A combustion efficiency test done on a gas-fired boiler indicated that the stack temperature is 750°F and the CO2 content is 7 percent.

a. Determine the excess combustion air and the boiler efficiency.

b. The boiler efficiency is increased to 83 percent by adjusting the fuel-air ratio so that the excess air is limited to 30 percent. Estimate the new stack temperature and the CO2 content.

c. Estimate the annual natural gas consumption (in therms) if a combustion gas analyzer kit, installed at a cost of $9,500, is paid back after four years. The kit helps maintain the boiler efficiency at 83 percent. The annual cost of natural gas is $1.40 per therm.