AC Tons needed, load calc done


  #1  
Old 05-31-09, 06:01 PM
S
Member
Thread Starter
Join Date: Jul 2007
Posts: 35
Upvotes: 0
Received 0 Upvotes on 0 Posts
AC Tons needed, load calc done

Wow, who ever thought there was so much to consider when sizing an A/C unit.

We are adding central air to our 3300 sq ft home. It's an open concept first/second floor, with a 500 sq ft playroom for the kids over the garage (fairly isolated from the main house).

I've had 3 A/C guys come out. None did a load calculation, that I know of, but all recommended a 5 ton system. This will be a single system, with ducts running through closets to the first floor.

I paid for the HVAC-calc and did my own load calculation. I believe I did it correctly and as accurately as possible, and I come up with 4 ton system based on the heat gain of 46k BTUs.

My biggest concern is buying too big, and having poor humidity management, or buying too small, and having poor cooling. Is a ton "extra" enough to ruin it? If I found it was too much, could we add a humidistat to control the run times? The air handler is variable speed, and the condenser is two speed. Is this an option?

I searched the forums and read a lot, but still can't decide if there planning for 5 ton is accurate. They all talked about it being a single system, not every room having a drop, etc, contributing to the size needed.

Any help is appreciated.
 
  #2  
Old 05-31-09, 06:18 PM
airman.1994's Avatar
Member
Join Date: Dec 2005
Location: VA
Posts: 5,491
Upvotes: 0
Received 8 Upvotes on 8 Posts
1st Id say that if they did not do a load cal Id find three more contractor. This would have taken at least an hour to do. 2nd If it is two stage then that helps a lot because it should run in 1st stage 75% of the time. 3rd are the duct sized to handle 2000cfm of air? Id bet not if you have never had ac in the home. 4th I would go with a separate unit or mini split for the playroom.
 
  #3  
Old 05-31-09, 06:31 PM
S
Member
Thread Starter
Join Date: Jul 2007
Posts: 35
Upvotes: 0
Received 0 Upvotes on 0 Posts
Would it be too much to ask if you explained the "stages" of the unit when running, and why it matters? I was able to choose between a unit that was single speed, and the two speed. I knew it was better to have the 2 speed, but wasn't sure of benefits why.

Interesting thought on the playroom. It's location in the front of the house makes it less than ideal for it's on system. No good place to locate the stuff. But it will have 2 feeds and it's own return of course.

Thanks.
 
  #4  
Old 06-01-09, 01:23 AM
S
Member
Join Date: May 2009
Posts: 111
Upvotes: 0
Received 0 Upvotes on 0 Posts
"Is a ton "extra" enough to ruin it?"

Yes 1 ton on 4 tons is a 25% issue. Too much.
Stop, take a breath, insist on a Manual J from a company who cares more about you than themselves.

I love the humidistat on my system.
 
  #5  
Old 06-01-09, 06:09 AM
S
Member
Thread Starter
Join Date: Jul 2007
Posts: 35
Upvotes: 0
Received 0 Upvotes on 0 Posts
Thanks guys. I'll talk to the contractor this morning. Interesting enough though, if I just tweak the house direction by 35 degrees, it pushes the recommendation to 5 ton. My house position is in between, but I thought that was interesting.

Also, how does the humidistat work? Does it just dehumidify the air without any additional cooling?
 
  #6  
Old 06-01-09, 06:19 AM
pflor's Avatar
Member
Join Date: Dec 2007
Location: U.S.A.
Posts: 976
Upvotes: 0
Received 0 Upvotes on 0 Posts
A high end modulating furnace with a variable speed blower, an outdoor (condensing) unit that is multi-speed, and a digital, multi-stage thermostat able to communicate with both, would take care of all your worries.

srercrcr hit it right on the nail when he indicated that anything over 25% oversizing is too much. This though, is quite true for single stage systems. BUT if you install all of the above, all you need to do is tell the thermostat what you want inside the house and the fella will communicate with the furnace and condensing units and do whatever it takes to maintain setpoints.

And we have to also think about you having done your calculations correctly.
 
  #7  
Old 06-01-09, 11:29 AM
S
Member
Thread Starter
Join Date: Jul 2007
Posts: 35
Upvotes: 0
Received 0 Upvotes on 0 Posts
Well, here's an update. I guess the guy I was leaning towards did do a load calc. I just didn't know it. He showed me today. He came up with 53,000 BTUs. Some of the differences were direction of walls and stuff, as his was more precise than mine, an dhe accounted for computers, large TVs, commercial gas cooktop, etc, stuff like that. It all seemed accurate. So he said he was borderline 4.5-5 ton, so goes 5, as we are using the two stage condenser and variable speed air handler.

Sound reasonable?
 
  #8  
Old 06-01-09, 12:49 PM
S
Member
Join Date: May 2009
Posts: 111
Upvotes: 0
Received 0 Upvotes on 0 Posts
That sounds sweet. A humidistat is wired into the system, only costs maybe $100 for the part. Check with your contractor to verify but with mine.....I set it to a certain humidity % and as long as it is wetter than that inside.. the unit will automatically run at slow speed to extend the runtime before reaching the desired thermostat temp setting. Works good in the morning when the unit may not go on cause there's little heat load but humidity is 85% outside. You have to play with it, cause if you have it set too low, it'll never shift up to high speed. During the hot day you want it running high speed for max cooling, and it will cycle enough that the air will dry out.
 
  #9  
Old 06-01-09, 02:04 PM
pflor's Avatar
Member
Join Date: Dec 2007
Location: U.S.A.
Posts: 976
Upvotes: 0
Received 0 Upvotes on 0 Posts
A top-of-the-line thermostat such as Honeywell's IAQ can take care of both, humidification "and" dehumidification, without so much as having to add an additional humidistat.

Just food for thought.
 
  #10  
Old 06-03-09, 07:06 PM
S
Member
Thread Starter
Join Date: Jul 2007
Posts: 35
Upvotes: 0
Received 0 Upvotes on 0 Posts
Can you buy those themostats as a homeowner, or are they only available to the pros? I found out my current tstat can't handle a multi stage cooling system, so I have to upgrade it anyway. Might as well get the one with the built in humidity stuff.
 
  #11  
Old 06-04-09, 02:10 AM
pflor's Avatar
Member
Join Date: Dec 2007
Location: U.S.A.
Posts: 976
Upvotes: 0
Received 0 Upvotes on 0 Posts
You can get, off the internet, just about everything nowadays. More importantly though, is knowing what you're doing once you have that baby on your hands.

Take a peek by clicking on the link below:
Product
 
  #12  
Old 06-04-09, 07:55 AM
S
Member
Thread Starter
Join Date: Jul 2007
Posts: 35
Upvotes: 0
Received 0 Upvotes on 0 Posts
I noticed that. After I posted, I poked around. A couple of hundred bucks, but I've never seem that "module" before. Wonder how hard it is to hookup.

Are those the only ones that do that, meaning, the Honeywell VisionPros? With humidity I mean.

Thanks for the link.
 
  #13  
Old 06-04-09, 10:24 AM
pflor's Avatar
Member
Join Date: Dec 2007
Location: U.S.A.
Posts: 976
Upvotes: 0
Received 0 Upvotes on 0 Posts
There are other manufacturers for sure. I just happen to be a Honeywell fan, so that's the control brand I hang-out with.
 
  #14  
Old 06-04-09, 10:53 AM
S
Member
Thread Starter
Join Date: Jul 2007
Posts: 35
Upvotes: 0
Received 0 Upvotes on 0 Posts
Sorry, I like Honeywell too. I meant within the Honeywell line, are the VisionPros, with the module, the only ones that have the ability to control the system based on humidity as well.
 
  #15  
Old 06-04-09, 12:58 PM
pflor's Avatar
Member
Join Date: Dec 2007
Location: U.S.A.
Posts: 976
Upvotes: 0
Received 0 Upvotes on 0 Posts
Click on the following link
Commercial Thermostat Tools - Honeywell Environmental And Combustion Controls

Then look for the link underneath where it says Commercial Thermostat Portfolio Overview Presentation
open or save the presentation. I hope it sheds some light.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: