i have a lennox G12Q3E-110-2 furnace with the serial number placing it from 1979. i am currently doing some renovations and while i have the ceiling open i would like to run the vent pipes for a new efficiency furnace for when i get to being able to replace that.
i think most places i can find say just run 2" pipes and you'll be fine, but due to the location of my furnace, i think the run is 30 feet with at least two 90's inside. plus whatever creativity needs to be done outside because it would exit the house 2' below a window.
if my furnace was 110,000 btu in 1979, do i replace it with 110,000 btu now as well? or do i use the bonnet cap of 88,000 btu? i cannot figure out the efficiency of this furnace, but if we assumed 60%, would i replace it with a 70,000 btu furnace? then i find calculators online that say zone 5 is 50-60 btu/sq foot where 1,800 sqft would put me at 90,000-108,0000 btu.
does anyone know how it works when you replace an old furnace with a new high efficiency one? as far as sizing them.
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