4.5 tons = how many square feet?

This house I just inspected has a 2 ton and a 2.5 ton heat pump for a 3 story 3100 square foot house. I am questioning whether the total tonage is enough for this spread out square footage. I thought a good rule of thumb was between 500 to 700 square feet per ton. Can any of you HVAC gurus help?

Was it zoned? was it well insulated? If in doubt refer it to a AC company. We are not in the business of sizing . A lot can depend on what it needs to do the job

Not zoned. Average insulation from what I can tell in the limited attic space exposed, about R-30.

Beyond the scope of a home inspection. Report the btus of each unit and you’re done.

That “is” a rule of thumb, but it is not a “good one”!

As Wayne was possibly trying to allude to, you cannot terminate tonnage by square footage. There are a whole lot of other factors that are more important.

When you build the same type of house in the same weather zone you can “assume” that a certain tonnage will handle a certain amount of square footage.

If you have a two-story house with 50% less square footage on the second story than on the first, would you put a HVAC unit on the second floor 50% smaller than the first floor?

The first floor has a buffer zone in the crawlspace or basement below. It has a condition space above, so the major heating/cooling load is the perimeter walls.

On the other hand the second floor as a buffer zone below (the conditioned first-floor). It also has the perimeter walls as a primary load (like the first floor). But the ceiling is against an attic that gets how hot? The roof above that attic is a solar collector and radiates, how much energy?

There is thermal bridging issues with the roof framing and thermal bypass issues with the ceiling penetrations for electrical penetrations (thus significant air leakage).

Hot air rises, so a significant amount of the first-floor cooling load may pass upwards into the second-floor zone and increased the load on that equipment…

With this in mind, do you still feel that square footage per ton on the first floor is equivalent to the square footage per ton on the second floor?

I would never question the tonnage without doing a proper Manual J (room by room) load calculation.

I have questioned it many times the Manual J is only as good as the person using it;-)

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These questions never have enough info…

Slab or crawlspace? Makes a big difference, slab is better for cooling.

Which floors did the 2.5 ton unit serve?

The sizing for a heat pump in FL is all about the cooling but up here its all about the heating when on a slab foundation. I had to select my heatpumps for proper heating which ended up being a little large for the cooling calcs but they still run enough to remove the humidity.

Well i have also lot of experience and i think your idea is right a good rule of thumb is between 500 to 700 square feet per ton.

Tonnage depends upon where you are located…
different geographic zones will vary the determination…
for here…
750 to 1000 square per ton…
your market may vary…