co-mingling separate air handlers

[kenvogt]

I want to know if there is a thermostat system that allows two air-handlers in two zones to work in unison.

I have two zones with two air-handlers on the first floor of my house. I want to connect the two zones with an electric baffle and install a thermostat system that would turn on one of central ac systems which would cool both zones. When the one system can not keep up (lets say I get a temp rise of .5 degrees instead of drop in temp) I want the second system to turn on. When the second system turns on it would also close the connecting baffle, as well as open an electric baffle at the plenum of the second system.

Why would I want to do this. I live in new Orleans, and my systems are sized for an 95 degree exterior temperature. When its only 80 outside my systems turn on for a short period of time, and stay off a long period. This has led to a humid environment, and I believe also reduces the efficiency of my units. Two units running for 5 minutes use more electricity than one unit running for 10 minutes.

[allan38]

Interesting problem. You want to set up a two stage system using two systems.
There are no off the shelf systems that will do this as far as I know. Many companies could build you such as system. I can think of many different ways of doing so.
A whole house dehumidifier may be a better choice to economically remove humidity during cool humid days.

[BaldLoonie]

A sharp tech can handle that. Friend has a pizza joint with a furnace & air that we added a cooling only separate system. If the first unit doesn't keep up, the 2nd one comes on from the same thermostat. Now if they share a duct system, that would get more interesting. Or let both air handlers run with a call for cool and just have the A/C for the 2nd one come on when the first can't keep up.

[motoguy128]

Its' sometimes called a "lead-lag" system. You can use a standard 2 stage thermostat with a lead-lag controller. There is a company that makes a single solutoin, but it's doesn't have a used interface, jsut a dial for temeprature. Jackson Systems has a lead-lag 2 stage controller that will do what you want. ITs' not cheap, but not super expensive.

Best of all, it will balance run time between both units. Meaning that if unit A cycled on first last time and then cycles off. The next call for cooling will bring on Unit B.

I'm doing this in a I/O room that needed 8 tons of cooling that used a water cooled heat pump. Rather than put in a single 8 ton water cool heat pump that often have 8-12 week leadtimes, I put in two "of-the shelf" 5 ton units and now I have partial redundancy in case one unit goes down.

If you don't care about balancing run times, then you could just use relays to isolate both systems and power the thermostat and relay coils indepedently. Then just use normally closed dampers and have the call for cooling for the repsective unit to open the damper.

Just make sure your ductwork is sized adequately to handle the combined airflow.

[motoguy128]

Or let both air handlers run with a call for cool and just have the A/C for the 2nd one come on when the first can't keep up.

That would work as well and avoid the need for dampers after the coils. So you'd just twin the two air handlers.

That's also done of course when you have a demand for more than 5 tons, but don't want to use a larger commercial split system (if you don;t have 3 phase power) and don;t care if its' 2 stage.

[kenvogt]

Its' sometimes called a "lead-lag" system. You can use a standard 2 stage thermostat with a lead-lag controller. There is a company that makes a single solutoin, but it's doesn't have a used interface, jsut a dial for temeprature. Jackson Systems has a lead-lag 2 stage controller that will do what you want. ITs' not cheap, but not super expensive.

Best of all, it will balance run time between both units. Meaning that if unit A cycled on first last time and then cycles off. The next call for cooling will bring on Unit B.

I'm doing this in a I/O room that needed 8 tons of cooling that used a water cooled heat pump. Rather than put in a single 8 ton water cool heat pump that often have 8-12 week leadtimes, I put in two "of-the shelf" 5 ton units and now I have partial redundancy in case one unit goes down.

If you don't care about balancing run times, then you could just use relays to isolate both systems and power the thermostat and relay coils indepedently. Then just use normally closed dampers and have the call for cooling for the repsective unit to open the damper.

Just make sure your ductwork is sized adequately to handle the combined airflow.

Do you know the model or specific name for that Jackson control lead-lag controller? That sounds like the perfect solution to what I want to do.

[kenvogt]

That would work as well and avoid the need for dampers after the coils. So you'd just twin the two air handlers.

That's also done of course when you have a demand for more than 5 tons, but don't want to use a larger commercial split system (if you don;t have 3 phase power) and don;t care if its' 2 stage.

My issue with running both air handlers simultaneously while only running one compressor is that the returns for the units are separated by about 25 feet. So the air-handler that was running without the compressor would be diluting the cooled air with warm, or un-cooled air. I've seen systems before that used that method, but they shared a plenum.