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I have followed this thread and I do believe trying to show the math to your TECH is a waste of your breath and effort. He has had several oppertunities to show his skill level and has done a great job of it. Time for the owner of the company to get involved or at least the SENIOR troubleshooter.
 

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the temp difference between outside and supply air doesn't tell you how much heat is being removed, what matters is change in temperature and humidity between return and supply, along with airflow rate.
Good point. But when you walk in from outside, you feel the difference.
 

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The calculation I used that is looking for sensible BTU's removed is calculated as Qs = 1.08 * CFM * Temp difference (return air vs supply air). The total heat calculation you provided earlier yielding 33.1 MBH and the ratio of sensible to latent 0.62 (which is not unreasonable). I suspect the contractor did not have a manual J and guess on the size. Combine the guess with a poor start up and you may have an underperforming unit.

Also, word of caution: design temps are 76F in cooling for codes and while the unit is maintaining 74F, while doing it very inefficiently, it does not have to go cooler from an energy code perspective. You may get push back because of this.
 

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My home unit can get to 22 degrees differential by running for more than 10 minutes when it is triple digits outside. Normal cycling is 20 degrees. I spent a grand on fixing the bottom dollar flexible duct work in the ceiling. When I replaced the unit 4 years ago.

I suggest that you get an cubic foot measurement device and measure at all of your ducts.
you can measure the room and based on cfm being delivered you can see if the air is close to the chart.

Every ac system I have worked on has a multi speed blower. It is in the chart from the MFG.
You might try moving the speed up one and see how it works.
Faster air across the coil will not be as cool. De humidification might suffer a bit as well.
It is a though that I have not seen raised. Have you checked the unit itself? Run your hand around where the duct meets the air handler? no leaks?

Sadly just changing equipment does not do what most people think it will do. Duct work and construction has a lot to do with it.
 

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Roughneck/user1234,

I added all measurements again below, including data from pictures as well as added AccuWeather info which varies slightly from measurements. I think this answers both of your questions. Tech only had one temp probe, hence 2 pictures above, one to capture SH and one for SC.

Airflow appears~1500cfm per earlier thread.
Outside ambient 93 deg, 49%RH (measured)
AccuWeather said 87deg, 58% RH outside
handler return 73.2deg , 49% RH
Handler supply 60.1deg, 86% RH
From picture, High side:
325 psi
8.8 SC
101.4 LSat
From pictures, Low side:
142.0 psig
2.3 SH
49.9 VSat
52.2 SLT
92.7 LLT

Please note that Goodman specs for this TXV based system are 7 to 9 for both subcooling and superheat. The Tech removed some freon yesterday to get subcooling in spec (he raised it last week to get it in spec. SC numbers aren't very stable. Normal?). Superheat is only 1/3 of spec. He couldn't get both SH and SC in spec at the same time. System performance didn't seem to vary with his freon changes last week and this week.
You have a major air leak or your measurements are wrong. You're picking up significant moisture.

Most hydrometers cheaply available are not accurate much above 60% RH. Your supply RH would need to be below 75% if you don't have a significant amount of upside air pouring into the return.


 

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Discussion Starter · #26 ·
The calculation I used that is looking for sensible BTU's removed is calculated as Qs = 1.08 * CFM * Temp difference (return air vs supply air). The total heat calculation you provided earlier yielding 33.1 MBH and the ratio of sensible to latent 0.62 (which is not unreasonable). I suspect the contractor did not have a manual J and guess on the size. Combine the guess with a poor start up and you may have an underperforming unit.

Also, word of caution: design temps are 76F in cooling for codes and while the unit is maintaining 74F, while doing it very inefficiently, it does not have to go cooler from an energy code perspective. You may get push back because of this.
You got a little over my head there. I see that you grabbed formula that I thought was only for heat. I thought the formula that I used was required for AC.

I don't understand manual J, etc, but will look it up. I get that the tech didn't do his due diligence, I wish I could back in time ... I would do several things differently.

3.5T is theoretically 42000. I assumed this is input BTUs and output BTUs will only be a fraction of that. Is that correct?

Using your formula with 21k btus, it is 50% efficient. Is that really low? What should it be, 80%? I have no idea how bad that actually is.
 

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You got a little over my head there. I see that you grabbed formula that I thought was only for heat. I thought the formula that I used was required for AC.

I don't understand manual J, etc, but will look it up. I get that the tech didn't do his due diligence, I wish I could back in time ... I would do several things differently.

3.5T is theoretically 42000. I assumed this is input BTUs and output BTUs will only be a fraction of that. Is that correct?

Using your formula with 21k btus, it is 50% efficient. Is that really low? What should it be, 80%? I have no idea how bad that actually is.
For temp change only (sensible) :
Q = 1.08 × CFM * ∆T

To consider humidity change
Q = 4.5 × CFM * ∆H

(∆H is the change of enthalpy. It is the total energy between humidity and temperature of the air. It is more accurate but harder to measure. Some processes like dry heating don't effect humidity, so help the sensible calculation is valid and easier.)
 

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Both Jamesy and Beenthere both calculate my output at 21k BTUs. I am trying to figure out how you calculate that so I can prove this to my Tech.

BTUs=CFM * deltaH * 4.5

CFM=1500 (or less)
Return=73.2/49%, H=44.7 (from online calc)
Supply=60.1/86% H=39.7

So, BTUs = 1500 * 5 * 4.5 = 33.7k

I know you are right and I am wrong, but could you please tell me what I am specifically doing wrong.
My calc was sensible BTUs only, not latent BTUs. Sensible is the BTUs that change the temp of the air.

I used CFMXTemp DeltaX1.1 for cooling sensible BTUs.

Using Carrier software, at 1500 CFM, it says you are getting 21,597 BTUs sensible, and 7,366 BTUs, for a total of 28,963 BTUs. So less than 2.5 tons of cooling.

Okay, where thos temps taken with an accurate thermometer right at the return of the air handler, and in the supply plenum. Taken at a grille, they are not a true indicator of what the A/C is doing. Poor duct insulation can throw the temps off, making it look like the A/C units are under performing, when it isn't.

A meat thermometer generally isn't calibrated to be accurate at temps below 100°F.
 

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In your other thread, by the temp rise and heater KW you posted, the air handler is moving 1,777 CFM. But that air handler can't move that much air, even at only .01" of static.
 
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My calc was sensible BTUs only, not latent BTUs. Sensible is the BTUs that change the temp of the air.

I used CFMXTemp DeltaX1.1 for cooling sensible BTUs.

Using Carrier software, at 1500 CFM, it says you are getting 21,597 BTUs sensible, and 7,366 BTUs, for a total of 28,963 BTUs. So less than 2.5 tons of cooling.

Okay, where thos temps taken with an accurate thermometer right at the return of the air handler, and in the supply plenum. Taken at a grille, they are not a true indicator of what the A/C is doing. Poor duct insulation can throw the temps off, making it look like the A/C units are under performing, when it isn't.

A meat thermometer generally isn't calibrated to be accurate at temps below 100°F.
It's actually adding latent heat, not removing it. Sign is backwards.
 

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Yeah it is. But I'm attributing that increase to an inaccurate hydrometer. And giving the latent the benefit of the doubt.

Probably shouldn't though.
 
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Yeah it is. But I'm attributing that increase to an inaccurate hydrometer. And giving the latent the benefit of the doubt.

Probably shouldn't though.
Either it's a really bad measurement or a significant problem.

Regardless, it's telling us something is wrong.
 

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Was the actual return air temp/humidity at the air handler inlet measured, or did you just measure ambient?
 

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Discussion Starter · #34 ·
I have followed this thread and I do believe trying to show the math to your TECH is a waste of your breath and effort. He has had several oppertunities to show his skill level and has done a great job of it. Time for the owner of the company to get involved or at least the SENIOR troubleshooter.
Thanks for your input. You have a point and I had to reflect on that for a bit before answering.

I guess the issue is that I haven't proved to myself yet that the AC itself is not working. Therefore I am not prepared to push back hard yet. Just because the AC doesn't cool the condo doesn't in itself mean that the AC is no good. The fact that it barely works better than the original 2.5T unit suggests that there is a problem, but that isn't proof in itself.

What I was hoping for out of this thread was proof that the AC itself is broken. The Tech tells me it is fine. How do I know that it is not. When I read the posts this morning that the system was only 50% efficient I was thinking that this thread was just about wrapped up.

However, when it was agreed that formula with latent heat formula is more accurate, I am less sure again. This formula suggests AC is 78% efficient (33k/42k). What is reasonable efficiency? 85%? If so, isn't this pretty close? If so, maybe the Tech is right.
 

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Discussion Starter · #35 ·
Using Carrier software, at 1500 CFM, it says you are getting 21,597 BTUs sensible, and 7,366 BTUs, for a total of 28,963 BTUs. So less than 2.5 tons of cooling.

Okay, where thos temps taken with an accurate thermometer right at the return of the air handler, and in the supply plenum. Taken at a grille, they are not a true indicator of what the A/C is doing. Poor duct insulation can throw the temps off, making it look like the A/C units are under performing, when it isn't.

A meat thermometer generally isn't calibrated to be accurate at temps below 100°F.
Meat thermometer accuracy. My first thought was that I don't care about accuracy in itself as long as the error doesn't appreciably change across this small 10 degree range since it is the delta that we are measuring. Regardless, it does seem closely aligned to thermostat temperature measurement in this temperature range.

Temperature measurement placement. Return placement is right in front of filter for both meat thermometer and, yes cheap, RH gauge. Supply temperature was taken by drilling a small hole in middle of ductboard plenum and using same meat thermometer. Measurement at first register is less than 1 degree higher. My cheap RH gauge was problematic for supply plenum, so it was placed at exterior of first register. I was shocked when RH quickly started to rise when I placed it there but a quick googling suggested that was normal. The 86% measurement did seem suspect and took a long time to go back down after removed from that location.

Less than 2.5T of cooling. 28k/42k=68% efficiency. What should I be expecting? 100%? 85%? I am trying to understand what value is an actual fail. I am assuming that 42k is input BTUs and output btus should be appreciably less similar to my boiler up north. Is that correct?
 

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Discussion Starter · #37 ·
Either it's a really bad measurement or a significant problem.

Regardless, it's telling us something is wrong.
I see your dialogue with Beenthere, but I don't fully understand. Which measurement is suspect? The supply 86% RH? I am not sure how to improve that one,please see my response to Beenthere regarding measurement methodology. It is definitely higher than return though, is that an issue?

Any other measurements suspect? I think I did a reasonable measurement on the rest but I will re-measure anything questionable. Clarification on "something is wrong" would be greatly appreciated.
 

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Discussion Starter · #38 ·
In your other thread, by the temp rise and heater KW you posted, the air handler is moving 1,777 CFM. But that air handler can't move that much air, even at only .01" of static.
Hmm, I believe that either you or Supers05 mentioned that in that thread. I used the tables in air handler manual and came up with 1600cfm. However, tables only went up to 230v but I had measured 247v. I also used a cheap DMM with ammeter clamp that I just bought at Walmart for this purpose.. For this thread, I simplified to 1500 because the tech told me that Goodman stated that range of ECM motor was 1400-1500 CFM using current motor tap points. I believe someone stated in my other thread that heater airflow measurements aren't that accurate, so I called it a success when no air restrictions were indicated. Should I re-measure all of that or is 1500 sufficient for this discussion?
 

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1500 CFM is too much for an A/C that you want to remove moisture.

42,000 is the nominal BTU rating, not what it can actually do.

The RH reading is very questionable.

It is NOT working close to how it should, as it was never set up to work right. Charge should have been added to reach factory recommended SC, then the TXV adjusted to factory recommended SH.

I you look in the manual that came with the air handler, you will see the blower CFM listings.
 
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