New HVAC 18 Seer2 Humidity???

[dieselscout80]

We just got a new HVAC system American Standard variable speed split system rated at 18 Seer2 and we live in South Carolina.

What should it bring the inside humidity down to?

We aren’t seeing the humidity get below say 48% at the lowest and typically its around 50% or above. The humidity menu is set at 45%.

 

[BigGG]

I've been recently going through this, so I'll share my learnings.

How are you measuring % humidity? I was going by an old Radio Shack meter and couldn't get below high 50s. I bought a new temp/humidity and figured out the old RS was off 6-7%. The new meter reads close to the meter on a dehumidifier I have.

48-52% in my opinion gives a pretty comfortable level. I can run the dehumidifier and get mine down to 45%; this would enable me to raise the temperature about 2° but I'm not sure the savings in AC cost would offset the cost to run the dehumidifier.

I have about 2400 sq ft, single story, in west Tn. It had ~11? SEER 1996 vintage 5T split system (nat gas heat). It cooled ok, but the house was not particularly comfortable. There is one room with 3 exterior walls, vaulted ceiling and a big arched window facing south-ish. It was always hotter or colder than the rest of the house.

I toyed with replacing the system (it was time) with a 4T and putting a mini-split in that one room. I chickened out and went with a York 5T 2-stage AC and 1-stage nat gas furnace. The distributor substituted a 2-stage heat pump. The downside there is I have an expansion valve and reversing valve I don't need, but I'm toying around with setting it up as a heat pump with gas back-up; I'll have to run the $s.

I don't currently have thermostat and wiring to enable the 2-stage switching. So, after trying different combinations, I'm running on the low compressor stage (should give me 3 - 3-1/2T) and the medium fan speed. It has kept the house at 74°/50% humidity during the hottest days we've had lately, running maybe 80% of the time during the day and 10%? at night. Running it so much keeps that odd room more comfortable.

You probably know that controlling humidity relies on running the unit more than a minimum amount; therefore the downside of oversizing a system. There is a sensible heat ratio (varies with coil construction and other things) that determines how effective a unit will be in dehumidification. The CFM circulated per ton also affects it.

I'm pleased with the performance so far. I'll probably replace the thermostat and wiring so I'll have better control.

 

[dieselscout80]

We are measuring the humidity with the American Standard electronic thermostat that they installed with the new system.

Current humidity reading is 52%.

The system was installed on 1 August 2023, so maybe the computer is still working on figuring out how manage our temperature/humidity.

Our house is right at 2100 square feet upstairs and the basement is about 900 square feet.

 

[bnold]

50% is generally the target.

72-74/50 for cooling design.

Sounds like things are working as they should.

 

[bnold]

I don't currently have thermostat and wiring to enable the 2-stage switching. So, after trying different combinations, I'm running on the low compressor stage (should give me 3 - 3-1/2T) and the medium fan speed. It has kept the house at 74°/50% humidity during the hottest days we've had lately, running maybe 80% of the time during the day and 10%? at night. Running it so much keeps that odd room more comfortable.

You probably know that controlling humidity relies on running the unit more than a minimum amount; therefore the downside of oversizing a system. There is a sensible heat ratio (varies with coil construction and other things) that determines how effective a unit will be in dehumidification. The CFM circulated per ton also affects it.

I'm pleased with the performance so far. I'll probably replace the thermostat and wiring so I'll have better control.

So in what I assume is the hottest days/months of the year for you, calling is about to be achieved with stage 1 cooling.

This generally means (and very common unfortunately), that your system is oversized and performance will suffer during lower load conditions.

Ideally, on a design day for your area, the unit would struggle to keep up at Stage 2 cooling, running that same 80-100% during the peak load. That way the unit could run longer at part time load and better manage humidity (stage 1).

 

[Yander]

Anything below 55% will prevent mold growth. American standard used to be made by Trane not sure if they still are. Very good units. I had one for 15 years in my last home.

 

[Sigarms]

50% is generally the target.

72-74/50 for cooling design.

Sounds like things are working as they should.

I agree with you.

That said, I sweat like a pig, and I find my own "personal humidity level" around 40-45% in the summer months in the home.

20 years ago when higher SEER, multi stage equipment with a variable speed contstant airflow air handler came out, these systems were sold for comfort by lowering the overall humidity level. From personal experince, I've seen average humidity levels brough down 3-5% with these higher SEER mutli stage systems, but that's when the home was running 55% RH before the new system was installed.

They question is for the OP is what was the humidity level in the home before this new system was installed?

The only way I'd promise a lower humidity level less than 50% is by installing a whole house dehumidifier and tie it into the ductwork.

 

[Torvy]

I'm about to find out. The system in my previous (still sleeping here until Thursday) house had true variable speed and runs at a very low speed to keep humidity in check. Works great here.

The new place is ~150 miles east in the Piney Woods. The temp RN in old place is 85 degrees and 60% humidity (outdoors, 6:30 am) The new place is 80 degrees and 76% humidity.

New place has a different brand and type of unit. It also, however, has a whole house dehumidifier. I'll try to let you know how that works out. It just got installed in the last 5 days or so. House isn't quite done, so air flows pretty freely inside and out. (No door hardware installed yet.)

 

[BigGG]

So in what I assume is the hottest days/months of the year for you, calling is about to be achieved with stage 1 cooling.

This generally means (and very common unfortunately), that your system is oversized and performance will suffer during lower load conditions.

Ideally, on a design day for your area, the unit would struggle to keep up at Stage 2 cooling, running that same 80-100% during the peak load. That way the unit could run longer at part time load and better manage humidity (stage 1).

The 4T (2-stage) + mini-split I considered would probably have been optimum. I agree with what you wrote. My experience suggests that most new units installed in my area are oversized.

I just hope that the efficiency on stage 1 is >= stage 2.

 

[Yander]

I'm about to find out. The system in my previous (still sleeping here until Thursday) house had true variable speed and runs at a very low speed to keep humidity in check. Works great here.

The new place is ~150 miles east in the Piney Woods. The temp RN in old place is 85 degrees and 60% humidity (outdoors, 6:30 am) The new place is 80 degrees and 76% humidity.

New place has a different brand and type of unit. It also, however, has a whole house dehumidifier. I'll try to let you know how that works out. It just got installed in the last 5 days or so. House isn't quite done, so air flows pretty freely inside and out. (No door hardware installed yet.)

Man you got a new tractor and a new house within a year of each other.

Was the new house part of the buying the tractor deal with the wife .

If so, wasn't that quite an expensive tractor?