My house needs a new furnace, the gas-fired forced air kind. The output of the old furnace was 84,000 BTUs. We must get it at the place where we have a credit account. Space to install it is limited so it must have a width less than 18 inches. The highest-capacity FAF furnace within the width limit is this. Its output will be 72,000 BTUs. My house has 1,300 square feet on the upper level and another 1,000 sq ft on the lower level which is half below ground level. We have a wood-burning stove which we use in extremely cold times.
Ehow says “Calculate the BTU output you need for your house if your old furnace is not available as a reference. To do this, multiply your home’s total heated square feet by a heating factor of 30 to 60 based on your climate. For example, if you have a 2,000-square-foot house and you live in the hot regions of the South, multiply 2,000 by 35 for a BTU actual output of 70,000. Multiply your square feet by 40 if you live in a slightly cooler area such as the Carolinas, Tennessee or central California. Use 45 for areas such as Kentucky, Virginia, Missouri and Kansas. Use 50 for the upper Midwest and 60 for the cold regions of the north.” I live in a cold place.
Elsewhere I read that one should halve the number of sq ft below ground level when performing this calculation. So, I get (1,300 + 500)60 = 108,000. But 432 sq ft of that 1,300 are in 2 rooms above the garage which are not part of the forced air system. They are 3 feet higher than the rest of the house and are kept warm enough by the effect of warm air rising. And, we have that wood burning stove. Eliminating that square footage yields a result of 82,080.
Can I get by with a furnace which produces 72,000 BTUs? If the furnace is overworked it could wear out too soon or something worse could happen.
How often does/did the old one kick in?
If it ran continuously trying to heat the place, a smaller unit will not work.
If it ran 10 minutes per hour at the most, a smaller unit might be adequate - it will just run more often, and longer.
Why the space limit?
If the space limit is because of flammable materials (house framing, for instance), read the installation instructions very carefully - they will specify the minimum clearance for combustible materials.
If it is going to run long, it will heat the surrounding materials.
The air return (rectangular galvanized metal duct) is on one side and house framing (wood) is on the other side. The gap is 19.5 inches. The old furnace is touching the duct and there is a 2.5 inch gap between the old furnace and the wood. To the best of my recollection the old furnace would run about 15 minutes ( maybe 20) per hour in very cold weather. Less when the stove was fired up.
We used to have a very helpful HVAC professional on the board. He talked me through replacing the furnace electronics ($500 list part - what the HVAC guy takes as base and adds percentage - was $300 online).
Since he hasn’t jumped in, I’ll try.
First: Disclaimer: I can’t imagine what you are talking about - is the air duct on the side of the house? 19" from the house? Is it top or bottom or side discharge? Where does the heated air go - the box on top of the (updraft) furnace with ductwork coming out is the plenum. The easiest way to replace a furnace is to jack it up and slide a new furnace with the same dimensions under it and seal up the gap (should be a small space - the plenum should rest directly on the furnace. If you have a side discharge furnace, make sure the new one is 100% side discharge.
And look into re-working the duct work to accept a top discharge - hot air wants to go up - not sideways.
Putting a furnace in the cold air duct is a very bad idea - you would be, in essence trying to reverse the function of hot and cold ducts.
If you are running 20 minutes with a marked larger unit, I’d be scared to try the smaller one - in almost everything, the last 20% is what makes it work.
You are looking for a larger unit to fit the plenum (not the cold air duct).
If money is a problem (I was used to calling the HVAC/plumber and pointing out the problem, then getting out of the way - the DIY project was a new experience), first find the unit you need, then look for the money. Going online with a mfg and model number will save close to half. Freight may be an issue - make sure to check S/H before finalizing a purchase.
As far as that goes - why are you replacing the furnace? The metal is probably still good, and the innards can be replaced. So why a complete swap?