Btu output

Does anyone know if there is a rule of thumb for heating BTU. To be more specific. How many BTU is sufficient for lets say say a 1800 sq’ house? This is what I believe it is SQ’ x 100 = BTU needed. (1,800 x 100 = 180,000 BTU)

Depends on your areas average low temps in Oklahoma I can heat a 1800 SQ FT home with a 100,000 BTU furnace. I always sized my furnaces by the CFM of air output required for the A/C BTU

It also depends on the age of the home. Just did a newer home, 2350 sq ft, great insulation. Furnace was 69k in, 56k out. Went from 65 degrees inside the home to 69 degrees in about 30 minutes. Newer homes are insulated and sealed much better than older homes. To try and compare that to a home that is more than 15 years old is like comparing apples and oranges.

Thanks guys I appreciate that. That relates why I’m always explaining the difference between efficient heating units and suficent heating units


As others have said, BTU requirements are unique for each house and location. You can look for clues or ask questions to see if the current system has been adequate for the current owner:

  1. Ask the current owner on the adequacy of the current system.
  2. Suggest the buyer have the seller show past utility bills.
  3. Supplemental heat sources in some rooms is a red flag the system may not be adequate
  4. Plastic over the windows or rooms with closed registers.
  5. Older houses that have been recently insulated may be an attempt to compensate for a poor furnace, however be careful with this one.

Many owners try to self-diagnose their HVAC system and through ignorance do not realize HVAC unit size, duct work size and configuration, air flow, insulation and architectural items, such as an open stairwell are all variables in the equation. Many home owners make the mistake of asking a salesman for help. The insulation salesman says you need more insulation, a HVAC saleman says you need a new HVAC system, same goes for the window salesman. IMO an independent assessment by a qualified person or company is the best solution.

Rodger that Randy Thanks for the info.

BTU for a house should be based on a heat loss calculation for that house, square feet of building envelope, square feet of windows and doors, R or u values of everything, the outside design temperature (average coldest 2 weeks), exposure (sheltered not sheltered), number of air changes from air leakage or ventilation. Insulation and and air tightness have a much larger influence than the square footage. A hundred year old 600 square foot house might need a bigger furnace than a brand new 3000 square foot house using the latest and greatest energy saving construction.

Look up Heat loss calculators on the internet, they often let you have one free try before you buy.

Looking at all responses you can see "there is no “rule of thumb” available…

You need to do it long hand…