Improving cooling efficiency?

[MN Renovator]

Originally Posted by rvCharlie

A search for improving my a/c performance yielded this 5 year old thread, so I thought I'd try here before opening a new thread.
I'm located in central MS, the heat/humidity capital of Hades. My new (2nd cooling season) Trane 16 SEER 3 ton 'pro'-installed system is incapable of de-humidifying when outside air temps go above 90 degrees, and a bit of interweb sleuthing has revealed that yeah, R410 systems basically go on permanent break when the condenser is operating in >90* temps. Just not enough delta-t across the evap coil to dehumidify properly, unless fan speed is so low that the house won't cool. According to the 'pro' installer *and* the Trane tech assist guy, 60-65% indoor humidity is now ok. (Apparently they've issued a memo to all the mold and mildew critters, who have agreed to not grow until humidity reaches 70% now....)

Anyway, to the point of this post: I'm pondering a self-installed minisplit of maybe 1 ton capacity set at a degree or two below the existing 3 ton central system, so it will run all the time during summer to dehumidify without pumping heat back into the house like standard dehumidifiers. I'm wondering how much ground loop I'd need to 'assist' the R410 in getting a really effective delta-t across the coils. The soil in my location is well saturated with water. I hit water when I dig a 30" deep post hole. The on-line info I'm finding all seems to be geared toward cold climate using ground source for heating instead of cooling, and I suspect that most have much drier soil than I do.
Anyone have a rough idea on whether I'd really need 500 feet of pipe just to assist the air heat exchange of a cheap off the shelf unit?

What level of CFM is your current AC set to? 60+% humidity is not okay and either your cycles are too short or the CFM going across the coil needs to be reduced.

A higher outdoor temperature should not reduce the dehumidication potential, but rather increase its potential to dehumidify because your cycles should be longer, possibly running for hours at a time with a properly sized system at those temperatures. Have you checked to see which blower tap/jumper setting is selected and checked that against the blower chart in the manual? In any case, it sounds like it is set too high and needs to be reduced to drop the temperature of the coil and allow the system to dehumidify properly. Even if your sensible capacity(temperature change) is reduced in order to increase latent capacity of the system(humidity change), the warmer temperature would be more comfortable once humidity is better controlled.

You aren't really providing enough information about the performance of the system otherwise, so I'll ask this question as a start. On a day roughly 90 degrees with full sun between 3pm and 8pm, how long does the system run for before it turns off? At what temperature outside on a sunny day does your system rise at least 2 degrees above the thermostat setpoint?

Also where is the air handler/furnace and ductwork located?

I think trying to band-aid this issue with installing a mini-split without troubleshooting the problem with the central system is an expensive hair-trigger band-aid involving some false assumptions about evaporator air flow in relation to system performance.

[rvCharlie]

To MN Renovator,

The old system was a 3ton SEER 12 Trane, single speed everything that was installed when we bought the house. At ~24 years old and after replacing the air handler control board, duct pressure sensor, condensor capacitor(s), condenser defrost switch under the compressor, condenser fan motor, etc over the years, and it starting to take 3 hrs to drop indoor temps from 74/75 daytime to 69 night (turned down around 9:30pm) I decided preemptive replacement would be a good idea.

New system ('pro' install) also 3ton single speed Trane SEER 16 (should have bought higher SEER in a cheaper brand) with air handler control board selectable speed taps, via pushbuttons on the board (no access from the thermostat that I'm aware of).

Pro installer made numerous mistakes in installation, from leaving packing/instructions in the A coil to refrigerant leaks. Multiple service calls to clear up problems, consult with Trane engineer (Trane refuses to talk to anyone except installers), fan tap slowed at least 2 increments, might be 3, below default speed. Installer's supervising rep finally claimed that his own system is running at 60% humidity; showed me his phone app to prove it (same thermostat as mine). Tried to convince me that it's 'normal'. (Obviously, it's not.)

Air delta-t (I've got my own probes in the return and supply, in their probe holes) is now 22-24F until outside temps get into the 90s, then it drops to around 18 degrees. Before *I* changed the fan speed, it was roughly the same as the 24 year old system; around 14 degrees.

Entire system is under the conventional foundation with about 90% vapor barrier coverage. Condenser is always shaded. I'm running a somewhat undersized 'basement' dehumidifier that typically shows around 60-63% humidity and mid/high 70s for temp.

House: 2" of foam insulation under metal roof; attic typically stays well under 120 and there's 12" of celulose in the attic. Low-E insulated windows (lots of them), 2" foam under fiber cement siding, etc on 1953 constructed house, cooling about 2000 of the 3000 sq ft floor space. Air leaks are almost certainly my biggest enemy since the house is such old construction with only the updates listed.

No good answer for you on run time during the day (usually keep thermostat on 74 during the day, as long as we can keep humidity around 50% with the dehumidifiers), but it never runs continuously, even on the hottest days. I suspect that it's a bit oversized for current insulation levels, but the 'pro' didn't do any calcs; just replaced with same size unit.

Everything I've been able to find about R410 systems vs the old R22 systems indicates that they really struggle when outside temps get above 90 degrees. I'm happy to listen to any input about that being incorrect, but that has been my experience with this system.

I *have* set fan speed even lower than it is now to get delta-t even better, but I then ran into it taking 3-4 hours to drop temps from 74 to 69-70 at night, just like the situation with the 24 year old R22 system. It seems obvious in hindsight that I should have bought a variable speed system (and self-installed), but it's what I've got now. I can 'bandaid' on a DIY basis for under $1K, and learn a bit about my personal last frontier, refrigeration, in the process (already acquired the tools). That's what brought me to this point; if $1k and some free labor will eliminate the dehumdifiers in the house and leave only 'heavy lifting' to the 3ton single speed, I'll be ok with that.

If exact model #s of condenser, coil & air handler would help analysis, I'll dig them up.

Thanks to all; I appreciate the help.

[rvCharlie]

To 3b3rg33k,

Air handler/furnace is Trane model S8B1B060M4PSCAA; no provision I can find in the install manual for any external control over fan speed; only the on-board pushbuttons. (Tempting to try to 'hack' that to allow external control; the motor leads are clearly labeled on the control board. But the thing is still in some semblance of a warranty, so not going there yet.)

Thanks for the tip on 'dry mode'; I'll look into that when selecting a unit. By 'dry mode', you you mean dehumidify only, pumping some residual heat back into the living space, or that it simply maintains the coil at colder temps to dehumidify more while cooling?

Still learning HVAC/refrigeration terms...

[MN Renovator]

I'll make a few snips to discuss.

Originally Posted by rvCharlie

Air delta-t (I've got my own probes in the return and supply, in their probe holes) is now 22-24F until outside temps get into the 90s, then it drops to around 18 degrees. Before *I* changed the fan speed, it was roughly the same as the 24 year old system; around 14 degrees.

14 degrees tells me there was way too much airflow for the cooling the condenser was providing to the evaporator. The 22-24f is within the recommended range for sure, I can expect that if you are dealing with high latent removal from high outdoor dew point coming through air infiltration that you'll be seeing less temperature drop across the coil along with reduced output from the condensers higher temperature. 69 degree indoor night setpoint with an 18 degree delta T should mean that the indoor dew point when the system shuts off is about 51 degrees, that's about a 53% relative humidity, which might be brief if the air infiltration continues and things inside the house release any held moisture. On the sub-90 degree days 22f temp drop and 75f indoor temps would be about a dew point of 53f or about 47% relative humidity when the system shuts down, of course that will rise until the system fires back up again. Lowering the temperature of the house will reduce the dew point further but there's an added energy cost to doing that.

Usually a deltaT of 19-25f across the evaporator coil what the aim is, higher than 25f and efficiency and issues such as coils freezing can become a thing. I don't know your system though, the manual for the air handler/furnace, the blower setting, and water column pressure measurements would give you an idea of the airflow of the system. I don't know if you have more headroom to drop blower speed further or not.
My own house has undersized ductwork and even with a brand new high flowing filter and every vent open an unobstructed, I can't get below 25f with my 1985 furnace that was only made for 2 tons of airflow maximum, but I'm not quite getting there, I've even seen 29f deltaT on my system which is just not enough airflow but the system seems to survive. The house was built with a furnace and the AC got installed the next year by a different company and I'm oversized for the ductwork and the furnace just can't handle pushing more than .5" water column of static pressure. I'll be downsizing my AC when it gets replaced but my house is also not too big, our days are slightly shorter, and our design load here is 88f. I don't have low-E glass yet because my windows haven't rotten out yet, but when I do the AC will need to be smaller or I'll have a humidity problem too because the AC will be running less with the reduced heat load through the glass. I've done some extensive air sealing though, so if the windows need to be replaced, I'll probably continue to ride things out with the same AC if it's still kicking.

Air sealing the penetrations in the house with the assistance of a blower door test and thermal imaging that one of your gas or electric utility companies might provide at a discount might be helpful in reducing the moisture load entering the house because apart from cooking and showers, like you said it's probably majority an air ingress issue bringing in the moisture. Your reduced cooling load from lowE glass also means you have less cooling to do which increases the latent(moisture) load relative to the sensible(temperature) load.

I *have* set fan speed even lower than it is now to get delta-t even better, but I then ran into it taking 3-4 hours to drop temps from 74 to 69-70 at night, just like the situation with the 24 year old R22 system. It seems obvious in hindsight that I should have bought a variable speed system (and self-installed), but it's what I've got now. I can 'bandaid' on a DIY basis for under $1K, and learn a bit about my personal last frontier, refrigeration, in the process (already acquired the tools). That's what brought me to this point; if $1k and some free labor will eliminate the dehumdifiers in the house and leave only 'heavy lifting' to the 3ton single speed, I'll be ok with that.

You are removing more moisture from the air with lower airflow so you can expect it to take longer for the air temperature in the house to go down because the capacity is more geared to reducing that latent load versus sensible load, it's called the latent sensible heat ratio.

I don't consider that to be a problem, but just a characteristic of how an air conditioner operates. If you want it 69 degrees when you go to sleep and it's among the hotter weeks of the year, either reduce the setback so it will not need to run as long when night comes or have the temperature setpoint on your thermostat happen an hour or two earlier to allow it more time to do its work and be comfortable for you when you want it colder. A 5 degree temperature drop in my house on a day when it was 90 at 2pm is probably about 4-5 hours and that is assuming the sun has set at least an hour before the time it turns off.

If anything, the longer runtime while recovering from the setback is probably helping your situation because the long uninterrupted cycle isn't giving any opportunity for the coil to evaporate its moisture every time it would otherwise turn off and the coil is colder for the longer period of time.

[rvCharlie]

Again, sorry for the long delay in replying. Preventing boredom in a visiting 17yr old grand kid is a full time job....

I agree with all the stuff you've written, but my current issues seem directly tied to R410's loss of efficiency with 90+ outside temps, vs the old R22 system. As I mentioned earlier, when temps are in the 80s the system easily handles humidity along with temps. It's only when the OAT gets in the 90s that humidity handling goes down the toilet, and that seems directly related to the delta-t going from low/mid 20s to mid/high teens. We never had the humidity issues with the old (same tonnage) R22 system, even though the old furnace/air handler was massively oversized and was moving a *lot* more air than this one (old furnace/air handler was around 100k btu, sized for uninsulated structure with single pane aluminum frame windows, vs new 60k btu furnace/air handler). Both old and new systems operating with improved insulation, new high efficiency windows, etc.

Given where I am at present (money spent; inefficient new system in place), my thought process is that for the price of a whole-house dehumidifier I can self-install a 1-2 ton mini-split, configured/ducted just like the dehumidifier would have been. If it's configured to maintain 22-25 degree delta-t and runs all the time (perhaps in 'dry' mode), it should use a lot less energy than the big 3 ton 16 SEER compressor that I paid too much for. Likely no more energy than a whole-house dehumidifier would use. Then the 3 ton system would only come on line when the mini-split couldn't keep temps at desired level.

[jeff5may]

If you are trying to dehumidify in the hot wet summer, look into rigging up a desiccant waterfall setup. I messed around with the setup in a previous house and it did the job well. Think of it as a reverse swamp cooler: indoors a little fountain pump circulates saltwater (brand name damp rid, I used road runner calcium) and it grabs humidity out of the air at room temperature. A simple system merely has an overflow drain leading to a bucket, but I ran a drain hose.

Outdoors, a solar desalinator disposes of the indoor humidity and regenerates the saltwater to refill back indoors. Strong saltwater goes back inside to move more harvested humidity outside. And Bob's your uncle.

Sounds too easy to be true, but believe me, if it didn't work well they wouldn't sell many damp rid Kleenex boxes of salt.