Table of Contents
Scroll Compressor Inside the Condenser
Two-Stage Scroll Compressors
Two-Stage Scroll Compressors - For many years HVAC manufacturers used two compressors in their higher efficiency equipment to stage the air conditioner or heat pump for added efficiency. They would use a small compressor for the lighter loads or demands and then when the air conditioner or heat pump needed more demand the small compressor would shut down and the larger compressor would kick on to satisfy the demand.
The HVAC controls were often complicated and the equipment was difficult to troubleshoot when there was an air conditioner problem. There were all sorts of ways to accomplish staging, including one manufacturer that would sell one whole unit that was two condensing units manufactured as one piece of equipment. The air handler for that condenser has a dual slab coil for each stage of the air conditioner.
A two-stage cooling thermostat would kick the second compressor when the demand called for additional cooling. Then new technology became available, and that changed how the staging works for higher efficiency models.
Two-Stage Scroll Compressors
Then HVAC manufacturers began using scroll compressors and later an engineer figured out that the design of the scroll compressor can easily be modified to allow for a simple way to stage the compressor for low and high demand situations whenever it is needed. A standard single-stage compressor will kick on when the thermostat calls for heating or cooling.
Heating for a heat pump in heat mode. Cooling for both the air conditioner and heat pump for the cooling season. The controls are not complicated, and a single compressor can now be used for staging the air conditioner or heat pump. An engineer figured out that you can put a plate in the compressor. The plate has two holes drilled in it — one big hole for high demand and one small hole for low demand.
A solenoid is used to shift that plate for whatever mode is called for from the thermostat. That depends on the demand. So now, nearly every HVAC manufacturer is using the two-stage compressor for staging in their higher efficiency equipment.
Life After Two-Stage Scroll Compressors
There will come a day soon when modulating compressors are the king of the day just as two-stage scroll compressors are now. The compressor will have an ECM motor controlling the speed of the compressor, and the modulation will be somewhere from 25% to 100% of capacity. The whole HVAC system of the future will be able to modulate based on demand.
These types of systems are already available in commercial refrigeration and mini-split air conditioners and heat pumps. The systems will be very efficient and save lots of money on utility bills, which what the two-stage scroll compressors do now. The modulating systems will offer more efficiency than ever in the future.
Modulating vs Two-Stage vs Single-Stage Cooling Systems
The following chart demonstrates the temperature swing characteristics of a modulating system, a two-stage system, and a single-stage system. The modulating system maintains more even temperatures closer to the desired set point while the two-stage system has a little more temperature swing and the single-stage has the biggest temperature swings simply because of how it operates. A single-stage, on a call for cooling, will operate at 100% of capacity while the two-stage and the modulating systems have stages based on demand.
An example of a modulating air conditioning system is a VRV or VRF system. The V stands for Variable. The R stands for refrigerant and the last V stands for volume. A VRF is exactly the same thing with a different name. It stands for Variable Refrigerant Flow. These are much more complex systems but they deliver more comfortable temperatures with added efficiency over the two-stage or single-stage systems. We have a planned future article where we plan to devote the entire article to the modulating cooling systems.
Two-Stage Scroll Compressors
Technical Resource: Refrigeration and Air Conditioning Technology
I currently have a 25 year old Lennox 3Ton AC unit and replacing it this month, pulling the trigger on an HVAC contractor next week. Looking at single stage Vs 2-stage or Variable compressor units, which is better?? (I was told that variable would not cool as fast and like variable furnaces, would supply just moderately cool temps to/from my registers? True or Not??). I was also told that there was some sort of inverter tech involved in the variable speed systems, which could result in reliability and longevity issues (And double the cost for little gain – SEER 17 Vs SEER 18)? Is any of this true and Why would you or I, choose one over the other?? I would appreciate any and all input. Thanks ahead of time. PS, I’m in Iowa and test e do have extremes on both ends of temperatures.
Carrier “WeatherMaker” 48TC. What does “two-stage/single circuit cooling capacity control”
mean.
It likely means secondary control is done in the Carrier system rather than the thermostat.
How does a TVX valve on a single stage a/c system change things? Single stage costs a lot less than variable systems – less than 1/2. So it comes down to comfort and operating cost.
Dave,
TXV’s work on demand – see more here – https://highperformancehvac.com/txv-thermostatic-expansion-valve/
I have a Carrier model 25HCB636A0031030. Does this unit have a 2 stage cooling and is it compatible with Nest thermostat Y1 and Y2 cooling?
From what I read in the Carrier specs it is a two-stage heat pump and will require a thermostat that will stage. I believe NEST offers that feature with their thermostat.
I have single stage R22 Scroll heat pump. I am converting to R410 and during this conversion want to change to Two-Stage Scroll heat pump. My thermostat can do 2 stage, and air handler can be wired for 3 speeds.
I have 2 questions.
1. Can I just swap scroll compressors to a R410 2 stage compressor.
2. Does this require metering, if so how should this be done?
Not recommended. The oils are different from R-22 to R-410A and any residual oil from the R-22 system will destroy the R-410A compressor. It will also require different sizes of coils from the condenser to the evaporator and the line set. You will also have to rewire all the controls. It would be cheaper to just just install a whole new system.
What causes the copper pipe to the outside unit of a 2 stage HVAC to have a knock similar to water hammer?
the only time I’ve ever had this happen I thought it was non-condensibles in the loop. After recovering and doing a thorough evac it was normal again. That was a single-stage system though.
Thank you for your very informative article. I also have a Lennox HPXA19 with a matching Lennox variable speed handler. Three years ago a technician came rewire the system as we needed to relocate the outdoor unit slightly. I recently noticed that only 4 wires are connected to the circuit board inside the outdoor unit, they are R,C,Y1 & O. Given you answer to Ben last September I now can assume that the second stage of the compressor has not been engaged as Y2 is not connected. I would like to you ask you a some questions if you wouldn’t mind:
(1) How would not connecting Y2 affect power consumption during winter and summer?
(2) Would this set up cause more frequent kick in of aux heat during winter?
(3) Any concern of reconnecting Y2 now after the second stage not being used for a few years?
(4) Noticing W1 and L terminals are also not connected on the board in the outside unit, what are those for?
(5) I also noticed that ambient temperature sensor was removed and the end of the wires are connected. What effect would this have?
Many thanks,
Frank
Honestly, I can’t answer without further info. Not connecting Y2 would mean you don’t have 2nd stage if it calls for the second stage so the system is not running at full capacity when it is called for from the controls (stat).
Not all HPXA’S have a second stage so make sure you are sure about it. A jumper between Y1 and Y2 would likely mean it does not have staging but double check to make sure.
Without W1 connected at the condenser, you will not get the backup heat to come on when the condenser goes into defrost. This means cold air because in defrost mode the system switches to AC mode.
My best advice to you would be to have a qualified Lennox tech to come out and give it a once over and make the necessary changes based on exactly what you have.
Reconnecting Y2 (if so equipped) should not cause any detrimental issues even if it has not been used for a few years.
I have 2 stage AC unit, Lennox HPXA19. is there a way to disable the second stage? Thanks, Ben
You disconnect the Y2 wire in the thermostat. This means it will only run at the lower stage which will likely mean more run time when the temperatures are extreme. Also, it likely will not maintain comfortable temperatures when it has a full load on the system.
Can you pair and install a variable speed fan coil with a 2-stage scroll compressor on a regular split system? If not, why not? Thank you.
Yes, you can do this. It is, in fact, done already in the industry. It is important to use the proper control however, as you want to match the fan (air flow) with the compressor. If the compressor is running at capacity you want full air flow. If the compressor is running at 2nd stage you want reduced air flow. The CFM’s need to match the output on the refrigeration side for maximum comfort. That means when the compressor switches stages the fan should either increase or decrease depending on the output capacity of the compressor. The bottom line, its all about control.
In commercial systems the fan can be controlled by a (variable frequency drive (VFD)) where you have two compressors with staging or unloaders. All this is controlled by a computer (Direct Digital Control) to match the compressor staging with the air flow. So in low demand situations you have less air flow. With that less air flow the compressors are staged to their least capacity output. In that scenario, you have four stages for output from the compressor. Thereby you would have four fan control sequences to match those compressor stages when operating in air conditioning mode.
Not enough air flow across the coil with maximum output by the compressors and you will freeze the coil or start making ice. Too much air flow across the coil and there will not be enough refrigeration capacity to remove the heat from the air.
Will an electricial outage of 5 or more days when restarting (surges)cause the Compressor to go out?
The unit was working before the outage and afterwards the Compressor was out. Choice Home Warranty Company has they will not cover because of poor maintenance which is not the case!
Not sure what to tell you about that one as it is impossible to tell without looking at the equipment. If you have documented proof of maintenance then submit it to the warranty company. The frequency of maintenance on an air conditioning or heat pump system is typically once a year for the air conditioner and twice a year for the heat pump. It looks like the problem is electrical related as the power was out for an extended period of time but again it is impossible to tell without looking at it. I have rarely ran into anyone that was satisfied with their home warranty.