We contracted for a 3 ton unit and just found out, 23 years later, that they installed a 5 ton furnace. Is this safe? Also will this will cut down life span of furnace due to short cycling? If so, approx. how many years am I losing here?

I meant to say 2 years later... not 23 years.

An oversized aircondition can cause too much humidity,therefore possible mold and mildew follows,, if you have a written contract, I would hold onto it

I am glad I do not have to pay your electric bills.

Five ton furnace? Do you mean five ton ac unit. Is this a heat pump? Any which way you look at it, five ton is way oversized for 1000 sq ft dwelling.