Zone Cooling Question

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  #1  
Old 07-07-03, 11:03 AM
tommyg12
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Zone Cooling Question

We have a two story house that is about 1,100 sf downstairs and 1,200 sf upstairs which includes a 12'x20' bonus room.

We have a nine year old Carrier Comfort Zone system. The system has one outdoor 3 ton unit and 3 zones (upstairs, downstairs, bonus room). The house has very little shade and gets sun most of the day. When the outside temp is around 90, the system runs all day (from about 10am to 10pm) maintaining 2-3 degrees over the thermostat setting. Does this sound normal? I've had the charge checked and the coils have been cleaned.

Also the bonus room is over the garage and has a attic/storage that runs parallel with the room. This room gets all of the afternoon sun and is very difficult to cool. It has its own return but only has one vent which to me seems awful small for this size room. So my second question, can bonus rooms like mine be difficult to cool and should I look at other cooling methods for this room ( a friend recommended a hotel style a/c)? Thanks in advance for any input.
 
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  #2  
Old 07-07-03, 02:28 PM
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A lot here is how far is the duct to this room from the ac unit?How well is it insulated?If its a long run on the duct work and lots of elbows. Im with you friend .You can get a slide in wall unit with AC and heat pump right in it. I think this is what I would do here. Just close off the return and outlet but let them there.You dont say if the knee wall of the room is insulated from the attic part? Id do that for sure. ED
 
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Old 07-07-03, 04:46 PM
tommyg12
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Ed,

Thanks for the reply. Yes there is insulation. Since I posted the message, I have received a phone call from the guy that cleaned my coil yesterday. He ran the numbers off the condensor and compressor. The condensor is a 3 ton Carrier but the compressor is a 2.5 ton Tecumseh. The compressor went out when it was two months old and apparently the HVAC company put in a 2.5. What kind of difference would this make ?

thanks
 
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Old 07-08-03, 08:39 AM
tommyg12
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Mismatched Compressor/Condensor?

After I posted the "Zone Question" thread below I received a phone call from the person that cleaned my coil. He ran the numbers and verified that the condensor is a 3ton unit but the compressor is a 2.5 ton. When the unit was only two months old (nine years ago) the compressor went bad and the installation company replaced the unit under warranty. Apparently they shorted me a half ton. The compressor is a Tecumseh. Shouldn't it have been a Carrier?

What improvements might I see if I have the compressor replaced with a 3 ton? Right now in hot humid weather it is very hard for the system to maintain all 2300sq ft at a comfortable temperature (90 outside, therm set at 75, house will stay at about 4 degrees higher through the hot part of the day).

Thanks again.
 
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Old 07-08-03, 02:43 PM
bigjohn
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Well, let's dicuss that issue, coz it's relevant to the SEER numbers game. I think the salient question is how has it been for the past 9 years since the replacement? When you had the failure, did they replace the compressor or the entire outdoor unit? If it was the compressor, then the replacement was the one that the manufacturer sent as a warranty exchange. It wouldn't necessarily have to be a Carrier compressor. Carrier, just like all the other manufactures, buys compressors for their units from Tecumseh, Copeland, etc. Now, it used to be years ago before the advent of higher efficiency units, that compressors in a/c units were about 1 hosepower per ton. But that is not so any more. Why? Because higher efficiency units run with higher evaporating temperatures and lower condensing temperatures. This translates to less horsepower being needed to produce the required cooling effect. You will also find that tonnage rating is a nominal rating. It's not unusual to find out that a nominal 3 ton unit is really only rated for 34,000 btuh or maybe even a little less. What does that mean? A smaller horsepower requirement which reduces the wattage consumption per X btu's of cooling. I went to the Tecumseh website and looked up some of their air conditioning compressors. I found model # AVA5532EXN. It's an R-22, air conditioning compressor with a 2-1/2 horsepower motor rated for 32,000 btuh at a 45* evaporator coil and a 130* condensing temperature. Therein lies the catch. What a manufacturer could do is put the condensser coil from their 3-1/2 ton unit on their 3 ton unit to increase the condensing surface. This would lower the condensing temperature which would raise the capacity of the compressor. So let's say that they use this particular compressor at a 120* condensing temperature. It could raise the compressor's capacity a couple thousand btuh to 34,000. Then they derate their 3 ton unit a litle down to 34,000 btuh total capacity and, voila!, you now have a 2-1/2 horsepower compressor on a nominal 3 ton unit. The gentleman who told you the compressor is too small may or may not be accurate. I haven't seen the thread about the coil cleaning. He removed the coil and cleaned it in the yard with a foaming action cleaner, right?
 
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Old 07-08-03, 07:28 PM
tommyg12
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bigjohn,

thanks for the reply.

Yes the coil was cleaned with a foming cleaner.

Being that the house was only two months old, I don't remember if the old compressor made a difference.

We've never felt like the house cooled well but was never really concerned until now. We want to begin using our bonus room (a third zone) and now we really notice a difference. For example today was 91 with a heat index of 96. Two zones are set at 74 and the third at 76. At 8:00pm the first two zones were still at 79 and the third was still 80.

Once temps drop to the low 80s, we don't notice much of a problem. I've always used the installer for service calls but maybe I should use someone else as the installer always defends the design.

what would you suggest , maybe having someone else perform a load analysis? I open for any suggestions. Thanks, tommy.
 
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Old 07-08-03, 07:41 PM
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Yes it would make a difference.Oh they will run alright but you got taken for 1/2 ton of AC thats 6000 btu of AC you dont have anymore. You said 9 years dont know what you can do about it now. I think it will cost you less to just put in a wall unit now. ED
 
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Old 07-08-03, 08:33 PM
bigjohn
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Wow, 2300 sq. ft. on 3 tons? Your house must be very well insulated. I agree with Ed, put additional cooling in the bonus room and close off the registers to that room. It will take some load off of the central system. I think you should consider looking at some ductless mini -split units for the bonus room Check out www.mrslim.com and www.sanyohvac.com These units are not cheap but they are very quiet and do a good moisture removal job.
 
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Old 07-09-03, 06:44 AM
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Moderator note: Second thread on same problem merged with first. Please post responses to the same equipment in the same thread. It's easier to follow.

tommyg12:

I agree that the compressor used as a replacement is likely the correct one.
Looking at everyone's comments and the original question it appears your system is undersized.
A suggestion is to have a qualified contractor come and give you a heat/cool load calculation and offer suggestions as to how things can be improved. You may also have some ductwork issues as undersized ductwork to second stories is common enough to almost be called an "epidemic".

The mini split in the bonus room would be a simple solution to cooling that room.
 
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Old 07-09-03, 06:04 PM
tommyg12
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Where can I check/verify compressor model numbers

Carrier Condenser (outdoor unit) model:

38ck036320

Tecumseh compressor:
aw55325 and/or aw108et-001-s7

I'm trying to verify if my condenser and compressor are mismatched (3ton vs. 2.5 ton).

thanks
 
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Old 07-09-03, 07:11 PM
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you have a

32000 btu compressor, right between 2.5 and 3 ton
 
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Old 07-09-03, 08:34 PM
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Moderator note:

Third thread on same equipment merged with first.
Please use reply button to keep all ideas together. It's frustrating for people who have an interest in your problem to follow what's going on.


tommyg12:

You may be beating this wrong compressor idea to death.

I think that the consensus is that the compressor is matched to the condensing unit!

Get a service company that is capable of doing a load and duct calc to tell you once and for all whether or not the original guy screwed up.

I'll repeat myself and say that it is rare that I see a two story house with a single hvac unit that has proper airflow to the second story to satisfy the cooling load.
 
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