Two firefighters assigned to the city’s fire department heavy rescue truck are sent out several times a day on all kinds of calls, from helping out senior citizens injured in falls to victims suffering from heart attacks or are involved in a serious motor vehicle crashes. But the rescue truck soon won’t be going out on quite so many calls. City Manager Sharon A. Addison has agreed to a new directive that would mean the heavy rescue truck will be used on far fewer calls, Fire Chief Dale C. Herman confirmed Saturday. Under the new deployment model, the two firefighters assigned to the heavy rescue truck will shift to fire engines.