HVAC Cooling Performance Questions

[Coolbreeze]

I have been in my newly constructed home about 18 months. There is about 2115 SqFt downstairs and 985 SqFt upstairs (3100 total). A single Lennox 5 ton model 14ACX-060-230-16 A/C compressor is installed. It is a split system with a bypass. The upstairs is not used much (all bedrooms are downstairs) so I set the thermostat to 81° to save a little energy. Downstairs, the thermostat is set to 78°. I have done some data collection and found that in about 3.5 hours (3:00 to 6:30) on a warm (92°) sunny afternoon, the upstairs will cycle about 16 times and the downstairs will cycle about 11 times. On average, the upstairs cycle is 4:15 on and 9:45 off (m:ss) and the downstairs is 5:10 on and 15:20 off.

So here are my thoughts:
1. Is seems to me, the "on" times are too short. No sooner than the system reaches peak cooling efficiency the system shuts off.
2. There are many times throughout the day that the system is running solely to cool the upstairs. That is effectively a 5 ton unit unit cooling 985 SqFt. Using the rule of thumb (yeah, I know about rules of thumb) of 600 SqFt/ton, that is 3 times the size needed. Seems to me this could be part of the reason the cycles are so short.
3. Also, there are times throughout the day that the system is running solely to cool the downstairs. Now it is a 5 ton unit cooling 2115 SqFt, which is 1.4 times the size needed.
4. Occasionally, both the upstairs and downstairs are calling for cooling at or near the same time but, during these times, the "on" times are only slightly longer.
5. The humidity in the house feels high. It is running in the mid to upper 50s most of the time (except when the outside humidity is low - not very often here in NW South Carolina)

So I guess my questions are, is my system performing acceptably? If not, what type of corrections/adjustments can be made (by me or an HVAC expert)?

Thanks in advance for your help.

Bill

 

[SShaw]

You need an accurate "Manual J" load calculation for your house to see what size AC is required. It sounds like yours is oversized though. I expect the design temp would be around 95 in most of SC. The system should run almost continuously in the evening if the outside temp is at or above the design temperature for your location.

Keep in mind that the typical design indoor temperature is 74 or 75 degrees, so running it at 78-81 will make the unit even more oversized for your conditions.

If you lower the inside temp to 74 (or lower) on both zones the unit will run longer and dehumidify better.

If the unit is oversized, I don't think there's much you can do to correct it, other than replace it with the correct sized unit. You might be able to have a technician reduce the fan speed a little.

A two-stage or variable speed-system would work better with two zones. Two separate systems would work even better.

I have a single 4T variable-speed A/C with two zones in my 2,900 SF house in VA. It peaked at 92 outside today and was 80 and 85% humidity at 10:30 PM. My system's been running non-stop since 11:30 AM and will log about 16 hours of runtime today. Downstairs is set at 72 and upstairs is set at 76.

 

[Sponsor]

 

[WorthFlorida]

You are way over sized. I have a home in Florida, (hotter and more humid), two story built in 2007 with a radiant barrier installed on the attic roof rafters with a 3.5 ton heat pump. It is sized just right. Most of the time in the hot months like now, I keep the thermostat at 77 or 78 degrees. Winter months I have to lower it to 75 just to keep the humidity down. Many companies will oversize just to keep the complaints down. For those few extremely hot days the AC may not be able to cool the home down sufficiently by what homeowner expect and that is when people complain. They expect a $5,000 AC unit to cool perfectly all the time.

You should be using a smart thermostat and most do read the ambient humidity. Honeywell sets theirs at 60% maximum and it sends an alert email of high humidity. I then lower the AC temp to get a longer cycle time to dry out the air.

Contrary to what many believe, you cannot size the second floor load based on sq footage alone. Heat rises so the second floor unit will be getting a lot of heat load from the first floor space. I cannot say of there is an equation for it but my last house was also two floors with two AC units. I had to keep the second floor thermostat set 2-3 degrees higher so the first floor unit would run more and take its share of the load. If both were set to the same the second floor 2 ton unit would run all day long. My current home is one unit with the air handler on the second floor. Works quite well.

 

[Dana]

Measuring the duty cycle during the hours when the outdoor temps are near (or even above) the 1% outside design temp for your location is effectively a measurement of the cooling load, even if you're keeping the upstairs at 81F instead of 76F.

Rules of thumb such as "a ton per xxx feet" are just plain stupid. A ton per 600' is usually enough to cover the 1% design loads of an uninsulated air-leaky tar paper shack with single pane windows. Hopefully your new construction home is built better than that. A more realistic whole-house rule of thumb would be a ton per 1500', but there are NO rules of thumb that come anywhere near working on a zone by zone, floor by floor basis, since the cooling loads of lower floors will have less load per square foot than upper floors, since the upper floors tend to have less favorable shading factors, and have the additional hot surface area of the roof above.

This graphic is the square foot per ton ratios of a few dozen Manual-Js performed by an HVAC consulting company in GA (some new construction, mostly already built homes.)

If you look at the cluster around the 3000' home size you'll note that even the very WORST performing house in that size range had a ratio of about a ton per 900' (not 600'), and the average was about a ton per 1500'. A 2 or 2.5 tonner could probably handle your load better than what you have now. A 3 tonner wouldn't necessarily be insane, but 5 tons is pretty ludicrous, especially given your measured duty cycles.

Running the load math ahead of time is the only decent way do deal with it. Rules of thumb must DIE!

 

[Fitter30]

Model of thermostat to what the set up is. Model number of the outside units.

 

[JerryR]

We put a 5 ton inverter system in with 3 zones in our 3000 sq ft Florida House. It runs at 25-50% of capacity most times. The RH is usually in the mid 40s. We actually keep the thermostat at 78-79 degrees during the day and turn down the bedroom zone to 75 at night and rest of house at 82 degrees.

Right now it’s 85dF and 63% RH outside and it’s a comfortable 79dF and 47% RH inside. The AC is running at 40% of capacity and air handler is outputting 1080 CFM (max 1800 CFM)

We don’t hear any AC or air rushing noise.

Then there’s our 1500 sq ft Florida vacation cabin with a 3.5 ton single speed system. My wife hates it. It goes from hot to cold. The volume of col air in the kitchen when the AC turns on freezes he out then when it turns of. She complains it’s getting too warm. The humidity struggles to get below 54 RH with short cycles

 

[Dana]

The modulated of output of inverter systems gives a bit of leeway and can offer GREAT comfort when either the turn-down ratio is large, or it's not too oversized. Sizing is still important with modulating inverter systems, but paying closer attention to the MINIMUM capacity is more important for comfort than it's AHRI "rated" capacity, or it's maximum capacity. Some only have a 2: 1 turn-down, and 2x oversizing delivers about the same comfort as a right-sized single speed. Others have substantially more range.

Most of Fujitsu's AOU/ARU xxRGLX "mid static" mini-ducted mini-splits have a 4:1 or better turn-down. They come in sizes from 1.5 to 4 tons- the 3 tonner can throttle all the way back to 9600 BTU/hr @ 95F outside/80F in, but can ramp up to 39,000 BTU/hr (more than 3 tons) at that temperature, a 4:1 turn down. The 4 tonner ramps from 17K to 50K @ 95F out/80F -in, a ~3: 1 turn down.

Some of Carrier's Infinity w/ Greenspeed series heat pumps have comparable or better turn-down ratios though some are only ~2:1- it's highly air-handler dependent. Carrier also has a series of multi-ton ducted mini-splits (re-labeled Midea units) with high turn down ratios.