Question 1

How do i wire this to 2 wire 110v hot(Black wire) and neutral(white) wire, want to use "C" connection.

Accepted Answer

You don't need the common wire if you have 2 wire 120. The black is your hot. And the white is your neutral.

Question 2

I have two wires on a line 120 v oil burner thermostat can this be used to install a nest e thermostat? And if so how should I connect it  thanks

Accepted Answer

you need to know '' the load '' on the circuit when 'oil burener thermostat' [I am not familiar with this, so let me say, this device may have a problem turning on and off an 'electric motor' [ there is a surge of current to energize the coils on an electric motor and this surge could be to much for the device] to me it does not sound like a thermostat [? but maybe we are not talking oranges and oranges but oranges and pears?] should draw much current, but if thermostat switched on a pump [like above] then need to know specs on the pump; I had a 120vac ''line thermostat'' controlling on/off water circ pump [very small hydronic circ pump] and I removed that thermostat and wired this device 'in between' [both the 'nest e thermostat and the 'rc840T-120' come with ''installation instructions'' and you might be able to youtube some videos [I do not know where or what ] but try '' nest e thermostat install in 120vac system you tube?'' goodluck .... I will explain the 'wiring diagram as shown on the RC840T-120 next: on the right are 3 contacts which are used to power the nest thermostat with 24 volts AC [verify your new device / thermostat operates with 24VAC input before wiring/buying] , when the 840T is wired to [on the left] the white [common/ neutral] and black [+110VAC hot wire/ 'line'] it will produce 24VAC current at contacts [on the right ] 'R' and 'C'  [you see in details , it says this" Compatible with 24V thermostats with 2-wire connections (R,W) or 3-wire connections (R,W,C)" so if Nest wants 2 wires or 3 wires they are provided [check Nest wiring diagram/instructions  to see what it requires ]  OR ? spring for an electrician ? [ maybe invest in a volt meter? to check for 'where is the 110VAC and where is the 24VAC , as you install ] [Caution 110VAC will kill some people, deenergize before wiring, reenergize before 'calling for heat' [ eg set Nest thermostat at low temperature so that it will not close its contacts], and it is always good to have a manual switch at the furnace [in series with the 'load' ] [the Nest thermostat is 'a switch' = its contacts turn on and off the load [whatever you wire to the load wires on left side , I am just guessing it is a low flow small oil pump ? and maybe a flame igniter? not at all familiar but the 840 switching 'contacts' [red wire] are rated for 18 amps 120vac [so verify your load is less than that] also 18 amps is 'resistive load' [ read up resistive vs inductance loads] [ if you try and start a  1HP inductance motor with this, it will probably [pop/ smoke/ break?] so your question can not be answered without knowing more details about 'what is the load' ? [It is working for me, but I have a very small load]

Question 3

which wires to use on the transformer/relay when i have a black and white wire only to call for heat

Accepted Answer

There is information missing from your question.  This device is meant to switch a 120 volt load when an independent (dry) contact is closed, say from a thermostat.  The thermostat will close a contact when heat is being requested.  These two contact wires should go to the "R" and "W" screw terminals.  The black and white wires should go to the 120-volt supply hot (normally black) and neutral (normally white).  The heater (load) is then connected to the red (hot) and white (neutral) wires from this device.  This results in 3 white wires being connected together - the power feed, the relay and the load.  When the thermostat asks for heat, its output contacts close, causing the relay to apply 120 volts to the heater.  Hope this helps.

Question 4

Will this work to convert a 120v 2 speed evaporative (swamp) cooler thermostat for a smart thermostat?

Accepted Answer

Just make sure amp of the swamp cooler does not exceed the amp rating for the relay with a little safety factor.

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Honeywell Aube Technologies RC840T-120 Electromechanical Relay with Built In Relay

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