Some of the best warm ups for choirs are the standard vocal lesson exercises. Start with something subtle to get the chords moving. A good hum exercise would be perfect. Hum up and down a one octave scale, ascending and descending as you continue.
This will loosen the chords. Next do a few sirens. Have you choir take a deep breath, when they breath out, have them make a siren noise in a descending fashion as low as they can. Then have them do the same back up the scale. Do that for a couple minutes. When done, do normal scales going through the sounds no, la, ha, nay, mo, neeh, new. Extend your scales to 1.5 octaves after a little. Next do a few Octave jumps. Pick a note, and sing that note for 1 second, then jump one octave up. Use the different sounds. Make sure you come down that scale after the jump.
Go up and down the scales doing this exercise. Helene Goldnadel would suggest using an ear training exercise as well. Make the choir do a counterpoint movement (have one part go down while the other goes up). There are many vocal exercises you can do to for warm ups and vocal lessons. In fact, the more variations you use, the better. Just make sure that you are warming up three important factors. Make sure you are warming up the diaphragm and breathing apparatus, the vocal chords, and the mouth and jaw muscles. At least one exercise for each should be done.