Alternating HVAC Loads with OmniPro II

[avjunkies]

Hi All,
 
I have a question for those that are experienced with OmniPro II controllers.  I am planning on integrating a OmniPro II controller and have a question about macro relay programming.  What I'm trying to find out is if there is a lot of capability in this controller to do PLC type programming.  What I would like to do is when my house is running off a 20k generator (backup power)  I would like to setup relay logic to control my multistage thermostats so that only one hvac zone can run at a time.  By the way this is a two zone system.  This is what I'm thinking.  When the generator is running and taking load I'll send a 12volt trigger to the OminPro II controller.  Upon detection of this 12volt trigger I would like to have the OmniPro II controller to toggle relays to allow or disallow a thermostat in activing the compressor.  So basically I would need alternating relay logic that detects power on and power off status through the compressor control wiring so when one thermostat powers off the compressor signal the relay alternates by blocking the compressor line and then allowing the other thermastat to send compressor turn on signals.  Likewise, when the second thermostat brings the room to temperature, the thermostat turns off the compressor signal then block that signal by the omnipro controller and allowing the other thermostat to send compressor signals.
 
 
 
 

 

[ano]

I haven't done that for a generator, but have during certain rate periods to keep peak rates low.  The easiest way is to just use OmniStat2 thermostats and have them controlled by your Omni.  They can even be Zigbee if you don't want to rewire.  The Omni can read a thermostat to tell if heat or AC is on, and its easy to put a trigger in, so when one thermostat goes off, you turn the mode to OFF, then turn the other thermostat to auto, heat or AC.  When that one is complete, you reverse it.  You can even enforce time limits, so if you want to force the AC/heat off after a set maximum period, you can do it. 

 

[avjunkies]

Hi Cocoonut,
 
I will be using the OmniStat2 RC-2000 Multistage thermostats.  Thats great to know that you can toggle the On, off or auto status based off triggers.  I could just switch the thermostats from off to auto to simplify things (assuming there aren't any significant time delays built in to the thermostat "delay on make").  Do you know if there is a way for the Omnipro II controller to know if the HVAC status is running or off (24v signal being sent to compressor)? If that's the case then maybe I could program a macro that looks for 12v from the generator and HVAC running status and alternates thermostats with a software configured toggle relay. Thanks!!!

 

[ano]

Yes, you can ask a RC-2000 if it is cooling, heating or off, and also you can get the mode its in, you can read its temp. and its heat and cool set points.  Unfortunately you can't trigger when its cooling or heating, but you could do something like ask it every 30 seconds for status. 
 
The only thing you can't do is see if it is stage 1 or stage 2 and you can't change the stage.  Other than that, you can get to almost anything.

 

[gatchel]

How about just using a relay or two to interrupt the common wire at the thermostat triggered by an output on the panel. (I dont know HAI programming or I'd try to cite an example)
 
 
If you want the corresponding air handler fans to continue to circulate air while its compressor is interrupted then use the relay on the common to the contactor in the outside unit.

 

[tmbrown97]

A twist on what Gatchel was saying - if the intent is to keep the thing on stage one, why not just interrupt the wire that controls stage 2?  That way even if both zones are calling, it'll still just be stage 1 power drawn... it'll never be able to call stage 2.

 

[avjunkies]

Yes, I totally agree the interrupt is the way to go to avoid any time delays built into the thermostats for HVAC startup.  However,  if I can I'd prefer the ability to use full stage when I need it.  Summers are really common to have power issues and in the event of mid july power outages I would likely need full compressor use to keep up with the temperature.  So with that being said I could just interrupt both stages using a simple pass-through relay and maybe use voltage sense relay to give me voltage status for both stages instead of doing status polling to the thermostats.  However,  what I'm trying to find out is if the hai controller supports condition based marco's similar to what I have below.  The conditions need to be layed out better, but might work.   
 
Trigger Macro on Generator 12volt trigger:
 
IF ZONE1 STATUS Went On to OFF
 
ELSEIF ZONE2 STATUS ON
 
THEN OPEN ZONE 1 relays and Close ZONE 2 relays
 
IF ZONE2 STATUS Went On to OFF
 
ELSEIF ZONE1 STATUS ON
 
THEN Close ZONE 1 relays and Open ZONE 2 relays

 

[tmbrown97]

It wouldn't be ELSEIF's it'd be ANDs - ELSEIF is what to do if the first condition is NOT true, with its own THEN statement essentially.  But that's beside the point.
 
What exactly is the point of what you're trying to do? What does it accomplish?  If you're not trying to prevent going into the higher-consumption usage of the air conditioner, then what does this solve?
 
Aside from that, I think what you're trying to do is possible directly through the OPII with HAI tstats without any external relays (if you aren't trying to affect stages), or with relays as you've explored already.

 

[avjunkies]

Your making a lot of assumptions work2play with regard to the hai thermostats and my goal.  Have you worked with the omnistats?  Do you know what the startup delays are when you switch from off to on?  Are they configurable?  Can the Omnipro controller combined a status of condition with a change of status?  Yes there are a lot of moving piece here and I'm trying to get insight to approaches.  The point..... I have two 3 ton HVAC units cooling a 6000sq foot house on a 20k diesel generator running everything.  Will the HVAC units heat the house in 30 degree temps or cool the house in 110 degree temps in single stage.  No... So load management is very important.....

 

[tmbrown97]

I didn't make ANY assumptions - I asked for more details as to what the goal is because it wasn't making sense.
 
Now to be honest I'm even more confused - originally you said it was a 2-zone system, not two separate systems; are they both multistage or does that even come into play?  You saying it was a 2-zone system vs. two separate systems had a lot to do with my answer because I know quite well how zone control boards work and how multistage systems work.  I've also worked with Omnistats, and yes, most of those things are configurable but if you have a zone control board in place (like your original post said) then the zone control board manages its own MOT/MRT as long as at least one of the thermostats is calling.
 
Now if you'd said that you have two totally separate AC units and your goal is to prevent them from both running simultaneously, that's much clearer and has a different answer; and I might even explore how to allow for both units to run in single stage and/or cutting one off so the other can run in dual stage when you need more cooling.  Let me ask though - if the whole house needs to come down a few degrees, how does alternating 3-ton units benefit over running both on stage 1?  I guess that depends on how your units split, if they're 1/3 or 1.5/3...  Either way, had you described the goal more accurately, your question would have made a lot more sense.
 
Lastly - yes you can poll a thermostat's status and use that in rules.  It may not always be so direct but it's nearly always possible.

 

[apostolakisl]

Your making a lot of assumptions work2play with regard to the hai thermostats and my goal.  Have you worked with the omnistats?  Do you know what the startup delays are when you switch from off to on?  Are they configurable?  Can the Omnipro controller combined a status of condition with a change of status?  Yes there are a lot of moving piece here and I'm trying to get insight to approaches.  The point..... I have two 3 ton HVAC units cooling a 6000sq foot house on a 20k diesel generator running everything.  Will the HVAC units heat the house in 30 degree temps or cool the house in 110 degree temps in single stage.  No... So load management is very important.....

 
Zoned HVAC and two separate units are very different things.  You said you had 2 zones at the onset and now it sounds more like you have 2 separate units, no zones.  Work2play has been quite correct in what he is saying and you seem to be harshly challenging his kind and free advice.  
 
Zones have zone control boards and zone control boards control the hvac system, not the thermostat.  The thermostat makes "suggestions" which the zone control board takes into consideration when deciding what to actually tell the hvac system to do.
 
If indeed the case is that you have no zones, but rather 2 separate units controlled by 1 each HAI Omnistat, then it would certainly be a better idea to simply send instruction from your Omni Pro II board to the thermostats.  It is generally best to let the thermostats run their proper course because the time delays are important for not damaging the equipment.  It gets even more complicated in the winter when you have reversing valves and defrost cycles to manage.  Putting relays on the various control wires to the hvac is essentially creating your own thermostat, and unless you know how to properly time everything, you probably should not be doing that.
 
If you have a generator and want to make sure the one unit is completely off before turning the second unit on, then I would suggest using current sensing switches to feed back info on actual operation.  First you send a command to the thermostat to turn off.  Then you monitor the current switch to see that it has actually turned off before turning the second unit on.  This would best be done by monitoring the blower motor since it should always be the last thing to shut off.

 

[ano]

Not to put words in anyone mouth, but if its two zones and not two systems, why would you want to alternate them? Must be two systems. I have two systems, and one system has three zones, so I sometimes say i have 4 zones, but really just two systems.
 
I'm not really sure what "delays" everyone seems to be talking about. You turn on an OmniStat2 and if the temp is such and it needs AC or heat, it calls for it. No delay. Each OmniStat2 does have a setting for minimum on time and minimum off time for heat and AC, but you can set those as you wish. Otherwise most compressors also have protection delays to prevent burnout if the AC is restarted too soon, but my AC guy said 30 seconds is enough with modern systems, so not a big deal. Also fan runs maybe a minute or so after AC or heat, and that delay is set in the HVAC unit, NOT the thermostat.

 

[Automate]

Also, stage 1 is usually more that half the power of stage 2 AC, it might be 65% of stage 2.  So two ACs running at stage 1 will draw more power than one AC on stage 2.  The two on stage 1 may be more than his generator will handle.

 

[apostolakisl]

Not to put words in anyone mouth, but if its two zones and not two systems, why would you want to alternate them? Must be two systems. I have two systems, and one system has three zones, so I sometimes say i have 4 zones, but really just two systems.
 
I'm not really sure what "delays" everyone seems to be talking about. You turn on an OmniStat2 and if the temp is such and it needs AC or heat, it calls for it. No delay. Each OmniStat2 does have a setting for minimum on time and minimum off time for heat and AC, but you can set those as you wish. Otherwise most compressors also have protection delays to prevent burnout if the AC is restarted too soon, but my AC guy said 30 seconds is enough with modern systems, so not a big deal. Also fan runs maybe a minute or so after AC or heat, and that delay is set in the HVAC unit, NOT the thermostat.

 
 
The minimum off time is the delay.  So your system has it.  It is what is called "purge" time.  It lets the high side pressure trickle down so the compressor doesn't have to start against it.  30 seconds is not an option as far as I have seen.  2 minutes is the shortest on the thermostat and zone control boards I have had the occasion to work on.  So, if you turn your system on, and it is calling, and it hasn't been 2 minutes (or whatever) it won't turn on.  If your unit clicked off because it reached temp, and you walked up to the thermostat and turned the temp down a few degrees, you would find that you do have a delay.  The AC units I have opened up do not have any built-in purge times at the compressor, but I don't deny its existence.
 
The heating side is more complex with reversing valves and heat strips to accomplish defrost.  Trying to screw around with interrupting the wires using relays is not advised unless you know what you are doing and are willing to write a lot of "if then" lines of code to make it work right.  You don't want to shut down a unit in the middle of a defrost cycle.