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Question
We live in Massachusetts. We will be replacing a 1973 Janitrol 100,000 btu gas furnace. Was told new furnace will be a smaller btu size.
I understand that new furnaces are much more efficient.

House was built in 1880. 1400 sq. feet. Some areas of the home are insulated, some are not. But we will be having blown-in insulation done through a state energy program.

How will or should the contractor/homeowner figure out what size the new furnace should be? I understand that a furnace that is too big is just as bad as a furnace that is so small.

I have yet to call contractors - I want to know what to look for.

VERY CONFUSED!

Answer
Anne,

Have the Contractors do a heat loss calculation to see what size you would need, you can buy a software program from hvaccomputers.com for $50.00 and do your own accurate calculations to compare . Gas units are upwards of 98 % efficient , those types use the existing chimney ( need a liner ) or the flue pipe can be exhausted horizontal ,not needing to use the chimney,  at 100,000BTU input furnace at best it's 80 % efficient  meaning your unit is rated at 80,000 BTU output. You go by the output rating. I would recommend a 92% -95% 2 stage furnace  your state may have some rebates for the 95 % furnace . I reside in a state next to you and the rebate for 95% or higher  natural gas warm air furnace is $600.00. Propane was a lower rebate.

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40+ years diversified experience.An HVAC/Refrigeration Self Employed Contractor since 1986. NATE Certifield. Answer questions pertaining to Residential and Commercial Air Conditioning , Warm Air Heating, Heat Pump systems.Mechanical and Electrical troubleshooting of these systems.Be as detailed as possible when describing problem.Packaged unit or split systems.No appliance questions.

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Installation and Troubleshooting, Mechanical and Electrical, in following areas, Refrigeration ( Walk In and Reach In Coolers and Freezers ) Commercial Roof Top and Packaged Heating/Cooling ( Natural Gas,Propane, Electric, and Heat Pumps ) Computer Room Air Conditoning Systems,Commercial Residential Packaged and Split Systems Air Conditoning and Heat Pumps.Warm Air Oil , Natural and Propane Gas Heating systems.

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Graduated from Private Technical School in 1975. An HVAC/Refrigeration Contractor . Have Unlimited Heating/Piping and Cooling Contractor License.Limited Sheetmetal Contractors License.NATE Certifield. York Certifield Master Heat Pump Technician 1986. Served 33 years combination Active Duty Air Force , Air Force Reserves and Air National Guard in the HVAC/Refrigeration Shop/Mechanical Shop. Troubleshooting/Installing HVAC/Refrigeration Equiptment Worldwide. Retiring in 2003 as a Senior NCO ,and a Mechanical Superintendent.

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