What size both heating and cooling are you replacing? What are the model numbers of furnace quoted?...not sure I understand about the new ductwork...

IMO

Two stage is, to put it simply, a system that can run at a low or high setting, depending on demand and conditions. The low speed would be less noticeable, and would likely control humidity better, but I don’t understand why they upsized the two stage unit, unless it isn’t available in a 2.5 ton unit. Generally, you want the a/c to be correctly sized or slightly undersized, versus oversized. An oversized unit will short-cycle, likely increasing humidity, and a two stage unit is a way to avoid this.

As tigerdunes says, to get some real feedback; current units (model numbers if possible) and if they “work OK” in your experience, the exact model numbers they propose to use (for all parts, including the coil), and are they running the ducts through the old slab locations, and if so how, or new ducts in the attic?

Neither of your concerns are founded. The 2 stage equipment will be more comfortable and this technology is nothing out of the ordinary. Some "old school" types will shy away from multistage/variable speed equipment and, while is will be slightly more expensive to repair, should this be needed, the technology is as reliable as the simpler stuff.

I would even go for variable capacity instead of 2 stage. But I would always go for 2 stage over 1 stage. Energy savings said, they provide much better temperature and humidity control. This is even more important if you live in a hot humid climate and keep your house warm - a single stage compressor sized for a design temperature of 72 will likely leave you house uncomfortably humid.

The two/variable stage unit, when coupled with a matching variable speed air handler, will run longer than a single stage, but with slower airspeed and warmer air, on all but the hottest days. You will not get the uncomfortable drafts/blasts of cold air that you get when a single stage turns on, rapidly cools the house, then turns off. Parts are a non-issue. These are now mature technologies and parts are readily available. It’s not like an oven where the old-fashioned alternative was a simple mechanical part.

Proper sizing, duct design and outlet selection are critical. Consider hiring an engineer to do a proper Manual J, D and S calculation. Most HVAC installers either done know how or don’t want to do this. They want to go by rule of thumb for simplicity or oversize to make sure they don’t get called back for the house not getting cold enough. Oversized equipment wastes energy and does not dehumidify properly. Poor duct and outlet design will cause uncomfortable temperature differentials and drafts. Make sure ducts are sealed with mastic, not duct tape, and insulated. Each bedroom should have It’s own ducted return.

Thanks. I do plan to ask about the returns. He mentioned using the same returns which I am not sure about. The whole point in having it all redone is not to use the old vents. (yes, the new vents will be in the attic)

The 2.5 ton is almost 10k while the only variable option was 16k. The 3 ton 2 stage is almost 13k which is why we are considering it. It's just hard to justify why we would spend so much more for even the 2 stage just to help with humidity and no idea when the 3k would pay itself back in energy savings.

Using the same returns is a poor idea. Modern systems require larger ducts because they move the air slower. I wouldn’t spend 2/3 more for variable speed, but the comfort advantages to 2 speed are often profound. I also wouldn’t go from up to a 3 ton outdoor unit just to get 2-stage as the low-capacity stage on most 3 ton units is 2.5 tons - so it would just be getting you a superfluous half ton of extra high-stage capacity. Without a proper manual J load calculation you don’t know if you even need 2.5 tons or if 2 tons would be fine.

You may want to investigate a ducted minisplit system. This would involve either multiple very small air handlers with short duct runs all connected to one outdoor variable speed unit, or someone of then are designed to work with a conventional air handler. This may cost the same or nominally more, but be much more efficient and give you zoning.

Carrier has a 5 stage unit that is less costly that the full variable-capacity. I know several contractors who don’t install their two-stage anymore and use the 5-stage instead.

Let's not confuse the word 'variable' with staging. There are multi-stage compressors out there, but I don't think you're considering them due to cost.

2-stage units only come in whole tonnage sizes. The first stage on a 3-ton unit will be between 1.5 and 2 tons of cooling capacity.

A variable speed blower with a single stage compressor would be a step up from a single speed blower. They start slow and then ramp up to full speed after a few minutes. The lower start helps with humidity removal.

I don't understand the $2700 going from a single stage to a 2-stage unit, even with the slight increase in size. That seems like more than twice what I would expect the price difference to be.

Our concerns:

While I realize we live in the South, we love the heat. I keep the AC on 78 during the day and work from home. I don't like it when the air is running. We have been told the 2 stage will be on more and blow out less and not bother me. Not sure if this is true??

Air conditioning as well as heating is something that has tiers of energy performance. This energy performance has to do with efficiency that in some cases saves you operating costs. The con in this arrangement is that you must be running the equipment. (If the equipment is off there is no energy drain... in that sense it doesn't matter what rated efficiency the equipment is.)

Because your preference is 78 changes the arrangement. The more efficient the equipment the more the set up will cost, the less you operate the equipment the less likely it is to save you much in terms of operational costs.

Minimum efficiency ratings for your area is 14 SEER. These are all single speed machines. However, since July 2019 the furnace blower efficiency standards have been raised to require no less than an ECM blower motor to raise efficiency.

Some manufacturer's may still have the older PSC style blowers and probably at a discount.

Because of your preference of a higher indoor temperature / how you prefer to operate the equipment it's not necessarily a bad idea to drop down to 14 SEER AC and a PSC blower motor / single stage gas furnace.

If you can't find a PSC blower (multi tap single speed) furnace / the ECM motor would raise the efficiency of the AC to 15 SEER.

You will most likely have to 'shop around' find a HVAC contractor that is willing to listen to what you want... as opposed to what they would prefer to install.

I service the Katy, Texas area.

Is this a single floor ranch style house? If it is then I am not sure 2.5 tons is enough cooling for a 40 year old 1760 sq foot house in Atlanta. In addition if you are planning to install the new duct work in an unconditioned attic, then your duct loss will be increasing. The cement slab helped keep the old ducts cool. Your contractor needs to take all of this and you insulation values into account to determine the correct size.

"Modern systems require larger ducts because they move the air slower."

This is an interesting comment. I would like to understand the physics behind this.

"Carrier has a 5 stage unit that is less costly that the full variable-capacity. I know several contractors who don’t install their two-stage anymore and use the 5-stage instead."

I am not sure I agree with this. Can you post the the Carrier model numbers so we can do a comparison?

Are you installing a furnace and AC or a heat pump?

gas furnace and ac

Thanks weedmeister. After reading everything we just don't think we can justify 16K for the variable and if the price difference should not really be that much between the single and 2 stage then we may as well stick with a single stage.

It's crazy because no one has listened to what we want and we are at the point where we need to make a decision because demo starts next week. The only thing I really had asked for was a system that included air purification which I was told was possible. Yet, each company came out and all gave me very similar options and none included anything about air purification. They were all concerned about the 2 stage or variable and discussed energy savings and decreased humidity. No one can actually tell us how much energy we would save (and like austinair said it it's not on then what's to save) and I could care less about humidity. We have never had an issue with it in a home before.

Honestly, energy savings are like calories...it’s all about how much you use/eat. It’s fine to cut the sugar in a recipe, but if it tastes bad, it’s no good...just like an efficient system is great, if it’ll work as well or better than the old system.

You still haven’t said what you are replacing...there’s a good chance a 2.5 ton, single stage unit will be too small for that size house, in that location, with new attic ducts. You need better advise from either us by providing details, or the companies by having them do a Manual J.

" I could care less about humidity. We have never had an issue with it in a home before. "

That could change.

I would like to hope that they would install a 4" filter media cabinet for you. These larger filters are now readily available at home stores.

Humidity isn't as much of a factor in a home with higher temps.

The problem with humidity is if the temp in the home hits dew point temp. This happens by dropping temperature.

I've said it many times mold needs 4 things to grow: damp, dark, cool, nutrients.

The nutrients are always present inside the home... so that leaves us with damp, dark, cool.

You don't create damp with a higher temp like 78. This temp is also not 'cool'. Mold can never grow under such scenarios. (In order for you to create dampness you must be hitting dew temps on a regular basis.)

Now if the home owner sells this home a new home owner moves in and runs the system to 70 --- there you go. To understand this HVAC is a 'cause and effect' situation.

The cause is what you have --- versus --- what you are doing (operationally speaking)