If the transformer is fed with two legs of power and not with 120V/common, you shouldn't ground one 24V side. 240-24V transformers give two 12V AC legs of live power.don't forget to have proper grounding to prevent a floating ground, this condition will cause extreme troble shooting problems.
you can not go wrong with a 150 va transformer, 75va is the lowest you can go on a small system, power draw combined with modifications for more functions can cause wierd problems, don't forget to have proper grounding to prevent a floating ground, this condition will cause extreme troble shooting problems. also use a 4 amp slo blow fuse on the 24 volt side
Use a 120V transformer and then you can ground the common side of the 24V wiring.What is the proper way to prevent a floating ground?