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Old 01-03-2018, 01:23 AM   #31
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Re: System Sizing


Quote:
Originally Posted by user_12345a View Post
ya the calc didn't look right to me either. .
It appears that it's not factoring wind. It can be 1.5 - 2x the "u" value of the wall, window, etc. (The inverse of R rating.) Which will significantly increase heat losses.

Cheers!
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Old 01-03-2018, 03:42 AM   #32
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Re: System Sizing


Quote:
Originally Posted by user_12345a View Post
ya the calc didn't look right to me either.

not sure why you quoted my post vs the original on this thread. didn't do that calc, op did.

Yes, in my area the design is something like 0F to -5F, and i would probably use as low as -15 since it can drop that low every 2 to 3 years, like it will later this week.

But op is south of that.
Its been several years since we hit any sub zero temps. In the old days, we had 2 weeks of sub zero, but haven't had that kind of weather for a long long time now. On those rare occasions we do go sub zero, its generally only for a few hours.

PS: Its currently 4°F in the OPs city.

Quote:
Originally Posted by supers05 View Post
I'm going to assume that's a typo, you actually reversed those.

Now with everything else people have said, you'll want to add about 20% to the calc for errors. (It doesn't look like that one has it built in.)

You'll also want to change the indoor temps a bit. People change, and so does the temperature they want. I'd put at least 72 for cooling and 75 for heating.

Cheers!
I hate most of those programs that pad the calc by 20%. Since it adds 20% to everything, including latent gain. An accurate Manual J, followed by using Manual S does a much better job.
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Old 01-03-2018, 07:35 AM   #33
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Re: System Sizing


Quote:
Originally Posted by beenthere View Post
...
I hate most of those programs that pad the calc by 20%. Since it adds 20% to everything, including latent gain. An accurate Manual J, followed by using Manual S does a much better job.
I don't normally add so much, but this one is measured by a novice, not a pro.

Cheers!
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Old 01-03-2018, 02:25 PM   #34
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Re: System Sizing


Thanks for everyone's input. I'll recalculate based on the suggested values and repost!
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Old 01-03-2018, 04:46 PM   #35
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Re: System Sizing


Here is an updated calculation. Please provide input and what size system should be installed. Thanks again!
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System Sizing-load_calc.png  
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Old 01-03-2018, 04:52 PM   #36
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Re: System Sizing


Are you working through a contractor? have you gotten quotes? what do they want to put in?
-------

Probably 2.5 tons* and 40 000 - 50 000 btu/hr input at 95% - granted many 40k may not come with a large enough blower for the 2.5 ton a/c and you may be nervous cutting it fine.

So can go up to 60k and go 2-stage to still get nice, long heating cycles.

*You have to get the actual performance data point for the unit you want to install, see if there's enough sensible capacity at 75F, 50%. I would be leary of getting 2 ton.
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Last edited by user_12345a; 01-03-2018 at 04:59 PM.
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Old 01-03-2018, 07:57 PM   #37
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Re: System Sizing


How did you arrive at that tonnage? Is there a calculation based on the heating/cooling BTUs? I have been getting quotes but want a second opinion on size.
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Old 01-03-2018, 08:06 PM   #38
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Re: System Sizing


one ton is 12 000 btu/hr, at 95F outdoor and 80f indoor.

But who sets their stat to 80? might as well not run it.

So, a 2 ton is 24 000 per hour, in reality, in real world at 75 indoor, closer to 22k than 24k - won't cover the sensible load you have. 20 to 30% capacity typically used to pull moisture out of the air.

So, I went up to 2.5 ton, which is 30k but in reality closer to 26-27k at 75f return.

The attached pic, taking from a spec sheet shows my point.

It's for a 2.5 ton goodman (30k nominal, 28k actual rating), you can see at design temp it supplies only 26.3k btu/hr cooling.

The s/t on the chart, is sensible heat ratio and it shows you how much capacity goes directly to cooling the air vs dehumidification.

0.83 x 26.3 = 21.9k sensible which meets your load.

The 2 ton despite having enough capacity overall wouldn't keep up.
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System Sizing-performance.png  
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Last edited by user_12345a; 01-03-2018 at 08:35 PM.
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Old 01-03-2018, 08:08 PM   #39
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Quote:
Originally Posted by shocker View Post
How did you arrive at that tonnage? Is there a calculation based on the heating/cooling BTUs? I have been getting quotes but want a second opinion on size.
He figured 12000 btu for a ton is what was figured if i'm guessing right..
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Old 01-04-2018, 05:25 AM   #40
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Re: System Sizing


A 2 ton would be able to meet your sensible load at 95 OD and 70 ID. But it would not be able to meet your latent load.

A 2 ton at 95 OD and 75 ID, will meet your sensible and latent loads, but just barely. So if a lot of cooking was being done, it would struggle to keep up while you were cooking.

A 2.5 ton AT 95 OD and 70 or 75 ID will need its blower slowed down to 870 CFM to hold a reasonable RH in the home.

A VS blower and a thermostat that can slow the blower would be the best solution for a 2.5 ton system. This would compensate for the extra capacity it would have when its only 80 outside.
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Old 01-04-2018, 12:10 PM   #41
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Re: System Sizing


Quote:
A 2 ton at 95 OD and 75 ID, will meet your sensible and latent loads, but just barely.
At what sensible heat ratio. the numbers i've seen paint a different picture.
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Old 01-04-2018, 03:19 PM   #42
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Re: System Sizing


Quote:
Originally Posted by user_12345a View Post
At what sensible heat ratio. the numbers i've seen paint a different picture.
95 OD, at 75 IDDB, 63 IDWB(51.1% RH), 83% SHR.
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Old 01-04-2018, 04:12 PM   #43
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Re: System Sizing


Thanks for all the great information. Based on the figures I assume you would all agree that there would be no possible way a system larger than 3 tons would be needed.
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Old 01-04-2018, 04:19 PM   #44
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Re: System Sizing


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Originally Posted by shocker View Post
Thanks for all the great information. Based on the figures I assume you would all agree that there would be no possible way a system larger than 3 tons would be needed.
You don't need 3 tons. A 2.5 ton is over sized. But can be made to work fairly good to keep the humidity down.
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Old 01-04-2018, 04:21 PM   #45
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Re: System Sizing


Putting a 3 ton could cause issues with airflow if your ducts aren't sized for it. Not to mention low humidity removal.

Quote:
95 OD, at 75 IDDB, 63 IDWB(51.1% RH), 83% SHR.
Were you using 24k capacity or was it de-rated for 75 return?

At lets say more realistic 22k and that ratio, i get 18.26k sensible, his sensible is just over 20k.
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