Relay box hooked into another relay box?

the current system is radiant? Or?

The additional system will be the same temperature or mixed down temperature?

Most relay boxes have communication connections for linking more than one together you can mix and match pump relay boxes with zone valve relay boxes

Piping and any additional pumps need to be consider, primary secondary for example

If the current setup has 3 zone pumps connected to a zone pump relay box you will need an additional zone valve relay box to oower two zone valves and the pump for those zones

If you have a box with communication connections connections buy the sane brand as a zone valve version

Or the TT connection from the current and the new box could call on the boiler

Bottom line, a new zone relay box gives you the transformer, pump switching relay, and connections for thermostats and zone valves. It is the simplest option

You said it is one zone currently. Why are there 3 zone circs? If you were indeed going from one zone (circ) to two (valves) as you said, you do not need a switching relay at all.

Is it possible you have a system like this that was built with future expansion in mind.

This would be a 3 pump, single zone example.

So there is a combi Mod-Con boiler that does both heat and DHW. It is currently operating with a low temperature for radiant floor space heating on 3 for 4 zones. The hottest water for space heating won't get much over 125°F. If the outdoor sensor is connected that temperature will ramp down to as low as 95° on milder days. You want to add a radiant floor zone the the unused circulator that is already piped in and connected to the boiler.

If that was the end of the story, then you have no problem.  There may be a little overheating as a result of the two different radiant floors… Staple up and concrete slab, but that is nothing compared to your additional zone requirements.  

In order to add a standing radiator, a panel radiator or a baseboard radiator or convector radiator to that system you will need a really big one if you don't get the water temperature up to 160° or more.  That means that you will need to have some sort of mixing valve or injection pump system to make 160° water that leaves the boiler down to the 125° or lower. You need the lower temperature for the system as it is operating now because that works.   Look at the water temperature output for simple baseboard radiators when you have only 120° water compared to the recommended 180° water temperature.   You will need at least 30 feet of radiator to do what 10 feet of radiator will do at 180° if you use the low temperature water.  Slant Fin Base/Line 2000

You need to rethink this idea, or get a really big radiator for the really small space.  

I had a customer request the same thing from me.  “Just connect this cast iron radiator in the shop to the radiant heat system you are installing so I can get some heat in there in the winter."   After telling him it won’t work multiple times, and trying to convince him, the additional $$$ for that project would be wasted money, I ended up doing it because that is what he wanted… and the customer is always right.   He called me after the first year and told me I was right.  Sometimes you need to tell them ten times and when they insist and you do it for them, and  get paid for it, you just have to wait for the phone call telling you that you were right. 

Everyone is an expert when it comes to their house. They know best until proven wrong (and paying $$$ for it).

Good luck with this project.

I can do the wiring diagram for you if they insist.

Is the radiator addition to supplement the staple up at the entry? If so a cast radiator will give off some heat at radiant SWT

That is some nice work in the mechanical room.

Very professional to see an actual balance valve on a recirc system, BFD with an actual vent tube. Good taste in hydronic components 😄

You could also master and slave the relay boxes with the communication connections.

If you pair two relay boxes together I would suggest using the COM connections. A 3 conductor thermostat wire between them. With Caleffi you can "daisy chain" an unlimited amount of relay boxes this way, of any variation ZVR (zone valve) or ZSR (zone pump)

The advantage to this is you dont tie up a relay, assuming you have a spare. A spare is always nice to have should one relay ever fail, especially with the soldered on relays.

But also by making the first box "master" any slave added to the first box will reconize priority on box 1.

An indirect or any load on relay 1, with priority triggered on, all loads on the slaves, valves or pumps, will turn off until priority is satisfied. Or the priority could be toggled off via the dip switch.

Priority relay is always relay number 1 in Caleffi boxes.

This also shows the 40Va transformer in the pump relay box, and it is protected by a self resetting fuse, the yellow disc next to the molex plug.

These features and functions are a result of contractor input when we first designed these relay boxes.

The snap in 40 Va in the zone pump relays was added by installer requests also. Two transformers can be installed for a total of 80Va across all connections on the ZVR zone valve relays. This can be helpfulmif you want to put multiple thermal actuators on one zone connection.

The 6 zone box comes with two relays. Theses are standard foot mount type transformers with molex plugs.