how much A/C tonnage do I need?

W

wolfman

1,000+ Posts
I am thinking of replacing my heating and cooling system. I get conflicting opinions from A/C people on how much A/C tonage that I need for a 2000 square foot house. Some people say 4 ton, others say 5 ton. I am leaning on going for 5 ton, but I also hear that it is bad to have too much tonage. Anyone here know what the proper size would be? Does size really matter?
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SF alone will not help you determine the proper tonnage. Other factors, such as the number of windows, whether they are high efficiency, the RF rating of your insullation, and the layout of your house all play a part. Your best bet is to have a trusted expert come to your house and do a thorough analysis. In addition to helping you determine the roper tonnage, they will also advise you on whether your vent configuration is optimal. Since you are replacing, you may as well do it right.
 
wolfman.... get someone to come out and do a system analysis...

for what it is worth, the house I'm remodelling is now about 2100 sq ft, and both A/C guys I had come out told me a 4-ton would be ideal... this is a 1960 built ranch style home with 8' cielings..
 
I just did the the city rebate program and it would only allow one ton per 500 sq ft.

Beware the monkeys that actually do the install.
 
I'm not disagreeing with the moist comment, but I don't understand it. I know A/c removes moisture and I'm not really connecting with the idea that more tonnage would add moisture or take less out.

The way I always understood the evil of too much tonnage was that it cooled too fast. Well, sorta. The way it was explained to me was that a huge chunk of your energy usage is just to start up the unit. The more starts per day meany more energy usage. Once it is going, supposedly it's not THAT expensive. The high tonnage unit would tend to short cycle because it would come on, blast an arctic freeze and then shut down only to come on 15 minutes later. It was supposedly cheaper to have "the little engine that could" struggle all day to keep it at 70 than it was to have the Big Bertha take it from 74 to 70, 18 times a day, even if it only took 5 minutes each time. I'm just making numbers up for illustration purposes so don't berate me about statistics.

That is why they invented the Trane systems that have a high and low where the low never shuts down. I know that doesn't answer the question about tonnage, but it might lend a little. A variable speed 5 ton system might not kill you on energy cost like it used to. I would think that 3 estimates of what you need ought to de a good start.
 
I think an oversized unit would blow a bunch of really cold air into the house, lowering the temperature, but before all the air in the house went through the system. Air that does not go through the system does not get the humidity squeezed out of it.

How's that for the scientific answer?
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That's the only thing I could think of, but doesn't the cooling process depend on removing the moisture? Or is it just a byproduct of the length of time that the cooling process takes?
 
Thanks for all the replies. I actually have spoken to more than on A/C person and one will say 4 ton and the other will say 5 ton. But that was just over the phone. I think that part of the equation is how well insulated my home is and at what rate the air is lost from the house. I think I really need to get some one to take a look at it to be sure.

They way that it was explained to me was that if you have too much tonage the A/C will cool the house much more quickly but the A/C will not run long enough to de-humidify the air and it will feel hotter and you will end up lowering the temp of the unit. Also as someone mentioned the unit will cycle on more oftem and pull more energy at startup more often.
 
Err on the side of too much. Those load calc guidelines are developed by engineers in Syracuse who think 85 is a heat wave.
 
We have an early 70s custom ranch with about the same square footage. Over half the house has has 16ft vaulted ceilings.
We went wiht a 4 ton 14 seer heat pump unit. Seems about right. I can't imagine needing a bigger unit.
 
Nick..
the reason for the humidity is indeed the speed at which the air cools... my dad has been dealing with this for quite some time down at his beach house..

the issue is that the air moves across the coils at a certain speed... too fast and the air hasn't had time to dry out... the A/C people have had to actually dial down my dad's system to remove some of the humidity.. seems crazy to have all that efficiency, but still be uncomfortable... the other thing they neglected to install was a thermal expansion valve which is supposed to help..
 
I had a A/C guy come out yesterday to look at my house and verify what tonage I need. He said that 4 ton was a good match for my 2000 square foot house. I even asked him if he thought I needed a 5 ton and he flatly said no. I would have bought 5 if he said so too.

He said that there are several factors other than square footage alone. Most of them were mentioned already in this thread but one big one that had not been mentioned was shade. I have mature trees on the east and west side of the house and except for high noon, a good deal of the roof is shaded for most of the day. He said that with the shading that I have I really could get by with only 3 tons of A/C. So I guess 4 tons is plenty. THanks for all the ifo from those who poste don this thread.
 
I am taking him for his word. He has been in the A/C business in DFW for over 30 years. Really, my house has a 4 ton in it now and it seemed to keep it cool enough. The furnace is old and has to be replaced, the AC is getting near the end of its life and I am gettng both replaced at once. I just wanted to be sure that 4 tons was the optimal tonage for my house. Looks like it is. I pretty much told him that I would buy a 5 ton if that is what it needed, but he flat out said no. Even if it get over 85 degrees
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Let an A/C person tell you then contact another for comparison

Depends on 1 story vs 2 and other factors

If you get a new system make sure it has three features regardless of tonnage--
1. digital thermostat
2. variable speed fans--humidity control and lower power usage
3. Zone cooling w/ damper system

Father in law owns an AC company in Houston
 
It's not a matter of knowing that it gets hot. It's a matter of those load calc guidelines being crap. They're based on the theory that you're more comfortable at 80 with low humidity than at 72 with some humidity. Bump up the tonnage and you get the best of both - 72 with low humidity.
 
Thanks for helping me be more smarter. From now on, I'll tell every a/c contractor in Texas I come into contact with that you are the only one who has figured that out, because I'm sure none of them have yet and they have nothing but dissatisfied customers.
 
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