I'm trying to help my son remote diagnose this problem in his loft in L.A. - usually I can spot the trouble but this one is pretty weird. He's got a fancoil in his exposed attic, does dual duty as heat and cool, I believe it's connected to a water loop connected to a tower on the roof. Here's what he's observing:
"AC hasn't been doing well for the past couple weeks. I'll set the thermostat to something really low, it'll turn on for a minute, then shut right off (and never turn back on). Same thing happens when I try to turn on the heat. I know that our filter is really dirty, do you think that would be the culprit?"
I'm guessing it's a safety shutdown due to no water circulation - but could also be a control problem (digital stat)?