Help with wiring ECOBEE thermostat with 18/3

you ran a New 18/3wire?

Another method illustrated here is to use a second transformer to power the Ecobee.

That is why I posted that diagtam where it is easy to find. Thanks @109A_5 for sharing it. But somehow there is a black line where some wires should be. Here is the corrected version.

I also went back to the original post and corrected it too. Thanks Again @109A_5

I was in an identical boat about 5 years ago. I installed an Ecobee wi-fi thermostat and had to replace my 2-wire thermostat with a 3-wire.

At the same time I installed a vent damper which required me changing the Honeywell S86F Ignition Module to a Honeywell S8610U which had the correct pinout for the vent damper I was using.

Long story short, I connected the "C" wire from the thermostat directly to the 24V side of the existing transformer. Below is a diagram I made so I wouldn't have to reverse engineer it again.

@AdmiralYoda I don't know what type of LWCO you have but the contacts portion are in a different place in the limit string as compared to @TheClevelandSteamer's boiler you may have removed a jumper and wired the your LWCO power and contacts independently. There are different ways of getting it to work depending on the system's equipment.

With @TheClevelandSteamer's WM boiler if his LWCO uses the 3 circuit connector isolating the LWCO power and the contacts may be more difficult.

@109A_5 I have a Peerless G-461-SP-1 installed in 1986. The G-461 was the naming convention before Peerless changed ownership in the 80's. It then became the 61 series and then eventually the 63 series. My old Peerless uses a float style LWCO. The contacts are closed when the float is up and open when the float drops to a certain level.

My main point was that he can just tap off of the 24VAC side of the transformer to power the C wire.

According to the Ecobee spec sheet it uses "Up to" 3.5VA, which on a 24VAC source is less than 150mA of current so there should be enough headroom on the transformer to handle the Ecobee.

Other options do exist. He could have another 24VAC transformer wired in just to power the Ecobee. If a 120VAC receptacle is nearby they make plug-in transformers to tap in to as well.

Ecobee also makes a "Power Extender Kit" which essentially taps off the 24VAC on the existing transformer for you and sends power to the C wire.

Back to basics. The Ecobee — and almost any other "smart" or wi-fi or otherwise wireless thermostat that I know of — requires constant power.

And, remarkably, they are pretty consistent about it. you need 24 volts AC power available between the R and C connections on the thermostat. For control, the thermostat makes or breaks a contact between W and, almost always, R.

Back at the controlled device, there are two classes of them: those that need power (zone valves, relay coils, that sort of thing) and those that don't — the most common example being the burner control.

And it's at that point where folks who don't follow basic electricity get caught up. If you are dealing with device needing power, your circuit is going to be power supply A to R connection, then the W connection to one terminal of the device, then from the other terminal of the device back to the power supply B. If the device wants a closed contact but not power, then you power supply is power supply A to R and C to power supply B. The device is connected device to R, out W and back to the device. Neither W on the thermostat or — perhaps more important — either of the device terminals! — connects in any way to the power supply.

All your other safeties and controls are also dry contacts,, except powered LWCOs, and can be on the controlled device — thermostat — device circuit somewhere, in series. A powered LWCO is treated the same way as the powered thermostat (but note that if you use the same power supply as is used for the thermostat you are going to need a relay to convert either it or the thermostat into a dry contact type control…)