# Oversized AC??



## DanGauer (Jul 14, 2010)

We recently bought a house with a new AC as of 2008. The humidity in the house is extremely high especially in the basement (between 70-80% throughout the house) while the temperature is extremely different from the main floor to the basement (75 degrees upstairs and sometimes 65 down). We live in a 1170 square foot bungalow and the basement is finished as well as has 4 supply registers and one cold air return that I just added). 

My concern is that the AC comes on often but only stays on for 5-7 minutes per cycle. I am thinking the AC is over sized from everything that I have read... Would I be right in assuming this, and if so, what can I do (other than replace the system) to help fix this problem.

Here is the AC info:
Goodman
14 Seer - R410A - 2 Ton Central AC with new evaporator coil and refrigeration lines
condensor model SSX140241
evaporator model CA30A24-130R


Any info or advice would be great!
Thanks in advance


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## unlvrebel (Jul 20, 2010)

I'm no A/C guy and I know that the calculations for A/C sizing are pretty complex, but it sure sounds like you might be oversized especially given your symptoms. I have 1800 square feet with a 2.5 ton unit. For a quick and dirty calculation that works out to 720 square feet per ton. Your quick and dirty works out to 585 square feet per ton. Again, there are far more complex calculations, but this is a quick method for comparision. According to my AC guy (when I had the unit installed a couple of years ago), his calculations had me right at the upper edge of 2.5 tons. My A/C cycles for 20 or so minutes. In the heat of Florida summer the best that I can cool my house is to 76 degrees. But the air is pretty dry.


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## DanGauer (Jul 14, 2010)

unlvrebel said:


> I'm no A/C guy and I know that the calculations for A/C sizing are pretty complex, but it sure sounds like you might be oversized especially given your symptoms. I have 1800 square feet with a 2.5 ton unit. For a quick and dirty calculation that works out to 720 square feet per ton. Your quick and dirty works out to 585 square feet per ton. Again, there are far more complex calculations, but this is a quick method for comparision. According to my AC guy (when I had the unit installed a couple of years ago), his calculations had me right at the upper edge of 2.5 tons. My A/C cycles for 20 or so minutes. In the heat of Florida summer the best that I can cool my house is to 76 degrees. But the air is pretty dry.



Not sure your calculations are exactly right though.... My house is a 1170 square foot bungalow.... I believe that square footage only counts for the main floor. The rest of the house is below grade, even finished it is not included in the square footage. So I believe that makes it well over 2000 including both levels.....


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## DanGauer (Jul 14, 2010)

I should probably add that the house is 60+ years old with block foundation. I am assuming that there is no insulation between the block foundation and the basement walls. 

I tried partially closing the supply vents that are close to the thermostat, this seemed to cause the AC system to stay on longer (just over 10 minutes), but now the difference in temperatures throughout the house are different (even the temperatures between the living room where the thermostat is located and the bedrooms on the same floor. 

Humidity levels did come down to 60% upstairs......


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## yuri (Nov 29, 2008)

Where do you live? The ONLY way to size an AC properly is to do a proper load calculation. Rules of thumb vary wildly from area to area due to different climates/humidity.
Load Calculator


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## DanGauer (Jul 14, 2010)

yuri said:


> Where do you live? The ONLY way to size an AC properly is to do a proper load calculation. Rules of thumb vary wildly from area to area due to different climates/humidity.
> Load Calculator



I live in Welland, Ontario, Canada


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## yuri (Nov 29, 2008)

Close to the Great Lakes, really humid eh? We don't count the basement as part of the square footage. You probably should get a large de-humidifier for the basement. It will de-humidify and warm the basement as it gives off heat. I have one in my basement. Your tstat may be in a drafty area and short cycling or even faulty. There is a CPS cycles/hr adjustment on some models, read the manual. I recommend the Honeywell Vision or Focus Pro type and no el cheapos. 2 ton sounds okay for your house at that age and type of construction ( I know cold weather and Cdn house construction, eh) but I would do the load calc for peace of mind.


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## beenthere (Oct 11, 2008)

Could be more of a thermostat problem, then a size problem.


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## DanGauer (Jul 14, 2010)

yuri said:


> Close to the Great Lakes, really humid eh? We don't count the basement as part of the square footage. You probably should get a large de-humidifier for the basement. It will de-humidify and warm the basement as it gives off heat. I have one in my basement. Your tstat may be in a drafty area and short cycling or even faulty. There is a CPS cycles/hr adjustment on some models, read the manual. I recommend the Honeywell Vision or Focus Pro type and no el cheapos. 2 ton sounds okay for your house at that age and type of construction ( I know cold weather and Cdn house construction, eh) but I would do the load calc for peace of mind.


Thanks for you help so far!!!

I have a FocusPRO TH6110D Thermostat which was installed the same time as the AC. I did notice that I can change the "Compressor Cycle Rate" and found out that it is presently set at the default - 3. What does this mean exactly. Is this how long the compressor runs and then it shuts off until the next cycle? Should I lower this or raise it? Is this in minutes or something else. I believe I can set it from 1-6 CPH.


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## yuri (Nov 29, 2008)

cph=cycles/hour. 3 would give you 3 sets of 10min on/10 min off. It is a heat anticipator and I don't want to go into all the gory details of how it works. Try set it at 2 to lengthen the cycles. You will always get different temps in the rooms in the house unless you run the furnace fan continuously for circulation. Unless you have a furnace with an ECM variable speed energy efficient motor that can get expensive for electricity. Try it for a few days to see if it helps. May be noisy and drafty. No perfect solution for comfort and temp control unless you have special ductwork and a expensive zoning system.


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## DanGauer (Jul 14, 2010)

yuri said:


> cph=cycles/hour. 3 would give you 3 sets of 10min on/10 min off. It is a heat anticipator and I don't want to go into all the gory details of how it works. Try set it at 2 to lengthen the cycles. You will always get different temps in the rooms in the house unless you run the furnace fan continuously for circulation. Unless you have a furnace with an ECM variable speed energy efficient motor that can get expensive for electricity. Try it for a few days to see if it helps. May be noisy and drafty. No perfect solution for comfort and temp control unless you have special ductwork and a expensive zoning system.


Thanks for all the info, and sorry for so many questions......

I am not that concerned with the difference in temperatures... I can figure that out later.... Mainly I am concerned with the system turning on for only 10 minutes or less, off for 10 minutes, and then on again... Everything that I have read says that at 10 minutes, the compressor is just getting to its peak performance, especially with removing humidity. And that is one of my problems with the house. It is min. 60% humidity upstairs and over 70% downstairs....... 

What can I do, other than closing the vents to make the system run longer..... what about changing the blower speed? Is this an option so that less air is pushed through causing it to cool the house more slowly? 
The thermostat keeps it at the same temperature rather than having a 2 degree differential.... Should I trade this Tstat for one that will have a wider range of temp differential?

I really think that my problem is an over sized system...... Even with changing it.... it recovers the temperature so quickly that it still only on for 10 minutes............


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## zootjeff (Jul 11, 2007)

DanGauer said:


> Thanks for all the info, and sorry for so many questions......
> 
> I am not that concerned with the difference in temperatures... I can figure that out later.... Mainly I am concerned with the system turning on for only 10 minutes or less, off for 10 minutes, and then on again... Everything that I have read says that at 10 minutes, the compressor is just getting to its peak performance, especially with removing humidity. And that is one of my problems with the house. It is min. 60% humidity upstairs and over 70% downstairs.......
> 
> ...


Set the cycles per hour to 1.

Consider slowing down your blower fan a notch.. You don't want the coil to freeze over, but the CFM may be set a little high if you are lucky, then turning it down will help..

Typical is 400CFM per ton. And some furnaces have dip switches to set AC blower mode to 350, or 400, or 450CFM per Ton of cooling. Your blower should nominally be set to 800 CFM. Not all HVAC pros even look at these settings.. If you do have these dip switches on your furnace and it is set to anything other than 700, try 700 CFM..



What is the make model of the air handler or furnace?

You might want to figure out what your CFM in AC mode is set to and post that on this forum..

-Jeff


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## DanGauer (Jul 14, 2010)

zootjeff said:


> Set the cycles per hour to 1.
> 
> Consider slowing down your blower fan a notch.. You don't want the coil to freeze over, but the CFM may be set a little high if you are lucky, then turning it down will help..
> 
> ...


I know the furnace is 12+ years old but that is it... I will get this info and re-post later.... Can I access this blower mode dip switch from the front panel?


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## DanGauer (Jul 14, 2010)

zootjeff said:


> Set the cycles per hour to 1.
> 
> Consider slowing down your blower fan a notch.. You don't want the coil to freeze over, but the CFM may be set a little high if you are lucky, then turning it down will help..
> 
> ...


The furnace is a Luxaire
Model # PCUHALD12N080B
Serial# EKGM364952

I am a little confused about "compressor cycle rate"... If I set it to 1, does this mean that the compressor will only turn on one time per hour? Will the condenser still turn on and off? Or will it just stay on the entire hour?


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## moondawg (Dec 17, 2008)

yuri said:


> cph=cycles/hour. 3 would give you 3 sets of 10min on/10 min off. It is a heat anticipator and I don't want to go into all the gory details of how it works. Try set it at 2 to lengthen the cycles. *You will always get different temps in the rooms in the house unless you run the furnace fan continuously for circulation.* Unless you have a furnace with an ECM variable speed energy efficient motor that can get expensive for electricity. *Try it for a few days to see if it helps*. May be noisy and drafty. No perfect solution for comfort and temp control unless you have special ductwork and a expensive zoning system.


This will definitely not help his humidity problem. Any water still on the coils will be re-evaporated by the fan instead of running to the condensate drain. 

Setting to reduce cph is good advice, though. With heat pumps, LONG runtimes with LOW fan speeds are best for dehumidification. Of course as another poster noted, the trick is to NOT freeze up the coils.


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## zootjeff (Jul 11, 2007)

DanGauer said:


> The furnace is a Luxaire
> Model # PCUHALD12N080B
> Serial# EKGM364952
> 
> I am a little confused about "compressor cycle rate"... If I set it to 1, does this mean that the compressor will only turn on one time per hour? Will the condenser still turn on and off? Or will it just stay on the entire hour?



The compressor (And fans) are the only active components in the system. The evaporator and the condenser are just passive coils.

Setting the Cycles per hour to 1 will mean that the compressor can only turn on and off 1 time in an hour. If it turns on for ten minutes the first time, then if after 20 minutes it wants to turn on again, it has to wait a full 60 minutes. During that waiting time the house is getting warmer, so in theory, the next run time will be longer and de-humidify more.

-Jeff


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## zootjeff (Jul 11, 2007)

DanGauer said:


> The furnace is a Luxaire
> Model # PCUHALD12N080B
> Serial# EKGM364952


I can't seem to find an online manual. If you pull the front panel you should see the jumpers.. If you read the stuff on the panel you may see if there are any jumper settings..


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## beenthere (Oct 11, 2008)

zootjeff said:


> The compressor (And fans) are the only active components in the system. The evaporator and the condenser are just passive coils.
> 
> Setting the Cycles per hour to 1 will mean that the compressor can only turn on and off 1 time in an hour. If it turns on for ten minutes the first time, then if after 20 minutes it wants to turn on again, it has to wait a full 60 minutes. During that waiting time the house is getting warmer, so in theory, the next run time will be longer and de-humidify more.
> 
> -Jeff


Not true.

On Honeywell thermostats. Temp always over rides CPH.

CPH is only accurate at the 50% load capacity of the equipment to the homes load.

So set to 1 CPH, it can still run 3 times an hour, if its hot outside.

His furnace has no jumper for blower speed. Its set by using the speed wire. He probably needs to use the low speed wire.


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## zootjeff (Jul 11, 2007)

beenthere said:


> Not true.
> 
> On Honeywell thermostats. Temp always over rides CPH.
> 
> ...


Oh. Thanks for the insight. Why would you ever change the CPH then? Seems like a pointless control.. Can you type up an example of where it's helpful or solves some compressor problem?


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## beenthere (Oct 11, 2008)

CPH is simply an electronic version of heat, and or cooling anticipation.

On a oversized system. You may have to use a 2, or 1 setting to get enough moisture removal from the unit even when its 85 outside.

On properly sized units/systems. Using a higher number can get you more comfort, if you like an very close temp tolerance(.5°F).


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## yuri (Nov 29, 2008)

" *You will always get different temps in the rooms in the house unless you run the furnace fan continuously for circulation.* Unless you have a furnace with an ECM variable speed energy efficient motor that can get expensive for electricity. *Try it for a few days to see if it helps*. "


This was to try even out the temps in his rooms. De-stratify the air not de-humidify.


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## DanGauer (Jul 14, 2010)

beenthere said:


> Not true.
> 
> On Honeywell thermostats. Temp always over rides CPH.
> 
> ...



Ok, so I looked inside and found what you were referring to... The blower fan can be set from 800 to over 2000..... It is done by way of wires.... I looked at the instructions and to have 800 it says to have the black wire in cool and the blue wire in heat..... actually for all it says this and then there are some park slots.... one with a red wire I think in it...... 
The actual setup however has blue going into cool and black going into heat.... Should I just reverse these then to slow it down?


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## yuri (Nov 29, 2008)

Are you sure? Usually the industry standard for motors is red for low, yellow for medium, blue for medium high and black for high speed. Can you scan the wiring diagram or get a good closeup with a digital camera and post it here? Luxaire is York and they do things differently. Post those instructions too.


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## beenthere (Oct 11, 2008)

Your model furnace won't do 2000 CFM. It only has a 3 ton drive. So basically 1200 CFM is max.
The install manual that you have isn't just for your model furnace. It is for all PUC line furnaces. From a 40,000 to a 120,000 BTU.

On yours. Use the Red wire for cool speed.


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## DanGauer (Jul 14, 2010)

beenthere said:


> Your model furnace won't do 2000 CFM. It only has a 3 ton drive. So basically 1200 CFM is max.
> The install manual that you have isn't just for your model furnace. It is for all PUC line furnaces. From a 40,000 to a 120,000 BTU.
> 
> On yours. Use the Red wire for cool speed.



OK great...... Thanks..... SO all I have to do is take the blue and red one and switch them.... Red wire in cool, Blue wire in park? Is the park just for the extra wires that are not in use then?

I will take a pic in a bit (when the kids go for a nap) and post it with the instructions as well just to be sure I am explaining it right


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## yuri (Nov 29, 2008)

I would recommend you get a Pro to check the freon level and the superheat reading after using red for cool. We do it all the time but it can be dangerous if you do not have enough airflow over the coil. Can slug liquid to the compressor and damage it and possibly void the warranty. DIY is fine but you don't want to create a BIG problem for yourself later.


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## DanGauer (Jul 14, 2010)

yuri said:


> Are you sure? Usually the industry standard for motors is red for low, yellow for medium, blue for medium high and black for high speed. Can you scan the wiring diagram or get a good closeup with a digital camera and post it here? Luxaire is York and they do things differently. Post those instructions too.



Here are those pictures..... The blue wire is in the Cool slot, but while taking pictures, I noticed a red clip like thing on the end before it enters the cool slot.... does this have anything to do with the speed???


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## DanGauer (Jul 14, 2010)

Forgot to include a close up of the wiring chart....


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## beenthere (Oct 11, 2008)

Yes. Park is just for the spare wire so it doesn't get shorted to anything.


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## DanGauer (Jul 14, 2010)

yuri said:


> Are you sure? Usually the industry standard for motors is red for low, yellow for medium, blue for medium high and black for high speed. Can you scan the wiring diagram or get a good closeup with a digital camera and post it here? Luxaire is York and they do things differently. Post those instructions too.



Since Luxaire does things differently, do you think that the installer may have assumed that blue was med and black was high and did not look at the instruction diagram..... could he have put them in the wrong slots not knowing that they were backwards?


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## beenthere (Oct 11, 2008)

He probably is from the old school. That the A/C can't use low speed.

It was wrong when it was taught.


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## DanGauer (Jul 14, 2010)

beenthere said:


> He probably is from the old school. That the A/C can't use low speed.
> 
> It was wrong when it was taught.



Ok, so I am safe to switch to the low(red wire) setting?... Even with an over sized system?.. Someone mentioned the risk of the ac freezing over? What should I look for if this happens so that I know to switch it back???


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## beenthere (Oct 11, 2008)

The A/C is oversized for your house. Its not over sized for the furnace, or it blower.

That furnace has a strong blower in it, and will move a lot of air even on low.


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## yuri (Nov 29, 2008)

Sometimes you need more fan speed for heating than air because of undersized ductwork causing the furnace to "ride" the limit control/cycle on/off on the control. You may be OK with AC on red/low but you will never know for sure unless a tech does a superheat test. Requires freon gauges and special knowledge to do that part. Low superheat will wash the oil from your compressor and shorten its life, could also freeze up on cooler days. A temp rise test should be done to determine which heating speed you need. Not everything is DIY friendly. Save $250 and do long tern damage to your equipment is a real possibility. Did you do the online heat load test? Even if your unit is borderline oversized (good possibility) are you going to get rid of it? A good large de-humidifier is a better bet to take care of a lot of your humidity problem. Under $400 solution vs a new $4000 AC.


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## DanGauer (Jul 14, 2010)

So I have gotten two completely different answers... One says lower it..... it'll be fine... the other says better not because I might damage the system.....

I tried lowering the speed to see what happens and I have kept a very close eye on the system...... The end result is that the air coming out of the register is a lot colder but the air is definitely not coming out as fast ..... More water is being pulled out from the dehumidifier (I know because I keep a 12 liter bucket under the drain and use the water for various things... before lowering the blower I would fill it 1.5 times a day... now it is more). I also kept track of on and off times and there was only a slight increase....... still around 10 minutes....... After all that, the system is still short cycling.... But at least the house is less humid.....

Any other ideas??


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## beenthere (Oct 11, 2008)

Seal your house a bit better.

Caulk around all of your windows. Replace or replace all worn door seals. Seal all wall recep and switches. Caulk all holes in the floor or ceiling plates where wires penetrate.


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## yuri (Nov 29, 2008)

*"So I have gotten two completely different answers... One says lower it..... it'll be fine... the other says better not because I might damage the system....."

*IMO you may be mis-interpreting our answers a bit. The devil is in the details. You can use the red low speed and I do it sometimes and also check the freon readings/superheat to make sure it is not going to damage the system. Another possibility is that it is overcharged because the installers did not check the freon level. Out of the box installs are very common. Lowering the fan speed compounds the problem. An experienced tech will check the freon level and superheat and make the necessary adjustments. The rest just_ let er rip_. New homes mostly have uncommissioned/unadjusted systems. Why is that? Installers get paid piece work $$ and Boss says owner has a warranty and will complain *IF* there is a problem. Nice eh.:yes:


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## DanGauer (Jul 14, 2010)

yuri said:


> *"So I have gotten two completely different answers... One says lower it..... it'll be fine... the other says better not because I might damage the system....."
> 
> *IMO you may be mis-interpreting our answers a bit. The devil is in the details. You can use the red low speed and I do it sometimes and also check the freon readings/superheat to make sure it is not going to damage the system. Another possibility is that it is overcharged because the installers did not check the freon level. Out of the box installs are very common. Lowering the fan speed compounds the problem. An experienced tech will check the freon level and superheat and make the necessary adjustments. The rest just_ let er rip_. New homes mostly have uncommissioned/unadjusted systems. Why is that? Installers get paid piece work $$ and Boss says owner has a warranty and will complain *IF* there is a problem. Nice eh.:yes:



Ok, thanks everyone for all you advice.... Looks like I am going to have to switch it back to what it was and call the company that installed it and ask what my options are.... 

Thanks again. I always like to attempt thing prior to paying someone to do something that I could have for free!!


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