Lennox Conservator III g16 furnace


Old 05-10-09, 11:45 AM
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Lennox Conservator III g16 furnace

We recently had our furnace inspected and were told that it needs to be replace due to cracks in the heat exchanger. We live in an older house (1928) and are trying to sell it due to a relocation. Our furnace is a Lennox Conservator III g16 furnace with an input of 100,000 Btu. It appears to be about the right size for the house, but I have no idea what efficiency the furnace is. Can anybody tell me what the furnace efficiency would be - it was installed in 1984. I'm trying to figure out what to replace it with, but the first estimate that came in wanted to use an 80% efficient 100,000 Btu unit. I doubt that the current one is 80% efficient, so I would think that we need a slightly smaller unit if we do go with an 80% efficiency.

Thanks for your help!
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Old 05-11-09, 07:05 AM
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If you look in the burner compartment of the furnace, you will find a rating plate with the furnace manufacturer, model and serial number on it.

It will also have the furnace input and output listed in BTUs. You can compute a reasonable estimate of the efficiency by calculating the percentage of the output from the input BTUs.
Old 05-14-09, 01:49 AM
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Most natural draft furnaces are 75-80% efficient when actually running; having a standing pilot and open draft hood drop the seasonal efficiency (average, including startup/shutdown/standby losses) to 55-65%. Input divided by output will tell you want the steady state efficiency is

The G16 was Lennox's first induced draft furnace with electronic ignition - it's around 80%. The third and fourth digits of the serial number indicate which year the unit with manufactured. If you must replace the furnace, have a load calculation done; chances are that the existing one is grossly oversized.

If you live in a cold climate, definitely get a high efficiency furnace. Also do everything you can to reduce heat loss prior to investing in new equipment; I doubt that a house built in 1928 has much if any insulation.
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