We replaced a Trane 15 ton rtu with a carrier rtu. It seams with the new carrier we dont have enough airflow across the evap. This is with only one 7.5 ton stage running. Its r410 a. My sub cooling is 3 my superheat is 3 and my suction temp is 27 degrees. Is there a work around to get my suction pressure up and superheat. I've tried turning the vfd all the way up but then have return air issues and it only brought my suction temp to 30 degrees. Any help would be appreciated. Hate to eat a 15 ton rtu.
Oversized RTU freezing coil
NMTech
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So Worthless when it comes to this!
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You'll lower your sub cooling.
Really I thought it would raise both? Never mind. Cant think straight today.
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As a permanent fix?
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..... and run like hell.
:D:
I have a Carrier RTU that I service that is way oversized. It has 2 circuits. It was freezing up due to lack of airflow. It was not on a VFD I maxed the motor amps by adjusting the sheave. What I did was disabled circuit 2 and blocked the upper coil. The coils were stacked, 1st stage lower 2nd stage upper. Worked like a charm. You will obviously need to make sure your coils are set up this way.
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Thanks. I'll look into it. When I turn up the motor speed all of the way I dont get enough return from the building. It starts pushing alot of air out the door and starts to whistle pretty bad.
This is the model # I am working withThis is the style I did it on.![Image]()
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50TC-D16A2A6-0A0G0 carrier
Whats your min outdoor air setting at? Might need to set it to a min of 50%.
Are you sure the old Trane wasn't a 12.5 ton, and that someone didn't read the 150 as being 15 tons instead of 150,000BTUs/12.5 tons?
Did the Trane have hot gas bypass on it?
Basically, your saying you have a 15 ton RTU that isn't able to move 2600 CFM through the duct work. What size is the supply and return duct.
Are you sure the old Trane wasn't a 12.5 ton, and that someone didn't read the 150 as being 15 tons instead of 150,000BTUs/12.5 tons?
Did the Trane have hot gas bypass on it?
Basically, your saying you have a 15 ton RTU that isn't able to move 2600 CFM through the duct work. What size is the supply and return duct.
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the next guy will be like .... wtf is this shit :eek2:
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They could run the power vent fans and crank up the min pos. That would help.
Is your fan spinning the right way?
Sounds like you need some HGBP or a RAWAL valve.
I would also install head pressure control / motor master and set for 400psi. This will keep the head up, discharge superheat up, and keep the suction up to a certain point.
Could even try a EPR.
Sounds like you need some HGBP or a RAWAL valve.
I would also install head pressure control / motor master and set for 400psi. This will keep the head up, discharge superheat up, and keep the suction up to a certain point.
Could even try a EPR.
LOL I did the same thing on the exact same type unit . Also worked well.
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And the guy before that looked at the “12” and thought it was 12 tons instead of 120,000!
Increasing airflow with what already seems like a low charge will not help. If you do anything to the charge it would be to add refrigerant, seeing how you have a 3 sub cool. I would really like to know your pressures along with indoor/outdoor temperature.
If you do anything with the VFD it would be to decrease airflow which I know seems counter intuitive because you already have a low suction temperature, but adding refrigerant or more fresh air can fix that part.
Before going all crazy and modifying the system even more, add some refrigerant and see what happens.
Adding in some fresh air, slowing down the blower(for SP only), and adding refrigerant can more than likely fix all of your issues.
And if this just so happens to be a 2 stage system you know what must be done.
Edit: Guess you know why the other unit ran the power exhaust.
If you do anything with the VFD it would be to decrease airflow which I know seems counter intuitive because you already have a low suction temperature, but adding refrigerant or more fresh air can fix that part.
Before going all crazy and modifying the system even more, add some refrigerant and see what happens.
Adding in some fresh air, slowing down the blower(for SP only), and adding refrigerant can more than likely fix all of your issues.
And if this just so happens to be a 2 stage system you know what must be done.
Edit: Guess you know why the other unit ran the power exhaust.
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i seriously question your advice.Increasing airflow with what already seems like a low charge will not help. If you do anything to the charge it would be to add refrigerant, seeing how you have a 3 sub cool. I would really like to know your pressures along with indoor/outdoor temperature.
If you do anything with the VFD it would be to decrease airflow which I know seems counter intuitive because you already have a low suction temperature, but adding refrigerant or more fresh air can fix that part.
Before going all crazy and modifying the system even more, add some refrigerant and see what happens.
Adding in some fresh air, slowing down the blower(for SP only), and adding refrigerant can more than likely fix all of your issues.
And if this just so happens to be a 2 stage system you know what must be done.
Edit: Guess you know why the other unit ran the power exhaust.
I don't know what I was thinking.
Tell me, what would you do with an oversized unit?
Get more air to it, slow it down, don't use 100% capacity. All seemed reasonable to me. Now tell me, what would you do?
And if you could tell by his first post, changing it isn't an option. Now tell me.
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