I have a 3,000 SF old farm house with a fairly new 85,000 BTU furnace. Last winter the furnace could not keep up. Often with the thermostat set at 70 it would not get above 60 in the house. I have noticed that much of the baseboard pipes do not have fins. Should I have fins the full length of the outside walls? Is there a standard calculation for length of coverage per SF of house?
With an 85,000 btu boiler at 80% efficient output, you would be able to put the entire heat output into 120 feet or more of baseboard. If there is less than 120 feet of baseboard, there is not enough baseboard to do the heating job. Take a thermometer at the outlet of the boiler and the return of the boiler, if the water temperature does not drop more than 10 degrees F, you can use more baseboard. If the temperature difference is greater than 30 F, you may need to pump more water to get all the heat out of the baseboard.
There is no magic rule-of-thumb for all buildings. The actual heat required can be estimated through a fairly simple process of calculation. The entire process is explained and can be done at: http://www.hydronic.net
Using a rule-of-thumb for 30-year-old homes, your 3000 sq ft building should have a boiler with an output over 120,000 btuh. As your building could be heated to 60F max with an 85,000 btuh boiler, you need a boiler with 100,000 btuh output to feed the proper amount of 172 feet of baseboard to reach 70F inside; so your barn is fairly well insulated – at least moreso than the houses build with 6 inches of fiberglass in the ceiling and 3-1/2 inches in the walls.