Appreciate response. Want to separate brand issue from basic question - does single stage AC benefit at all from variable speed furnace and if so, how? Still considering brand but models provided for reference. 13acx very well reviewed from my research and pricing attractive. Warranty also good here.

I am not an HVAC PRO, however I do not believe you will not see any benefit. The blower speed will be set to run with the appropriate tonage. Single stage is just that, one stage. If you desire multi stage unit for cooling you will need both a new furnace and multi stage condensing unit. If the two stage furnace is not controlled by the thermostat, then that means it is most likely timed or some simple algorithm to determine how long to run the 1st stage before engaging the 2nd stage. Make sure your stages are thermostat controlled or it's pretty useless IMO. If I am wrong I am sure someone will let us know.

Single stage AC systems are low end entry level. So there is no 'real' benefit to the AC system per se except the variable speed blower of the furnace can 'somewhat' enhance the performance of the system.

Most of this enhancement comes via the way of better air flow... provided the duct system is properly designed and return air is adequate.

In terms of dehumidification a 2 stage AC paired with variable speed furnace is adequate for most areas with *minor* humidity problems provided of course the humidity doesn't come during low temperature times of the year.

This is achieved because most 2 stage AC's first stage is around 70% of capacity of the machine. Because the machine is driven by temperature the lower capacity of first stage forces the machine to run longer thus provides *some* dehumidification.

However a 2 speed AC is not really dehumidification control... it is a 'feature' that merely helps in that regard.

Better dehumidification control via AC equipment would be completely variable AC compressor paired with variable speed furnace. Some manufactures have variable AC compressor but due to controls may limit them to 5 speeds. So equipment selection can make all the difference depending on what your comfort goals are.

If you have high humidity with low ambient temps --- trying to dehumidify with an air conditioner will probably not work to your satisfaction.

I service the Katy, Texas area --- well known for hot + humidity problems all year long. Math problem? Hot + Humidity = Hodity.

Also wonder if the single stage AC I plan to get (Lennox 13acx) gets any benefit from new 2 stage variable speed furnace if I don't change my conventional older honeywell programmable thermostat?

I don't think you will see any benefit if you have a conventional programmable thermostat. The furnace will be set to provide the same air flow when the thermostat is calling for cooling.

basic question - does single stage AC benefit at all from variable speed furnace and if so, how?

If you may see a small benefit if you bought the communicating thermostat that is the same brand as the variable speed furnace. This assumes your installer knows how to configure the thermostat and furnace properly.

The way it works is the thermostat needs to be able to speed up or slow the variable speed blower in the furnace. This is done by communicating the desired speed to the furnace. Let's say the you have a 3-ton condenser. Normally the air flow would be set up at 400 CFM per ton, which would be 1200 CFM this your condenser. In cases when the indoor humidity is high (>60%), the thermostat could slow down to blower to 325 CFM per ton, or 975 CFM for a 3 ton. It is not much of a reduction in cooling, but it helps. Slowing the air flow any lower risks the freeze up of the coil.

Thermostats with cool to dehumidify will continue to call for cooling in order to reach the target humidity level. Most will go 3 degrees below the set temperature, and some will go even lower. If you want good humidity control then invest in a 2-stage condenser. You will get a higher SEER rating which may qualify for a local rebate to help offset the higher cost.

From what I continue to read, 2 stage AC for climate like ours would be overkill, I think. It would be like getting two stage or modulating variable speed furnace for southern Texas. Just hoping to better understand if I get some benefit to single stage AC with a variable speed furnace. Seems that it depends on the internal programs of the AC unit, and that as an example, I could use a setting which would start AC at about 50% for 30 seconds, then at 82% for 8 minutes, then 100% until temp reached. This is a bit of a benefit, as it would provide more dehumidification than 100% on/off functionality. All input has been helpful and thanks to those who have taken the time to respond.

A single stage furnace with high eff blower motor and a var speed 2 stage furnace as a rule of thumb will bump up a condenser's SEER rating by 1 point. Always good though to get the rating of a matched system from AHRI Directory.

TD

Always good though to get the rating of a matched system from AHRI Directory.

BUT, this number means nothing if the unit is installed by piece meal hacks with little to no experience.

How could a number guarantee that your unit was installed properly? Hint: It can't.

I know, I know lets all sit back and sing 'Comb by ah, my Lord'.

This is the same kind of stupidity in which there is a small air leak in the attic coming out of the drain line connection from the coil, but the coil has a Freon leak in it.

Oh, we need to seal the air leak to improve efficiency!

Uh, sir your coil has a leak in it. It will never be efficient until the coil is replaced and the refrigerant leak is fixed.

Well I read on the internet that the air leaks cause inefficiency... sir that is not the only thing that causes inefficiency... it's also foolish people spouting nonsense things they couldn't understand --- because --- they aren't really professionals. They are home owners with too much time on their hands.

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A single stage air conditioner will not provide any 'real' dehumidification by simply lowering blower speed. It's driven by temperature -- not humidity. If you want to fall for gimmicks, that is your God given right.

Same as such as relying on a number to determine how efficient your system will 'supposedly' be with the lowest skilled people on the planet installing your 'good deal' HVAC system.

It is what it is Austin and while I agree these numbers come from a controlled environment and not real world testing, a matched system is still desired. The numbers probably are more of a guide. I luv the way HVAC markets their products, up to 18 SEER, up to 96+% efficiency.

TD

The contractor is at least as important as the equipment, which is why I have spent as much time researching that before making a decision. Your first answer confirmed that the extra run time on AC does provide some additional dehumidification but now you say it is a "gimmick" that I "fell for". Ok. Not sure where all that anger came from. Doing research is a good thing.

I always recommend finding a HVAC contractor will do a load calculation whenever replacing equipment. Proper sizing will improve the efficiency, give you better humidity control, and save money on the initial installation costs. I would not be surprised if both your current furnace and AC condenser are over sized for your size home and climate.

Let's say the cooling load is calculated to be 2.1 tons. If your goal is to achieve lower humidity then I would recommend rounding down the equipment size to 2 tons. The smaller equipment will produce longer run times which will result in lower indoor humidity. The heating load will be determine the right size furnace.

Efficiency ratings are meaningless if the equipment is over sized and constantly short cycling.

This is what I have it it’s works well. Trane XR16 paired with a Trane S9V2–In cooling, fan runs slow at start for about 10 mins to get coil cold and allow for better dehumidification—fan then ramps up after about 10 mins. I think most variable speed airhandlers have this feature.

Yes var speed motor software with slow ramp up, run, and then ramps down...most of the motors were GE, not sure if that is still the case...

TD

The slow ramp up is I think what I am looking for as it gives some of the benefit to the AC side. Appreciate that and I guess what I need to do is make sure that the variable speed furnace I select has the right ramp up profile setting and it is properly set up by installers.

The furnace ramp is a not controlled by thermostat and not a function of temperature and humidity. It is not as effective as control by the thermostat, but it will in some situations. It is also nice in the winter so you don't get a blast of cool air when the furnace first starts up.

Your first answer confirmed that the extra run time on AC does provide some additional dehumidification but now you say it is a "gimmick" that I "fell for".

Only with a 2 speed air conditioner. BUT it's still mostly a gimmick.

WHY? a 2 speed air conditioner's first stage (most brands) is approximately 70% of capacity of the machine.

It is more or less in dehumidification mode when the unit runs in 1st speed. (It must be a 2 speed AC condenser.) However, 1st speed turn on is not determined by humidity it is determined by temperature.

Do you see the problem with that?

The kick to 2nd stage is often times determined by time. Usually in the realm of 8 to 10 minutes or the temperature difference from set point to current temp of the structure (usually greater than 3 degree difference will initiate stage 2 within a few minutes). Humidity again plays no importance / no determination from ramp up or down --- it's all decided around temperature set points.

How about now? Do you see a problem with that?

So when in cooler seasonal temps --- the system will nearly always over cool... very much like a single stage air conditioner does all the time.

PS: I'm not angry. But you should be for being 'duped', by the super slick advertising nonsense of the HVAC manufacturers. If I was angry why would I then turned around and explain things to you further so you understand the point being made here?

What does 'SERVICE' mean to you? This is a service business, you will ALWAYS need service... better hope it's the kind you really want.

I service the Katy, Texas area.

most of the motors were GE, not sure if that is still the case...

Nope. GE is a pigeon compared to it's former heyday self. Nearly every piece of GE manufactured 'anything' is merely a licensed product that is made by some other Chinese / Malaysian manufacturer. For example: small household appliances like dishwashers, clothes washers / dryers & refrigerators.

It's been a number of years since I've seen a motor made by GE. If they're still being made it's most likely a licensed product. -- but because these types of things are out of public view I doubt GE actually manufactures these any more.

General Electric Company (GE) is an American multinational conglomerate incorporated in New York City and headquartered in Boston. As of 2018[update], the company operates through the following segments: aviation, healthcare, power, renewable energy, digital industry, additive manufacturing, venture capital and finance, lighting, and oil and gas.[2][3]

I knew GE's appliance division had been sold, it's been on the market for several years. The GE name is still being used though by the new ownership. I purchased all GE for kitchen remodel in 2018.. I believe a Chinese company, Haier, is the new owner. Sad.

TD

GE sold their motor business to Regal Beloit. For several years Regal was allowed to use the GE brand name. The motor subsidiary is now called Genteq Motors.