I have a hydroair system with an air handler for the main floor in the basement next to the oil burner and an air handler for the second floor in the attic. Since it's regularly below zero here and since we have occasional power outages, I got worried about freezing in the attic loop and had glycol put in the system.
After that I noticed that the fan in the air handler in the attic was running essentially all the time, even when the thermostat upstairs was not calling for heat and the circulator had been off for a long time. I checked and found that the pipe is hot on the supply side of the attic air handler but cold on the return side. It's slow circulation due to convection, I thought, just enough to make the hydrostat turn the fan on. Sure enough, if I switch off the power to the air handler, in a few minutes, the return side starts to warm up.
Then I got to thinking -- I wonder why they put the hydrostat on the supply side of the air handler instead of the return side? When the circulator turns off, the temperature drop on the return side is much bigger than it is on the supply side. So, I had a look at the air handler in the basement and discovered that its hydrostat *is* on the return side.
Now I'm confused. Which side of air handler should the hydrostat be on? Should I just turn up the set temp for the one in the attic until the fan shuts off? Why did this start happening after putting glycol in the system?
Any help or enlightenment would be most welcome.