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Furnace sizing

Mmkmmk's picture

We've a bit of on odd existing condition and are trying to confirm the correct size for a new furnace:

  • 2 story house with basement, 800 1 SF 1sf fl and 600 SF 2 nd fl
  • in Chicago
  • very old, 1870s
  • masonry, poorly insulated exterior walls
  • a boiler that feeds first floor radiators
  • An old furnace, 70000 btu// hr that feeds 2nd fl only
  • 2 ton exterior condenser that works with furnace

We need to replace the old furnace and need it to heat the 2 nd floor. It is very old but also up cycles too much. condensing unit can stay. What size unit should we be looking at if we need it to heat the 2 nd floor but are also looking for it to cool the 1st floor as well during the 2 to 3 weeks when it's super hot?

Thank you,







If the old furnace is (post #213968, reply #1 of 1)

If the old furnace is properly sized at 70,000 BTU (output, not input), and the new unit will be heating the same space, it needs to have the same heating capacity.

However, the new furnace might need to have to have a larger, more powerful fan to handle the airflow needs of the first floor.  (And, of course, you'd have to somehow provide ductwork to the first floor.)

(You're probably better off investing in a condensing furnace with a DC-drive variable speed motor, even though it will be quite a bit more expensive than the cheapest "meets requirements" unit.  The energy savings and increase in comfort will be well worth it.  You might even consider ditching the old boiler and using the furnace (with zone controls) for the whole building.)

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville