some where on your furnace it has a name plate. it tells a lot of things including the b.t.u.s’ with a number in front or behind the BTU. this is the size of your furnace. if you think you need to replace it get an energy efficient one. if it works but not well then try this. believe it or not i have seen some mobile home furnaces not work just because the squirrel cage fan was dirty. if the fan is dirty it won’t move the air away fast enough and it either won’t start or will quite right away. shut off the electricity. pull out the fan assembly and clean the squirrel cage. good luck.