installing Tekmar 256 to a weil mclain cg5PI boiler

T0nesea

Thanks in advance, Im a homeowner trying to install the 256 to my boiler, its a single zone unit,

slots 1-4 are straight forward and complete, slot 9 also is good with 24v.

Now with 5,6,7,8 I'm have a tough time where these need to connect.

5&6 boiler control ? is that the circ pump relay?

7-T ?? for thermostat? one diagram shows it connected to 9?

8 is r+ is that also from thermostat

any insight would be helpful, my big issue is terminology of parts

I cant get thru to anybody online

Tonesea

EdTheHeaterMan

I just helped someone with this yesterday. they also had a Weil McLain (but it was steam)

Give a minute to look for that wiring diagram

EdTheHeaterMan

If you have an indirect water heater, then there are some more wires to connect.

If you have an indirect for DHW, is it controlled by a zone valve or by a separate circulator?

T0nesea

Hi Ed, this unit is different than my 256, if the boiler connects are the same where am i connecting from 256 to where on boiler?

EdTheHeaterMan

OOPS. that is a 260 not a 256. Sorry

Let me look into the 256 for you.

EdTheHeaterMan

OK here is the diagram and there is one thing that gets most folks confused.

The C from the transformer needs to be connected to both the R on the thermostat and the C- on the Tekmar for this control to work. I don't know why, it just is.

And that makes it very confusing when you need to connect a smart thermostat that requires a C terminal for the thermostat because the ends up connecting to the R on the transformer.

T0nesea

Thank you!…1 more question your showing the "boiler output as T/T at the boiler…what does T/T stand for ?

quick note this whole Tekmar thing started when I upgraded to a "smart" therm!! lol

Thank you

EdTheHeaterMan

When a boiler system is prewired as a package unit the thermostat gets connected to the terminals T and T on the system or control.   T and T are assigned the thermostat terminals for a 2 wire thermostat. It is the most basic wiring diagram from the earliest days of automatic heating. 

When you add something extra to a boiler system like zone valves or outdoor reset controls then the thermostat(s) will get connected to a different control so that other control gets the call for heat signal from the thermostat.   When that other control gets a call for heat, that other device then does the job it is intended to do.  But the heating boiler does not get the call for heat from the thermostat anymore because it is going somewhere else like the outdoor reset control or the zone valve.   

So what tells the boiler to operate if the thermostat is no longer doing that job?  Usually that falls to an end switch of some kind that will get connected to the place where the thermostat would have gone if it was only one simple zone… that is two wires or two terminals that have the thermostat designation T  T   

Does that help explain the T T on the boiler?

EdTheHeaterMan

You made mention of using a smart thermostat. Are you attempting to connect a smart thermostat to that Tekmar 256? you have to be careful of the C wire because it is different on the Tekmar than it is on the smart thermostat as I mentioned in my comment from November 1

T0nesea

Thank you!

..and yes I already installed a smart thermostat. So the 256 cant or wont work with a smart thermostat? or is there a work around so they both work?

EdTheHeaterMan

It can work, it is just a little confusing because you use the C on the Tekmar terminal to connect to the C on the transformer. But you use the C on the thermostat terminal to connect to the R on the transformer. Counterintuitive but it works. Here is the diagram.

The other option is to wire the thermostat correctly and put wires on the Tekmar backwards. Either way it is going to be confusing to someone at some point.