The “loose” general rule of thumb for AC is 1 ton/500sqft. What is the loose general rule for BTU’s, or where can I find this info?

EX. how many heat furnace BTU’s are needed or recommended for 500 sqft?

Mark, I have absolutely no idea but I’d guess 20,000 to 25,000 btu’s per 500 sq. ft. based on what I normally see up here.

I’m sure there are many variables that go into figuring that.

HVAC contractors have been putting in so many pieces of equipment into the same type house in the same geographical location that they can divide the BTU by the square footage of the house and come up with a close rule of thumb that will work.

To use this rule of thumb nationally is ridiculous.

“Heating load calculations” is another area of the HVAC that should be kept away from. Unless you have a whole lot of time, and a whole lot of money to spend on school, keep away from it.

American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) has and is still attempting to identify comfort levels. Comfort levels change between age, sex, ethnic background, physical condition, and a whole assortment of other criteria. You can not satisfy the comfort levels of everyone with the same setting. ASHRAE has also set the standards to determine heating/cooling equipment requirements of buildings of all sorts and sizes.

Heat load calculations consider the color of the house, a number of doors and windows, the construction of the doors and windows, the construction of the roofing and wall framing, the orientation to North, the amount of wind flow, the normal heating and cooling days for the particular location, number of people entering into the building, the number of people occupying the building, the appliances to be utilized, the amount of lighting expected to be utilized, and a whole bag full of other criteria to come up with the approximate size of the equipment to be utilized. After they determine this, they must then address the impossible task of determining the comfort of the inhabitants of the building. These folks are engineers. This is their job. Leave it up to them. If you wish to perform this service you need a whole lot more education, purchase a whole lot more test equipment, and you must charge about 15 times more than you do for a home inspection.

Simply put, turn on the heater or air conditioner while you’re inspecting the house. If you are excessively hot or cold by the time you’re done, the system is working. If you find that there is inconsistent comfort levels in the house while conducting your inspection you do not need more than your opinion that it is not comfortable.

In SC we are required to report btu’s when available.

I report the output btu’s which is lower than the input btu number.

So this brings up another issue with btu’s per SF, which btu do you use in the calculation?

Gas furnaces around here are not a problem, its the older heat pumps that might not work in some houses very well in really cold weather. The newer heat pumps and the ones that use R410 are usually sized ok.

Verifying a unit will function well in summer and winter in a house with hidden wall insulation or lack thereof is not something you can do in “3 hours”.

But, if you see a system size that just does not seem right it is a good idea to report it as needing further testing by an hvac tech.

I don’t know, I can tell a lot in three hours.

What do we base


I know what feels right, thoes numbers stamped on a unit don’t mean a thing.

Again, determining the right unit for the job is not OUR JOB.

I stay away from sizing A/C’s but here’s a couple links that you can recommend.

I also do HERS testing and heat load calculations for architects in California. I can tell you 1st hand that the heat load calculation is only the tip of the iceberg when trying to decide if the right combination of HVAC hardware has been installed.

If the ducts aren’t tight, you can expect an average leakage rate of 30%. How that affects the HVAC system also depends on how much the supply and return side of the ducts are leaking.

Throw in the infiltration level of the house, you have a very complex problem.

Stay away from trying to determine if the HVAC system is adequete for a house. You’re putting yourself up to more liability than you need to.

I’m not trying to determine anything. I just know that someday someone is going to ask, Is this furnace the right size for this house? Just like I get asked almost every house about the ac.

Mark, my reference material indicates 35 to 55 BTU’s psf depending upon the quality and type of construction, i.e. amount and type of insulation, air tightness of the home, etc. So, I’d say a typical Southern/South Western 2,000 sf 1-5 year old home would be probably around an 80,000 BTU output heater. That could be as much as 110,000 BTU on an older, less well built home.

The size of furnace depends on the climate. The formula to calculate heat loss is BTU=(A X (T-t))/R + A… +A…
BTU= Btitish Thermal Unit
A= Area of that pictular wall or window or etc.
T= Inside temperature Usually 70 F
t= Outside temperature (set by local weather conditions -40F in Alberta)
R= Thermal resistance of Area being calculated. 2x4 wall with fibreglass insulation R12 for the insulation and R1 or 2 for the sheathing). R1.5 for single pane galss. R2.5 for thermal pane glass. and so on. You need a book full of R ratings and do a lot of calculations to calculate each area of each thickness of structure between the inside and the outside.
Anyone that asks me for an answer to “Is furnace the right size” better have a $1000,00 for me to provide an accurate answer. The total of all areas is the output rating for the furnace required. All furnaces are rated for sea level. As you go up the furnace output drops.
There are rules of thumb but they apply only to the pictular area and climate you are in. :cool:

Im not sure anyone answered your question but after reading it several times I came to the conclusion you wanted to know how many BTU’s = tonnage. The formula is quite simple…12,000 BTUs per ton. From that you should be able to extrapolate what you want to know.

Doug, He used the A/C rule of thumb as an example. He wants to know if there is a similar “rule of thumb” (wild a$$ guess is more like it) for furnace sizing.

Vern, Now your talking!

Thanks guys