2 thermostats on one input on a TACO SR504??

OcrRzr

I'm an idiot, so keep that in mind please.

First the short, bottom-line question: can I connect two thermostat inputs to a single input/zone on a TACO SR504?

I'm adding two hydronic unit heaters in our basement. We have one extra output/input on our manifolds, so the plan was to use a single new circulator pump to deliver hot water to both units on a single loop (i.e. hot water will run thru both units in series). So I need to turn the same circulator pump on with two different thermostats. Possible? Dumb?

Thanks in advance

EdTheHeaterMan

Do the thermostats connect to the unit heaters, then the unit heaters operate the pump?

OR

Are the unit heaters in separate rooms?

OR

Are you hoping for an average of the two thermostats set points to be the time when the unit heaters operate?

OR

Is there a different reason for such a request?

The answer is YES you can connect two thermostats or even 10 thermostats to one zone on a Taco SR-500 series zone control. Just so you know it will only take one of them to operate the zone pump of they are wired parallel, both must be calling if you wire them in series.

GroundUp

Sure, but why? If the heaters are going to run together, there is no sense in having two separate thermostats.

OcrRzr

Thanks @EdTheHeaterMan & @GroundUp. How SHOULD I wire them?

- The heaters are in separate rooms with their own thermostats.
- The hot water will run thru both heaters, but the fan should not run unless the tstat is calling for heat.
- I have aquastats to put on each heater to delay fan until water is warm.

To complicate things a bit I am using Nest tstats, and the unit heaters are Modine Lodronic with 120V tstat control. So I have (2) 120V to 24V relays (Aube Technologies RC840T-120).

What outputs on the thermostat should I use and in what order should I wire the heaters and SR504?

EdTheHeaterMan

OK got the picture now. you will want the use the 24 volt NEST to operate a switching relay like the Taco SR501 attached to each unit heater. The Taco SR501 has 2 sets of NO contacts. One set of contacts will bring the fan on in series with the Aquastat that will prevent blowing cold air. The other set of contacts will bring on the circulator pump by connecting the that zone thermostat terminals on the Taco SR504.

Diagram to follow

By the way, the Aube Technologies RC840T-120 is the wrong relay for your situation.

EdTheHeaterMan

You will need two of these: https://www.supplyhouse.com/Taco-SR501-4-1-Zone-Switching-Relay

The relay you mentioned has only one set of contacts to operate the fan on the Unit Heater. there are no extra contacts that will operate the TACO SR504 zone
The TACO SR 501 has the needed 24 VAC R, W and C terminals you need and it has the necessary contacts to turn on the 120 VAC fan motor and to operate the 24 VAC Taco SR 504

GroundUp

What Ed said.

OR, if they're close enough to each other, a separate SR502 switching relay near the unit heaters with the 120v outputs running your fans with the XX end switch being your "thermostat call" to the SR504 which would fire the boiler and spin the pump.

OcrRzr

@EdTheHeaterMan you are an absolute legend! Thank you!

OcrRzr

@EdTheHeaterMan clarifying question: the output on terminals 5 and 6 on the SR501 (to the Sr504 to turn on the circulator pump), is that 24V or 120V? I can't find a reference for that in the manual...

EdTheHeaterMan

5 and 6 are dry contacts that have a rating from 0 to 240 volts up to 12 amps. on some models up to 20 amps of those contacts

Think of 5 an 6 like this: a simple switch from home depot that you might connect to a wiring system in your home to turn on a light has no power in itself. if you connect that switch to you home 120 VAC circuit to operate a light or fan in the ceiling, that switch is operating a 120 VAC circuit

If you put that same switch on a 1.5 volt battery to turn on a flashlight bulb, then that switch will be operating a 1.5 VDC circuit. Same switch different volts.


The thermostat connected to the Taco SR501 is like your finger operating the light switch. The Thermostat and the Relay connected to the internal Transformer create an automatic action based on temperature to act like your finger turning on the switch 3 to 4 to run the motor and 5 to 6 to operate the boiler

I have attached a page from the Taco SR501 brochure with the ratings for the load contacts (in Red) and the Voltage (in Green) for your observation and to keep on file if needed.

OcrRzr

Thank you @EdTheHeaterMan. Somehow I'm too dumb to grasp this. I get the switch analogy, but which voltage is being switched? It's the 120V input, not the 24V thermostat input, right? Just wanting to make sure the Zone 4 inputs on the SR-504 can handle the 120V in. Don't want to release any magic smoke...

EdTheHeaterMan

5 and 6 on the SR 501 is switching 24 volts to run the zone on your existing SR504. 3 and 4 on the SR501 are switching 120 VAC for the fan. The contacts can handle both 120 and 24 volts.

The 24 volt thermostat contacts CAN NOT handle 120 VAC. that is why you are going to use the 5 and 6 on the SR501. Those contacts are not connected to any voltage from anywhere

EdTheHeaterMan

You need to look at each piece of the puzzle separately to understand what is happening. In the Taco SR501, the 5 and 6 are not connected to anything except for the contacts shown on the wiring diagram. I have faded out everything in this close up of SR 501 for the one fan coil so you can clearly see that 5 gets connected to 6 and no other parts of that control are going to add or subtract any other power. It is just a switch that is controlled by something other than your finger.