OT question but hopefully you can still enligten

v1rtu0s1ty

v1rtu0s1ty

Senior Member
I'm looking for an A/C for my 6 month old house. I already contacted 2 vendors and they came up with different size. The Carrier vendor quoted a 5 ton and the Lennox vendor quoted a 3.5 ton. One vendor says that the other vendor is quoting me a small one, and the other vendor is saying that the other a/c is too much for my house.

I'm in IL. House is 3,057 sqft, 2 levels and an unfinished basement. Both guys are claiming that they did a manual j and heat gain/loss calculation. LOL, they came up with different values.

Any help would be greatly appreciated.

Sorry for this off topic question.

Neil
 
ask them for their J load worksheets and check them yourself. Last time I went HVAC shopping I had trouble finding anybody who knew what a J load was!

To big a unit is bad as it won't run long enough to give you humidity control. Too small a unit is bad because you won't have sufficient cooling on a hot day or with lots of guests/traffic.

Or spend $50 and do it yourself. http://www.hvaccomputer.com/
 
Yeah, just like Wayne says . . .

Another good resource is http://www.descoenergy.com/

The HVAC industry has had a history of over-sizing forever. As you might learn from the Desco site, it is better to under-size than oversize.

Said another way: It is better for your air-conditioner (and your pocketbook) to struggle with the heat 10 days a year then it is for it to run inefficiently the other 355 days.
 
When you ask for a "Manual J" the rep will probably tell you that they use a rule-of-thumb. That's garbage. If you must, tell them that your locality requires you to submit the Manual J calculations with your permit. Also, note that there are multiple revisions of Manual J. (I can't tell you which one is current). If they aren't willing to perform a Manual J, they aren't serious about doing their jobs, and you should be serious about finding someone else to do the work.

A few key discussion topics:
-- Sensible heat includes humidity. A unit that turns on an off often because it's oversized will not remove humidity. You will not feel comfortable and will set the temperature lower, which only wastes power
-- The keys to energy efficiency: Sensible Heat ratio and the SEER ratings. Check out both.
-- It is not better to undersize or oversize. You should match your total tonnage to your heat gain. I believe "correctly sized" means that the unit should be the next smallest increment tonnage that's higher than your heat gain. eg: if your heat gain suggests a 2.3 ton unit, you should get a 2.5 ton unit. I'd think if you calculated a need for a 2.05 ton unit, you'd do just fine with a 2.0.


If you are a DIY, I have to links that you can use:
This link is a mini-Manual J. It's about the best you'll find that's free:
http://www.acdirect.com/spacepak2/spacepakapp.html

This is another version.
http://www.mrhvac.com/index.asp?load=http:...lcshortform.htm
 
Yep, those calculations vary quite a bit depending on where you are. In Texas, you generally need about 1 ton for every 800 square feet. I would suspect that IL would be less.
 
politics123 said:
When you ask for a "Manual J" the rep will probably tell you that they use a rule-of-thumb. That's garbage. If you must, tell them that your locality requires you to submit the Manual J calculations with your permit. Also, note that there are multiple revisions of Manual J. (I can't tell you which one is current). If they aren't willing to perform a Manual J, they aren't serious about doing their jobs, and you should be serious about finding someone else to do the work.

A few key discussion topics:
-- Sensible heat includes humidity. A unit that turns on an off often because it's oversized will not remove humidity. You will not feel comfortable and will set the temperature lower, which only wastes power
-- The keys to energy efficiency: Sensible Heat ratio and the SEER ratings. Check out both.
-- It is not better to undersize or oversize. You should match your total tonnage to your heat gain. I believe "correctly sized" means that the unit should be the next smallest increment tonnage that's higher than your heat gain. eg: if your heat gain suggests a 2.3 ton unit, you should get a 2.5 ton unit. I'd think if you calculated a need for a 2.05 ton unit, you'd do just fine with a 2.0.


If you are a DIY, I have to links that you can use:
This link is a mini-Manual J. It's about the best you'll find that's free:
http://www.acdirect.com/spacepak2/spacepakapp.html

This is another version.
http://www.mrhvac.com/index.asp?load=http:...lcshortform.htm
Click to expand...
Thanks for the advice.

Also, I'm going to try the link now. I will let you know the results later. :)
 
I have tried the java application. It recommended a 2 ton unit. I think, I should also add and consider kitchen, laundry, loft, livingroom, diningroom and familyroom as rooms. I only entered the bedrooms.

I will keep you posted.
 
And also folks, does it help if I put the condensing unit in the northside so that it doesn't get full sun?
 
v1rtu0s1ty said:
And also folks, does it help if I put the condensing unit in the northside so that it doesn't get full sun?
Click to expand...
Not much. Not enough to change you from a 2 ton to a 1.5 ton :) The most important thing is that you get good air-flow through it. However, if you have a choice on location, put it in the shade.
 
v1rtu0s1ty said:
I have tried the java application. It recommended a 2 ton unit. I think, I should also add and consider kitchen, laundry, loft, livingroom, diningroom and familyroom as rooms. I only entered the bedrooms.

I will keep you posted.
Click to expand...
Do you only have one unit for the entire house? Is it zoned? If not, I'd recommend investigating the extra cost to zone it. Over the long-haul, you'll save money by only conditioning the areas you need.

If you only have one unit, then you need all the rooms included. There's usually an extra-heat gain for the Kitchen too, to allow for cooking temperatures.
 
politics123 said:
v1rtu0s1ty said:
I have tried the java application. It recommended a 2 ton unit. I think, I should also add and consider kitchen, laundry, loft, livingroom, diningroom and familyroom as rooms. I only entered the bedrooms.

I will keep you posted.
Click to expand...
Do you only have one unit for the entire house? Is it zoned? If not, I'd recommend investigating the extra cost to zone it. Over the long-haul, you'll save money by only conditioning the areas you need.

If you only have one unit, then you need all the rooms included. There's usually an extra-heat gain for the Kitchen too, to allow for cooking temperatures.
Click to expand...
I just did some research. I found out that I cannot do zoning. It's because, currently, I only have one furnace. And also, for zoning to work, I read somewhere that I should have separate ducts for each A/C. Is that true?
 
There are many types of zoned systems. In my house, I have two units, one for upstairs/one for downstairs. That's two zones.

eg: In other houses, they only have one unit. Within the ducts they have dampers that control the rate and direction of airflow. Your house probably has a central stack going to the upstairs registers. You can put a damper (controlled by separate thermostat) that opens and closes to heat/cool the upstairs separately from the downstairs.
 
politics123 said:
eg: In other houses, they only have one unit. Within the ducts they have dampers that control the rate and direction of airflow. Your house probably has a central stack going to the upstairs registers. You can put a damper (controlled by separate thermostat) that opens and closes to heat/cool the upstairs separately from the downstairs.
Click to expand...
I think that explanation is a bit simplified. Yes, a zoned system can be made with a single furnace and dampers on the main runs (e.g. up/down stairs) To do this it is likely that the duct runs will need to be over sized and/or bypass dampers installed. A zone controller is also required that takes the inputs from the zone thermostats and makes the furnace/dampers do the right thing as well as protect from over/under temp conditions and various forms of short cycling.

Here is a good link:

http://www.zonefirst.com/products/DesignManual.pdf

It's for a commercial product but I think it gets the highlights mostly correct.
 
Thanks hucker.

Here is an update. I spoke to the company that designed and installed our furnace. He told me that our house needs a 42,000 BTU for cooling. Since the rule of thumb is 12,000/ton, that would be a 3.5 ton. However, combining all the other information I have gathered, the perfect size is 4-ton.

Now, I just need to decide if it's Puron or R-22 based A/C that I should purchase. I know that R-22 is going away but there is NU-22 that will be coming out. What are your thoughts folks?

Since I have an ELK system installed, what do I need to buy so I can control cooling and heating?
 
politics123 said:
There are many types of zoned systems. In my house, I have two units, one for upstairs/one for downstairs. That's two zones.

eg: In other houses, they only have one unit. Within the ducts they have dampers that control the rate and direction of airflow. Your house probably has a central stack going to the upstairs registers. You can put a damper (controlled by separate thermostat) that opens and closes to heat/cool the upstairs separately from the downstairs.
Click to expand...
Looks like zoning can be done even if we only have one furnace like what you just have mentioned guys. That's a good news. However, does having zone more efficent in single furnace house?
 
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