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g/hvac • Posted by u/duct_doctor •

Manual J load calc showing 5-ton for a 2,000 sq ft house — am I doing this right?

Doing a Manual J for a 2,000 sq ft home in Phoenix, AZ (design temp 110°F). The calc is spitting out 58,000 BTU cooling load which rounds to a 5-ton system.

That feels large even for Phoenix. Existing system is a 3-ton that the homeowner says "struggles in July." I'm using Wrightsoft, inputs are:

  • R-30 ceiling insulation
  • Double pane windows, medium solar gain
  • 9-foot ceilings
  • West-facing orientation

The window area is large — about 450 sq ft total. Is the calc reasonable, or am I likely making an error somewhere?

💬 3 replies

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u/minisplit_mike •

Also verify your design temp assumptions are actually for that ZIP code, not just "Phoenix." Some Wrightsoft databases use nearest weather station which can be 5-10°F off for suburban locations. A few degrees matters a lot in a heat-load-dominated climate.

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u/hvac_hero •

450 sq ft of windows in Phoenix is a massive solar gain load — that's what's driving your number. West-facing makes it worse. The calc is probably right. Rule of thumb is 400-600 sq ft per ton in Phoenix, but that's before accounting for large window area and orientation. 5-ton for your description sounds defensible.

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u/freon_frank •

Double-check your infiltration rate. Phoenix houses are often leakier than owners think, and a high ACH50 (air changes per hour at 50 pascals) can add a ton or more to your latent load. Ask if the house has ever had a blower door test. If not, be conservative on your infiltration assumption.

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