Molly, A warm air furnace in itself is a fairly simple piece of equipment. It basically has 3 parts. 1(burner),2(blower),3(heat exchanger). All furnaces do have heat exchangers. It is the part of the furnace that the plenum (metal box that your trunk line or pipes are connected to) sits on above the burner section. The way this works is your t-stat calls for heat and the furnace comes on and heats up the air in your heat exchanger which then turns on the blower through a switch. The blower pulls your return air in and over your heat exchanger and sends it out through the top of the furnace as warm air to the registers. That's the basic premise. Though technology and design they have found ways to send less heat up the chimney and raise effeciency. The 90% furnace are what they call a condensing furnace because the heat temp. going up the chimney is so low they can use plastic pipe instead of sheetmetal to vent it. Although it sounds good and is, it depends on the price difference they're charging if it is actuslly worth it to you. Where warm air heats up fast to begin with I don't know how much you would save. Different manufactures have different warranty's. Usually at least 20 yrs. on heat exchangers. Repair vs. replace is a tough call whitout inspecting it. Fifteen years may sound old but if everything else is fine I wouldn't rush into a replacement because of a small repair. The only moving part you have is your blower that may fail or some part in your burner. The new ones today are all electronic with so many bells and whistles, depending on the knowledge of your repair tech can be a nightmare. As far as heat loss calculation or load calc. goes you can get it done anytime and should be done if your getting a new furnace. It is something the contractor should do anyway. That is the only way to know if yours is undersized. Part of how you do this is get the size of your trunkline and pipes coming off of it to determine the cfm's(cubic feet per minute)of air you need. For example a 12x8 trunk is rated for about 500 cfm's . A 6" round pipe is good for about 110cfm's. A 12x8 is good for about four or five 6" runs. Every piece of ductwork has a value. Say you have 7 rooms and they have 6" round pipe delivering heat. That's 7x110=770cfm's worth of air that have to be delivered by the proper size trunkline and also imperative to have the proper size return line or you will not get all the air your supposed to. That would mean that your starting trunkline coming off the furnace would be 18x8 and you would decrease it as you went along. You only get out as much air as you pull in so the return is important.Improperly sized return can also shorten the life of your heat exchanger by overheating it and causing it crack which would most likely mean a new furnace. Also determining the cfm's will tell you the proper size blower you need in your furnace. There are different size blowers depending on the btu's of the furnace. The bottom line is that with warm air the furnace is fairly simple but the ductwork must be properly figured to operate everthing correctly. I realize this has been long but I hope it gives you some help. I will look for you if you have any more questions.

Thank you so much Spot09 for your answer. Give me a little time to chew over this information and perhaps try to measure the trunkline and pipes and I probably will have some more questions. We are still waiting on a service man so although it has warmed up some we are still having to use the wood stove. We at least have some form of heat, right? Thanks again, Molly