I get asked this question lots in my physiotherapy practice, what is better for back pain, heat or Ice!! The general rule for using ice would be for a hot, swollen, angry area. For example if you twist your knee and it swells and feels irritated and inflamed ice would be the best thing to use. Adding heat to an already hot swollen area will most likely make it worse.
For an area that feels achy and stiff but that isn't swollen or angry then heat is your best option. When it comes to back pain if you have had an episode of your back 'going' then you tend to find muscle spasm associated with this and therefore heat can help relax muscles that are tight or in spasm. I usually recommend heat for back pain whether it be an acute episode or chronic pain, however if you don't get any relief with heat try ice! Hot water bottles or wheat bags are a great tool to use to provide heat to your back.