A few questions:

- How many sqft is your home (and do you have a lot of windows?)

- What was the old unit? Was it the same size (3 ton?) - what was the age on the old unit? Or?

- What's the model # of your outdoor unit, and Air Handler?

- If it was the same size as the old unit, did your HVAC contractor perform a Manual J/D calculation?

Your HVAC system needs to be designed for the range of outdoor conditions in your climate zone. Run times will vary accordingly. An 8-minute run time and 27-minute off time are both shorter and longer, respectively, than I would expect for the stated outdoor conditions. I'm surprised you're able to drop the indoor relative humidity as much as you can.

The minimum indoor design temperature for cooling per code is 75 F. You may need to decrease the setpoint temperature on your thermostat to 75 F to get closer to your desired 50% indoor relative humidity. That will also extend your run time.

@G. The old unit was an off brand made in Texas, also 3 ton, over 30 years old. Near the end of its life it barely cooled our 2000 sqf brick ranch with lots of large windows, including a sunroom with 3 side glasses and 4 skylights on its roof. The contractor did the calculation after taking measurements of house footage, duct, window sizes and came up with 3 ton. Although he cautioned me that the sunroom may need its own split ductless heat pump. Outside unit model #24acc636a310 Evaporator coil #cnphp4221alaeaaa

@Charles Ross Homes.

I'm surprised you're able to drop the indoor relative humidity as much as you can.

Is that because the house is quite airtight, no leaks in ductwork and the contractor installed a large capacity N-type evaporator 3.5 ton matching with 3 ton condenser unit, so it is able to draw more water?

The minimum indoor design temperature for cooling per code is 75 F. You may need to decrease the setpoint temperature on your thermostat to 75 F to get closer to your desired 50% indoor relative humidity. That will also extend your run time.

That makes sense to lower the set point temperature on the thermostat to let it run a bit longer for better humidity control. I’ll try that on next opportunity when it’s hotter outside. Right now it’s 72 F inside the house.

Another point is that I took the recommendation from the contractor to add more attic insulation a week prior to the HVAC installation. He said extra insulation there could effectively give me 3.5 ton system as he did not want to go higher than 3 ton for the outdoor AC unit.

"He said extra insulation there could effectively give me 3.5 ton system as he did not want to go higher than 3 ton for the outdoor AC unit."

Insulation does nothing to increase or decrease the capacity of an HVAC system. A 3-ton system provides 3 tons of cooling capacity. What adding insulation does is to reduce the heating and/or cooling load the system needs to meet.

What was the old insulation value and what is the new value? Did the contractor use the new insulation value in the calculations?

Mike, it went from about R15 to R38 approximately. I don’t know if he factored it in as I didn’t decide at the time we spoke.

You did the right thing to upgrade the insulation to R38. I think what the contractor was trying to say the increased insulation reduced the cooling load by 0.5 ton. This is something the Manual J calculation can generate without too much difficulty.

You might be oversized by 0.5 ton. However you house is a one story brick ranch with lots of windows, 4 skylights, and a sun room. Given that, and if the duct work is in the attic, the cooling load is probably in the 2.5 - 3 ton range. It is possible the AC will short cycle at night, but operate with longer cycles during hot sunny days.

You have a Carrier Performance single stage 3-ton AC. At a 78 degree setting you may not get be able to go below to a 50% humidity level. Try lowering the temperature to the 75-76 range to get better humidity control.

A change from R-15 to R-38 attic insulation is a significant change to the heating and cooling loads of a rancher. Hopefully your contractor figured the change before doing the manual J calculation.

Indeed, I was glad I had the attic insulation added to raise the comfort level. In 100 degree afternoon the day after that was done, even without AC the ceiling remained cool to the touch. It was quite warm before. So now most incoming radiated heat sources are from the glass windows, skylights, from the thermal mass of the brick veneer in late afternoon, and cooking. As you both suggested, I do have an option to lower the thermostat to better control the humidity. I think the extra insulation lightens the cooling load significantly, and causes my AC running on the inverse (if there is such a term) of the typical short cycle, as Tim noted earlier of long off-time and short on-time. But at least in this situation the compressor is not on so often in an hour to shorten its life.

And what do you all think about the pros and cons of matching the 3.5 ton evaporator coil to 3 ton condenser unit that he installed? Is this a common practice contractors do when they don’t have 3 ton coil to go with 3 ton compressor unit? My contractor had to wait an extra week on shipment of the 3 ton condenser unit while they already had the condensing gas furnace and the 3.5 ton evaporator coil in their warehouse.

@palisades_ Normally an oversized evap/AH is used to get higher EER/SEER ratings. However, higher FPM/CFM reduces the dehumidification capacity of a system.

For example, with your outdoor condenser:

https://www.ahridirectory.org/NewSearch?programId=68&searchTypeId=3&productTypeId=3500

Look at the differences in SEER/EER ratings when using different indoor evap/AHs. (Here is your equipment)

Personally, to me, your equipment is probably oversized by a ton. Need a HVAC engineer to do a proper field Manual J.

So you have a couple of options:

- Dedicated dehumidifier like a Santa Fe

- Something like a Honeywell IAQ Vision, which has dehumidiy on demand. (it's one of my favs).

- If possible, lower the CFM on the air handler to increase the dehumidification potential.

Mike,

…..Given that, and if the duct work is in the attic, the cooling load is probably in the 2.5 - 3 ton range.

My duct work and air handler are under the house (crawl space). Would that change the cooling load a lot more (lesser load)?

G,

So you have a couple of options:

- Dedicated dehumidifier like a Santa Fe

- Something like a Honeywell IAQ Vision, which has dehumidiy on demand. (it's one of my favs).

I’m glad you mentioned dehumidifier. I will look your models up for future use. I have had one GE dehumidifier in the sunroom (lots of plants), and one in the living room that I usually run during mild weather periods (between summer-fall, spring-summer) when AC or furnace is not running. I used dehumidifier more often during this past summer as well because my older AC unit was on its way out. It’s surely a good option.

- If possible, lower the CFM on the air handler to increase the dehumidification potential.

I will look at the control board of AH to see which jumper speed he set it. The evaporator installation manual recommends minimum 350-400 CFM/ton to avoid icing. So the minimum would be around 1050-1200 CFM for my 3 ton AC.

Normally an oversized evap/AH is used to get higher EER/SEER ratings. However, higher FPM/CFM reduces the dehumidification capacity of a system.

I see. Thanks. That’s a cool link to AHRI directory site.

The contractor told me this setup would give SEER 16 so he must also set CFM high to achieve SEER 16? And the AHRI Certificate you did for me gave SEER 14.5 (no CFM info) - am I missing something here?

If possible I’m ok to trade off a bit of SEER for lower humidity, by lowering CFM [within range 1050-1200] of the evaporator coil.

Among other shortcomings, calculation of a SEER rating doesn't consider dehumidification. You can boost system efficiency by pairing an evaporator with a larger nominal capacity than the condenser unit (i.e., oversizing the evaporator) and end up with a more efficient system that doesn't adequately dehumidify. That's one reason I recommend matching the capacity of the evaporator and condenser unit.

The first consideration is getting the system-- as designed and supplied by the HVAC contractor-- to perform as needed. That should be their job--not the OP's.

Supplemental dehumidification may be needed during "shoulder" seasons when there is little, if any, call for air conditioning. I have two Honeywell dehumidifiers integrated in my HVAC systems which serve that purpose--one for each level in my home. They were not a small investment, but they are cheap compared with the cost to deal with mold and mildew which is common in my (coastal VA) climate zone. The Santa Fe units would be my choice if the budget allows.

My duct work and air handler are under the house (crawl space). Would that change the cooling load a lot more (lesser load)?

It doesn't change the cooling load. But it does affect duct loses. Ideally all duct work is installed inside the conditioned space to minimize losses. The SEER testing does not take into account duct losses. Up until this year the static pressure used in the testing is 0.1 which is unrealistic. It is being changed to 0.5. In 2023 you will see a SEER2 rating which reflects a more realistic static pressure of 0.5. Even that is optimistic for homes with 5 ton condensers.

The AHRI certificate shows a SEER rating of 14.5. I don't know why the contractor is telling you it would be 16 SEER. Changing the air flow may have an effect on overall efficiency. You can rest assured the test engineers at Carrier will adjust the air flow to make the SEER rating as high as possible.

What is the location of the house? We can look up the summer and winter design temperatures.

Did the contractor discuss a 2-stage condenser and air handler? The 3.5 ton coil size is fine. It is an allowed match on the AHRI website. We can debate whether a 3 ton coil would give a better dehumidification. The size of the condenser and the run time play a bigger role in my opinion.

"My duct work and air handler are under the house (crawl space). Would that change the cooling load a lot more (lesser load)?"

Duct work located in a hot attic tends to gain heat from the attic which adds to the load on an air conditioner. In heating mode, there is heat loss from the duct work. This is why best practice is to install ducts in conditioned spaces. If your crawl space is a conditioned crawl space that is an ideal place to locate duct work and the air handler. If your crawl space is unconditioned, locating duct work and the air handler can result in condensation in air conditioning mode which can give rise to mold/mildew growth.

My crawl space has been cool and dry, sort of naturally conditioned that way, and I haven’t seen mold/mildew under there or around the system. This condition likely helps my previous furnace, AH to last such a long time, and hopefully the same for this new installation, although now they don’t build things to last like they did back then.

Up until this year the static pressure used in the testing is 0.1 which is unrealistic. It is being changed to 0.5. In 2023 you will see a SEER2 rating which reflects a more realistic static pressure of 0.5.

I think static pressure 0.1 is more like a clean-room setting where everything is perfectly designed and precisely controlled . I wonder if CFM value would be taken at 0.5 for most residential or commercial setups, although it can be higher or less than 0.5 depending on air filter types, ductwork design and sizes, etc.

He did mention 2-stage for both which I recall the Infinity series. We’re in central NC so 2-stage could be more helpful but I read mixed reviews about them so decided on the 1-stage Performance series.

As for the SEER value, I just looked on Carrier website. This is what it says for my model#: “Performance™ 16 Central Air Conditioner 24ACC6 Up to 17 SEER for enhanced energy savings with standard comfort features”. Further searching on the nomenclature of model#24ACC6, 6=16 SEER. Obviously the matching of coil and condenser determines the actual SEER value after installation as I understand. But it is far from 14.5 SEER obtained from AHRI calculation ??? Maybe it has to do with CFM setting as well?

I have yet to check the control board to see which CFM speed he set for the AH for claim of 16 SEER. Now getting ready to tie up loose ends around the yards and garden before Ian arriving tomorrow.

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It should be criminal to not do a TESP check of the equipment after it is installed.