Ton to Square Foot rule of thumb?

[thes]

Guidelines rarely, if ever work out optimally... but they will most likely get you close if set correctly for a given region/building construction.

1ton/1000sq. ft. is laughable in my region, where homes are built cheap, and the weather is quite hot. I've got 1.66 ton/1000sq ft, and it doesn't quite maintain on the hot days. (seriously tempted to get a single 1 ton window unit to help out on the hottest days).

hey crazy eddie, i feel your pain, i lived out in lemon grove, calif. with my uncle years ago,1 ton per 1000sq. ft. is for a house that is buttoned up tight. but you go to the nyc tri state area with capes built in the 50's and 60's and johnny homeowner on a budget, 1000 sq. ft. per ton, disconnect your phone, they'll be calling!

[54regcab]

you say 3 tons for 1587sq.ft. is way too big? you say a 1.5 ton seems quite small, what do you think is the right size for your house as it sits this minute w/o improvements.one observation i'll tell you right now, and at least this is in the nyc tri state area. if you go into a guys house who's owned his house,20 years , and say he has a 3 1/2 ton unit and he's happy, you do a load calc, you decide to give him a 2 1/2 ton unit! w/o any other modifications! and then he runs into a 97 degree day! do you think you'll get a call? do you think he'll be ######@? do you think he'll want to hear, it's running all day and look at all the humidity, he's taking out? good luck. these are tough areas your getting into when you start down sizing peoples units after the've lived with the other unit for years. tread gently out there!JMHO.

I did a manual J a few years ago and it came in at just over 2 tons. A 2ton unit would be fine, I don't mind if it can't keep up when we get extreme weather, I'm not going to "fall off the edge of the earth" if it's 80 in here for a few hours while it's 100+ outside. I'll close blinds/turn off appliances to reduce load at the hottest times of the day. manual J doesn't account for thermal mass storage of brick in the house. The brick "spreads the load" by absorbing heat at peak outside temps and releasing laster in the evening. instead of a 30k gain for 2hrs and a 20k gain for 4hrs the house would have a 24k gain for 6 hours.

On the heat side were at TWICE the needed capacity 88,000 BTU furnace that didn't even come close to "maxing out" when it got into single digits last winter (unusual for Oklahoma). Man J calls for 40K, a 45K input should do it. We can put more clothes on if the 45K can't keep up for a night or two.

[luskys a/c]

-- So how many people actually listen to us when those of us who actually give more than a straight tonnage to them. Recommendations go as far as the front door for most. {Yeah, we know our house is old} {We our planning on fixing that down the road} { I just want to get a new A/C , not a new house}Then they stare at me when I tell them I wont put in a higher tonnage system and that what I recommend won't cut it when it gets 90+ deg outside because of the extra money they WON"T spend now.

So here we are trying to give the most comfort for the right system for the right price and making their home more energy efficiant. The customer 9 times out of ten will just look at the quickest way to be comfortable @ 95*amb temp even if it means higher energy bills later. But hey, maybe its just the people around here.

[thes]

I did a manual J a few years ago and it came in at just over 2 tons. A 2ton unit would be fine, I don't mind if it can't keep up when we get extreme weather, I'm not going to "fall off the edge of the earth" if it's 80 in here for a few hours while it's 100+ outside. I'll close blinds/turn off appliances to reduce load at the hottest times of the day. manual J doesn't account for thermal mass storage of brick in the house. The brick "spreads the load" by absorbing heat at peak outside temps and releasing laster in the evening. instead of a 30k gain for 2hrs and a 20k gain for 4hrs the house would have a 24k gain for 6 hours.

On the heat side were at TWICE the needed capacity 88,000 BTU furnace that didn't even come close to "maxing out" when it got into single digits last winter (unusual for Oklahoma). Man J calls for 40K, a 45K input should do it. We can put more clothes on if the 45K can't keep up for a night or two.

just so you know, i went to a rheem training class on load calcs back in 2000 in gwinett county, ga. the rheem guy said if you go over say, 24,000 btu just a little, you go to the next size up. why, say you came in at 25000 btu, and give the guy a 2 ton 24000btu unit. YOU UNDER SIZED THE EQUIPMENT. it's like speeding, if the speed limit is 30 and you do 31, your speeding, also when loading out a house, and there are trees, around load it out, like there are no trees, why., the guy can cut them down tommorow, that's another thing they told us.

[dijit]

The guy that is supposed to be installing my system didn't like the 3.5 tons that HVAC Calc came up with. He and the counter guy at the supply house think that I really need 6 tons. He will install the 4 ton unit, but he won't guarantee that I'll like it. He won't guarantee the 5 ton unit he bid either. The only way I'll get a guarantee is if I get the 6 ton?

Adam

When all else fails, just ask the counter guy LOL! Counter guys have dubious advice at best (don't mean to offend any knowledgeable counter guys as I'm sure they exist). Why is his calculation so much different that his recommendation? Does he suck at typing in the correct info or what? Get a second opinion. Rules of thumb that many people use are bs. It is like saying how many horsepower per square foot of automobile do I need. Depends on lots of variables.

[udarrell]

What would your opinion be of your tonnage performance if attached to a typical/old duct system?

Those little room A/Cs wouldn't work on ductwork; that's also where a lot of loses happen.

The only reason those little Half-Ton Room A/C cool larger areas is the two large 20" Wind Machines that move the cold air throughout the rooms.

Let me clarify; the attic is 623-sf so if 1st floor is 623/ .5-ton is 1246-sf per ton.
The insulated attic area is less sf than the other floor areas; think they're closer to 725 to 750-sf 1st floor

Up on the 2nd floor I cool only one bedroom hallway & bathroom; but I don't run it much during the daytime so its always a lot warmer & with more grains of moisture than the first floor area. First floor cooling is separated from 2nd floor.

My home is a 1937 two story farm home with a lot of windows & a deep basement. See photo:
No links to any sites that provided DIY Diagnostics, darrel.

The spruce trees on the south-side were all cut down last year, there is not much shade.
NO weatherization until just last week; still have considerable air infiltration.

When those with money invest wisely in the energy economy our future will improve; however, if we simply create debt ceiling uncertainty crises, that will be the end of economic & job recovery & will gravely threaten our economic futures...

[udarrell]

hey crazy eddie, i feel your pain, i lived out in lemon grove, calif. with my uncle years ago,1 ton per 1000sq. ft. is for a house that is buttoned up tight. but you go to the nyc tri state area with capes built in the 50's and 60's and johnny homeowner on a budget, 1000 sq. ft. per ton, disconnect your phone, they'll be calling!

Yeah, I imagine so...

[tedkidd]

Guidelines rarely, if ever work out optimally... but they will most likely get you close if set correctly for a given region/building construction.

1ton/1000sq. ft. is laughable in my region, where homes are built cheap, and the weather is quite hot. I've got 1.66 ton/1000sq ft, and it doesn't quite maintain on the hot days. (seriously tempted to get a single 1 ton window unit to help out on the hottest days).

If you don't have the tools to diagnose, design, and cost/benefit the options then of course you get blank stares. "If you are clueless to what I should do, then how am I to figure this out, I called you?".

If nobody in your area has the tools, then you won't be made to look the chump, for now. But watch for them, they are coming and you might want to be in front instead of behind.

Remember, you make your buck and move on. They stay & call someone else about problems you didn't solve. Someone coming behind you saying "for less than the extra ton cost you could have FIXED the problems that still persists, and lowered your energy cost over the life of the equipment by $13,000."

How do you think they'll feel about YOU if you grabbed the equipment sale without mentioning "if you think you'll fix this house, equipment replacement is the time to figure it out."

If you give them the option and they make the decision, the problem that persists is not on you. Many will do nothing, but your A is covered. If someone like me shows up after you and you didn't tell them, your credibility will be crushed. They'll ALL say "we would've" after the fact. They'll all blame you.

Again, if nobody in your area has the tools, then you won't be made to look the chump, for now. But watch for them, they are coming and you might want to be in front instead of behind.

[jimj]

If you don't have the tools to diagnose, design, and cost/benefit the options then of course you get blank stares. "If you are clueless to what I should do, then how am I to figure this out, I called you?".

If nobody in your area has the tools, then you won't be made to look the chump, for now. But watch for them, they are coming and you might want to be in front instead of behind.

Remember, you make your buck and move on. They stay & call someone else about problems you didn't solve. Someone coming behind you saying "for less than the extra ton cost you could have FIXED the problems that still persists, and lowered your energy cost over the life of the equipment by $13,000."

How do you think they'll feel about YOU if you grabbed the equipment sale without mentioning "if you think you'll fix this house, equipment replacement is the time to figure it out."

If you give them the option and they make the decision, the problem that persists is not on you. If someone like me shows up after you and you didn't tell them, your credibility will be crushed.

Again, if nobody in your area has the tools, then you won't be made to look the chump, for now. But watch for them, they are coming and you might want to be in front instead of behind.

Very well said! Most in the HVAC trade have NO IDEA WHAT YOUR TALKING ABOUT.

[AC Bad Dog]

When all else fails, just ask the counter guy LOL! Counter guys have dubious advice at best (don't mean to offend any knowledgeable counter guys as I'm sure they exist). Why is his calculation so much different that his recommendation? Does he suck at typing in the correct info or what? Get a second opinion. Rules of thumb that many people use are bs. It is like saying how many horsepower per square foot of automobile do I need. Depends on lots of variables.

Counter Guys Know How to size equipment,

The load comes out to what he has in stock.

Perfect System.

[tedkidd]

Boy ac, my post and jimj's totally teed you up to talk about comprehensive cost/benefit analysis software, did you miss that?

[adamt]

Okay, let us hear what U have that U even need 4 or 5-Ton; let alone 6-Tons.

When 4-Ton & larger is installed it seldom has adequate ducting, usually low airflow per ton, & not nearly enough RA filter area, etc.

My 1978 house is 3200 sq ft, 2700 finished. Walls are r13, weak vapor barrier with brick veneer. Thermal pane metal windows. R38 in the ceiling(except "cathedral" ceilings). I was pretty impressed with the HVAC Calc program, it generalized and itemized in the right areas. All of my supply ducts were exactly what the program called for and my return drop was almost exactly half of what it called for. The previous unit was 5 tons and didn't usually make it more than 1 night before freezing solid. I purchased a new 16.5" x 20" drop to be installed with the new furnace. Additional returns will be added later.
The installer insists that 600 sq ft per ton works for this area and this altitude. He was also concerned about my "water problem". I'm still a little confused about this comment, I was planning on installing a humidifier with the ac. He said that the 100 grains that the program defaulted to for Denver was unrealistic.
I am a Carpenter(Local #55) and he is a Tinner on the same sight. I like his work and his work ethic. The price is also unbeatable, even though he keeps increasing it.

Adam

[Shophound]

What I did not see in the article, nor in this thread, is any discussion focusing on nominal tonnage vs. actual delivered tonnage when the house is at or above design day conditions.

I take that back somewhat: Udarrell picked up on it by stating the following regarding his house a/c configuration:

"Those little room A/Cs wouldn't work on ductwork; that's also where a lot of loses happen.

So he mentions duct losses being one (significant) contributor toward not seeing actual tonnage delivery approach nominal tonnage rating.

Another would be the condensing unit itself. If ARI's nominal rating parameters for, say, a three ton matched system is 95 degrees outdoor dry bulb at 80 degrees indoor dry bulb at 67 degrees indoor wet bulb, even with excellent ductwork in a house with a good envelope, will this same system performing under 105 degrees outdoor dry bulb with a target indoor condition of 75 degrees dry bulb, 62 degrees indoor wet bulb deliver nominal tonnage? Probably not.

Well, my three ton nominal frankensystem can keep my house at 75 degrees and under 62 degrees wet bulb when it is 105 degrees outdoors, but I still doubt it is actually delivering three tons of refrigeration, including dehumidification, to get me there. My house is 1800 square feet slab on grade, so we're talking 600 square feet per nominal ton. I won't go into the numerous envelope improvements I've done to this place, but without those improvements I doubt I'd be getting 75 wonderful degrees of comfort when it's 105 degrees outside.

BTW my city's design day conditions are 100 degrees dry bulb at 74 degrees wet bulb. On Monday, July 24 of this week, we had 105 degrees outdoor dry bulb at 71 degrees wet bulb. Meaning at design outdoor dew point is 63 and on 7/24/2011 it was 54. Which also means that, typical for my region, when outdoor temps exceed 100 degrees, outdoor humidity levels decline. Which might translate to a boost in sensible capacity of the a/c, just when you need it!

So, when does an operating system actually deliver nominal tonnage? Only at ARI conditions? Or never? Bottom line: I don't think a discussion of this topic is complete without this important aspect of nominal vs. actual delivered tonnage. Going by calculations and nominal tonnage ratings alone, one might be tempted to conclude an a/c system is oversized for a house, when in reality it could be barely adequate on design or worse days.