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Discussion Starter · #1 ·
Hello, I got my new system installed(Lennox 16ACX, ML296V) last month, and Nest wired as 2-stage cooling. What concerns me is I double it the system is wired as 2-stage cooling. I never had 2 stages of cooling before, but it feels like the 1st stage is pretty strong cooling and I don't feed the comfort improvement with 2 stages at all.
I checked the installation manual on both the furnace and ac unit and found 2 suspecting points.

1. The furnace manual says:

"The unit is set up at the factor for single stage cooling. For two stage cooling operation, clip the jumper wire located between the Y to Y2 terminals on the integrated ignition/blower control board"

So what does this "clip" mean here, does it mean to cut the jumper wire that is there for the default single stage? I opened the furnace door and found a jumper wire between Y and Y2 is there, no cut. I doubt the crew forget to do that and left my used as a single stage AC?

2. Next question is about the wire from the furnace to the outdoor unit, my old system had 18-2 cables, so only 2 wires in that cable, and my 16ACX manual shows there should be 3 wire to the outdoor unit, Y1, Y2 and C. However that 18-2 wire goes outside now connects to the Y1 and Y2, and there is no wire for the C connection. Seems like they should change out that cable to a 3 wire cable and they somehow cut corners to have a C wire from elsewhere in the outdoor unit?
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Thanks in advance!
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There are wires at the furnace connected to both Y1 and Y2, but based on what you said, not at the a/c.

They need to run a wire with enough conductors for the a/c.

They also need to cut the jumper between Y1 and Y2 on the board, and I see that they did not. Without the jumper cut, the fan won't run at a lower speed when on low cooling, which will really negatively impact dehumidification

They also need to set the board's jumpers or dip switches governing cooling airflow so airflow is within correct range for size of a/c you had installed.

To start, pull the nest off the base and check the wiring -> you should see W1, W2, Y1, Y2, R, G, C hooked up at the stat.

W2 is actually optional with the 2-stage furnace as the board has a timer which can be set to bring on high after x minutes, but it is far superior to have the t-stat control the stages.

Considering they made elementary errors, I doubt the unit was ever charged right and so that should be checked too.

What size a/c and furnace did you get, and what size is the house? Where are you located?
Describe the house.
 

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Discussion Starter · #3 ·
There are wires at the furnace connected to both Y1 and Y2, but based on what you said, not at the a/c.

They need to run a wire with enough conductors for the a/c.

They also need to cut the jumper between Y1 and Y2 on the board, and I see that they did not. Without the jumper cut, the fan won't run at a lower speed when on low cooling, which will really negatively impact dehumidification

They also need to set the board's jumpers or dip switches governing cooling airflow so airflow is within correct range for size of a/c you had installed.

To start, pull the nest off the base and check the wiring -> you should see W1, W2, Y1, Y2, R, G, C hooked up at the stat.

W2 is actually optional with the 2-stage furnace as the board has a timer which can be set to bring on high after x minutes, but it is far superior to have the t-stat control the stages.

Considering they made elementary errors, I doubt the unit was ever charged right and so that should be checked too.

What size a/c and furnace did you get, and what size is the house? Where are you located?
Describe the house.
Thanks for replying. I checked the thermostat to furnace connection, they have those y1 y2 w1 w2 wires connected to the furnance board.
I am in Kansas, 2700SF 2 story home, equitments are:
1-ML296UH110XV60C-58 / GAS FURNACE
1-CX35-48C-6F-20 / 4-TON / COIL
1-16ACX-048-230A01 / 4-TON / 16-SEER / AC

I also did a test so I stand outside by the condenser unit and watch it kicks on and feel the compressor and blower speed, after 2 or 3 minutes, I set my thermostat to be much lower set point and it shows it is now in COOL STAGE2, however, I did not observe any change the compressor noise level and blower fan speed. That's how I see the 2 stage cooling is not in effect.

What can be the consequence if the unit not charged properly? too cold? I did not see ice build up in the liquid line by the evaporator coil.
 

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It won't run efficiently and at rated capacity if it's not properly charged in accordance with manufacturer instructions, if really off, could end up with more major issues.

The unit comes with refrigerant for 15 ft of lines, more or less and very high probability charge has to be adjusted.

The compressor noise most likely will not change much when on low cooling - but the indoor fan airflow should drop. Cutting the board's jumper which links Y1 and Y2 is mandatory to get the fan to run at lower speed on low - says so in pics of manual u posted.


The furnace has a 5 ton/2000 cfm rated drive blower, so the cooling blower cfm has to be dropped from factory. It should be set to 1600cfm on high/1200 on low or 1400 on high/1050 on low for better humidity removal.

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Looks like they screwed you too by grossly oversizing the furnace - that should have been 90k absolute worst case with zero wall insulation, and perhaps easily 70k. You're not so north, needing enough heat for -40F and lower.



The a/c seems large but could be warranted and would have had to pump up to a 90k to get a blower capable for 4 tons of cooling.

With such a big furnace, I would be very tempted to outright disable high fire, would be about 70 000 on low.
 

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why are you not asking the people that put it in for a proper function demo?

you touch it you own it
(n)

...to get armed with information to be able to talk to the contractor and get the install errors corrected.
The more equipment owners know, the better and the lower the risk of being taken advantage of.
 

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So isn’t the purpose of 2 stage cooling … better efficiency? How would you be able to tell if it’s running in stage 1 or stage 2? Isn’t the theory that it runs at lower power when there is less demand, but in any case it runs as needed to satisfy the t-stat set point.
 

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So isn’t the purpose of 2 stage cooling … better efficiency? How would you be able to tell if it’s running in stage 1 or stage 2? Isn’t the theory that it runs at lower power when there is less demand, but in any case it runs as needed to satisfy the t-stat set point.
Tell by fan speed/airflow and length of cycles.

The primary benefit is enhanced comfort due to longer run times - more consistent temperature. Providing less cooling over a longer time in itself doesn't save much energy, the same amount of heat must be removed to maintain setting.

It just so happens low cooling is more energy efficient than high cooling- larger indoor/outdoor coils relative to capacity, compressor has to do less work relative to amount of cooling. But that improvement is already reflected in the efficiency rating.
 

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Discussion Starter · #9 ·
It won't run efficiently and at rated capacity if it's not properly charged in accordance with manufacturer instructions, if really off, could end up with more major issues.

The unit comes with refrigerant for 15 ft of lines, more or less and very high probability charge has to be adjusted.

The compressor noise most likely will not change much when on low cooling - but the indoor fan airflow should drop. Cutting the board's jumper which links Y1 and Y2 is mandatory to get the fan to run at lower speed on low - says so in pics of manual u posted.


The furnace has a 5 ton/2000 cfm rated drive blower, so the cooling blower cfm has to be dropped from factory. It should be set to 1600cfm on high/1200 on low or 1400 on high/1050 on low for better humidity removal.

View attachment 694552



Looks like they screwed you too by grossly oversizing the furnace - that should have been 90k absolute worst case with zero wall insulation, and perhaps easily 70k. You're not so north, needing enough heat for -40F and lower.



The a/c seems large but could be warranted and would have had to pump up to a 90k to get a blower capable for 4 tons of cooling.

With such a big furnace, I would be very tempted to outright disable high fire, would be about 70 000 on low.
Thanks for pulling the cfm data to help me out. I actually grabbed a mat and sit in front of the furnace and watched the cfm led blinking when y1 calls, for the first 7.5 minutes, it has 15 blinks and after 7.5 minutes, it blinks 18 times. And installer did not touch the factory preset cfm jumpers, so that proves I am in single stage cooling mode. The furnace manual page 53 described it, see my pic.

For the unit sizing, I got multiple bids and they all give me 4-ton AC(the old unit was 4-ton) and 4 or 5-ton furnace. A guy said it's their practice to size the furnace 1 ton up than AC. And BTU-wise, the old unit(28 years old) was 125k, so all bids give me 96% with BTU lowered to 110K. Maybe that's a regional preference here?
(Kansas City) I do have large windows though.
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Discussion Starter · #10 ·
why are you not asking the people that put it in for a proper function demo?

you touch it you own it
They did give me demo before they go, but I wasn't vigilant enough to ask them to show the 1 stage and 2 stage cooling. They were cranking the thermostat to 60ish degree and unit was on Y2 all the time when being demoed.
 

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Discussion Starter · #11 ·
(n)

...to get armed with information to be able to talk to the contractor and get the install errors corrected.
The more equipment owners know, the better and the lower the risk of being taken advantage of.
Exactly, I used to write software for commercial retrofit controllers in this field so I can easily understand the manual and stuff, but for most homeower they many not notice this problem if they have it.
 

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Discussion Starter · #12 ·
Tell by fan speed/airflow and length of cycles.

The primary benefit is enhanced comfort due to longer run times - more consistent temperature. Providing less cooling over a longer time in itself doesn't save much energy, the same amount of heat must be removed to maintain setting.

It just so happens low cooling is more energy efficient than high cooling- larger indoor/outdoor coils relative to capacity, compressor has to do less work relative to amount of cooling. But that improvement is already reflected in the efficiency rating.
So help me understand this, for the 2 stage ac(16ACX for me), and once it is on a low stage, other than the lower indoor cfm, what will happen at the outdoor unit? Will the compressor lower its speed? and how about condensing fan, will it get lower speed? How, as a homeowner, not opening the control panel knows an outdoor unit is on low stage or high?
 

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So help me understand this, for the 2 stage ac(16ACX for me), and once it is on a low stage, other than the lower indoor cfm, what will happen at the outdoor unit? Will the compressor lower its speed? and how about condensing fan, will it get lower speed? How, as a homeowner, not opening the control panel knows an outdoor unit is on low stage or high?
Today's 2-stage scroll compressors run at the same speed on low and high, but use an internal bypass to cut capacity and solenoid which enables and disables that, energized based on Y2 signal.

Some 2-stage ac units use a single speed outdoor fan motor, fancier ones use 2-speed or variable.

Straight from emerson:
"How does a Copeland scroll two-stage compressor work? Two-stage modulation adjusts compressor capacity by bypassing a portion of the gas in the scroll back to suction. By doing so, the compressor can closely match the heating or cooling demand allowing it toRead More... cycle less frequently. "
 

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Thanks for pulling the cfm data to help me out. I actually grabbed a mat and sit in front of the furnace and watched the cfm led blinking when y1 calls, for the first 7.5 minutes, it has 15 blinks and after 7.5 minutes, it blinks 18 times. And installer did not touch the factory preset cfm jumpers, so that proves I am in single stage cooling mode. The furnace manual page 53 described it, see my pic.

For the unit sizing, I got multiple bids and they all give me 4-ton AC(the old unit was 4-ton) and 4 or 5-ton furnace. A guy said it's their practice to size the furnace 1 ton up than AC. And BTU-wise, the old unit(28 years old) was 125k, so all bids give me 96% with BTU lowered to 110K. Maybe that's a regional preference here?
(Kansas City) I do have large windows though.
View attachment 694571
The ramping profile is for humidity control, the fan runs at a lower speed for the first few minutes to get the indoor coil cold fast.

The feature can be disabled.

Sizing a furnace based on size of the old is absolutely wrong, as most old furnaces were grossly oversized - back from when the thought was "bigger is better" and load calculations were not mandatory on new construction.
The house may have changed since the last furnace was installed - new windows, air sealing and insulation reduce heat loss and heat gain.

The correct way to size is to do a heat loss calculation.

Further, new furnaces of the same capacity have higher airflow requirements compared to those made 28 years ago, so it is very unlikely your ducts are sized large enough for a new 110k 96%. Your new one moves almost as much air on high as a 5 ton a/c does at standard 400 cfm per ton -> at factory setting, in high heat mode, fan tries to move 1800 CFM, greater than what your new a/c needs.

Installing a furnace with 1 ton higher blower capacity than size of a/c makes sense when dealing with a multi-speed blower motor and undersized air ducts, as airflow drops below rated/nominal cfm as duct pressure exceeds rated.

Even the newer multi-speeds do better in this respect, as they're constant torque - electronically controlled.

The motor in your new furnace on the other hand is a variable speed/constant airflow type that automatically ramps itself up or down to directly deliver required airflow.
Within reasonable limits, a 4 ton drive variable speed is capable of maintaining full rated airflow at higher duct pressures compared to multi-speed motors.

As well, some 80 and 90k furnaces are available with a 5 ton/2000 cfm rated blower, eliminating the need to as grossly oversize the furnace to meet cooling airflow needs.
 

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The outdoor unit has no circuit board and 3 low voltage wire connections -> Y1 turns it on, Y2 enables high cooling, and C is common.

They probably tied Y1 and Y2 together in the outdoor unit so it runs on high only and the effect on the furnace blower operation in cooling mode is the same without the jumper cut -> full cfm only.

A real piss-off because it just shows the installers don't care and think you wouldn't notice. Really, the sales-person should have noticed you don't have enough wires going to the outdoor unit.
 

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Discussion Starter · #17 · (Edited)
The outdoor unit has no circuit board and 3 low voltage wire connections -> Y1 turns it on, Y2 enables high cooling, and C is common.

They probably tied Y1 and Y2 together in the outdoor unit so it runs on high only and the effect on the furnace blower operation in cooling mode is the same without the jumper cut -> full cfm only.

A real piss-off because it just shows the installers don't care and think you wouldn't notice. Really, the sales-person should have noticed you don't have enough wires going to the outdoor unit.
If they tied Y1 and Y2 outside, then there should be a Y1 and C going out, right? But from my furnace board, it is Y1 and Y2 that are connecting with the 2-conductor cable going out, not Y1 and C. That part I don't understand. And also, I am not sure if the w1 and w2 are wired correctly for 2 stages, I see a w1 and c wire goes up into the furnace chamber, shouldn't it be w1 and w2 then c? I do want my t-stat to control the staging, not furnace internal timer.
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That is very strange if common was not connected somewhere a/c would not work - so must be another splice/change along the way - possibly right outside the furnace.

Having wiring between W1 and C - that's to power an accessory like a humidifier that energizes when there is a call for heat.

If you want to solve this yourself, you'll need to trace all the wiring you have access to.
Really, the contractor should be fixing this - ask for a service tech to come, not the original installers who screwed this up.
 

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Discussion Starter · #19 ·
That is very strange if common was not connected somewhere a/c would not work - so must be another splice/change along the way - possibly right outside the furnace.

Having wiring between W1 and C - that's to power an accessory like a humidifier that energizes when there is a call for heat.

If you want to solve this yourself, you'll need to trace all the wiring you have access to.
Really, the contractor should be fixing this - ask for a service tech to come, not the original installers who screwed this up.
That is very strange if common was not connected somewhere a/c would not work - so must be another splice/change along the way - possibly right outside the furnace.

Having wiring between W1 and C - that's to power an accessory like a humidifier that energizes when there is a call for heat.

If you want to solve this yourself, you'll need to trace all the wiring you have access to.
Really, the contractor should be fixing this - ask for a service tech to come, not the original installers who screwed this up.
yes, that w1 and c wire goes to the power humidifier, very positive.
I did call the company about this issue and am waiting for them to come out, and thanks to the help in this forum, I will not be in the dark when they arrive.
 

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That is very strange if common was not connected somewhere a/c would not work - so must be another splice/change along the way - possibly right outside the furnace..
My guess is that they used ground as common. Not legal but done sometimes. I don't think that board would allow enough current to pass through the y²/y¹ terminals to engage the contactor.
 
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