furnace size

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Old 04-23-13, 05:29 AM
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furnace size

will a goodman 70000btu 95 percent 95070 and a 2.5 ton a/c heat and cool a 1300 sq ft 2 story house built in 1930 with masonary walls r 13 in attic and
 
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Old 04-23-13, 10:10 AM
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house is in pittsburgh pa
 
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Old 04-23-13, 10:38 AM
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Without knowing much more about wall insulation, type and number of windows, doors etc. it is hard to say what size you need.
You would do best to get a reputable firm to do an analysis. They should have the tools to determine air infiltration, heat loss etc and be able to advise the correct size furnace.
 
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Old 04-23-13, 12:22 PM
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it has masonary walls 3 doors and12 new er vynal windows
 
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Old 04-23-13, 02:09 PM
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Your house is exceedingly unlikely to have large enough air ducts for a modern 70k furnace. (unless the existing furnace and duct system was grossly oversized - ie 100k+ btu. Airflow requirements have increased dramatically with the new high efficiency furnaces)

Undersized ducts cause excessive noise, high energy consumption, and greatly reduce lifespan.

The best strategy is to reduce heat loss prior to replacing the furnace; for example, R13 is minimal for an attic (you want at least r-30, preferably R-50) and a house of that vintage could probably benefit from air sealing.

The correct way to size is to have a heat loss/gain calculation done.

Also, 2.5 tons of cooling is crazy for a 1300 sq ft home -> might only need 1.5 or 2 tons depending on the way it was built.
 
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Old 04-23-13, 11:18 PM
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I forgot to add that 70k may not be out of line if the house is very leaky, but the air ducts need to be able to handle the 1200+ CFM that furnace needs to move to operate correctly.

A 60k model (have to use another brand or model for that) which needs 900-1000cfm may do the job; 40-50k (800 cfm +/- 50 needed) is likely to be too small given that the walls have no insulation. If the house was reasonably insulated and air sealed, even the smallest model available would have plenty of capacity to spare.

The ducts in your home may only be suitable for 600-700 cfm if they were put in a long time ago because old furnaces didn't need to move much air -> a/c, for which good duct design is critical, was also out of the equation.

Only an experienced pro can properly evaluate the duct system.
 
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