Nope. You need to do a proper heat loss and heat gain calculation based on the specific location, building envelope data and solar orientation. Rules of thumb are just wild a$$ guesses that bear no similarity to reality.
There is a "rule of thumb" for A/C systems: 1 ton per 500 square ft of floor space. In general, this is gross overkill, however, trying to do the "exact" calculations are not that trivial:
Rules of thumb are useful but not for design purposes. Someone may be putting together a quick budget estimate for a capital budgeting purposes or for a sanity check on someone else's sizing of a system.
I have posted this link many times before. There is free modeling software called eQUEST available at
This software is made available courtesy of California's electric utility ratepayers and the California Public Utilities Comission. You can even get free training on the software in California through the electric utilities. Using the schematic design wizards is relatively easy and if you are familiar with modeling software it should only take you an hour or two. Since I use it a lot I probably could do a block load for this house in fifteen minutes. It is used in LEED assessments very often.
If you do decide to use it I would click on the "multifamily dwelling low rise" to use its default settings. There is no single family residiential since it is not intended for a residential tool.
Heating load in the Southeast is approximently 2.5 times cooling load. So if you assume 3 tons for 1800sf home in Atlanta, assume your heating load will be around 90k btu/hr.
Rules of thumbs are just a guideline -- they're nice to compare your engineering calcs for comparison; then, you can rethink your assumptions to derive a professional calculation.
I run a load calculation on Every project I design. Like stated earlier, lawyers are waiting for ya' using only Rules of Thumb numbers.
But I DO use a self imposed check of 550 - 650 sq.ft. per ton for residential condo calculations. It seems that the majority (Florida coast condo's excluded)of the condo's run upward of 800 sq. ft/ton per load calculations. While this is what the condo unit may require to meet the cooling load - at this value not enough air is being delivered to assure good air circulation throughout the space. That's when I will increase the tonnage by using the 550 - 650 number.
By your estimate, a 3-ton heat pump system would require as much as 90,000 BTU's of heating? That is a 69 degree delta T!! How you going to get that out of a heat pump? Gas Furnace maybe......
Heating is always much less than cooling here in the southeast . Best advise I can give is to size your supplemental heat for a 25 degree rise or 95 degree discharge. With this, you are covered even if the compressor fails.
Wow... on a house with a 3-ton AC unit I'd expect to see an 80,000 Btu/hr furnace... but I'm currently surviving a cold snap that has seen us drop to -45C.
For reference, on a typical home even when the outdoor temp dropped to -45C not one furnace ran continually. Mine ran ~1/2hour on the hour.
It is amazing Chris at how low the heating capacity of a furnace in a Canadian Home is with respect to those in the southern states. You must be a minimum 100F temperature differential in the Praires, NW Ontario was not much warmer.
A lot of times when the southerners use gas heat, they end up using a GROSSLY oversized gas furnace, just so that they can move 5 tons worth of air.
When I was contracting in Canada, the most common size furnaces I sold for NEW construction homes was 60K and 75K, in both condensing and non-condensing models
Take the "V" out of HVAC and you are left with a HAC(k) job.
If a 3 ton HP can provide 24k btu/hr of heat on a 10 degree day, the rest has to be made up in auxillary heat.
Think about it -- during the Summer the difference between ambient and indoor is around 20 degrees -- what is the difference between 70 degrees indoor & a cold winter day???
Rough total: 17345 Btu/hr ... which is more than I would size any furnace in Florida for.
In summer the ambient contribution to cooling load is 1/2 (~8000 Btu/hr). which means the rest of the 30,000 Btu/hr cooling load is solar (and humidity).
I've ignored ventilation because 1)most of my projects use HRV's 2) This post was getting way to long anyhow. I've also guesses on R values... but even if you half the R values we've got a long way to go to 90,000 Btu/hr.
