Dry Bulb Temps 38F Discharge 66F Return- House Not Cooling Down


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Old 05-16-13, 07:42 PM
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Question Dry Bulb Temps 38F Discharge 66F Return- House Not Cooling Down

House is 3000 SF. The A/C is Lennox 10ACB60-11P AC Condenser with Lennox C33-50/60C-2 Evaporator Coil sitting atop Lennox 80MGF4/5-140A-12 Furnace.

Just replaced main blower motor and installed Honeywell Comfort 2.0 with EIM and D & R sensors. The blower wheel is clean, the Evap coil seems clean and the condenser outside is clean. Clean media filter and pretty strong airflow on all vents (none closed or blocked). The supply line gets surface frost and the return line is slightly warm.

The A/C was barely keeping up before. Now the house won't cool down below 74F even on 80F days and with all blinds closed. I double checked the main blower connections on the SureFire furnace board and they seem correct. The heat is on Medium, the A/C is on Medium Low and the fan is on Low.

Did the setup diagnostics on the Comfort TS. Here are the numbers:

Run Time - 30 min

Discharge Temp - 39F
Return Temp - 66F
Delta-T - 27

Indoor Humidity - 26%

Outdoor Humidity - 25%
Outdoor Temp - 84F

The A/C was serviced last year, but I know that the tech was not good. The drip pan was clogged and the condensate was leaking all over the furnace. He did not said anything at all. At that time I was clueless about HVAC so I was not aware of the importance of that detail.

What I need now is some advice as what may be wrong so when I call service I will have some idea and know if I am being BS'd or not.

Thanks.
 
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Old 05-16-13, 08:05 PM
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Your sensors should probably be moved further from the evaporator coil.
You posted that it is 74 degrees in your space but the sensor display is 66 degrees.

This can also occur with a stuck bypass damper, but you made no mention of a zoning control.

Have your unit serviced and the refrigerant charge checked.
 
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Old 05-16-13, 08:21 PM
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The heat is on Medium, the A/C is on Medium Low and the fan is on Low.
I'm assuming the blower runs on medium for heat and medium low for AC.

Not quite sure what the fan on low means.

Could you have the fan running too slow in AC mode ? Usually the AC is high speed and the heat is slower.
 
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Old 05-16-13, 08:36 PM
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Continuous fan has a separate terminal on the board.
It is odd to have the cooling speed so low.
 
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Old 05-16-13, 08:40 PM
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That furnace blocks the blower assembly with a panel.
Did you verify the blower rotation?
 
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Old 05-16-13, 09:43 PM
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There are no zones.

The blower is running in the right direction.

I did hook up the Medium High lead from the motor to the A/C terminal on the board and did get 37F discharge temp which is 2F lower, however the return was still 66F and the return temp did not change at all.

This makes me think that I may have a faulty return sensor. I will switch the two sensors and see if that's the case. I will also try to hook up the A/C terminal with the High lead on the motor and see what happens.

Anybody from Denver area know of a reputable and certified HVAC tech i could call?
 
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Old 05-17-13, 05:24 PM
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It sure sounds like supply air is getting into your return air ducting.
A 37 degree supply air temperature could also point to an airflow problem.

Did you see it spinning the correct direction, or are you assuming that the rotation is correct because you feel air at the supply registers?

Did you install the new blower motor yourself?
Was it a universal motor?
Universal motors usually have a couple of plugs that you can switch to reverse the rotation.

Let us know what you find.
 
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Old 05-17-13, 06:33 PM
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The supply line gets surface frost and the return line is slightly warm.
Are you talking about the refrigerant lines?
The large tubing is the suction vapor line coming from the evaporator. Is that the one that is frosting?

The small tubing line is the high pressure liquid line going to the evaporator, it would normally be a little warm; it would not frost-over unless restricted.

With that low a latent load & Denver's high elevation the evaporator is not getting enough heatload to absorb. With that low of indoor humidity it would need 2100-CFM to get an adequate load on the evaporator coil.

Even with an indoor 50% RH & 75F Return air, with a 5-Ton system, it is difficult to get enough airflow through the coil & through the duct system to the rooms...

If it has a Thermostatic Expansion Valve (TXV) metering device, it will throttle the refrigerant flow down to keep the superheat near its setting.

Check the heat-rise of the air discharge from the outdoor condenser; & list what it is.

With those low outdoor & too low a dry Return Air temp, the system will tend to have an evaporator lack of heatload problem.

Try it TO SEE WHAT IT DOES when it gets much warmer, both outdoors & INDOORS...
 
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Old 05-18-13, 04:28 AM
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The A/C was barely keeping up before. Now the house won't cool down below 74F even on 80F days and with all blinds closed.

Discharge Temp - 39F
Return Temp - 66F
Delta-T - 27
Why is the Return Air at 66F, when the room temp (or RM/TH) is say, 74F? That is a huge 8F differential.

Perhaps, the Supply Air is too near the RA register; or you have a very imbalanced SA duct system.(?)
 
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Old 05-24-13, 10:25 PM
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Since your a/c is 5 tons, the black high speed lead MUST be connected to the cooling terminal. The blower in your furnace is rated for up to 5 tons of cooling on the highest speed. Even with the blower on high, it may not move enough air. Raise the speed and re-check the return and supply air temps.

27F delta-t = low airflow; low indoor humidity also increases the split.

I also recommend actually measuring the supply air/return air temps with a digital thermometer to verify the readings.



Things that can cause a low return air temp:

- Humidifier bypass damper left open
- Too much r/a coming from basement
- Returns are at floor level; ceiling level returns are better
- Supplies too close to returns
 
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Old 05-27-13, 09:12 AM
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Thanks for all your replies. Sorry for not replying earlier but was quite busy with other stuff.

I did hook up the HIGH lead to the AC terminal and it is performing better.

However I believe that there is a problem with the return ducting. There is a section that runs parallel to the engineered wood floor I beams. The return uses two I beams that are enclosed by piece of sheet metal on the bottom. There is no mastic sealing it so I believe that air from the crawl space gets sucked in and therefore the hot air from inside of the house does not get evacuated enough. I felt the return registers and the air movement is almost undetectable. Another hint is that I always had problem with dust.

I did get digital thermometer and measured the temps at the sensor placement and then again at the supply and return registers. The temps were 40F and 67F at the furnace and 40F and 73F at the registers.

I have appt scheduled with certified tech who also has lots of experience with sheet metal fabbing. We'll see what he suggest, to seal the ducts with mastic or to replace the I beam section with ducting. I will post the results once it's done.
 
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Old 05-27-13, 01:17 PM
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Where is the ductwork located? where are the returns?

One of the greatest problems with your system is probably a low load on the coil due to a return air problem. The lower the return air temperature/humidity combined (removing moisture is a significant part of the cooling load), the lower the capacity will be. (the same goes for airflow across the indoor coil)

A 5 ton unit only provides 5 tons of cooling at 80F indoor, 50% humidity.
 
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Old 05-27-13, 03:14 PM
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The ductwork is in crawlspace. The house only has partial basement; just one small room. The rest, which is the majority is an insulated crawlspace.

As for the returns, the problem is that four are at the ground floor approx four inches from the floor. Two are on the first floor just below the ceiling.

Another problem is that the house has open plan with cathedral ceiling in the living room and four approx 6 feet x 6 feet windows in the family room. The radiant heat gain is crazy even with closed blinds.

Regarding the humidity. I have consistent 30 % humidity inside with the humidifier shut off and outside humidity lot less (15 - 25 % on most days as this is Colorado). Would turning on the humidifier help the A/C?
 
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Old 05-27-13, 10:13 PM
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Humidifying in the summer would make your system perform worse.

I was just saying that humidity is a significant part of the cooling load.

66F air with 60% humidity has a lot more heat in it than 66F air with 30% humidity even though the temperature is the same. Moisture in the air has a form of heat in it which can't be measured by a thermometer; that heat is extracted by the a/c when moisture condenses on the indoor coil.

But anyway, the more (total) heat and there is in the return air, the greater cooling capacity becomes. Cold return air combined with other problems could be reducing the capacity of your system by 10% or more. That's what HVAC RETIRED was alluding to in his first post.

So, the trick in your case is to figure out why the return air is 66F instead of the room temperature.

High supply air leakage into the crawlspace (making is cold down there), combined with high return air leakage can cause that problem, as can floor level returns (or leaving a humidifier damper open during the cooling season.)

Try measuring the temperature near the floor where the returns are. If there's a significant temperature difference between what the returns are pulling in and the return air at the furnace, air is bypassing somewhere or something else is wrong.
 

Last edited by user 10; 05-27-13 at 10:35 PM.
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Old 05-27-13, 10:31 PM
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Edit: I didn't even see the part about replacing the blower motor - you sure it's the right RPM and HP?

I would think that if it was spinning in the wrong direction, it wouldn't be moving much if any air.
 
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Old 05-28-13, 07:18 PM
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Muggle, the motor is OEM Lennox 28F01 same as the original. I knew that it was really important to get the OEM replacement in this case because the motor is rated at 3/4hp but it's actually under-rated and stronger than that. What is ironic is that the old motor turned out to be OK. The problem was that there was no bolt on the blower wheel hub that secures the wheel to the motor axle and prevents it from slipping. Talk about a shoddy work whoever worked on this system before.

Just to confirm, there was no markings on the capacitor leads, and no markings on the capacitor contacts. Can you confirm that it does not matter how they are connected?

I am starting to believe that I have problem with the supply and return ducting not being sealed in the crawl space as you mentioned. As I stated there is lots of dust in the house and when the A/C is on there is always a little musty smell.

If that's not the cause then it must be the location of the return registers. Four of the six are on first floor next to the floor which means that they are sucking in a lot cooler air that is actually at eye level or at the ceiling.

I have whole house humidifier but it is not the bypass type so there is no damper. It is mounted directly to the supply plenum.

I have HVAC tech coming over in couple of days and he will do maintenance of the furnace and A/C and check the ducting for disconnects or leakage. We'll see what he says.

Muggle and all. Thanks for your great replies. This is one great forum. I did learn some new stuff and appreciate that greatly. I will post later what the outcome will be.
 
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Old 05-28-13, 09:05 PM
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I'm always surprised to see a 40 microfarad cap on Lennox blower motors when most brands use a 10 to 15 microfarad cap. Did you get a new cap?

If your tech shows up with a manometer(He should have a manometer) ask him to get the static pressure of the return air entering the furnace and the supply air leaving it but before the evaporator coil.





A thermometer reading at the filter grill and at the furnace return air side would also be a good idea. A temperature reading on the leaving air side would help establish if your sensors are okay.
 
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Old 05-29-13, 10:47 AM
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Yes I did get new cap and I asked for static pressure check and leak check on the ducting when I scheduled the appointment. Hopefully we'll get to the bottom of this problem.

Interestingly I talked to my neighbor cross the street and she has the same problem. The A/C won't go below 72 no matter what. I really am starting to believe that this is a design flaw that is exacerbated by the fact that we are at 6200 feet ASL and the houses have huge windows facing South.

I am looking at outside shades to mitigate the sun problem to some extent. We'll see.
 
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Old 05-29-13, 02:41 PM
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That's not really a problem; 72F at low humidity should feel freezing. Central a/c is designed to maintain a reasonable setpoint.

Creeping up above 78F in spite of running continuously on the hottest day of the year is a problem.

In the low to mid 80s (outside), a 5 ton unit shouldn't be running much at all to maintain a reasonable setpoint, so there's definitely a problem with your system or there's excessive heat gain.

Poor attic insulation/ventilation might be part of issue. (example - soffit vents blocked with wood)
 
 

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