Oversized Central AC


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Old 01-17-08, 11:07 PM
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Oversized Central AC

I have been meaning to ask a question for a while and hear it is I have a 5 ton Rheem Central AC which was installed in 1989 it still works great knock on wood but the thing i have noticed in the past is it short cycles a lot mostly on days which are not to hot and at night. it runs for maybe 5 or less minutes and is off if i am lucky for maybe 10 minutes if i am lucky. But on hot days lets say over 85 degrees it runs constantly without shutting off which is good. I am looking for a way to increase the time of the off cycles to over 10 minutes maybe even 20 minutes. Is there a device i can tie into the wiring which will do this?
I came across this online what do you guys think of this or do you guys have another device i can use?

http://www.intellidynellc.com/pdf_pr...elliCon-AC.pdf
 
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Old 01-18-08, 03:30 AM
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Please indicate if this central A.C. unit is able to cool off your house whether it is just lukewarm outdoors (or at night time) and it seems to short cycle, or whether it is quite hot outdoors and seems to run constantly.

The reason for this inquiry of mine above is that...if you tell me that the unit cools the house regardless of the length of its running cycles, then it may very well be oversized.

This is not a magic number that necessarily will apply in your case (it all depends where in the U.S. you live, the construction materials in your house, its orientation, etc.)...but a 5-ton unit would be a unit size needed for a house about 2,800-3,000 square feet. How big is yours?
 
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Old 01-18-08, 04:09 AM
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Sorry...
I should be more careful when reading... I skipped the heading of your message. You seem to already know your unit is oversized and you concern, I guess, is the short cycling. Short cycling is not good for the compressor. I take it that this is likely your concern, that the compressor may be affected in the long run.

With an oversized unit, there's not much (that is affordable or practical) you can do to increase the running cycles on lukewarm days or at night, but you can take care of the short-cycling and for the most part eliminate it if you add to your contactor coil in the condensing unit an adjustable "time delay relay", and set it up for, say, 5-to-7 minutes or so. that will space out the start-ups of your compressor.
 
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Old 01-18-08, 08:03 AM
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home is about 2200 square feet i am on long island new york. On hot days unit runs continuiously and seems as if the unit is sized good for house unit runs and runs does a good job of removing humidity of house but temp drops very slow. The problem is on the mildier days the unit seems as if it is only cooling the house and can't remove humidity because the run cycles are to short. My thermostat has a built in time delay which will not allow the unit to come on for 5 minutes which is ok but i think it would be better to have longer off cycles and in return the run cycles would be longer. Is there a device i can tie in which lets say i can make the time delay lets say 15 minutes?
 
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Old 01-18-08, 09:50 AM
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You seem quite knowledgeable regarding what an a.c. does and does not when it's oversized. And you're quite right, the most noticeable sympthoms on an oversized unit are (a) short-cycling on days that are not hot, and (b) poor humidity control.

I thought you were concerned about the compressor taking a hit b/c of excessive short cycling...which is obviously not the case with what you reported below. I do not recommend longer time delays...that will not solve your humidity problem. This is what I'd try:

The blower of your unit is likely a multi-speed one (4 speeds? 5-speeds?) And from factory units come typically wired for their highest speed when in the cooling mode...So...change the speed. If your blower is a 5-speed one (hi, med-hi, med, med-lo, lo), choose "med". The slower airflow will help with humidity control and will extend the running time of the system. Of course there may be a caveat: freeze-up developing in the evaporator coil...maybe yes, maybe not, but it's worth the try. And if you do not set your thermostat too low and replace your air filter periodically, coil freeze-up may never happen.

If you need help with the wiring, just bring it up
 
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Old 01-18-08, 11:09 PM
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I am more concerned about the compressor being damaged then i am about my humidity. The AC does a good job of keeping me comfortable i just don't like them way it cycles. My theory was to increase the off cycles which will in turn allow the house to warm a little and allow more humidity in the air which will in turn make the run cycles longer. It sounds interesting what you were saying about lowering the blower speed. How do i know if my blower is a multi speed motor or not and what speed it is wired at now. I believe the blower motor on the air handler is the original from rheem which was installed in 89 if needed i can get you a model number later on today. I am always good about changing my filter. i change it every 3 weeks
 
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Old 01-19-08, 04:46 AM
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Given its age, that Rheem of yours likely has a 3-speed blower motor (4-and-5 blower speeds are a more recent development)...but we will know for sure when you provide me with a "complete" model number.

In the meantime, if you are of the hands-on type, turn the furnace OFF and remove it's service and blower compartment doors. For an upflow unit the big house blower is at the bottom. You'll see a few wires coming out of its motor...tell me what they are. For cooling a black wire is for high speed. For heating, a red wire is for low speed. You should have a white too (Neutral), one (or possibly up to two) brown wires connected to a capacitor, and at least one more wire (blue? yellow?) for the third speed I mention here, capped and not being used.

Lengthening the OFF cycles could be easily accomplished by adding to the contactor of the condensing unit an adjustable time-delay-relay, which of course you can set for 7 minutes.

But longer ON/OFF cycles "could" also be accomplished by having a lower blower speed. Remeber, it's the cool air coming off the registers that cools off your house...if there is less of it circulating, cooling the house will take longer, and you'll reap the benefit of better dehumidication.
 
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Old 01-19-08, 06:50 AM
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Ok i will have to crawl up in the attic later on today i will get you a model number of the air handler and take a look at the wires from the motor. The thermostat i have now gives the system a 5 minute time delay. I am curious is there another time delay which i could set longer then 5 or 7 minutes?
 
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Old 01-19-08, 06:54 AM
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There's this company called ICM that makes controls...one is this TDR (Time Delay Relay), adjustable, from a few seconds up to 10 minutes. Not that you'll need that many, but to show that there is out there what you're looking for.

Those ICM TDR's are installed by the contactor of the condensing unit.
http://www.icmcontrols.com/products/....php?cat_pk=11
 

Last edited by pflor; 01-19-08 at 07:03 AM. Reason: timing given incorrect
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Old 01-20-08, 09:50 AM
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Finally made it up to attic a lot of fun.

Model # of air handler is RHQA-2000B
Motor HP 1/2 FLA 4.1

I saw 4 wires coming out of motor

I believe they were Black yellow either green or another yellow and orange i believe.

Does this sound like a multi speed blower?
 
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Old 01-20-08, 07:15 PM
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Here's what I got from the schematic for your unit:
RHQA-2000-B


4-speed blower that has a running capacitor.
Motor is 240V, single-phase

Hot line #1: Orange wire (permanent)
Hot line #2: from factory - Black [high speed, switchable]

Unused Speeds [parked]:
BL = blue - medium high
Y = yellow - medium low
R = red - low

Capacitor wires: BR, O (brown and orange)
Likely, regardless of the picture above, there is only one orange coming from the motor and connects to one of the terminals on the capacitor, and from the same terminal another orange goes and connects to the hot line.

To select a different speed than the factory set, disconnect and cap the BLACK wire and connect one of the others instead (the BL, Y or R)...I would not recommend using the RED (too slow, coil may freeze)...try the YELLOW first, if the coil ices-up hook up the BLUE; if the coil does not ices-up give a shot to the RED
 
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Old 01-21-08, 03:59 PM
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Thanks for your help i will try and challange this project over the next few days I didn't remember seeing any blue wire coming off the motor i did remember seeing 2 black wires connected togather with a wire nut under the motor so all i have to do is disconnect the black wire coming from the motor and hook up one of the other colors?

What are the chances that the unit will start freezing up? Would it be better to be on safe side and hook up the blue wire (If There is One) First to see if it increases my run times and decreases the short cycling?
 
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Old 01-21-08, 04:09 PM
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It is possible that there was no blue wire...double check. Those manuals go through revisions, and I can't be 100% sure of that wire...so pls check out the wires again. And if you have the manual of your unit handy, it shall be right at your fingertips

If you keep the t-stat setpoint at 75F chance for coil freeze-up is small (maybe it just won't happen at all). But if you like keeping your home at or below 70F is more likely...the colder you like your home, the longer the thermostat will keep the system running, and that increases the possibility of a coil ice-up...but so does too having a very dirty (clogged) air filter even with the blower at high speed.
 
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Old 01-21-08, 05:28 PM
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Wink

There should be a wire schematic On one of the doors there on the blower coil unit. This should show you the lower speed wire colors
 
 

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