VisionPro IAQ thermostat - strange behavoir at midnight?

[garya505]

I have a VisionPro IAQ thermostat and I've noticed this strange behavior twice in the last few days. I am running 3 CPH on heat. Right around midnight, it did 3 very short runs with very little OFF time between runs, during a period of about a half hour, and then resumed normal cycling at about 3 CPH. Load is about 50% at this time, so normally runs are about 10 minutes ON and 10 minutes OFF.
11:56pm to 12:04am (8 minutes ON)
1 minute OFF
12:05am to 12:12am (7 minutes)
3 minutes OFF
12:15am to 12:23am (8 minutes)
After this, it ran several cycles of about about 10 minutes OFF and 10 minutes ON.

Is this strange or what?

[motoguy128]

Did you feed it after midnight?

[beenthere]

Thats strange. Did you have any power outages during the day when this happened.

[kdean1]

Have you checked the programming? Could there be any setback/setup changes within that 1/2 hour?

[motoguy128]

Have you checked the programming? Could there be any setback/setup changes within that 1/2 hour?

That was my thought. There's a couple of periods mashed in there.

[kdean1]

That was my thought. There's a couple of periods mashed in there.

If two of us think so, then it must be right.

[garya505]

Thats strange. Did you have any power outages during the day when this happened.

Not that I know of. The first time I thought it was just a one-time thing, or maybe I imagined it. Then it happened again just like the first time.

Have you checked the programming? Could there be any setback/setup changes within that 1/2 hour?

I have all periods programmed for th same temps, and the "sleep" period starts at 10PM anyway.

[kdean1]

Not that I know of. The first time I thought it was just a one-time thing, or maybe I imagined it. Then it happened again just like the first time.



I have all periods programmed for th same temps, and the "sleep" period starts at 10PM anyway.

Maybe you have already done so, but I'd still check all 7 days on the program.

[TwincamDave]

Do you have a back up generator? If so it maybe going through it's testing at that time and causing it to short cycle going into and coming out the testing due to loss of power.

[garya505]

No back up generator.

I have a suspicion that this is caused by the weather here. We usually have a large day-to-night temperature difference. Since I have been watching the cycling behavior, I have noticed what appears to be some short cycling well before midnight, which seems to correct itself after a while. Could it be that the PID algorithm does not handle the large drop in temperature very well, and takes some time to adapt?

[garya505]

OK, there it goes again. 11PM
10 min ON
1 min OFF
9 min ON
1 min OFF
3 min ON
11 min OFF
10 min ON
15 min OFF

Got any ideas?

[beenthere]

Looks like its trying to do 4 CPH. Set CPH to 2 and see if it still does strange cycles.

Could be a faulty stat.

[garya505]

Looks like its trying to do 4 CPH. Set CPH to 2 and see if it still does strange cycles.

Could be a faulty stat.

Yes, I was wondering about that, but it seems to work fine most of the time.

Actually, the OFF times I listed above are all shorter than stated, by 90 seconds, because the furnace runs the blower for 90s after the call for heat ends.

[garya505]

Well I've been reading up on PID control and I think this problem is due the derivative part of the PID algorithm. From what I've learned, PID control can be the best but it appears that with thermostats, under certain conditions it can become unstable.

[motoguy128]

If it's become unstable in it's control response, the issue is likely not the controller, it's probably something like thermostat location. Garbage in = garbage out. Maybe a water heater tuns on at that time creatign a negative draft that draws air into the wall cavity behind the thermostat, creating a draft across the temprature transmitter that creates a small shift in the local air currents. You'd be suprised. Always make sure any wall penetrations behind a thermostat are sealed. Poor thermostat location can cause a lot of issues. I've relocated on of mine for just that reason and need to relocate another that lcoated too far from the return duct and in a corner. I think the PO wanted it in a less conspicous location.

[garya505]

If it's become unstable in it's control response, the issue is likely not the controller, it's probably something like thermostat location. Garbage in = garbage out. Maybe a water heater tuns on at that time creatign a negative draft that draws air into the wall cavity behind the thermostat, creating a draft across the temprature transmitter that creates a small shift in the local air currents. You'd be suprised. Always make sure any wall penetrations behind a thermostat are sealed. Poor thermostat location can cause a lot of issues. I've relocated on of mine for just that reason and need to relocate another that lcoated too far from the return duct and in a corner. I think the PO wanted it in a less conspicous location.

That could be the case here. I was using a wired remote temperature sensor in an interior wall, which I have stopped using. It's possible that this wall could get cold inside. I'll check it out with my IR meter. The thermostat is also on an interior wall, but is closer to the returns than the remote sensor, so maybe air mixing is better there.

[garya505]

Oh, and I see that the VisionPro uses P&I, not PID. Oops. But I"ll take a guess that it's continually adjusting the integral period. Maybe there's some lag there when the outdoor temp drops as fast as it does here at night.