Thermostat with 2-24v inputs?

BigEHead

I have 5 thermostats in the home, and one of them has 2-24v inputs. I'm trying to find out if it's necessary or not.

I have a Taco Zone switching relay with 24VAC out. Not sure why, but the 24VAC is connected to the Rh and W1 on the thermostat. Rc is connected to R on the air handler, and I recently used an unused wire (brown) to connect to common to power the thermostat. W1 and R on the air handler is going to an aquastat.

Anyone have any idea why they would have wired in a 2nd 24v input? None of my other 4 thermostats in the house have them and I'm wondering if I could remove the 24VAC coming from the Taco Relay and use it for a Outdoor reset control instead.

Thank you for any input!

Jamie Hall

Make and model of the thermostat, please

BigEHead

4 are Nest Learning thermostats, and 1 is a Honeywell T6 pro. The one in question is a Nest. None of the other Nests are wired similarly. The Honeywell does have 2 - 24v inputs but that's expected for that. Thanks!

EBEBRATT-Ed

are your running heat and cooling?

ch4man

leave it. your boiler has a transformer and the air handler does too. they need to stay separated or else the can cause all kinds of headaches. that's why there is a RH and RC, to keep the two apart

BigEHead

Yes it does both heating and cooling.  But why do all other nest thermostats not have both then even though they control both heating and cooling?

HVACNUT

The systems with one power source (R) have isolation relays in the air handlers, or near that will energize the heat circuits. 
The other thermostat is using the aquastat to operate the fan once the coil is up to temperature. 

BigEHead

Thanks HVACNUT, is there a way to power it up via the air handler transformer instead of the one from the switching relay?

HVACNUT

Is the Brown wire from the air handler or from the boiler?
You would typically use the air handler transformer to power a thermostat, so if there's an extra wire, use it.