Simple air conditioning question:
@5 years ago my ex replaced the outside air conditioning compressor. It was replaced by a ‘friend’ of hers that was starting a heating/cooling business. The new compressor is about 3x’s the physical size of the older unit. Looks like it belongs on the roof of a larger office building.
My house is @ 3,200 sq feet. (I inherited this house as part of divorce) I noticed – now that the air is on – that the unit cycles about every 10 to 15 minutes. My belief, and please correct me if I am wrong, is that the unit has too much capacity and hence is not on long because of its too large capacity.
From what I have read here that is bad – it does not remove the humidity from the house as it should and its constant on/off is hard on the unit itself.
Is there any way I can change the capacity or the cycle time or the thermostat characteristics to modify how it functions short of replacement?
There is no name on the side of the unit and no instruction booklet and no id number I can find. It stands 4″ tall and is square. My old model was 2x3x3.
Thanks, Mike C.
Replies
Actually, if it's only four inches tall it's a pretty small unit. ;)
The frequency of cycling isn't as significant as the % of time the unit runs on a hot day. On the hottest days it should be running maybe 80% of the time.
There has got to be a nameplate on the thing somewhere, usually down near where the pipes enter. If you can find the brand/model it's POSSIBLE that an HVAC guy can figure out a different A coil or some such that will SLIGHTLY reduce the overall capacity. But probably by no more than 10-20%,
Thanks Dan - you're right it is a bit taller than 4 inches. 3 ft, 8 inch. taller to be exact. I will look tonight when I am home and see if I can find some indication of who made this monster and what the stats are - then I can write back.Mike C
Uhh, a smaller capacity indoor coil compared to the outdoor unit should never be installed.
You send liquid back to the compressor if the charge isn't set right.
Shorting the charge to "make it work" significantly reduces efficiency.
It works within limits, but you've got to have the condenser unit specs to see what those limits are. They'll often list different capacities with different A coils.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
None of the specs I have list a outdoor condenser with a *smaller* capacity indoor coil.
There are listings for combinations like a 24k btu outdoor unit and a 42k btu indoor coil, though.
Sizes of the newer condensing units are bigger in order to meet the higher SEER ratings, i.e., more surface area = better efficiency = bigger units. But, the fact that you hear it cycling every 10-15 minutes probably means you have an over-sized unit with its problems of poor humidity control and excessive wear. Oversizing is the perennial mistake by incompetent or uninformed HVAC guys who don't do load calcs. Homeowner ignorance plays into the problem, as well.
But to answer your Q more directly, there is not much you can do to significantly lower output, as DanH says.
Is the house cooling throughout? Or is it too hot in some rooms, like bedrooms farther from the t'stat? You might be able to modulate the temps a little better by simply having the blower run constantly, not just when the unit cycles. The blower will help average out the temps. Re-locating the t'stat can make a big improvement, if that's a practical option for you at this point.
Assuming that you have a system that does both heating and cooling with one duct system and one blower, the best locations for the t'stat are the rooms with the smallest difference between the cooling cfm value and the heating cfm value. Without an accurate load calc, you would have to make your best guess as to where that might be.
Many of the digital setback thermostats have a cycle setting them.
Now what that can do is to force the unit to run longer at a time, but you will get larger temp swings in the house.
William the Geezer, the sequel to Billy the Kid - Shoe
Thanks for your responses. I have a 'little' additional information. It is a digital thermostat - installed this winter when I had to replace my furnace. For furnace size the HVAC people did a calculation and determined the correct size. I kept the old air conditioning unit.A small tag on the very back and bottom of the unit gives the following info:Standard
5/99 Mfg Date
240V 38 amps
HCFC - 22
7B0060C100A0Bedrooms are considerably warmer than the basement or first floor. In fact there is a noticable temp gradiant between all floors. Thermostat is on the first floor and cannot be moved - at least without a considerable expense.I will try the blower on at all times solution and see if that helps. If you have any further suggestions let me know.I loved the line - "Homeowner ignorance helps as well". I will pass this along to my ex - NOT!Mike C.
Did they do a load calc, or the "look at what's there" calc?
Temperature gradient between floors is indicitive of crappy ductwork incapable of moving the required air and/or leaky ductwork letting air out everywhere *but* where you want it to go.
Do you have a powered attic ventilator? If so, turn the dam thing off. All they do is depressurize the attic and suck the conditioned air out of your home. Try it. Would you leave the windows open with the AC on?
If you have a pleated filter on your furnace, try pulling it out and put a fiberglass one in there. I bet you will notice an improvement. Then turn off that attic ventilator.
I have yet to test a system with a pleated filter that is capable of moving enough air.
You can have a wireless thermostat installed and put it anywhere.
Load calculations are good, but still give bogus numbers. I installed a 2 stage 16 SEER heat pump in my house with a 95% variable speed furnace. The calculated cooling load is 30k btu's at 93*F outdoor and 77* F indoor design. My thermostat allows a dehumidification function with a variable speed blower. The AC has yet to (1) stay on all the time even at 90+ outdoor temps and (2) run on the second stage.
My house has the original R9 in the walls, single pane windows, and maybe R19 in the attic.
Edited 6/25/2009 8:13 am ET by danski0224
That is a 5 Ton (60,000 btu/hr) American Standard AC unit, 10 years old.
You have a 3200 SF house with 5 tons in Michigan, unless your house is uninsulated, I would say its oversized. For a house in the upper midwest with decent to good insulation, you can easily cool effectively with 1 ton/1000 sf, with good air distribution. I have about 1500sf/ton and it works quite well.
Personally I disagree with some of the posts about percentage runtime and cycling. On a 95 degree day, while the sun is shining, I want my AC to run continuously (and it does). If it gets to 100 degrees in N.IL, I'll be happy if its 80 and 45% rh in my house because 85 degrees outside and 95% rh is much more common conditions for the midwest.
One easy way to lower the capcity and raise the dehumidifying capacity (actually lower the sensible/total capacity ratio) is to lower the speed of the furnace/air handler. On many units, this is simple. On some, practically impossible. If it is a variable speed unit, blower capacity is set with a set of dip switches. If it is a multi-speed unit, there will be taps on the fan control board, maybe.
Need to find the same data plate for the inside unit (make, model number, year of manufacture). I was able to pull up a data sheet on your condensing unit, I should be able to find one on the air handler, too.
i don't disagree that his unit is plenty big. but if that 3200 is in a 2 story,that second floor makes it work a little harder. i don't see that his unit is so big it needs jerked and replaced. it's a good unit, guessing a 12 seer,i'd leave it.
now as far as a how often it cycles,to me thats house function. if i cool 2 houses to 73 degrees,shut the units off.
one has no insulation and the other is insulated like a refrigarator with foam ,great windows etc.
in 15 minutes the one poorly insulated house will be to 75-76 the other may still be sitting at 73.no need for the air to kick on. YOU ONLY NEED TWO TOOLS IN LIFE - WD-40 AND DUCT TAPE. IF IT DOESN'TMOVE AND SHOULD, USE THE WD-40. IF IT SHOULDN'T MOVE AND DOES, USE THEDUCT TAPE.
Many dynamics affect how quickly the thermostat will be satisfied and how long it will stay that way. Setbacks, thermal mass, construction, siting, shading, wind, control deadband(s), air distribution effectiveness AND appropriate capacity being among the few that come to mind immediately.
heres something that happened to me when i built our house.
put in a good honeywell thermostat. holds within 1 degree +-.
i fall into the category of half way well insulated. it was driving me nuts,set the thermostat at 73,click digital sense 74,system on. runs about 5-8 mins ,thermostat hits72 ,off. 6-10 minutes same song.
i bought a old style honeywell [round] it has a 2 degree swing. today it's 98,rh in the 60 % it will kick on about every 20 mins,run 20 and shutoff.
so look at your thermostat,see if it's trying to hold the temp in a tight range.
YOU ONLY NEED TWO TOOLS IN LIFE - WD-40 AND DUCT TAPE. IF IT DOESN'T
MOVE AND SHOULD, USE THE WD-40. IF IT SHOULDN'T MOVE AND DOES, USE THE
DUCT TAPE.
What I call the throttling range of the stat (old Honeywell pneumatics stuff) ... the difference between cooling coming on and when it shuts off. Some stats have an adjustment ... spread it out a bit to avoid the frequent cycling due to a tight/small throttling range. Normal might be like 3-4 Fdeg, I think. MAYBE 5, but that may be a bit much. If you have a 4 Fdeg throttling range, the setpoint is in the middle; temp goes up 2 Fdeg and it kicks the unit on until it senses 2 Fdeg below the setpoint and kicks off.
It's time to actually call in an HVAC guy... or three... to diagnose your system. I agree it's probably the wrong unit, but a good quality rep will know what to do.
Tu stultus es
Rebuilding my home in Cypress, CA
Also a CRX fanatic!
Look, just send me to my drawer. This whole talking-to-you thing is like double punishment.
Where is the thermostat relative to (1) supply air registers and (2) returns? It should be 1/2 way between the floor and ceiling in the room in which it it located and near a return, away from supplies. If old you could replace with one of the Honeywell VisionPro units which are excellent.
All of that being said, the remainder of the comments about it being oversized are probably correct.
Jeff
Thanks for all the responses. The house is @60 years old. New Anderson Replacement windows 4 years ago. Insulation is probably mid-line. 4" studs with batt insulation. The thermostat is in the LR and is a new - last year with replacement furnace - Honeywell. Rec from furnace installer. It is halfway up the wall and near the largest return duct in the house and 10 to 12' away from any register.The new furnace is a Payne with a variable speed fan. At least when it was in 'heat' mode the second speed rarely was utilized. Seemed only came on when very very cold outside and lots of traffic in/out of house.I live in Michigan. Today 90+ degrees. Probably @ 80% rh. It cycles like clock work but then again in the winter my furnace did the same thing - cycled on/off every 10 minutes. Dealer told me that was as it should be. I could try an older round mercury switched thermostat if everyone thinks that is a good idea. Mine is a 1+/- degree now and the additional 2+/- that an older style offers might do the trick. Do they even sell those?Myabe my best bet is to call an expert. Any suggestions on how to locate one? My furnace installer turned out to be a disaster. Good price but call backs were almost impossible.Mike
Most likely you can do the same thing with the Honeywell by adjusting the cycle control.Some of the "pro" models you need to get into a setup mode to change it..
William the Geezer, the sequel to Billy the Kid - Shoe
Nobody mentioned it yet, but may not be something you feel comfortable with doing?
Easiest way to drop the capacity (assuming charge, etc is OK) would be to go to grainger or the web and buy a variable frequency drive for the compressor (a few hundred $$).
Run the unit at half speed, vary to suit. Your COP will also rise dramatically and save you $$$.
I'm not a professional HVAC person, however, you may also want to have the refrigerant measured. Too much & too little are both hard on the system.
I agree ... improper charge and your system isn't going to work well. A colleague of mine and I've heard this MANY other times ... improper charge is VERY common ... particularly split systems charged in the field. Not sure how to ensure that the guy you get in there will give you the right charge ... because that is the common problem in the first place ... the contractor not doing due diligence to ensure the proper charge.
So, the outside unit was replaced and the indoor coil remained?
How big is the indoor coil?
5t condenser + 4t indoor coil = bad.
Odds are, your ductwork will not move the air required for a 5t unit to function properly... especially if there is a single filter on the side (a 5t unit should have a filter on each side, or the furnace is set on a box allowing a large filter on the side). If you are using a pleated filter, stop now and remove it, and replace it with a plain old fiberglass filter.
Slowing the airflow will reduce efficiency as long as the original system is capable of moving enough air. BTU's delivered is a function of temperature difference (latent for AC) and airflow. Residential systems need 400 cfm per ton to work properly. If your ductwork is small, then reducing fan speed may help because a residential fan is incapable of developing much static pressure, so it actually moves *less* air while consuming more power... but there is a point where the airflow drops off and the coil freezes.
Essentially, a 5t blower on high may functionally deliver the same airflow on medium and consume less power due to duct restrictions.. but that does not *fix* the problem because you are under the 400 cfm per ton guideline.
The ac probably isn't charged right, either. Anyone measure superheat and subcooling?
Unfortunately, the best solution is to replace the whole system.
Keep in mind that R22 equipment is being phased out next year, suppliers are not stocking much now, and deadlines for ordering some stuff may have passed.
Yes, a new system costs money.
So does cycling a 5t compressor and indoor fan. That cycling also reduces equipment lifespan.
"New" 410A equipment needs to be installed properly or else it will have a short life... but it lasts the 1 year warranty so they are off the hook. Watch out for the low bid guys and make sure they use nitrogen while brazing and use a micron gauge to measure vacuum.
"Residential systems need 400 cfm per ton to work properly."
This is wrong. Stupid, blind, rule of thumb garbage. You should know better.
Seldom, if ever, does a residential system perform at nominal ratings. When the do, the latent removal by the evap coils is terrible. In the midwest (where you work) 350 to 380 works much better for lower s/t and is more common than not. HVAC enineers that use 400cfm/ton are generally thought of as ROT (rule of thumb) idiots. I know you're not an engineer. Slowing the airflow will not reduce the efficiency by any significant amount. It will reduce the capacity and will do so at a lower energy consumption.
Ok, so who is going to take the time to engineer a residential system? If you take the time, who pays for it? Plenty of residential work involves 4 and 5 ton units with one 16 x 25 filter. Lots of low bid "engineering" there.... Worse when the homeowner slaps one of those MERV 20 filters in there...
Yes, lower airflow = more latent removal. Works great on my 2 stage AC with a variable speed blower.
Improperly charged AC systems with low SH or high SC don't work as designed, either. "Beer can cold" doesn't cut it. Charge them right and they will pull out the humidity as designed, all else being workable.
Last time I looked, plenty of residential specs are based on 400 CFM per ton (2000 cfm / 5 tons = 400 cfm/ton), so it isn't totally rule of thumb. Only the variable speed systems are close to capable of actually delivering the CFM you set them up for and are capable of fine adjustments.
Yes, slowing airspeed reduces capacity. Delivered BTU's involves a temperature difference and multiplication by measured CFM and a constant. Lower energy consumption can be true as long as the AC is oversized to start with. If the AC is sized right, how can you have lower energy consumption when the unit runs longer to deliver the required BTU's?
Edited 6/25/2009 9:26 am ET by danski0224
This is clearly getting interesting. I understand about 40% of what has been discussed and so let me add a few facts:The furnace is less than a year old. It is a 97% eff 85,000BTU Payne. The blower - according to Schaafsma H/C is twp phased.I do use pleated filters - they told me that was a necessity. Should I use something different in the summer? The filter is located on one side of the furnace - just one filter - and is 20x20.The coil is the original coil I assume - never paid for a replacement. The 'old' AC unit was probably a 1 ton??? It was much smaller and ran on 120V. I had to have the house rewired for 240V when this unit was installed. Actually my ex did all of that BEFORE I inherited this cash black hole.I suspect the duct work is in fact inadequate. At one time - many years ago - when the basement was redone a mechanical engineer wanted to put some separate fans in the duct work controlled by butterfly switches to boost eff or air supply to upper BR. Never done and cannot be done now without destroying a lot of new drywall in basement.I do operate an attic fan that is heat controlled - on at 95 degrees. Attic is unused space. Maybe I should build a new house - boy would I love to if I was in debt for this house that I cannot sell in this market.Mike
Turn off the attic fan.
Take out that pleated filter. If the furnace isn't on a box, then part of the filter is unused. Can't get a 20" tall cutout on the side.
If you really have a 5t condenser connected to a 1t evaporator, you have problems.
I will check furnace tonight to see if on a box. I'm not at home right now. What is best filter type? I have a whole house fan and an attic fan - you mean the attic fan right?How can I tell if the coil is a one-ton type?Thanks, Mike
Just reading the thread and I'd like to make a note of what you said.
"I have a whole house fan and an attic fan"
So your going to be drawing from the house unless that hole house fan is sealed up max, which I kinda doubt.
But the fans should for a short time lighten the load on the ac. So if you ran the hole house to drop the max temp a little. Then back to AC. And from what you say if the fan was doing alot you would not have the stratification your describing.
Spread the thermo setpoints to 2 or 3, reset or check the ac fan/cooling setting. and slow it down. Variable speed will have the options to do this and if you can't or are not sure about it get the help you need to set it up. Once over of the system would resolve to the best of the ability of the system to function. Won't cure design flaws but work the best with what you have.
I use a whole house fan(and AC) in the summer and seal it off for winter/heating season.
It's foolish/short-sighted to remove the filter.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
Remove the pleated one and put a fiberglass filter in its place.
From:
danski0224 <!----><!---->
Jun-25 10:44 pm
To:
DanH <!----><!---->
Remove the pleated one and put a fiberglass filter in its place.
I have a 5t Trane system with a 5" thick 20"x28" pleated filter. Yes, they are expensive, but are rated for six months, so the cost equals six good pleated 1" thick filters. When the inside coil began leaking two years after the install, the Trane warranty replaces the whole coil and duct assembly. As they carried the old one to their truck, I made a point to follow them out and look at it. After two years, the coil had no visible lint or fuzz and looked brand new.
Down here in Texas, those fiberglass "filters" do very little filtering. The A/C guys down here know that and call those fiberglass filters "Rock Catchers"......
Bill
Yeah, I agree. The fiberglass filters are not worthless, but not all that effective. The extra-thick pleated filers are the way to go.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
Ah, but what is the static pressure drop of that filter?
It is very easy for a pleated filter to restrict 30% of the airflow of a system. The 1" pleated are especially bad.
The pleats create more surface area, so if 1/3 of seal over with fuzz and dust, there's still more open space.
I replaced the old Lennox A/C system (2 speed outside compressor) and the High Eff. pulse heat system, it was 15 years old, but those were built like tanks...
The installer deal was to remove the working, but twitchy, Lennox system, install the new Trane system, and then install the Lennox system in a corner closet in my workshop... ;>) They agreed to run two lengths of ducting, Freon up the A/C and install the PVC furnace intake and exhaust piping for just $200.
Before they could do that, due to using the fiberglass filters, the old Lennox inside evap. coil had to be acid washed because 1/3 of it was clogged with fuzz and dirt, plus the condensation tray that catches it and sends it to the PVC drain had to be replaced...add another $250... I'm running the normal 1" electrostatic filter in the system, plus two 1" pleated filters. One tilt-out filter-grill on each side of the corner closet so it slows down the incoming air and filters it better, too.
With about 1/2 a day's work on the workshop's system, I can install 4" thick pleated filters found at the Box stores and a lot cheaper than the Trane's special size.... The shop A/C runs on low compressor and the double Pleated filtering I have lets the blower push clean air without stopping-up the coil... I bought a set-back thermostat and can have the A/C or the heat stay on for finish/paint drying or 30 minutes on before I go out to it...it'll spoil you for sure!!
The 15yr. old Lennox system on its 9th year in it's second life and still working like it was new. Owe that to the pleated filters, vacuuming the once or twice during the month, and to using Dura-Lube on all the fan/blower motor bearings....that's another story... ;>)
Bill
Edited 7/22/2009 12:02 am ET by BilljustBill
Take a total external static pressure reading at the air handler, reference a fan chart... then get back to me :)
I understand your point on the pressure difference, but it's really a moot point... You either loose at the air intake side with a good pleated filter or you lose a lot more with a corked-up evap. coil... ;>)
Bill
Edited 7/22/2009 12:17 am ET by BilljustBill
Just as a counter point they just pulled out my old systems.They where 30 years old. But the drip pan on the orginal coil rusted out and had to be replaced. That was at least 10 years ago, but I think that it was close to 15.Strike that I just found the check book entry. It was in 01 so it was only 89 years old.Any I am not exactly a very neat housekeeper. And I have a hair doggy that is even less neat.And most of the time it has only had a fiberglass filter.And the coil looked as clean as new.Might be the different in dirt particals in the air.
.
William the Geezer, the sequel to Billy the Kid - Shoe
And most of the time it has only had a fiberglass filter.
And the coil looked as clean as new.
Might be the different in dirt particals in the air.
That is interesting. What was the thickness of the air filter. Being 30 years old, I'd bet that they started with the wire frame that you rolled the filter media over and then changed to thicker fiberglass inside the usual framed filters.
What did the blower cage blades look like, clean, fuzzy, or caked with a layer of hardend dust?
Bill
The old unit is still out in the driveway. I had to go out and check it this morning to see what the blower looked like. It was fairly clean.There was a small amount of dust/dirt on the fan support brackets which are right in the air flow where the return connects to the unit.The filter was always a the basic one inch. In the old days they used to have a light weight metal "shield" with large holes punched in it on the upstream side to hold the FG in place.But for a number of years they have just been the cardboard case with FG.And the whole filter setup was a PITA. Had to remove two covers and get down on the floor with a flash light and hope that you could side it in place with tearing it up on the screws that stuck through joining the return to the side of the furnace or it popped out of the track in the back and 1/2 the air went around it. That probably happened more often than not.The modern setups are so much nice with holders where they just slide in the ductwork.About 2 months ago I got a camera and a few weeks ago I got a new lap top that will work with it.So this is my first up load. It is a picture of the 30 YR blower..
William the Geezer, the sequel to Billy the Kid - Shoe
I'm willing to bet, (and I don't gamble) that your air flow was reduced by at least 10%. Lots of dirt on the fan blades.
Mike,
Sorry for the sidebar.
I prefer pleated filters. I use 4" MERV 7 in my old AprilAire media filter box. IF your system is moving close to the a nominal airflow for a 5 ton system (1800 to 2000 cfm) then the 20x20 filter is undersized. That is not a cause of the unit short cycling.
Without a make and model on the inside coil it would be had to guess its capacity.
I disagree the attic fan is not beneficial. You have eave or soffit vents, right? Just removing heat that will otherwise increase your need for cooling. That's not bad to do.
Aside from the mystery of your system components, does it cool the house? The cycling is indicating that something is not ideal, but is it working? Sometimes the can of worms that HVAC systems can be, mean the best option is to live with it.
I have multiple sofit vents. I also have some roof vents - three.The house is basically a box. basement - and two additional stories. There is insulation on the floor of the attic. It is very small - height and is not used for anything. No one ever did a load calc or anything else for that matter when the AC was installed. There was such an animal done when the furnace was installed.Even though the unit 'cycles' often it does keep the basement and first floor cool. The BR are tolerable but much less cool. Then again we live with this heat for only a few months total. So your advice to live with it is probably very good advice. But each time I write about a home repair question I not only learn how to but learn new vocabulary as well.Mike
> But each time I write about a home repair question I not only learn how to but learn new vocabulary as well.And whenever I start a home repair project I discover vocabulary I didn't know I had.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
If wall (top plate) penetrations (electric, plumbing, HVAC) aren't sealed... or things like can lights aren't sealed (drywall to fixture for example)... or register boots to drywall isn't sealed... the attic access hatch/door isn't sealed...
Then that powered attic ventilator will suck air from the home before the fan will suck from the soffit vents.
Path of least resistence. If the leaks in the envelope are so bad that the fan pulls interior air out before air through the vents/soffits, there is trouble, and the power exhauster is the least of it. But I would expect that not to be the case. Vented attics are a good thing. Shiddy construction in all its versions, always causes trouble.
who is going to take the time to engineer a residential system? If you take the time, who pays for it?
<G> own residential system had a 12 point mollier diagram for the hp before any metal bent......
obviously, only way to 'pay' was diy
think propane enthalpy
Way over my head :)
I understand that engineers are rarely involved with residential systems. My point is that 400 cfm/ton is only one of many operating points and is not necessary for "proper operation". Lots of examples of poorly done systems and components abound, don't need to go there. All the published data in my files has at least three speeds listed for every coil/condenser combination in the single phase (aka resiidential) products.
I do not disagree that a system should be properly charged to work as designed.
As far as reduced energy consumption at a reduced capacity, consider the two energy draws of the system: the fan and the compressor. The fan runs at a slightly slower speed, reduces the work done, the friction due to flow and consequently the fan motor draws less current and uses less energy. With a TXV controlling the refrigerent flow to maintain a constant suction temperature, lower air flow means less refrigerant is evaporated, less allowed by the TXV to pass through the coils and the compressor. With less refrigerant passing through the compressor (at more or less the same pressures), it does less work and draws less current/uses less energy. Both of these changes in the mass flows involve in the energy transfer does lower the thermodynamic efficiency of the system, but the last time I crunched the numbers, it was in single digit %'s. Delivered BTUs in any heat exchanger is based on mass flows, relative energy of the fluids and the effectiveness of the heat transfer surface(s).
Start a system under a load and measure the amp draw of the fan and the compressor. Change the speed of the fan and measure again, if you're curious.
The stuff I have doesn't list multiple speeds.
I get lower fan speed = lower amp draw = lower energy usage.
Isn't there a point when the airflow is too low?
I also understand that the TXV valve is the best option, but unless they come with the coil, they do cost extra money... and most people don't want to spend it.
Published data usually stops at 320 or so cfm/ton depending on the manufacturer, but that's pushing it a bit. You see a lot more data for commercial products, but the coil doesn't know if its in a house or an office building, does it? With an orifice as the meter, freezing the coil is very likely below 340 cfm/ton.
In reality, it depends on the load and controls. Since we talking about primarily residential systems with minimal/simple control, I would limit lower air flows to 360 cfm/ton or more. This allows some margin for closed registers, dirty filters, etc.