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Old 04-26-2011, 10:30 PM   #1
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Rooftop A/C for 3,000 sq ft shop


***** Prologue 4/29/11 4:40 Eastern time ********

To those reading this thread:

I've presented a number of questions, only one of which had to do with getting educated ~rough~ estimates of how many tons of cooling capacity I might need. Some people were willing to give one, others were adamant about not doing so. To those who might be willing to offer one, I'd like to provide some additional information. I do this in large part because I have yet to find any software that is geared toward steel buildings.

My three 1,000 sq. ft. units are in the middle of a single steel building that contains 10 virtually identical units. The 3 units are all open to one another. The back walls are to the south and have no windows. The front walls are to the north and have 3 12 x 12 insulated overhead doors (R - 16.04) with a total of 24 sq ft of insulated (double-pane) lites in them. There are also 3 steel entry doors on the north side. One side wall of the 3,000 sq foot total area abuts an unheated unit, and the other side wall abuts a heated unit. Air infiltration is minimal. Lighting consists of sixty 8' 95W HO T-12 fluorescent tubes powered by 30 electronic ballasts.

If you are inclined to offer a rough estimate, great. If not, please don't tell me to "do a load calc", because one or more will be professionally done prior to purchasing any equipment. I am interested only in getting rough estimates from those who are willing to provide one.

Lastly and most importantly:

The sizing issue is really secondary to what has evolved into my main question: how do you guys approach the apparent trade-off between having enough cooling capacity to achieve a target indoor temperature on the hottest day of the year, and not having so much capacity that the compressor(s) short-cycle(s) during the less demanding part of the cooling season, thereby reducing system efficiency and negatively affecting dehumidification?

I've personally experienced the opposite situation; a residential split system that could not keep the indoor temperature below 80 on the hottest days of the year (around 100 F.), and had to be supplemented by a window unit. It was a 2-ton system, and probably should have been 2.5 tons.

But even if it had been sized more properly, it still seems to me that there's a trade-off between cooling capacity and cycle times. Is it simply undesirable and/or impossible to have a system that will keep a given space at a certain target temperature on the hottest day of the year, and still offer acceptable SEER and dehumidification during the rest of the cooling season? If it is a trade-off, which way do you go? Do you tend to tell your customer that if they're willing to be a little warm on a few days, that they'll save money and have better dehumidification the rest of the season? If you had a customer who insisted on having enough cooling capacity to handle the hottest days, would you try to talk them out of it?

Also:

In this thread, I've tried, with limited success, to put forth a hypothetical situation in which it has been competently determined that my cooling load calls for exactly 5 tons. I'm on a limited budget (like most of the rest of the world), and will be using a used packaged roof unit or units. I would like to get some opinions on whether or not I would be better off, under this hypothetical situation, to go with a single 5-ton unit, or a combination of smaller units that could be controlled in such a way as to allow for greater flexibility for achieving a better compromise between raw cooling power and cycle times/SEER/dehumidification during periods of varying outside temperature. So I pose the hypothetical to try to achieve a real-life result; namely, not sweating in my shop this summer. I've received some very helpful answers involving two-stage and variable-output systems, but as a practical matter I'm probably going to be limited to plain old one-stage packaged units, so any input on how to divide the load between multiple units, or to not try to divide it at all, would be greatly appreciated.

Thanks.

***** Original thread follows.


OK, it's finally warming up in CT and I don't want to sweat through another summer. I'd appreciate some advice on what brands and sizes of rooftop units and air handlers to get.

I have three adjacent 1,000 sq ft units in a Butler steel building. Average ceiling height is 13.5 ft for a total of 40,500 cubic feet. Insulation is minimal; R-11 in ceiling and walls, three insulated 12' X 12' doors on north side, no windows on south side, concrete slab floor. No major heat sources to speak of; refrigerator, 30 HP air compressor that will probably run 1% of the time, computer, metalworking and woodworking machines that will run no more than 2 at a time, and nowhere near continuously; two people on average, 1 dog, 1 cat. (Dog has been instructed to pant outside).

So, a series of questions:

1) How many tons do I need? I know I don't want to over-size, but I want to be moderately comfortable on 100-degree days. The concrete slab helps a little, but with no A/C, if it's 100 outside it's probably 90 inside. Can I have a system that will keep it at 70 inside when it's 100 outside, and not be too big?

2) Should I get one system and duct it to the two adjacent 1,000 sq ft shops, or am I somehow better off with separate systems?

3) Should I get a "packaged" system (compressor, condenser, air handling all-in-one), or separate compressor(s) and air handler(s).

4) What specific brands/models are good?

5) I'm going to be buying used if possible. What kinds of prices would be fair? What should I look out for to avoid getting junk?

6) What am I leaving out?

Thanks in advance.


Last edited by John D in CT; 04-29-2011 at 03:50 PM.
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Old 04-27-2011, 04:58 AM   #2
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Rooftop A/C for 3,000 sq ft shop


Do a load calc on each one, so you know what size A/C each one needs.

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Old 04-27-2011, 11:44 AM   #3
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Rooftop A/C for 3,000 sq ft shop


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Do a load calc on each one, so you know what size A/C each one needs.
On a scale of 1 to 10, with 1 being not at all helpful, and ten being incredibly helpful, that is a 1.
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Old 04-27-2011, 03:33 PM   #4
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Rooftop A/C for 3,000 sq ft shop


Anyone?

Yeah, maybe I was a little hard on the first answerer, but I think that if I knew how to "run a load calc", I wouldn't be asking this question.

I'm thinking that a single 5-ton packaged unit might be the way to go. I want it to run nearly continously on the hottest days of the year.

Can anyone answer any of my questions? Thanks.
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Old 04-27-2011, 03:46 PM   #5
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Rooftop A/C for 3,000 sq ft shop


http://www.hvaccomputer.com/hvac/newfeatures.asp
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Old 04-28-2011, 10:50 AM   #6
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Rooftop A/C for 3,000 sq ft shop


Would still love to get some educated estimates of how many tons of cooling I need.

Also: Best brands of packaged units, should I get just one for all three units and run ductwork, or get separate units.

Thanks.
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Old 04-28-2011, 02:54 PM   #7
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Rooftop A/C for 3,000 sq ft shop


Quote:
Originally Posted by John D in CT View Post
On a scale of 1 to 10, with 1 being not at all helpful, and ten being incredibly helpful, that is a 1.
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Old 04-28-2011, 02:59 PM   #8
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Rooftop A/C for 3,000 sq ft shop


Quote:
Originally Posted by John D in CT View Post
Would still love to get some educated estimates of how many tons of cooling I need.

Also: Best brands of packaged units, should I get just one for all three units and run ductwork, or get separate units.

Thanks.
Beenthere already gave you the best advise as what to do. Then you bash him with it.

Good luck
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Old 04-28-2011, 03:07 PM   #9
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Rooftop A/C for 3,000 sq ft shop


Sorry, but not knowing how to do one, I didn't find "do a load calc" to be helpful.

If I asked how to rebuild an engine and someone told me to "do an engine rebuild", I wouldn't find that helpful either.

That said, my question is nowhere near as complicated as how to rebuild an engine. I supplied quite a bit of information, and I have to believe that someone with sufficient knowledge could give a reasonable answer if they were so inclined.

It would also be great to get some input on my other questions, like whether a packaged unit or units would be the way to go, and what brands are desirable.
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Old 04-28-2011, 03:32 PM   #10
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Rooftop A/C for 3,000 sq ft shop


Quote:
Originally Posted by John D in CT View Post
Sorry, but not knowing how to do one, I didn't find "do a load calc" to be helpful.

If I asked how to rebuild an engine and someone told me to "do an engine rebuild", I wouldn't find that helpful either.

That said, my question is nowhere near as complicated as how to rebuild an engine. I supplied quite a bit of information, and I have to believe that someone with sufficient knowledge could give a reasonable answer if they were so inclined.

It would also be great to get some input on my other questions, like whether a packaged unit or units would be the way to go, and what brands are desirable.
If you want the best, then you need to GET a load calc. If you can't do one yourself, then hire someone to do it. If you go the 'rule of thumb' route you will not be happy with the performance. You could either be under sized or over sized.

We are not there to look and see what you have, but me personally I would do more than one unit.

Too do it right.... Get a load calc. The money you spend to have it done, could save you money in equipment cost, elec, and gas bills.
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Old 04-28-2011, 03:50 PM   #11
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Rooftop A/C for 3,000 sq ft shop


Quote:
Originally Posted by John D in CT View Post
OK, it's finally warming up in CT and I don't want to sweat through another summer. I'd appreciate some advice on what brands and sizes of rooftop units and air handlers to get.

I have three adjacent 1,000 sq ft units in a Butler steel building. Average ceiling height is 13.5 ft for a total of 40,500 cubic feet. Insulation is minimal; R-11 in ceiling and walls, three insulated 12' X 12' doors on north side, no windows on south side, concrete slab floor. No major heat sources to speak of; refrigerator, 30 HP air compressor that will probably run 1% of the time, computer, metalworking and woodworking machines that will run no more than 2 at a time, and nowhere near continuously; two people on average, 1 dog, 1 cat. (Dog has been instructed to pant outside).

YOU NEED A LOAD CALC


So, a series of questions:

1) How many tons do I need? I know I don't want to over-size, but I want to be moderately comfortable on 100-degree days. The concrete slab helps a little, but with no A/C, if it's 100 outside it's probably 90 inside. Can I have a system that will keep it at 70 inside when it's 100 outside, and not be too big?



We can not answer without having done the calculation of load.




2) Should I get one system and duct it to the two adjacent 1,000 sq ft shops, or am I somehow better off with separate systems?

You need the answer 3 first.

3) Should I get a "packaged" system (compressor, condenser, air handling all-in-one), or separate compressor(s) and air handler(s).

Someone has to evaluate the architecture

4) What specific brands/models are good?

You need the answer 3 first.

5) I'm going to be buying used if possible. What kinds of prices would be fair? What should I look out for to avoid getting junk?

You need the answer 3 first

6) What am I leaving out?

**************LOAD CALC***************

Thanks in advance.
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Last edited by JJboy; 04-28-2011 at 03:56 PM.
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Old 04-28-2011, 04:51 PM   #12
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Rooftop A/C for 3,000 sq ft shop


JJBoy, I appreciate where you're coming from, and it's admirable that you're sticking up for your fellow members, but I find it hard to believe that I need to "do a load calc" before someone can give me an opinion as to what brands of packaged air conditioning units have a good reputation.

Let's try, shall we?

I say JJBoy, what brands of packaged air conditioning units do you like?

And as for "someone would have to evaluate the architecture", it's the most typical type of steel building you can imagine. I'm pretty sure that a packaged system is the way to go, and since there's no earthly reason for the air handler to be any distance at all from the compressor, and I have a feeling that you are also pretty sure that a packaged system is the way to go, but are attempting to make things more complicated than they actually are.

Look, before this gets any further out of hand:

1) Official apologies to beenthere who was only trying to help.
2) I seem to recall hearing somewhere that it might be good to have a load calc performed, so I will do that.
3) In the meantime, I really would appreciate some input from anyone on the parts of my question that don't depend on a load calc.

In my opinion, they are:

In my situation, is a packaged unit or units the way to go?
What brands have a good reputation for reliability?

And probably most importantly:

Is there an advantage to having more than one unit?

For the purposes of this last question, let us assume that the exact amount of cooling required for my situation is 5 tons.

A single 5-ton unit might be just right for the hottest days of the year; hardly cycling. However, for the majority of the time, it might cycle on and off too frequently, diminishing the amount of dehumidification that longer cycle times would provide. Might it be better to have, say, two 3-ton units, or three 2-ton units, or some other combination that could be programmed to run for desirable amounts of time in order to maximize dehumidification, and also provide the flexibility to keep my shop cool on the hottest days by running simultaneously?

Thanks in advance.

Last edited by John D in CT; 04-28-2011 at 05:00 PM.
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Old 04-28-2011, 05:06 PM   #13
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Rooftop A/C for 3,000 sq ft shop


Quote:
Originally Posted by John D in CT View Post
JJBoy, I appreciate where you're coming from, and it's admirable that you're sticking up for your fellow members, but I find it hard to believe that I need to "do a load calc" before someone can give me an opinion as to what brands of packaged air conditioning units have a good reputation.

Let's try, shall we?

I say JJBoy, what brands of packaged air conditioning units do you like?

And as for "someone would have to evaluate the architecture", it's the most typical type of steel building you can imagine. I'm pretty sure that a packaged system is the way to go, and since there's no earthly reason for the air handler to be any distance at all from the compressor, and I have a feeling that you are also pretty sure that a packaged system is the way to go, but are attempting to make things more complicated than they actually are.

Look, before this gets any further out of hand:

1) Official apologies to beenthere who was only trying to help.
2) I seem to recall hearing somewhere that it might be good to have a load calc performed, so I will do that.
3) In the meantime, I really would appreciate some input from anyone on the parts of my question that don't depend on a load calc.

In my opinion, they are:

In my situation, is a packaged unit or units the way to go?
What brands have a good reputation for reliability?

And probably most importantly:

Is there an advantage to having more than one unit?

For the purposes of this last question, let us assume that the exact amount of cooling required for my situation is 5 tons.

A single 5-ton unit might be just right for the hottest days of the year; hardly cycling. However, for the majority of the time, it might cycle on and off too frequently, diminishing the amount of dehumidification that longer cycle times would provide. Might it be better to have, say, two 3-ton units, or three 2-ton units, or some other combination that could be programmed to run for desirable amounts of time in order to maximize dehumidification, and also provide the flexibility to keep my shop cool on the hottest days by running simultaneously?

Thanks in advance.

You just don't get it. How can you expect us to design a system for you without prints or a load calc.

Spend the money and have a company come out and look at it

As far as units go. They are all good. All depends on the installing co
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Old 04-28-2011, 05:24 PM   #14
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Rooftop A/C for 3,000 sq ft shop


I think it's pretty obvious from my last post that I do "get it".

If you don't want to give me straight answers to direct and valid questions, fine; I certainly can't make you.

That said, you really have no opinion whatsoever on the possible advantages of multiple units over a single unit? Again, for the purposes of that question, we're assuming that a team of the best load calc'ers in the world have independently come to the conclusion that 5 tons of cooling capacity would be required.

I realize that sizing air conditioning systems is very complex. I'm also aware that it's very difficult to achieve a "perfect" balance of cooling capacity and cycle times. What's good for the hottest days of the year will not be so good for the slightly less hot days. I think it's perfectly valid for me to ask a general question about the possible merits of using multiple units to achieve greater flexibility in cooling capacity versus cycle times. If you can't see the merit in this question, I would submit that it is you who "doesn't get it".

I'd still appreciate input (from anyone) on any or all of the questions I've asked, including even a wild guess on the amount of tons that might be required. Yes, I'm going to get expert opinions locally. In the meantime, I'd like to have a rough idea so I can get some idea of how much some good used units might cost me. I would bet $1,000 that most experts would examine my shop and give a number somewhere between 3 and 6 tons. I'm not asking for an exact number, just a rough estimate based on the fairly extensive information I've provided. By the way, the dog weighs about 15 lbs.

Anyone?

Last edited by John D in CT; 04-28-2011 at 05:27 PM.
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Old 04-28-2011, 05:30 PM   #15
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Rooftop A/C for 3,000 sq ft shop


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By the way, the dog weighs about 15 lbs.

Anyone?
LOL, You need 1T for that


Check this link: http://www.google.com/url?sa=t&sourc...sregcmOK80eyBg

http://www.mrhvac.com/loadcalcshortform.htm

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