Re: Why doesn't water in fire hydrants freeze?

There are two types of fire hydrants used in the  U S. of A , on called a
wet barrel, used in the south, not prone to freezing due to milder
temperatures. This is used mostly in the the southeast and Texas areas 
where water mains are buried above the frostline.

 In the Northern areas  a dry barrel system is used whereby the water
 mains are buried at least 3 to 4 feet below soil grade and they are
also equipped with a fairly elaborate  draining system whereby any water
remaining in the hydrant after use is able to drain off beneath the system 
to keep problems from happening freezing temperatures 
for many days in a row.  Once the water is brought above grade, it can freeze 
but this danger can be minimized by keeping water moving through the system
rather than allow it to stand stagnant or still, thereby allowing 
it to freeze up.

Consult further the NFPA Handbook or the International Fire Service 
Training Association's Handbook, Essentials of Firefighting.

Hope your question is adequatetely answered.

thank you for the query

Donald E Duggan