$5\mathrm{.}18$ The building in Example $15\mathrm{.}5$ can be heated either by the boiler specified in the example or by two boilers, one with one$-$third the example boiler capacity and one with two$-$thirds the capacity. Both have the same part$-$load characteristics as the boiler in Example $15\mathrm{.}5.$ Since the boiler in the example has more capacity than needed most of the time, less energy may be used with smaller boilers operating more closely to their full capacity. Add the columns needed to the spreadsheet in Table $15\mathrm{.}5$ to study this option. For simplicity, assume that the smaller boiler operates up to full capacity before the second is activated.

$(4)$