One or two AC systems for my house?

[steve f]

I have an old 2100 sf house with 11 foot ceilings in Atlanta GA. The current system that was there when I moved in, a Rheem 2.5 ton unit, doesn't come close to keeping the house cool in the summer.

The living room, dining room, kitchen and breakfast room are all on the west side of the house and there are no trees on that side, it pretty much bakes in the evening sun. There is not a lot of airflow thru the vents on that side of the house and there's only one vent per room to boot. When the outside temp hits the mid-90s, the temp in the west side of the house is in the low 80s.

There is actually good airflow thru the vents on the east side of the house, and it's fairly comfortable in either of the 2 bedrooms and the bath on that side.

I was wondering if adding another 2 or 2.5 ton unit, with maybe some additional ductwork, for the west side, and having the existing unit dedicated to the east side, would be practical.

The existing unit is around 10 years old, and will need to be replaced at some point, but it actually hasn't given any trouble and the air is still cold coming out the vents, there's just not enough of it coming into the house.

I'm going to look at getting more insulation in the attic, under the house, etc, but I'm pretty sure that's not going to cancel the fact that I don't seem to have adequate cooling. The question is whether to get one big, new unit for the entire house (along with some additional ductwork and vents), or go with dual units, getting a new smaller unit now, and eventually replacing the older unit when it fails.

[hyper_racing]

What size is the ductwork comeing off the airhandler? Do you have access to the ductwork to where you can see if there is any transitions makeing it smaller. The unit is located on the east side right?

[dan sw fl]

TWO!

[airworx]

1 system with a zoning kit installed = lower installation costs and lower utilities.

[bornriding]

2100 sf, 11 ft = about a 4 or 5 ton system
no, I would not go two systems - one system will cost less
per month - unles you can completely seperate the two sides of yur home.

[RoBoTeq]

I completely disagree that one system will cost less to operate.

Two systems are less costly to operate and provide much better comfort.

I prefer to zone systems by day/night usage if possible. This way you don't have to over cool all areas of your home when you are not utilizing those areas.

I like it supercool when I sleep. Cold air falls and warm air rises. Upper floors are usually for night time usage and have a higher heat gain during the day.

For all of these reasons it is great to not pay to drop the upper floor temps to 74º during the day when the system would be battling the higher heat infiltraiton.

Conversely, when the cooling system is in operation in the upper floor rooms in the less heat load evening, the heat load of the lower floor drops tremendously and that system most times does not even need to operate while you are not using it.

I only suggest duct zoning of a single system when it is impracticle to equipment zone. If one system of a multi system home breaks down, you still have a conditioned area of the home to retreat to.

[bornriding]

Been there, done that Robo,
Chenged two, 2 ton, units to one 4 ton unit, saved customer over 60 dollars a month on his electric bill - he came by my office to show me the electric bill results & to thank me. Oh yea, the electric charge per kw had gone up and he still saved the $60.
I will agree with you that zoning would be ok. Though I'm not sure the additional cost would be offset by the gain.

[RoBoTeq]

Every situation I have had has been just the opposite. Glad to know it worked for you and your customer in that instance, but two smaller systems will almost always be more efficient and cost effective then a single system.

I have a feeling that there was something wrong with the two systems you replaced with one.

[airworx]

i sell a lot of zoning systems. and all i ever get is how much the electricity bill has dropped (usually around 50.00 a month)
and how well the system works. zoning systems will keep each zone right at thermostat setpoint. plus the installation cost is almost half that of installing another system.

[jimlockwood]

I agree with robotec. But you really need someone to come out and survey your house. I am not a fan of one system with zone damper. They can get expensive and you have more part to fail.If you replace the small unit with a much bigger one you might have to replace the entire duct thats allready there.

[jimbo48236]

I vote if it is a single story zone and resize the unit (a 3 ton might be more correct but for goodness sake get an HVAC person to do a Manual J calculation). If it is a two story I say buy another one! You should know I am not an HVAC technician, but I am a Mechanical Engineer with a master's degree and I have not enough cooling in the upstairs. Since heating isn't an issue, I went with a new AC and new ducts and returns for the upstairs.

Like most engineering problems there are many solutions, but only one is best when you take cost into account.