Ecobee randomly running my fan

DKirch

I have a 4 zone taco controlled hot water system. 

Zone 1: hot water coil in air handler forced air 
Zone 2: main floor radiant heat
Zone 3: basement floor radiant heat
Zone 4: garage floor radiant heat

Primary heat for main floor is radiant heat (zone 2) and the secondary is (zone 1) forced air. It’s set up so the forced air should only come on when a 3 degree temperature differential is sensed…. like opening our large from door, etc. however, it seems the air handler has a mind of its own and comes on randomly when zone 2 comes on. It’s annoying. I’ve done some research and went into advanced settings and have the fan run time set to zero and also have it set in HVAC mode and not Ecobee thermostat. Thoughts on how to set it up so the fan is only on running when zone 1 is calling for heat? I’ve yet to really determine when it’s actually coming on honestly. Seems to have a mind of its own. 

EdTheHeaterMan

It may not be the thermostat. There is a possibility that the air handler has a thermostat (aquastat) that measures the water temperature in the air handler and turns on the fan when the temperature in that coil or pipe increases. It depends on where it is located. Then there is another phenomenon commonly called Ghost Flow. That is where hotter lighter water might rise inside a pipe while colder heavier water flows down inside the same pipe. This will increase the water temperature in that pipe without a pump circulation water thru the zone.

If you are lucky enough to have both those things happen on a semi regular basis that would explain your fan operation. You actually have a ghost turning on the fan. In order to resolve that situation I have always included a relay and a aquastat in the air handler to operate the fan on heat cycle. Both the relay and the aquastat need to be calling for the fan to operate. This way if you do geet hot water at the aquastat when there is no call for heat, the relay will not allow the fan to operate.

If you discover that to be the case, I can offer you the wiring diagram to accomplish the aquastat/relay configuration for this to work. I would need the air handler model number and the type of hot water coil you have. Whether it is integral to the air handler or an aftermarket add on coil.

HVACNUT

Is the Zone 1 circulator energized and the air handler is delivering heat, or it's fan only?

How is the fan set to energize during a Stage 2 heat call? Does the thermostat control the fan? Is there an HVAC relay at the air handler with a temperature sensor?

DKirch

Zone 1 is not calling when the air handler is on. In the t-stat stage 2 is set up to come on when a 3 degree temperature difference is sensed. Neither of these are met and it still comes on. Not sure about your other questions. My HVAC guy said he might have to set it up as a heat pump or emergency. It would rarely be used as the radiant does a perfect job at keeping up. 

Ironman

Check the ventilation settings. Higher end stats have it and will attempt to bring in fresh air by running the fan periodically. It’s assumed that you have a fresh air duct or an ERV or HRV.