What size AC for St. Marys, PA? 2 tons for 2,000 sq ft

What size AC a 2,000 sq ft house in St. Marys needs

2 tons central AC or heat pump, about 21,700 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 93% more than this climate needs. An oversized unit cools the air fast, shuts off before it pulls moisture out, and leaves the house cold and clammy.

If a contractor quotes you more than 2 tons for an average 2,000 sq ft house here, ask to see the Manual J load calculation behind it before you sign. That works out to one ton per 1,105 square feet in this climate. For a single room, size by the room: 10.9 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Glen Hazel 2 Ne Dam (8.4 mi away). Jul is the hottest month, averaging a high of 78.5°F and a low of 55°F, with about 1 day a year at 90°F or more. That gives a design temperature of 83°F against 75°F inside.

What it costs to run

About 314 kWh a year at SEER2 14.3, or roughly $66 at Pennsylvania's average residential rate of 21.13¢ per kWh. Cooling runs 2 months of the year here.

What drives the load in St. Marys

Of the 21,700 BTU/hr, 5,760 comes through walls, roof and windows on a 83°F afternoon (27%), 13,000 is sun on the house (60%), 2,120 is four people and their appliances, and 800 is moisture the coil has to wring out of St. Marys's summer air (4%).

Sun is the biggest share here, more than heat through the walls, so shading west windows and a light roof cut the size you need more than extra insulation would. Sensible heat totals 20,880 BTU/hr before the moisture allowance.

Other house sizes in St. Marys

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 1,600 BTU/hr; a 350 sq ft living room needs about 3,800 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 6,500 BTU/hr.

Month by month in St. Marys

Jul alone uses 38% of the year's cooling energy. The unit runs the equivalent of 207 hours a year at full output.

St. Marys against nearby cities

Of 68 Pennsylvania cities here, St. Marys has the lowest cooling load. The range runs from St. Marys at 21,700 BTU/hr to Philadelphia at 34,300.

Where St. Marys's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Glen Hazel 2 Ne Dam, 8.4 miles from the center of St. Marys at 1,720 ft above sea level. That is close enough that the station's summers are St. Marys's summers.

No other city in this set uses Glen Hazel 2 Ne Dam, so this sizing is St. Marys's alone.

The sun over St. Marys in late July

At 41.45°N, the noon sun on 21 July stands 68.9° above the horizon over St. Marys and the day runs 14 h 33 min from sunrise to sunset. A lower noon sun puts more of the day's heat on east and west walls than on the roof, which is why shading west glass pays off here. Long days mean the house keeps gaining heat into the evening, well after the outdoor peak.

12,361 people live in St. Marys, the 52nd largest of the 68 Pennsylvania cities sized on this site. Coordinates used: 41.4550, -78.5202.

St. Marys AC sizing questions