Load Rule of Thumb

[the_dude]

Dude here,

I have heard of the rule of thumb "400CFM / ton of cooling".

Is there a similar rule of thumb that uses SQ FT per ton (or CU FT per ton of cooling")?

[bama 101]

no, not really but would recommend you to have a load calculation done.


to many variables to set a number of square feet per ton.

[hvac hero]

It has a lot to do with where you are in the US. The only exact way is to do a load calc. I admit I have sized probly hundreds of houses before I ever got the load calc program. When I got into this trade, I was taught 500 sq ft per ton as long as the ceilings werent super high, etc. I never had a problem with that. I do have the load calc program & when I run the program, it just about always still works out to 500 sq ft per ton. I live in Memphis. I can assure you this wouldnt apply to many other states.

[tinmantu]

For air flow, I calculate the duct to deliver 100 cfm per 100 sq. ft. and always round up instead of down...tonnage matching the airflow required via the load calcs to determine whether I recommend a 3 or 5 ton blower and make adjustments accordingly on the keyturn. Haven't had a complaint yet, only compliments.

[drwiley]

500 sq ft per ton works well in north Texas as well

[NormChris]

You better read this son.

http://hvac-talk.com/vbb/showthread.php?threadid=33866

So should some of the rest of you.

[Senior Tech]

To assume so many tons per square anything is improper. You are assuming the heat loss or cooling load of any given home of the same square footage is exactly the same. What about different types of windows, doors, insulation, etc...doe's every home face the same direction?
Save yourself alot of frustration and run a load on EVERY home.

[beenthere]

After your familiar with the houses in your area, theres nothing wrong with using sq ft to do the estimate for your customer.

Do the load calc after you have the signed contract if your worried about wasting time on estimates your don't get.

Keep in mind that with some customers, the measuring for a load calc during the sales call is a valuable sales tool.

[Midwest]

This rule of thum is OK... that is unless you like accuracy, doing things correctly for the customer, savinge them energy and improving their comfort. If those things are unimportant, go with the rule. As Dan Holohan of heating help dot com fame says, "the urge to oversize HVAC systems is stronger than the average sex drive". Calcs show many customers you are professional and others who fel it's unnecessary need educating. Too many factors will cause variances in loads for a structure- orientation, insulation, windows, shading, ventilation, local climate, customer habits, etc. Example- just take a typical rancher and turning it 90* can make a 1/2 to 1 ton difference due to loading from east/west windows.

[rickboggs]

"the urge to oversize HVAC systems is stronger than the average sex drive"

Now that explains everything. I thought it was just me.

I can see those emails now, "Want a bigger system" and oh no!!! those tv commercials... "Meet Bob..."

[elkhvac]

I only use "rule of thumb" to give an estimate. I will MJ8 every home. The typical rule of thumb here is 600 sqft per.

After load calcs I find 500 to 900 much more accurate. We are doing a new custom home that will require one ton per 1250 sqft!

Sure the 600 rule will work most of the time. Of course the system will be over sized most of the time.

Start running the loads on every home you do. You may be surprised how much over sizing you are doing. After doing MJ on every home you will be able to "guess the size" much more accurately for giving prices to customers.

[drwiley]

I propse we all read this
http://dictionary.reference.com/search?q=rule+of+thumb

[allstar08]

my favorite rule of thumb for residential sizing is 4 vents per ton. a guy from another company told me he uses 1.5 btu's per sq. ft of living space. i prefer my method, but i like the 500 sqft method also. with all the programs out now you don't even need to break out the man. j. i have found that a load calc is crutial in some cases to getting that bid.

[arc8]

I guess i use rule of thumbs as well, but it works for me.
Do you guys use any rule of thumb for estimating a bid price for residential based on sq.ft.?
I take total sq.ft.(if ranch double it) multiplied by 2.65 rule.
Anyone else?

Oh by the way, this is for a standard gas forced air with 13 seer equipment.

[txborn]

Originally posted by elkhvac
After load calcs I find 500 to 900 much more accurate. We are doing a new custom home that will require one ton per 1250 sqft!

Sounds like a tight house! Using foam insulation?

[stevensonmech]

I find things become tricky when you run into homes with dual heat sources - ie. forced air and radiant floor heating on one or more levels...

[elkhvac]

Originally posted by txborn

Originally posted by elkhvac
After load calcs I find 500 to 900 much more accurate. We are doing a new custom home that will require one ton per 1250 sqft!

Sounds like a tight house! Using foam insulation?


Not foam insulation but, yes it is very tight.

[dan sw fl]

Originally posted by elkhvac
We are doing a new custom home that will require one ton per 1250 sqft!

Could you provide some building details, features, and location that enables such a low cooling load?

I believe that
windows area must be very small And
infiltration an absolute minimum
to achieve > 800 Sq. Ft. per ton.


Elkhvac,

Would you start another thread to address how to develop energy efficinet houses?

Thanks.

[Mr Bill]

Originally posted by beenthere
After your familiar with the houses in your area, theres nothing wrong with using sq ft to do the estimate for your customer.



I totally agree with you beenthere and I bet after doing this in Houston for 30 years I can come closer to the needs of a home in "Houston" than any "general" fit all load calc can. I have got butt into some really harry situations, when I used to depend totally on a loac calc. from now on it's a load calc along with some good old common sense and experience in Houston when I bid a job or I would just walk.

[Mr Bill]

My favorite rule of thumb is how many tons of air do you want installed in your home sir?
Ok that will be XXXXX when would you like us to get started? and will you be paying cash? and also by the way we are not responsible for to much air or not enough when the Home Owner decides and that will be documented on the invoice, so are we good to go? :D

[dan sw fl]

Originally posted by beenthere
After your familiar with the houses in your area, theres nothing wrong with using sq ft to do the estimate for your customer.


I assume that with your EXPERIENCE
you Know
and somewhat inherently account for
the effects of house orientation,
insulation, window area and infiltration.

I believe that you have a different sizing method to address New and Old home
which can not be addressed the same.

[Mr Bill]

Originally posted by dan sw fl

Originally posted by beenthere
After your familiar with the houses in your area, theres nothing wrong with using sq ft to do the estimate for your customer.


I assume that with your EXPERIENCE
you Know
and somewhat inherently account for
the effects of house orientation,
insulation, window area and infiltration.

I believe that you have a different sizing method to address New and Old home
which can not be addressed the same.

I think you can if your just giving an estimate which means in Texas "to determine roughly the size, extent, or nature of, to produce a statement of the approximate cost of "
Now I don't know what an estimate mean in your state language but if you had not read so much into beenthere's statement he did say estimate, and I like him am not leaving my load calc there on an estimate for another "CHEAPER" company to use as I have been there done that way to dam much, when and if I get the job I will come back and do a load calc and write my price and tonnage required in stone.

[beenthere]

Originally posted by dan sw fl

Originally posted by beenthere
After your familiar with the houses in your area, theres nothing wrong with using sq ft to do the estimate for your customer.


I assume that with your EXPERIENCE
you Know
and somewhat inherently account for
the effects of house orientation,
insulation, window area and infiltration.

I believe that you have a different sizing method to address New and Old home
which can not be addressed the same.

As I said, to do the estimate.

You go down a street in the city that has about several sets of row homes, the inside the homes are about the same size, and the ezposed walls are the same.
You look at the windows if they are original or new.
Ask if they have or are going to do any weatherization in the near future.

With this info you will be close enough to give an estimate.(As in a price)


The other side of the street may need a 1/2 ton more or less depending on orientatin of the street your on.

But I've never had it be more then 1/2 a ton in either direction, from one side of the street to the next, and that is by a load calc.



I don't keep my laptop with me all the time so I can't don't run a load calc at the house most of the time.
But i make more sales giving the estimate on the call then if I mail it.



I'm not talking new construction.

[pulaskihvacr]

I tell my students to just get a picture of a house in their mind, place it in Phoenix and imagine the AC it needs. Now hook a lowboy under that sucker and haul it to Fairbanks.. think it will need the same AC?? You didn't change ANYTHING but the location and the load changed. Wouldn't you also expect things like orientation, occupants, insulation, windows, color and numerous other things would have an effect?

[classicrock4you]

my favorite rule of thumb is making a square with your thumbs and then steping back and framing the house between your index fingers - with a single story home, if it fits from 30 feet away it requires 2 tons, 40 ft. = 2.5 tons, and 50 ft = 3 tons.

This always works perfectally, except the time that I sprained my thumb and all my loads were 1/2 tons short.

Now, at the risk of being serious, a real professional is needed for a load caculation. How many times have we seen different load calcs presented by different contractors, and then find out that Joe Newbee used wrong U values, didn't take into account certain variables (age, leakage etc.) or just didn't care enough to do it right.

[Mr Bill]

Load calc without experience and common sense is about as useless as tits on a bore hog. :D

[beenthere]

Originally posted by mrbillpro
Load calc without experience and common sense is about as useless as tits on a bore hog. :D


ROFL

[turtle]

Originally posted by the_dude
Dude here,

I have heard of the rule of thumb "400CFM / ton of cooling".

Is there a similar rule of thumb that uses SQ FT per ton (or CU FT per ton of cooling")?




This is Turtle.

Going by Sq. Footage per ton is just a good guess at best. now if you want to know exactly get a heat load done on your house. yes , guessing in Louisiana you go 500 sq. feet per ton but using it alone may get you in trouble.

TURTLE