Opinions please on sizing a replacement system

[sumdumgai]

Am replacing a 28 year old Carrier (38EB030300) heating and cooling system. Stills cools and heats but it's time. The Carrier is a 2.5 ton and handles the main floor and downstairs finished (heated and cooled) basement & office. The mainfloor is around 1200 sf and the downstairs around 500 sf. Above the mainfloor, upstairs, are the bedrooms, which are handled by another 2.0 ton Carrier system.

The upstairs has never been a problem with heat or cooling. The mainfloor has always been difficult to cool on hot days. Normally, I leave the thermostat at 77-78. The house is surrounded by trees so there's plenty of shade. Four normal size windows on mainfloor face West and get afternoon sun.

I've gotten 5 different estimates from different contractors. Three suggest a 3.0 ton unit and two say a 2.5 ton is enough. One contractor explained that 2.5 tons is enough because the mainfloor is 1200 sf, and there is another heated and cooled floor above and one beneath, and the basement, around 500 sf, does not need that much cooling capacity. Another contractor explained that 3.0 tons should be put in because a little too much (.5 ton too much) is better than not enough, and the only time too much is a problem with cycling is when there is way too much (e.g., 1 ton too much).

Both contractors are offering Lennox units. The one proposing 3.0 tons is actually less money.

I know there are other factors involved, but from this information, can someone please offer an opinion as to what size should I go with, 2.5 ton or 3.0 ton?

Thanks.

[Punteam]

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[sumdumgai]

Thanks. Both are offering Lennox XC14's with SL280V. I thought the staging applies to the furnace. The XC's come in 1/2 ton increments. Neither of these two contractor (both reputable and in business a long time) has done a J calc, just general inspection of home constructon, duct work, etc. The J calc aside, which of the two explainations seems to make more sense; i.e., a 1/2 ton too much is not a problem vs. 2 1/2 tons is enough because the 1200 sf mainfloor can be handled by 2.0 ton and the 500 sf basement doesn't warrant a full ton?

[udarrell]

There could be a lot of reasons why the main floor system is not easily cooling 1200-sf down to say, 75 or 76-F & 50% or less RH. With the situation you described I would think that normally a properly installed efficient operating 2-Ton system ought to cool that area.

Get the load calcs performed & figure out ways to determine why the old 2.5-ton did not cool better; also figure out ways to reduce the main floor's heat-gain. I'd consider doing something to reduce the heat gain of those west windows...

Basements with very little above ground-level normally require a small amount of cooling.

What climate do you live in; where is the nearest large city so we can look up the summer design estimate?

[sumdumgai]

50 miles north of Atlanta. Last summer, we installed awning over deck that shades 3 of the kitchen windows. I'm not ignoring your suggestion to get J calc. I'm just wondering what the down side is in getting a 1/2 ton larger unit for less money.

[udarrell]

Atlanta, GA 2.5% summer design is 92 dry bulb & 74 wet bulb or 43% RH.

If you get infiltration down low, the sensible ratio of the system will be rather high which should allow the A/C unit to bring down the temperature to comfortable levels. By not oversizing the RH% should be kept rather low, with adequate runtime, for excellent comfort without going below 76-F when it's an intermediate temperature outdoors.

[motoguy128]

50 miles north of Atlanta. Last summer, we installed awning over deck that shades 3 of the kitchen windows. I'm not ignoring your suggestion to get J calc. I'm just wondering what the down side is in getting a 1/2 ton larger unit for less money.

The downside is poor humidity control, especailyl on newer equipment. Older equipment had smaller coils which made them less efficient, but they ran a lot colder. Add more surface area and you get more effceincy, but but the coil is warmer. SO sizing is very, very improtant in a humid climate. Atlkanta is more humid than hot. You'll need the smallest furnace available with a large enough blower capacity for cooling airflow.

I'm not sure why 2.5 tons is struggling. Could be the condition of the coil and equipment. Overtime fins get bent, dirty and you lose capacity. The upstairs may be comparatively better insulated and shaded. Strange that 2.0 tons gets is donw upstairs but struggles downstairs. The 500 sqft basement is irrelevant to cooling load. It only adds a little heating load. With good shade and normal construction, 1.5 tons should cool a 1200sqft downstairs in your climate. For all we know the coils are all beat up, its' dirty, low on refrigerant, all sorts of possible reasons. Maybe the previous owners had a dog and never used an air filter and it's packed full of dog hair. I think the techs on here have seen it all. That's why existing equipment size, while it can provide somewhat of a baseline if it's too big, shouldn;t be used for sizing. You never know who or if calculations were done.

Let a load calculation determine the sizing and make sure it's installed right including adequately sized ductwork. I defintiely would not go larger.

[sumdumgai]

Thanks for the information and suggestions.

[beenthere]

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[BaldLoonie]

We find that usually the ducts there are borderline or undersized for the capacity you have. So if someone says go bigger and doesn't address the ducts, odds are your performance will be compromised or with a variable speed blower, high sound level. I know, they'll say the ducts are fine...

[dan sw fl]

Am replacing a 28 year old Carrier (38EB030300) heating and cooling system.

Stills cools and heats but it's time. The Carrier is a 2.5 ton and handles the main floor and downstairs finished (heated and cooled) basement & office.
The mainfloor is around 1200 sf and the downstairs around 500 sf.
Above the mainfloor, upstairs, are the bedrooms, which are handled by another 2.0 ton Carrier system.

The upstairs has never been a problem with heat or cooling. The mainfloor has always been difficult to cool on hot days. Normally, I leave the thermostat at 77-78.
The house is surrounded by trees so there's plenty of shade. Four normal size windows on mainfloor face West and get afternoon sun.

I know there are other factors involved, but from this information,
can someone please offer an opinion as to what size should I go with, 2.5 ton or 3.0 ton?

Thanks.

....................
The house is surrounded by trees so there's plenty of shade.
Four normal size windows on mainfloor face West and get afternoon sun.
.....................................

It's either shady or sunny BUT NOT both.

Clear glass - 80 square feet - might have 4,000 + BTU/hr heat gain.
Tinting might reduce the heat gain by half.
... There's a possible 1/4 ton reduction in mid-late afternoon
after consideration of effective Sensible Heat Ratio (SHR) in a humid climate.

I would say the upstairs unit should generally be the larger of the two A/C units.