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Old 04-28-2011, 08:23 PM   #31
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Rooftop A/C for 3,000 sq ft shop


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Originally Posted by beenthere View Post
You didn't ask how to do a load calc. You asked what size you need, and really didn't provide enough info for anyone to tell you what size. And the info you need to provide, is the same info you need to do a load calc.

Brand doesn't mean anything. A good contractor means everything.

I asked a number of things, not just what size. From my original post:

"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?"

If you can address any of those questions, and/or the specific additional ones I've asked, particularly about how to address the problem of having enough capacity to stay cool on the hottest days, but not having too much capacity for the rest of the cooling season, I would genuinely appreciate it. Thanks.

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Old 04-28-2011, 08:28 PM   #32
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Rooftop A/C for 3,000 sq ft shop


You need to know what size you need first, before you can determine the answer to some of the other questions.

The pricing question can't be answered.
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Old 04-28-2011, 08:38 PM   #33
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Rooftop A/C for 3,000 sq ft shop


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You need to know what size you need first, before you can determine the answer to some of the other questions.
OK, but how about the questions that you are implying ~can~ be answered without knowing size requirements?

From one of my other posts:

"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".

I don't think you're contending that I need to know the exact size before someone can offer an opinion about whether I should go with a system with a separate compressor and a condenser/air handler, or a packaged system. And the running question about the tradeoff between cooling on the hottest days and cycling times is most definitely independent of sizing.

Since you've "beenthere", I'd appreciate knowing some of what you found out when you got there. That's what these forums are for.

Thanks.
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Old 04-28-2011, 09:37 PM   #34
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Rooftop A/C for 3,000 sq ft shop


Package unit or split, depends on size needed.

All brands have good and bad reputations.

As for an advantage of having more then one, depending on size yes, depending on size, no.

You NEED to know what size before you can determine most of what you are asking, and speculating before you know what size is some what like counting your chickens before they hatch.
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Old 04-28-2011, 10:11 PM   #35
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Rooftop A/C for 3,000 sq ft shop


I presented you with a hypothetical, for the purpose of obtaining an opinion on how to address the problem of having ample cooling capacity for the hottest weather while not having so much capacity that the system cycles excessively during cooler weather.

This hypothetical has nothing to do with the actual "optimal" cooling capacity required for my situation.

Once again, for the sake of exploring this question that comes up time and again in the science of engineering cooling systems concerning the trade-off between cooling capacity and cycle times:

"Is there an advantage to having more than one unit?" (slash "compressor")

"For the purposes of this last question, let us assume that the exact amount of cooling required for my situation is 5 tons [as hypothetically determined by competent application of the laws of thermodynamics and the best technical expertise available in determining cooling loads and sizing cooling equipment].

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?"

The answer to this question is independent of whether or not the systems are split or packaged, and size is stipulated for the sake of discussion to be 5 tons. Heat is heat. You know what I'm asking. I know that you know what I'm asking. Again, this hypothetical question has nothing to do with the actual cooling needs of my shop, which will be determined before I actually purchase any equipment.

I'd appreciate it if you could share some of your expertise on how you address the problem of balancing cooling capacity and cycle times.

Thanks.

Last edited by John D in CT; 04-28-2011 at 10:13 PM.
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Old 04-28-2011, 10:21 PM   #36
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Rooftop A/C for 3,000 sq ft shop


I deal with real world situations. Hypothetical will give you a false thought/direction, so I didn't talk about your hypothetical building/sizing.. Until you know what size. No real or good advise can be given to you. So do the load calc, so that you can help us help you.
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Old 04-28-2011, 10:35 PM   #37
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Rooftop A/C for 3,000 sq ft shop


Can you please tell me what strategies you use when designing air conditioning systems to balance the need for cooling capacity on the hottest days of the year versus excessive cycling (and commensurate loss of dehumidification capacity) on cooler days?

Naturally, these strategies are regardless of the total tonnage of the systems involved, since the principles involved have little or no relation to scale.

Thanks in advance.

Last edited by John D in CT; 04-28-2011 at 10:38 PM.
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Old 04-29-2011, 04:17 AM   #38
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I use to do design and builds when I worked for a commercial company. And the strategy I used, was to start by finding the load, so I knew what size equipment I needed. Which then helped to determine what type equipment, as in single or 2 stage equipment, single or multiple units.
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Old 04-29-2011, 10:05 AM   #39
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Rooftop A/C for 3,000 sq ft shop


Quote:
Originally Posted by beenthere View Post
I use to do design and builds when I worked for a commercial company. And the strategy I used, was to start by finding the load, so I knew what size equipment I needed. Which then helped to determine what type equipment, as in single or 2 stage equipment, single or multiple units.
And after that fine start , did you ever install a system that was either:

A) Incapable of achieving an indoor temperature of 72 degrees F. on the hottest day of the year, or

B) Cycling excessively during any other part of the cooling season, thereby reducing the SEER and/or dehumidification capacity?

If no, then congratulations.

If yes, what did you learn from that experience that you might share with me?

Or is it your opinion that there is always going to be a trade-off between cooling capacity and cycle times? If yes, then which of the two do you tend to optimize - the ability to achieve an ideal indoor temperature on the hottest day of the year, or maximum system efficiency during the majority of the cooling season?

Thanks.
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Old 04-29-2011, 03:37 PM   #40
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Rooftop A/C for 3,000 sq ft shop


I was taught by people that had already made the mistakes of undersizing commercial, and learned from it, and already learned how to avoid high humidity. They looked over my first designs, and pointed out what was wrong, and what needed changed, and why.So although I did design some that wouldn't work, my mentors saved my butt a few times.

So it still comes back on what size is needed, to know how to address short cycling possibilities, or humidity problems.

Can't select equipment until you know what it has to do.
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Old 04-29-2011, 03:58 PM   #41
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"So it still comes back on what size is needed, to know how to address short cycling possibilities, or humidity problems".

There are general principles involving trying to achieve a balance between maximized cooling capacity and system efficiency that have little or nothing to do with the size of the system.

I would appreciate it if you could share with me some of those principles.

You say that you have learned how to deal with the problem of high humidity. I would appreciate it if you could share with me some of what you have learned about that.

Thanks.

Last edited by John D in CT; 04-29-2011 at 04:03 PM.
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Old 04-29-2011, 04:03 PM   #42
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Rooftop A/C for 3,000 sq ft shop


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There are general principles involving trying to achieve a balance between maximized cooling capacity and system efficiency that have little or nothing to do with the size of the system.


There are? Please tell me what they are.
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Old 04-29-2011, 04:20 PM   #43
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OK, I probably should have said "From my understanding of physics, and in particular, thermodynamics, it seems to me that a reasonable person would conclude that there are general principles involving trying to achieve a balance between maximized cooling capacity and system efficiency that have little or nothing to do with the size of the system".

But that's fairly irrelevant now, because I'm finished with trying to get any helpful information from you.

I will rely on members of this forum who might be more willing and able than you to share some of what they've learned.

Last edited by John D in CT; 04-29-2011 at 04:22 PM.
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Old 04-29-2011, 05:03 PM   #44
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Rooftop A/C for 3,000 sq ft shop


There are guys that post answers on these websites that have no clue and give bad advice......

Beenthere is not one of those guys..... His knowledge far surpasses 99.9% of the posters out there. Your questions are impossible to answer without knowing the load. If anyone gives you "answers' I don't think I would rely on them.
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Old 04-29-2011, 05:11 PM   #45
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Rooftop A/C for 3,000 sq ft shop


"Your questions are impossible to answer without knowing the load".

I respectfully disagree.

Is there anyone who is willing to share some thoughts on some general principles and strategies involved in striking a balance between maximized cooling capacity and acceptable cycle times?

Thanks.

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