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KLS

On 3 Sep 2006 10:23:44 -0700, wrote:

Why does anyone need a manual J calculation done when they already have
actual data and experience from their existing unit? Certainly you
need to do it for new construction, but if you have an existing 3 ton
unit, the experience with how that has performed IMO is going to be a
lot more accurate than a theoretical calculation.

Your points are entirely valid, but some of us are dealing with
indestructable and persevering units like the Williamson forced air
gas furnace from 1963 that we acquired with this 1930 house in 2003,
when we moved in. That baby, lovely and unstoppable as it was, was
significantly oversized for the 3-floor (excluding full basement)
3,000 sf house it was heating, so we decided to remove it and put in a
smaller, but much more efficient furnace. I'm so happy we changed it
out for the 92% efficient 2-stage York gas furnace we're using now
because I really think in just one more year or two we will have
recovered our investment what with the natural gas prices skyrocketing
as they have over the past three years. So, often, a manual J is
needed.