New furnace and load calculations

Old 08-16-22, 03:22 PM
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New furnace and load calculations

In a previous thread several people recommended calculating the load when looking at new furnaces.

The house involved is 1 1/2 stories with a finished basement. The supply ducts have no dampers. In the winter (sometimes down to 25 below F), electric baseboard heating in the basement does a good job keeping those rooms warm as needed. The gas forced air furnace warms the main and upper floors adequately in the winter, but my thinking is that manual dampers in the ducts would be nice.

In the warm months, the basement is always cool, never needs AC. So much cold air gets down there through gravity and registers (even if fully closed) that it is too cold whenever the AC is on. The main floor can be kept comfortable by the AC, but when quite warm the 2d floor requires so much cooling that the main floor is overly cool and the basement frigid.

When calculating load, shouldn't you take into account that the basement needs no heat or AC from the furnace and coil?

Also, do you need more tonnage to push more AC to the 2d floor, or can you accomplish that with dampers or motor HP? When looking at new furnaces, what feature would you focus on to get enough heat to the 2d floor? The ductwork lacks dampers and the supply registers are in the floor and returns in the upper walls; I expect these issues affect efficiency quite a bit. All of the room registers can be opened and closed, but wouldn't in-duct dampers help?

P.S. Are there tricks to adding manual dampers, so that it could be done DIY? All the basement ducts (rectangular and round) are reachable.


Last edited by GaryMN; 08-16-22 at 03:47 PM.
Old 08-16-22, 08:29 PM
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The typical problem with a furnace system doing A/C duty is the registers are low in the walls and the same with the returns. A true A/C would have registers high in the wall and ceiling returns.

You can't push enough cold air up to the second floor without removing the heat from the ceiling. Attic insulation is key to decent second floor A/C performance.

It is not easy to install large dampers in existing duct work.
One end needs to be open to get the damper in.
Old 08-17-22, 09:53 AM
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How is the sealing on the duct joints? An older house likely has little if any duct sealing, so you're getting the equivalent of a register or two in just duct leaks into the basement.


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