Two AC compressors and one air handler

[Mike Lamb]

This house has 2 outside split system compressors w/ separate refrigerant lines going into one furnace air handler plenum.  I have never seen this.  No zones. One thermostat kicks both compressors.  Both compressors are operating fine. Any thoughts as to why this was done?

Edited June 13, 2019 by Mike Lamb

[Marc]

19 minutes ago, Mike Lamb said:

This house has 2 outside split system compressors w/ separate refrigerant lines going into one furnace air handler plenum.  I have never seen this.  No zones. One thermostat kicks both compressors.  Both compressors are operating fine. Any thoughts as to why this was done?

I've seen this only once and it was field modified, not factory.

Are you sure there's only one evaporator coil?

Edited June 13, 2019 by Marc

[Mike Lamb]

[Mike Lamb]

There may be two coils. I don't know. I just thought it was weird.

[Marc]

Looks like each set has its own expansion bulbs so I'm guessing two coils.  The vapor lines look small.  Do you know what tonnage was each outdoor section?

[Mike Lamb]

2.5 ton each.  A 2800 sq. ft, ranch.  

[Marc]

Perhaps they had to replace an original 5 ton system and thought at one time that they could get the equivalent of a two-speed AC by paralleling two smaller systems.  This would work AFAIK.

If they were smart, the 2 air flows are in parallel because they were each engineered for that sort of delta temp.  If they're in series, I'd write them.

[Jim Katen]

In my area a 2,800 foot ranch would do fine with a 2.5 or 3 ton unit. 5 tons of AC would be absurdly over sized. Chicago can't be all that different, except you probably have a larger de-humidification load.

From the air handler's perspective:

• If the coils are in parallel, then the air handler would simply see this as a regular 5-ton air conditioner. If it's too large for the house, the air is going to be chilled without adequate de-humidification. 
• If the coils are in series, then the air is going to spend more time in contact with cold coil surfaces and this will provide great de-humidification. 

From the AC perspective: 

• If the coils are in parallel, then they'll work within their normal parameters, but they'll have very short cycles. It'll just behave like any oversized air conditioner. 
• If the coils are in series, then the 2nd one is going to be seeing lower temperatures. You might get freezing and the efficiency of the system will drop. 

I don't suppose you measured the temperature drop across this thing? If they're in series, the drop should be really high. 

Bottom line, I have a hard time imagining how this arrangement can be a good thing. It goes back to my old adage, "Unconventional installations behave in unconventional ways." My guess is that this system is not doing what the installer hoped it would do.