Relays on steam boilers

What relays do you mean?

You have to remember that there are — for the purposes here — three types of thermostats. Those which are simple switches and require no power. Those which have some digital electronics, but are battery powered (they often have a relay built in to provide the on/off thermostat contacts) and those which have more electronics, particularly wi-fi or similar capability and which require constant external power to operate (this category would include Nets and Ecobees and the like). These usually do NOT have real relays, but instead solid state switches (usually triacs) instead.

Now a boiler usually has 24 volt AC power for its various trim and safety devices. It is very tempting, for the third class of thermostats above, to use that same power supply — but usually the boiler transformer (or zone panel transformer) does not have the capacity to run the thermostats as well, since they take a good deal of power. In those cases a separate power supply is required for the thermostats. In most instances this is not a problem — but it is possible, in some installations, to get the separate power supply wired in such a way as to burn it, the thermostat, or the boiler electronics out.

To avoid that, use an isolation relay…

Looking at the Burnham Independence manual, it seems they used a relay to solve a few awkward situations with the probe type LWCO and other limit devices. If the thermostat was the very left most device (connected to the transformer) on the control and limit device string, when there is no call for heat the LWCO would shut off. Also with the actual thermostat connected directly to one side of the transformer and operating a relay's coil the thermostats that are parasitically powered by the boilers transformer will not loose power if a limit device opens. Using a relay resolves these issues.

Much the simplest way to add a smart thermostat to a steam boiler — or indeed most hot water systems — is a separate power supply for the thermostat or thermostats, and a simple (and cheap!) relay for each thermostat to send the heat signal to the rest of the system, particularly if it is an existing system.

@eliasjj12 , You are correct with your assumption on a 24 VAC operated Steam Boiler. As long as you have a simple 2 wire thermostat you do not need a relay. you just place the thermostat in the series circuit to break the line of limit controls like the LWCO and the Pressure high limit and the pressure operator control(s) The problem with many of the steam boiler OEM wiring diagrams is that the Thermostat is not the first or last in line with a relationship to the transformer. This throws a monkey wrench into the gear works of the smart thermostat that requires a constant 24 volt power supply with a common (third) wire back the the transformer.

Are you attempting to add a smart thermostat to your steam boiler? (or your customers steam boiler? if you are doing this professionally)

If the answer is YES then you have to safely rewire the OEM wiring to accommodate the thermostat to be the first or last "Limit Switch" on the series of limits that include the LWCO, the Pressure switches, any spill switches or roll out switches and the safety switch that is part of the automatic vent damper.   The problem with that idea is that the probe type LWCO and the Auto Vent Damper that require constant 24 VAC to properly power their circuits, also require that they be the first or last on the limit circuit in order to properly operate.  Since there are only 2 places that can be either first or last, that leaves no place for the smart thermostat that requires a Common wire back to the source (24 vac transformer)  

So in order to make the thermostat that requires a C wire to work properly, you add a transformer and relay to the thermostat circuit in order to leave the OEM wiring as is and use the relau NO contacts to do the job of the thermostat while the actual thermostat can be wired as needed to operate properly.  

Hope this helps to relieve any confusion on why there are so many RELAY posts on steam boilers for thermostats.  Only on Smart thermostats that require a C wire to be connected. 

YOU DO NOT NEED A RELAY ON A STANDARD 2 WIRE THERMOSTAT 

Here is a typical Steam boiler OEM wiring diagram. See how the thermostat location is NOT connected directly to the transformer? If you connected the smart thermostat in the recommended location and connected the R to the side with the LWCO between the transformer and the thrmostat, when ever the LWCO did it's job as intended, the thermostat would get no power to the R terminal and that would cause an E72 or other code that indicates No Power to R terminal. that may cause you to need a reset in person to get the system back up an operating. If you are on vacation skiing in Vermont of Colorado when that happens, you may come home to frozen pipes.

I think we may be a little confused here… the Honeywell T87 mercury thermostat is close to bulletproof, and very unlikely to fail. It only takes two wires. Now the question is, what do those two wires connect to at the boiler? Do they go to some sort of relay box, or do they go directly to the T-T contacts on the burner control?

Perhaps some photos of the controls on the boiler might help here…

Taking time to look as some basic electricity YouTube videos will help in your understanding of electricity and how it works.

Many plumbers and pipe fitters can get to understand how electricity works by using plumping analogy for form basic circuits. Like on a flashlight there are the three basic parts of the circuit. the Source, the Load and the return path. Without at least those 3 basic parts you have no electric circuit.

Source could be a battery in the flashlight. or the boiler in a heating system. The load is the light bulb in the flashlight or the radiator in the heating system and the return path is the wire that goes from the light bulb back to the battery in the flashlight and the pipes are the return path on the boiler system.

The tough thing for the plumbers to get around is the difference between the open valve and the open switch. the open valve allows water to flow while an open switch stops the electric from flowing. Once you get over that hurdle electric is easy as piping in a radiant floor system with 4 different temperatures and an indirect water tank.

The relay probably has a part number on it that could be researched and options developed.