Thoughts on controlling a DX air handler

[kirkH]

We've got this old building we are retrofitting, the existing Trane air handler with 2 Carrier CU's is staying. Don't have M#'s not sure it would matter after 20 years of hackery not much is original anymore anyway. I'm looking for thoughts on the best way to control the DX. Each CU has an O6E I believe and is supposed to get 2 electric unloaders to replace the current mechanical units. We have 2 evaps and LLSV per CU. My thought was to stage cooling up and down via the 4 LLSV's and load/ unload the comps to maintain suction pressure. I'm trying to keep this a simple as possible. Thanks

[coolerik]

Sounds like your on the right track, load and unload based on suction, control the llsvs threw relays from the stat

[Betabass]

Honeywell has a retrofit economizer control that's easy to install, also you may want to consider hot gas bypass depending how low your compressors can unload.

Beta

[ryan1088]

Is this ahu doing ventilation air? Single zone? Multi zone? Trying to just make 55 degree air?

[kirkH]

Thanks for the replies, it's a multizone all new VAV's, DAT will be reset based on demand we will control econo also. Hot gas is already piped also not sure it is really needed but I'm sure we will make that work as well.

[air1]

Thanks for the replies, it's a multizone all new VAV's, DAT will be reset based on demand we will control econo also. Hot gas is already piped also not sure it is really needed but I'm sure we will make that work as well.

Is there a BAS system controlling the VAV boxes? If so, why not use the BAS system to control the AHU? The only thing different from a built-up system would be the control modulation of the DX system. You will need to insure that the DX system has all the necessary safeties in place and interlocked with control system.

[ryan1088]

We have a few that are setup similar to that. Multi zone air handlers can be a trick though on a dx unit, if you have one zone call for cooling all the time it's possible to keep a call for cooling on for a really long time and run unloaded.

[coolerik]

We have a few that are setup similar to that. Multi zone air handlers can be a trick though on a dx unit, if you have one zone call for cooling all the time it's possible to keep a call for cooling on for a really long time and run unloaded.

Oil return probems? Properly sized double risers should take care of that

[kirkH]

Is there a BAS system controlling the VAV boxes? If so, why not use the BAS system to control the AHU? The only thing different from a built-up system would be the control modulation of the DX system. You will need to insure that the DX system has all the necessary safeties in place and interlocked with control system.

It's all going to be managed by the BAS we are installing, just trying to nail down the best control scheme for the DX.

[air1]

It's all going to be managed by the BAS we are installing, just trying to nail down the best control scheme for the DX.

That can be a little tricky. You'll need a PID loop to control the DAT setpoint. The loop output of the DAT PID will stage the DX. The tricky part is the time delays to prevent short cycling of the compressors. If you have a good programer with a solid understanding of DX he/she should be able to figure it out.

[kirkH]

If you have a good programer with a solid understanding of DX he/she should be able to figure it out.

That would be me. After looking at this thing it occurred to me that I never gave much thought into how something like this was being controlled back in the day when I was a service tech. Dusting off some pretty old brain cell on this one.

[ryan1088]

Oil return probems? Properly sized double risers should take care of that

We lost one compressor to that and being low on charge. The other one that died short cycled so fast that the compressor shorted to ground.

[dlove]

On an old system like this using DAT is not the best. depending on how many zones you are dealing with, i would either use a stat located in a common area or use RAT. and than use reheats in zone dampers. Of course using unloaders along with suction pressure will help. but again not great when humidity comes into play.

But if your going to use a BAS why not just install zone dampers and bypass.