Background
You are rent/own a condo/apartment and you want a smart thermostat to keep track of your spending on heat/cool, or want a thermostat with remote sensor support. You pulled the current thermostat off the wall and looked at the wiring and you have a 4 wire system (R, Y, G, W) but no ‘C-Wire’ and the thermostat you want to use requires a ‘C-wire’ but you don’t have one. You look at the Ecobee3 that supports 4 wire systems but you don’t have access to the HVAC unit because its in a locked closet/on the roof and you don’t want to have to deal with your landlord/association to get access to it as it probably means having to spend a few hundred more bucks to have a professional installer do the work. Don’t worry though you can still have that smart thermostat you want, but it’s gonna be a little ugly but you can still have what you want.
What is a ‘C-Wire’ and why do thermostats need them?
HVAC control systems are based on 24V AC control signals, in a combined heat/cool unit you have 4 or 5 wires (we are going to concentrate on the 4 wires here because if you had 5 then you probably would not be searching for a solution for this problem. In a 4 wire system you have one common supply wire (R) when the thermostat wants to request a function from your HVAC unit it connects one of the other 3 wires to the common supply wire to complete the circuit for that feature. 4-Wire ‘pins’
- R – Common supply line
- G – Fan request
- Y – A/C Request
- W – Heat Request
The 5th wire in the system is C this is the second common wire and is connected to the other side of the 24V transformer in the HVAC control system, its used to power accessories such as a thermostat. Your system probably does not have a C-Wire as your thermostat is battery powered. ‘So how does the Nest manage to work without the c-wire?’ I hear you ask, well it scavenges power to charge an internal battery/capacitor by using G wire in short bursts to complete its charging circuit. ‘Ok so why don’t other thermostats do that?’ Well if your HVAC system is to sensitive it can cause your fan to turn on and off for very short periods of time, this can be very bad for the longevity of your blower motor.
Imagine this simplified control board diagram:
So can’t I just hook up a second PSU to the thermostat and hook it up from R to C?
Short answer, no! Long answer if you are are VERY VERY VERY sure that the phases of your HVAC unit and the outlet you use are in sync it is a very bad idea to just hook up a second PSU to the system, you will damage both your thermostat and HVAC control board.
So what is the solution?
To get the system to work you have to create an interface that will mimic a thermostat to the HVAC control board and mimic a 5 wire HVAC control board to your smart thermostat. To do this we need 3 relays with a coil voltage of 24V AC, a 24V AC PSU, some screw terminals, a proto-board and some form of case. I used the following relays and PSU:
- Relay RT314524 – Newark P/N 03P2422
- Triad Magnetics WAU24-200 – Newark P/N 96M0310 (Note this item is discontinued once stock levels are depleted so you may need to find an alternative)
Once you have the parts you need to assemble the following circuit:
Put it in a case and hook it up to your system. Now you can enjoy your 5-wire requiring thermostat with your 4 wire controlled HVAC system.
You should also design a better wall mounting solution into your case so you don’t have to use blue painters tape to hold it to the wall, this will be being rectified with some double sided foam mount tape in the future.
