BTU's per square feet

[beenthere]

[QUOTE]I got a 4000 sqft house made of Styrofoam 4 feet thick and can cool it with a ice cude and a fan.QUOTE]

LOL...

[qssfl]

Here in South Florida we get people all the time kicking the tires asking for quotes. Depending on the age the house was built... if it is well insulated or not...depending if it is a rancher or a cube shape...depending on exposure...etc...etc... I usually go average 600 sq feet per ton...

I quote a price....get a down payment and fine tune from there....otherwise....I let them keep shopping....

My 2100 sq foot rancher built in 1977 with a South West exposure... CBS construction ...not the best insulation...some original windows with lots of sliding glass doors ....called for 4.5 ton....I installed a 4 ton...corrected the duct work which had a properly sized supply and a return grill only good enough for a 2.2 ton....

My electric bill went down 50%.....as we upgrade the windows and insulation....I might just be a pinch oversized... with humidity control.... all should be o.k...

[hcong]

500 sq.ft. per ton

2000 sqft house 4 ton unit. In most cases after you do a study you will fall somewhere more or less close to that.

Did i just hear an ME curse?

*curse*!!! Actually I do use rules of thumb to check my work and for bids since I don't want to do the load calcs before the project is awarded.

This is what I got from ASHRAE Principles of HVAC 1994 (attached is a longer list of buildings).

Houses:
Cooling: 17 BTUH/Sq. FT
Heating: 3.0 BTUH/Sq. FT

[dlove]

*curse*!!! Actually I do use rules of thumb to check my work and for bids since I don't want to do the load calcs before the project is awarded.

This is what I got from ASHRAE Principles of HVAC 1994 (attached is a longer list of buildings).

Houses:
Cooling: 17 BTUH/Sq. FT
Heating: 3.0 BTUH/Sq. FT

Thats what im Talking about

I know it don't take much to do a calcs when the job is yours. But, how many times are you going to do a calculation or give a detail estimate than have the customer turn around and handed to the next guy, before you start doing round about estimates?.. call me when your serious...

[hvacker]

I think ROT's are used much more often than actual loads. So called "older guys" will say they've been doing it this way for years and why should I tie up my time with a load just to lose the job to another contractor.
I think younger contractors are much more likely to do a load because they are more familiar with computers and more comfortable with the results. Computers seem like a natural place to get information for them.
I often see posts here looking for load software when it's just as ez with a simple paper form but they've learned to trust computers. I'm sure it's easier to get an older contractor to use the paper form than a computer. Different generations.
I wouldn't expect many older contractors to change any time soon. Their system will probably work ok and no one's going to yank it out to show how a different system will be better as long as the house is fundamentally similar to those where ROT worked before.

[courttech]

What is a computer and where can I get one, i hear they are the way to go....

[hcong]

I think ROT's are used much more often than actual loads. So called "older guys" will say they've been doing it this way for years and why should I tie up my time with a load just to lose the job to another contractor.
I think younger contractors are much more likely to do a load because they are more familiar with computers and more comfortable with the results. Computers seem like a natural place to get information for them.
I often see posts here looking for load software when it's just as ez with a simple paper form but they've learned to trust computers. I'm sure it's easier to get an older contractor to use the paper form than a computer. Different generations.
I wouldn't expect many older contractors to change any time soon. Their system will probably work ok and no one's going to yank it out to show how a different system will be better as long as the house is fundamentally similar to those where ROT worked before.

Here in California, plan checkers are becoming strict and asking for Manual D & J. For energy purposes, the state won't let you install a new HVAC without load calcs and making sure the unit is not oversized.

[courttech]

yes, I am experiencing the same thing here in the west coast of canada...basically same climate as seattle, inspectors wanting heat loss/load calculations...not that they know if they are right or not. But to get down to the accurate numbers takes getting so much info from the general contractor in regards to the materials used in the construction of the building that a day can be spent on just gathering info..adding cost to the quote that the puts your bid over the competition. If it was a blanket policy throughout each district/province/ state then we would all be on the same page for costs...until then...get the quote out based on past experience and carbon copy the heat load calculations unfortunately.

[hvacker]

I think the fair way to do a design/load estimate would be for the responsibility to fall on the Architect/engineer, house designer. They have the responsibility for the integrity of the building and have been putting the responsibility on the trade for too long.
It's done in commercial buildings, at least larger ones and as long as building performance is an issue I think the design should be where it belongs, with the designer.
I'd bet one thing that would disappear if design/performance were an issue would be flex duct snaked all over an attic.

[Juan Madera]

My rule of thumb in sizing is use manual j and then pick the smallest system that will fit the bill! I tend to (slightly) undersize as it adds comfort, reliability, economy of operation and lastly, customer satisfaction. I think we ALL tend to add just a little to make sure we are covered. This, i believe, includes the #'s you get from man J.

Have installed 50k BTU furnaces in 1800 foot split entries (900' per floor) in Montana and had nothing but praise for how even the temperature was throughout the home in the coldest weather.

Keep in mind that Man J does not include internal gain in heating calcs.

[ICanHas]

Can this question even be answered?

Isn't this like asking.. the person weighs 150 lbs
how many Calories does he need to eat every day to maintain his weight?
There are so many variables that you can't possibly give one simple answer.

[dlove]

I think the fair way to do a design/load estimate would be for the responsibility to fall on the Architect/engineer, house designer. They have the responsibility for the integrity of the building and have been putting the responsibility on the trade for too long.

Diamond in the rough. I deal with A&E's a lot. Not all have a clue how to design a system outside of a text book. But we need them. And most of the time they get by. But you would be mistaken if you don't have the HVAC vendor and GC look over the drawings as well.

Its a shared responsibly.

[hvacmike85]

what program or method do you guys use for load calculations? Are commercial calcs easy once manual j is learned

[meplumber]

Mike, actually, Manual J is for residential only. For commercial, there are (2) accepted methods, ASHRAE RTD and ASHRAE CLTD. These methods factor in required ACH and required ventilation applicable to the commercial and institutional market.

[beenthere]

ACCA manual N is for commercial.

[meplumber]

True. I forgot about Manual N. I use WrightSoft's RightSuite Universal. We also have HVACalc. It is a much simpler residential/light commercial heat loss/gain program. We only use it for little projects that don't need all the bells and whistles that we get from RightSuite.

[Willserve]

I think ROT's are used much more often than actual loads. So called "older guys" will say they've been doing it this way for years and why should I tie up my time with a load just to lose the job to another contractor.
I think younger contractors are much more likely to do a load because they are more familiar with computers and more comfortable with the results. Computers seem like a natural place to get information for them.
I often see posts here looking for load software when it's just as ez with a simple paper form but they've learned to trust computers. I'm sure it's easier to get an older contractor to use the paper form than a computer. Different generations.
I wouldn't expect many older contractors to change any time soon. Their system will probably work ok and no one's going to yank it out to show how a different system will be better as long as the house is fundamentally similar to those where ROT worked before.

What is the Rule of Thumb to determine if you fall into the "older guys" category? Is that an actual age or mentality Lol

[Mr Bill]

I think ROT's are used much more often than actual loads. So called "older guys" will say they've been doing it this way for years and why should I tie up my time with a load just to lose the job to another contractor.I think younger contractors are much more likely to do a load because they are more familiar with computers and more comfortable with the results.

This is becoming the biggest line of BS I have ever seen, hacks are not determined by age, their determined by mentality or capabilities, you come to Houston and I can show you two old dudes, to one young guy, that actually does a load calculation. I saw a sting on Channel 11 here in Houston about a month ago, guess what! all the crooks and hacks in the sting were probably under 30, go figure! This "old school" crap about the old school dudes is really getting old, some of us have come around, quit the dang stereotyping.

[zxcb]

What do you guys use for a wall R factor in old houses when the HO has no idea what insulation is in the walls?

[54regcab]

A load calculation is best IF you don't fudge input numbers to get the answer you want. Newer homes in our area can be as little as 1,000 sqft per ton, 800sq ft per ton is common. Older homes and upstairs units can go as high as 350sq ft per ton. Unfortunately "rule of thumb" 500sq ft per ton is what typically gets installed regardless of actual load...

On the heat side pick the smallest capacity furnace that has the blower size you need for A/C, it's more than enough heat for just about any southern home.