I'm seeing two radically different temperatures in the supply plenum


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Old 08-07-22, 04:30 PM
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I'm seeing two radically different temperatures in the supply plenum

I've had an odd problem for the last couple of summers where the exit air temperatures are different at various vents. The vent temperatures are radically different in two bedrooms even though the ducts are the same length and run parallel to each other for most of the distance--one vent has ~67deg air the other has 85deg air. Both vents seem to have the same air flow. Even odder is another room that is farther away has ~57deg air. I checked for leaks in the duct going to the hot room and didn't find any.

One clue to the issue was related to how the the ducts for the two bedrooms were connected to the supply plenum. They were stacked one above the other near the far end of the plenum. The room with colder air is connected to the lower part of the plenum. The other duct was near the top of the plenum. The duct for farthest room (the coldest temp) was connected to the opposite side of the plenum near the bottom and close to the evaporator coil.

I had a technician check the system. He drilled two holes in the supply plenum--one on top and one on the side near the bottom. The temperature at the top of the plenum was ~82deg while the bottom temperature was ~55deg. He checked the compressor pressures and they were ok.

Now the question: WHY? What could cause this? I'm asking here because the HVAC company is suggesting a new system. I'd like to see if there is possibly a simpler/lower cost solution.

Thanks,
Steve

BTW, I am in the Dallas, TX area; outside temperature 100deg; thermostat set for 81deg;
 

Last edited by stevep10; 08-07-22 at 05:04 PM.
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Old 08-07-22, 06:06 PM
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Are the 2 bedrooms on the same zone? Do the ducts to the bedrooms have manual flow controls?
 
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Old 08-07-22, 07:13 PM
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Are you sure you weren't confused and the warm one is the air return?
 
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Old 08-07-22, 09:17 PM
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Yes, the two bedrooms are on the same zone. Both ducts are attached to the same supply plenum with one duct physically attached right above the other duct. All ducts have manually controlled dampers. The two bedroom dampers are full open.

Also, all of the ducts I talked about are on the supply plenum which is right after the evaporator coil. The system is a horizontal setup in the attic with the return air coming in on the right, passing through the filter and blower, the gas-powered heat exchanger, the evaporator coil and then out through the supply plenum on the left.

Is there a condition that can cause air from the top of the evaporator coil to be significantly warmer than air from the bottom?

Thanks for taking a look at this,
Steve

BTW, the humidity has been very low here for about two months. We had one small rain a week ago then the humidity went right back to below 10%.
 
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Old 08-08-22, 10:48 AM
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The temperature may vary by a few degrees depending on where they are tied in to the supply plenum but that's it. Not wide temperature swings.

Get a thermal imaging gun where you point it and shoot at an object to measure the temp.
Check the duct work from end to end to see where the temps change.
 

Last edited by PJmax; 08-08-22 at 01:58 PM.
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Old 08-08-22, 01:44 PM
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The temperature difference is definitely inside the plenum. The air temperature going into the top duct is 82deg (as measured with a probe into the duct). The room register is essentially the same. The temperature into the bottom duct is about 57deg with the output register temp about the same. I also measured the temperatures very close to the evaporator coil by inserting the probe through a couple of screw holes on the side panel of the evaporator coil box. The top temperature was 82deg the bottom was about 50deg.

The evaporator coil is a flat design. I opened the side panel and felt the side of the coil on the supply plenum side (the airflow output). The upper area was warm (uncooled) and the bottom area cold.

Thanks,
Steve

Edit note: The return air to the system is about 82deg (we keep our house warm to save money). So, the upper part of the evaporator coil is not cooling at all since its output is about 82 deg.
 

Last edited by stevep10; 08-08-22 at 02:03 PM.
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Old 08-08-22, 01:58 PM
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If the coil is not a uniform temperature.... you are low on refrigerant.
Surprised it's not freezing up on you.
 
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Old 08-08-22, 02:32 PM
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Thanks!

Supposedly the technician who came out checked the pressures and said they were fine--maybe they weren't. He thought the expansion valve (piston type) might have debris in it. He said they could try evacuating the system, remove the valve to clean it out, then reinstall. A limiting factor is the system uses R22 and they don't have any left in case some is needed.

I suppose it's time to put in a new system. I hate to do it because of all the negative comments I've heard over several years of people having problems with their newer systems. My system is very old but has been almost bullet proof. Both of our systems are Janitrol units that had SEERs of 9 or 10. Even with the low efficiency, our electric bills have always been below $200 in the summer in Dallas. The compressors have needed little help--a couple of capacitors on one and a couple of fan motors on the other. The attic units were replaced years ago because of cracked heat exchangers, but they have only needed one control card changed since being installed.
 
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Old 08-08-22, 03:50 PM
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It's no secret that the newer units are nothing like the older ones.
I just lost my 33 year old Rheem system. The compressor gave up the ghost.

I could have gone ultra high efficiency but the return is very slow and the repair history is sad.
I went with slightly higher efficiency than I had.
Remember... the energy money you save with an energy efficient appliance is spent on the repair.
 
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Old 08-08-22, 03:59 PM
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the energy money you save with an energy efficient appliance is spent on the repair.
Just adds to the throw away society we've become, or forced to become!
 
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Old 08-09-22, 08:06 AM
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Could a partially clogged expansion valve cause the warm/cold issue with the evaporator?

Mytwo Janitrol compressors are in their 39th summer and still running fine. 38 years ago when we were considering which units to install, I went with the lower SEER units because I thought they might not be as stretched to their limit like the higher SEER units might have been. Currently I'm looking at the mid to lower SEER units with the same reliability assumption.

Is there any information that proves or disproves my assumption that higher SEER units are less reliable?

Thanks, I really appreciate the feedback,
Steve
 
 

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