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Omni Pro II Fresh air when furnace runs

pct88

Member
Given the particularly cold weather in recent days I decided I should automate the fresh Outside Air fan that warms outside air through a heat recovery ventilator unit, then into the return plenum of the furnace.  I found that temperatures below 0 are low enough that the HRV is unable to preheat the OA enough to avoid a few minutes of particularly cool air as the furnace deals with the initial 30-40 degree air.
 
Yes, I have experimented with lowering the OA fan speed, (suggestions for a UPB Fan Speed control?) added insulation, turning off the fan entirely, etc...  My concern is that due to the radon levels in the area we have two sub slab radon fans that continuously pull in outside air.  The HRV warmed OA helps balance this, and also keep the radon levels down.
 
I have set up the following code that is intended to poll the thermostat to determine when the furnace is running, see if it is below 40 outside, wait two minutes (or more depending on when the heat turns on during the two minute polling period), double check that the furnace was not turned off again in the interim, then start up the OA fan.  When the furnace cycles off the OA fan turns off immediately.
 
Does this code look like a reasonable way to provide this functionality?  I realize there are always multiple ways to get the same results from programming-  I would prefer not to discover that perhaps the OmniStats lock up if poled too often, or repeating the UPB on and off signals every two minutes will cause other issues and I should add code so that only a single on and off UPB update is sent per furnace cycle.
 
CODE
52.    //    OA Supply
53.    EVERY 15 MINUTES
        AND IF Outside Temp CURRENT READING IS LESS THAN 45 BY MORE THAN 5
            THEN OA < 40 Deg. ON FOR 16 MINUTES
 
54.    EVERY 2 MINUTES
        AND IF Living Room SYSTEM STATUS IS HEATING
        AND IF OA < 40 Deg. ON
            THEN Wait 2 Min. ON FOR 2 MINUTES
55.    WHEN Wait 2 Min. OFF
        AND IF Living Room SYSTEM STATUS IS HEATING
            THEN Outside Air Intake Fan U50 ON
 
56.    //    Turn off OA Fan immediately when call for heat turns off.
57.    EVERY 2 MINUTES
        AND IF Living Room SYSTEM STATUS IS NOT HEATING
            THEN Outside Air Intake Fan U50 OFF
/CODE
 

Desert_AIP

Senior Member
I think you're going to run into trouble with those UPB commands every two minutes. 
That sends a lot of traffic on the network.  You may see delays or missed commands.
 
 
This section has a logic error:
 

54.    EVERY 2 MINUTES
        AND IF Living Room SYSTEM STATUS IS HEATING
        AND IF OA < 40 Deg. ON
            THEN Wait 2 Min. ON FOR 2 MINUTES
55.    WHEN Wait 2 Min. OFF
        AND IF Living Room SYSTEM STATUS IS HEATING
            THEN Outside Air Intake Fan U50 ON

 
The first run through the code, Block 54 turns on the 2 minute timer.
Two minutes later, just as the timer is expiring, Block 54 resets the timer to 2 minutes.
So the timer never turns off, so Block 55 never executes.
 
If you juxtapose those two blocks the timer will expire before its reset and the now block 54 will execute.
 
If you can find something else to initially trigger on trigger on you can lose the Every 2 minute triggers with something like this to reset the timer and execute the On command:
 

Code:
54.   WHEN Wait 2 Min. OFF
        AND IF Living Room SYSTEM STATUS IS HEATING
        AND IF OA < 40 Deg. ON
            THEN Outside Air Intake Fan U50 ON
            THEN Wait 2 Min. ON FOR 2 MINUTES
 
55..   WHEN Wait 2 Min. OFF
        AND IF Living Room SYSTEM STATUS IS NOT HEATING
            THEN Outside Air Intake Fan U50 OFF
 

pct88

Member
Thanks for pointing out my timing logic flaw.  I adjusted the timing there and added two flags to (presumably) limit the duplicate UPB commands being sent.  Testing the code now with the furnace.  Ordered a UPB fan speed control to add to the mix.
 
Code:
54. EVERY 2 MINUTES AND IF Living Room SYSTEM STATUS IS HEATING 
      AND IF Main Furnace Call For Heat CURRENT VALUE IS 0 
      AND IF OA < 45 Deg. ON
         THEN Warmup 90sec ON FOR 90 SECONDS

55. WHEN Warmup 90sec OFF
      AND IF Living Room SYSTEM STATUS IS HEATING 
         THEN Outside Air Intake Fan U50 ON
         THEN INCREMENT Main Furnace Call For Heat

56. //    Turn off OA Fan immediately when call for heat turns off.

57. EVERY 2 MINUTES
      AND IF Living Room SYSTEM STATUS IS NOT HEATING 
      AND IF Main Furnace Call For Heat CURRENT VALUE IS GREATER THAN 0 
         THEN Outside Air Intake Fan U50 OFF
         THEN SET Main Furnace Call For Heat TO 0
 
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