Dual Two-Stage Split System with a Dual Slab Coil. Huh?

Dual Two-Stage Split System with a Dual Slab Coil
Richard - Very nice site!- Dual Two-Stage Split System with a Dual Slab Coil - I have a question; my home was fitted out by the previous owner with a twinned system, and a thermostat (Totaline P374-1900) which was not designed to control discreet, paired units. I confirmed this with Totaline. The result is only one unit runs, and the second unit is never activated. It functions, but not automatically.

Your website mentioned dual compressors in one systems, and mine is similar, two independent compressor/condensers. Seeing the separate condenser units, I first thought the house was zoned, and later discovered both units feed into one Aspen BHA40 4-roll coil.

Besides the issue of the wrong thermostat, I’m having problems finding an HVAC person who understands these hybrid systems and will service it. Is this configuration that uncommon?

I live very near Houston Texas, so if you know of a local HVAC person with the skills to help me out – let me know.

PS – I checked with Honeywell about the possibility of their thermostats working with my setup. They emailed me – they would only work with a HVAC professional. I thought this was strange since they sell them in the hardware stores.

Dual Two-Stage Split System with a Dual Slab Coil

Richards Response yo Dual Two-Stage Split System

I think you have dual compressors and I suspect if you check the indoor coil in the air handler you will find that the coil has a dual design also. In other words, there should actually be two coils there being fed by different line sets from the two different compressors. Usually, in HVAC, when you refer to a twinned system you really mean you have two air handlers hooked up in series next to each other and the blowers work simultaneously while the heating and cooling is staged from a dual compressor or condenser system like you have. These systems, with two air handlers serving one duct system, usually have problems with one of the blowers failing or never working or providing airflow unless they are equipped with a twinning kit. The reason a twinning kit is necessary is because if one of the blowers starts before the other it will make the other blower turn backwards and when the blower control calls for it to start it will run backwards never allowing any airflow. Do you have two blowers or just one?

Or do you have a double coil in the air handler?

If you do then the answer is simple for what you want this system to do. You simply need a thermostat that has two-stage cooling. It should have a Y1 and a Y2 terminal connection. One wire will go to stage-one condenser and the other wire will go to stage-two condenser. I’m not exactly sure if this is the way the system was designed without looking at the system myself but you can send me some pictures (including the indoor coil and line set(s) and some more information about the system and it would help me identify the system better. If you have any technical booklets or model numbers this will also help. Simply scan them and email them to me and I will see what I can find out for you.



Ok, my error, just one air handler and one coil slab; Aspen BHA40 48×1 50-50SP (split). It has two suction and two liquid lines – one part number. So twinning is not the correct nomenclature. Would it be “split system“?

The condenser units are Amana, and the thermostat is a Totaline P374-1900. The totaline rep told me the 1900 would not work with my dual condenser setup, and suggested I obtain a lead/lag controller. He may have been misleading me into thinking I had a “twinned system“, my error.

If I recall, the 1900 has Y1 but no Y2.

Dual Two-Stage Split System with a Dual Slab Coil

Richard’s response:

The slab coil you refer to actually has two different coils in it. You need a thermostat that has Y1 and Y2 and you can make this work. You use one system under low load conditions and when the temperature is really hot and you have a bigger heat load the other system kicks on and provides more cooling capacity. There are many different Honeywell thermostats that have Y2 which stands for 2nd stage cooling. Make sure the wiring is correct. I’d call an HVAC company and make sure they send a good technician who is familiar with staging and control wiring. Most residential technicians understand staging when it comes to heating especially heat pumps but do not know about staging with cooling. Commercial HVAC technicians know staging with air conditioning because many commercial air conditioners have staging for better economy.

Have you had a problem keeping the house cool with just one condenser running (especially on a very hot day)?

If you have a problem with humidity then the system is either oversized or it is not running enough to remove the moisture. If it is oversized then you definitely do not need the other stage but you have a backup in case the one side goes bad or breaks down for whatever reason. The humidity sensor in the thermostat probably only controls a humidifier for winter operation which is a necessity – see the humidifier page for more information. The air conditioner removes humidity but it has to run a certain amount of time. If it’s oversized it will run for a short period time and you will have more cooling and less humidity removal which means you will have humidity problems like you have now. Call a good HVAC company to have a look at the system because there could be other problems which cause high humidity with air conditioning systems and it could easily be fixed (unless it’s oversized).


We don’t try to keep it below 74-75, and so far it has not been a problem. We also have enough trees that the sun rarely hits our roof. We do have a serious humidity problem, and though our Totaline P374-1900 has a humidity sensor, it doesn’t help. I believe it is not wired correctly or configured/set incorrectly; we’re seeing a lot of mildew on books, mold on the vents, etc.

Richard – thanks you’ve been a lot of help.

Is there an industry term for the setup I have? It’s not twinned, or split, might there be a buzzword for this configuration?

Dual Two-Stage Split System with a Dual Slab Coil

Richard’s response:

It is a dual split system with staging. Rare in residential but I have seen these. There are different variations to this; Lennox has a condenser that has one condenser with a two stage compressor. Some new Trane’s have a two-stage scroll compressor and some manufacturers have two compressors in one condenser, a small compressor and a big compressor. The thinking behind this is to use only what you need to satisfy the load so when you only need a little bit of cooling you only get one compressor running. When it is really hot and you need more the big compressors kick in or both compressors run. Some commercial systems have ten compressors and all these compressors are staged depending on the heat load they need to satisfy. All of this is getting ready to go by the wayside as Variable Frequency Drives are being used now to control the speed of the compressor so you will need only one compressor with a frequency drive to slow and speed up the compressor based on demand from a complex controller which controls the speed based on demand of cooling needed and a few other factors. This hasn’t been introduced into residential yet but it’s coming.

Good Luck.


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