Quote:
Originally Posted by NiHaoMike
An oversized heat exchanger would be more efficient in steady state operation, but blanking off one of the burners will not give you an oversized heat exchanger since you'll only be using part of it. What would is downsizing the burner nozzles (or otherwise reducing the fuel flow) and then somehow adjusting the airflow to maintain the correct mixture. (I think you would then have to slow down the air handler to avoid condensation and corrosion, but I'm not sure.)
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That's the whole kicker on a natural draft furnace like the OP has, no easy way to do this.
I've tried downsizing orfices, delta T across the plenum drops faster than BTU input. Dropping input in 1/2 drops Delta T by about 2/3. 80K to 40k would take Delta T from 50f to 20f. Excess air in the combustion chamber accounts for a large portion of the efficiency loss.
I ended up just finding a good buy on a smaller furnace on Craislist. Easier to replace the whole furnace than experiment with different downsizing strategies. Bought a 2004 44,000BTU 90% furance still new in the box for $250, sold the 1999 88,000BTU 80% furnace for $175. Deals are out there, just have to look for them.