Undersized AC - Options?

Question

Background: Our two-story house is about 3,000 square feet. When we bought it last year, it had two separate AC units, a 2.5 ton and a 4 ton. They were both old and died last month. We had them replaced with a single 5 ton unit, with two zones controlled by dampers (the original ductwork was merged into a single supply). The contractor insisted that 5 tons (versus the original 6.5) would be fine since the new system was much more efficient.

It was very hot today (pushing 90) and I noticed that regardless of what I had either thermostat set to, it just kept getting hotter and hotter. I felt cool air blowing out of the vents, but it simply wasn't cooling the house.

I got an emergency technician visit and he checked everything and asked me about the previous system. He came back inside and said, "We screwed up." It seems the 5 tons is not adequate for the heat load this house experiences. Although the system is working perfectly, it just can't cool both zones at once. As a temporary workaround, he suggested running just one zone at a time. The warranty manager is going to call me tomorrow.

I want to understand the options. I thought the technician said the 5 ton unit we have is the "largest residential unit available", so replacing it with something bigger might not be possible. What else could be done here? Could a second condenser be added? Is there some other way to beef up this system?

Hopefully the company will "make this right" as the technician said. But given that they apparently made a mistake on sizing it, I'm not confident they'll make the best recommendation.

Any ideas? Thanks!

Answer 1

I'm not a HVAC expert but a 5 ton unit sounds pretty large for a 3000sqft house. You can probably approach this in one of two ways. One is probably what the previous owner did and just throw more cooling capacity at the problem (two units, larger units, etc.). Alternatively, you can start looking at ways to get more out of your existing unit. Some things that might help:

• Improve attic insulation
• Improve attic airflow
• If your windows are old, consider replacing them with more efficient windows
• Find and plug sources of air leaks - doors, windows, electrical outlets and other exterior openings
• Plant trees and shrubs to help shade the house
• Install ceiling fans to help circulate the air
• Add insulation to the exterior of the house (requires replacing siding usually)
• Seal all duct work with proper HVAC tape to prevent leaks
• Insulate duct work
• Replace your air filter
• Install an air exchanger

The benefit of trying some of the above is that they also help when heating.

Answer 2

I have a 3,200 square foot home (2,800 down and 400 up) that has a single 5-ton unit with three dampered zones and one "dump" zone. I live in South Texas and we have dozens of 100+ degree days every summer. My system is able to hold the house easily at 74 degrees in all zones even when it is 100 degrees outside. A few thoughts: First my home is relatively new (6 years) and has efficient windows, radiant barrier in the attic and is well insulated. Our dump zone that runs any time one of the dampered zones calls for air is in the kitchen - always the hottest room in the house and this helps to keep the busiest area in the house cool. This also helps bleed off supply from the other three zones making sure the unit runs long enough to adequately cool and dehumidify the house. It is a simple but elegant solution to the challenge of too many zones running for short periods of time. I also have the zones set where the dampers don't close all the way - providing a bit of a "leak" to keep air moving in the entire house when one zone is running but the others aren't. I had the builder put in lots of return air. I have five ceiling mounted filter grills in the house to keep the unit well supplied with return air and to maximize filter effectiveness. The slower the air moves across the filter the quieter and more effective each return will be. This also helps to even out the temp in the house since it isn't all going to one large filter. I have three 12X12 filter grills, one 12X24 and one 20X30. Finally - I have a Z type evaporator coil in the air handler rather than the standard V shaped one. This allows my 5 ton unit to maximize efficiency and increase the surface area of the large volume of air moving across the coils. I didn't even know this existed when we built six years ago - but it works great. Like I said - I easily keep our house at 74 even when it is 100 outside - and could keep it cooler - I just don't for budget reasons. Our unit seems to have no trouble at all with intense South Texas Heat.

For your install - I would check to make sure your return air is properly sized in addition to all the insulation questions the earlier poster inquired about. My in-laws have a 2,100 square foot house built in 1978 with poor insullation and they have a five tone unit that barely keeps up. So the age and efficiency of the structure will play into how well a 5-ton unit can keep up with the heat.

Many builders and HVAC specialists default to "add capacity" for their answer - because that is easier. Building in intelligence into the system and efficiency into the house is better long term. Good luck.