction Rate for Residential Duct Design
This article is written by one of the smartest guys I know online, Neil Comparetto. Thanks, Neil! Recently, I posted a question in the HVAC School Group on Facebook that went like this: “When designing a residential duct system, what friction rate do you use?” As of writing this, only one answer was correct according […]
Read more
Tunnel Vision and How to Avoid It
How many times has the following situation happened to you? You're on your way to that final service call. While you're listening to the customer explain their complaints over the phone, there's this precise moment where you've thought: “I know what it is already. This will be a quick one.” Sometimes, intuition proves to be […]
Read more

To continue you need to agree to our terms.

The HVAC School site, podcast and daily tech tips
Made possible by Generous support from